OF 1944 - Earth and Planetary Scienceeps.berkeley.edu/people/lunaleopold/(008) The California... · 2012. 10. 30. · OF 1944 Morrie pelburger Assistant Professor of Meteor University

THE CALIFORNIA STRATUS ~ ~ T ~ G A T ~ O ~

OF 1944

Morrie pelburger Assistant Professor of Meteor

University of California at Loa

Charles G.P. Beer l e t Lieutenant, Ar

Luna B. LeopoId Ist Lieutenant, Army A i r Forces

A Report on a Cooperative Research Project of the U. S. Anny A i r Forces, U.. S. Navy,

U. S, Weather Bureau asld University of Cal i fornia a t b e Angelee.

I

w i n t e a '$9 the U. S . Department of Co

Waehington, De @. Apri l 1945

Weather Bureau

TABLE OF CO1ocE14Ts

List of Figure8

List of Tablee

sunrmasy

Introduction. . . . . . . . . . . . . . . Page 1 Section I Relation between ltratue & the Temperature Structure

o f t h e A i r . . . . . . . . . . . . . . . 21 I1 Dletribution of Invereion In Space . . . . 34 I11 Diurnal Variation of Invereian . . . . . . . . . 41 IV The Structure of the Sea Rreeze in the Im Angeles

Bmin. . . . . . . . . . . . . . . . . 55 V Variation of Mixing Ratio 66 . . . . . . . . . . .

V I Day t o Dsy Variation6 of the Teaperature Structure and the8ccurrsnce of Stratus . . . . . . . . . . 7 1

VI1 Forecasting Implicatione of the Stratus Inveetigatlon. . 80 . . . . . . . . . . . . . . . . . 02 Conclusion

Acknowledgement . . . . . . . . . . . . . . Bibliography . . . . . . . . . . . . . . .

a3

84

l l i t

Figure 1. b c a t i o n Map Showing Raob and. Ravln Statio- e . . I Page 2 opography of the s h g e l e s Basin . . . . 4

t i c T 1Soundings e e . . e . . ., . 6 Section, 30 August-8 September . . e e e . 10

i a Time Section, 30 August-8 September e e . . . 12 7, C a l , Tech, Time Section, 30 August - 8 September . . . . 14 8, $ants Ana Time Section, 30 August-8 Septermber e . e e . 16 9, San Clemente Island Time Section, 30 August-8 Septermber . . 18 10, Frequency of Observed Relative Humidity in Stra tus Clouds

OB Recorded by the Radiosonde. . . . . e . 26 11. Frequency of Observed Relative Humidity In Heavy Surfme

Fog a t UCIA. e e . , . . . . . e . . 26 12, Frequency of Relative Humidity i n S t ra tus at Observation

T i m e s . . e e . . . e . e . . . s . . . . 28

13, Xllustration of Processes Changing Temperature of Inversion B a s e . I e a e e e e . e . e . . . . * . 3 2

14. Average Crosa S ~~~~~ of the Inver8ion Bme normal t o Coast. . 36 I I 3

ion Baee Over the Atlantic . e . . 3 6 Inversion Base Along Coaet .) e 37

38

f the Inversion Base and Top. a e 42

ighte of the Inversion Base During t he ship Period . e e e . . . . . . . 46

20, Resul tmt Height of the Inversion 13 e After Subtpaction of Eefght at the Ship e a . . . . . . . e e 46

2 1. uency of Occurrence of hum an& Minimum Heights of' Inversion Base a t Observation Time. e (I . e . . e , 4'7

22. Frequency of Various Heights of the Inversion Base byHours . . . . . . . . . . . . . . . . . 48

23. Effects of various Processes on Inversion Heights . . . . 51 24. Average Profi les of Inversion Base From Ocean Inland

(Ship Period) . . . . . . . . . . . . . . . . 52 25. Wind %wee shrming Direction Frequency During August 1944 e . 56 26. Direction and Duration of Land and Sea Breeze, Southern

Cal i fornia Coast . . . . . . . . . . . . . . 57 27. Wind Time Section, 23 August-14 September . . . . . . 58 28. Prevailing Diurnal Wind Regime . . . . . . . . . . 63 29. Schematic Picture of Sea Breeze Regime . . - . e e 64 30. Mixing Ratio Time Section, UCLA and Santa Ana . . . . . 67 31. Mixing Ratig a t Surface and Inversion Base. . . . . . . 69 32. Frequency of Occurrence of Various Mixing Ratio Values at

Ground and Inversion Base, UCLA and Santa Earia. . . . . 70 33. Day-to-Day Variations of Inversion Character is t ics . . . . 73

75 34. Winds and Pressures Aloft . . . . . . . . . . . 79 35. Occurrence of S t ra tus a t Six Stat ions . . . . . . .

LIST OF TABLES

Table le

2,

3.

4,

5 .

6 .

7 .

0 .

9.

List of Upper Air Obeeriations . . . . . . . Page 9 Aircraft Obeervatione of Cloud Top and Inversion Height . . . . . . . . . . . . . . . .

MaximLoa Relative Humidity in Soundings through Stratus . Mean Lapse Rates in Marine and Inversion Layers . . . Average Percentage of Marine Iayer Occupied by Stratus

Average Celliags for Various Dew Point Depressions . . . Frequency of Occurrence of Ioweet Inversion . . . . . Diurnal Variation of Inversion Base . . . . . . . . Diurnal Variation of Invereion Top , . . . . . . . .

.

22

24

27

29

30

40

44

84

V i

SUMWLRY

To study the fac tors t o be considered in forecasting the ce i l i ngs In r s t r a t u s over Southern California, a Close network of upper air sbeerv- ta t ione wae established during 1944. Four-hourly radiosonde .observations

ere taken at f i v e s ta t ione i n the area, and for a shor t period captive balloon soundings were obtalned from a sh ip j u s t off the coaet. These obeervations were augmented by airplane SO~nding8 and upper wind observatione. of the upper air observations is given i n Table 1, and. the locatlone of the s t a t i o n s shown In Figures 1 and 2.

A summary

Study of the data ehorred tha t the s t r a t u s top is a h o a t invariably a t the base of the s t rong InVer8iOn which is preeent over the region throughout t he eummer (Table 2). considerably below lo*, 90$ being the most frequent maximum value In s t r a t u s over t h e Ins Angelee area, 118 shown i n Table 3 and Figure 10. from lo@ Is a t t r ibu ted t o the presence of hygroecopic nuclei , due pr incipal ly t o Induetr ia l pollution.

The r e l a t i v e humidity in the clouds w a s frequently

The departure

The lapse rate i n the marine layer below the invereion var ies from s l i g h t l y sub-adiabatic at night t o super-adiabatic i n the &ternoon (Table 4). posi t ion of the cloud base le determined by the temperature and mixing r a t i o of the marine layer. baee is lowest and thickneee grea tes t (Table 5). sion le cloeely related t o the ce i l i ng height, except f o r the higher c e i l ~ e (Table 6).

The

A t night, when the marine layer is coolest , the cloud The surface dew-point depres-

The lnvereion, on the average, 8lOpe6 upward from the coast toward both but ehms no d e f i n i t e slope i n a direct ion land and 886 (Figure6 14 and 17)

p a r a l l e l t o t h e o o w t (Fi-e 16s until late In the surmer. a definite d iurna l oec i l l a t ion which var ies i n a d i rec t ion normal t o the ehore (Flgures 18 and 19). Over the sea and near the ehore the inverelon is higheet in the mrning and lonest i n th8 evening, while inland I t 8 maximum height occurs in ths aftarnoon. the sea it l e caueed primarily by the divergence of t he diurnal wind oeci l la- tion, t o which I s added over land the influence of Ineolat ional heating and the advection of changes in inversion height due t o i t e slope.

Its height show6

Analyeie of the d iurna l var ia t ion showed that over

Study of the diurnal w i n d oec i l la t ion In the region indicated that the d-sea breeze at the shore merges w i t h the valley-mountain wind t o form

continuous flow (Figures 25 add 26). t h that of the theo re t i ca l model, w i t h the land breeze aloft persisting ter the eea breeee sets in at the ground t o form the counter current t o it,

Its observed v e r t i c a l s t ruc tu re agrees

vice versa (Figures 27-29) . The diurna l wind osc i l l a t ion is not limited t i c a l l y by the lnvereion. Both the sea breeze and the land breeze start

e t r a tus at UCIA the land breeze had reached the inmn%lon before s t r a tua ear t h e ground and increase in thickness w i t h time. In about half the cme8

Ooaurred, so that advection could not

be responsible fo r its occurrence.

The mixing r a t i o shows some gradient from the surface t o the inversion bme and a rapid decrease i n the inversion layer, except t h a t occasionally high mixing r a t i o is present near t he top of the inversion (Figures 30-32). High moisture content above the inversion baee is probably injected in to upper leve ls by heating near the mountains during the afternoon.

In addition t o the d inrna l o sc i l l a t ion of the inversion, t he re are marked var ia t ions i n its da i ly average height and other charac te r i s t ics (Figure 33). of s t r a tus , days with no s t r a t u s having lm inversions (Figure 35). inversion height is strongly correlated with the coast-inland pressure gradient. the pressure f ie ld .

The da i ly average height is associated with the occurrence The

Prediction of pressure gradient is dependent on forecast ing of

Since the pressure changes i n Southern Cal i fornia i n summer a re small and subtle, understanding of the mechanism producing them I s necessary t o enable one t o forecast them. The hypothesis t h a t they are waves in the southerly f l o w a lof t seems inconsistent w i t h the observations. Further investigation of t h i s question is essen t i a l t o t he solution of the problem of s t r a t u s forecasting. For the present, kinematic methods using pressure tendencies or 24-hour pressure changes must be used.

From the forecast of the pressure f i e ld , t he t r a j ec to r i e s of the air which w i l l reach the s t a t ion may be evaluated, and the s t ruc tu re of the a i r deduced. deviations therefrom, together w i t h t h e re la t ionships between s t r a t u s and temperature s t ruc tu re w i l l yield a forecast of the s t r a t u s duration and cei l ing.

Consideration of the average d iurna l var ia t ion and probable

Further study of t he data is planned, with emphasis on f inding the causes f o r the deviations from average diurnal behavior, and devising methods f o r forecasting the t radector ies of air reaching t h e region and t h e effects of them on the s t ruc ture of the air.

Y i i i

INTRODUCTI(BI 1

The principal forecasting problm on the California coast d u r a the This summer months is the predi0ticm of ce i l ings bue t o coae ta l s t r a tus ,

s t r a t u s is present prac t ica l ly every night along most of the coast, but In Southern California it m y be absent at any one statim f o r periods of severa l days. It usually is observed over land before midnight and d ias i - pates during the morning. Although s t r a t u s is seldom responsible f o r accidents s ince a l t e rna te airports a re p rac t i ca l ly alwap avai lable inland, it const i tute8 a serioue r e s t r i c t i o n t o aviation, and considerable econamic advantage would r eeu l t if it were forecse t more mcurately.

This type of coae ta l stratus is present i n other parte of the world where anSlogouf3 geographic s i tua t ions prevail , namely the w e s t coaete of continents In subtropical la t i tudes . For example, Cseablanca haa c a d i - t ion8 similar t o b e Angelee and the heavy air traffic there is affected in l ike fashion. thus would have applicaticme in various areas thraughout the world. more, the some physical f ac to r s which p h y a role i n this phenomenon are present i n d i f f e ren t combinations in other types of s t ra tue , such as the "Gulf'" s t r a t u s which is frequent over t h e Southesetern states. Analyeis of these f ac to r s in Cal i fornia may contribute t o the understanding of their ro les i n other regions, and thus be of a id in forecast ing wherever 8 tr atus w c UT-

Solution of the loca l problem in Southern Cal i fornia Further-

In order t o study th ie problem, there w a s established during the summer of 1944 what mrry be ca l l ed a three-dimensional micrometearolagical network i n the U s Angelee area. The cooperation of the Weather Bureau, the Navy Bureau of Asronautlcs, and t h e Army A i r Forces Weather Dlvision md Weather W i n g wa8 enl i s ted by the h i v e r s i t y of Cal i fornia Department of Meteorology through t he Joint Meteorological Camittee. Weather S te t ion on the b e Angeles cmpw of the University of California became the center for thes co l lec t ion and processing of the data accumulated

The A.A.F.

by the cooperating - agencies. - This mlcrolaeteorological system was probably the c loaeet network of

serologioal retations eetabliehed t o date, takins obeematione at amal l in tervale I n t h e and epace. deelgned f o r large male phencmena may be used t o take such obaervations, they barely give su f f i c l en t detal1 and accuracy far a mlcr~etaorologioa~ study. The Investigation brought out the Inadequacy of exletin8 method8 to give a quick, precise, orid detailed evaluation of the temperature and b d U 1 t y s t ruc tu re in the lowest two kilormeters. was, it did not eliminate the necessi ty f o r interpolat ion and interpre- t a t i o n in determining the changes which curred In the free atmosphere over th ie mea. clarification of the complex proceesee at work during the s t r a t u s seaeon In Southern California.

It waa found that while the inetrumente

Close aa t he network

Nevei!theless, the observations have reeul ted i n a

2

Sania Barbara Islands

I

-- 1 -

Legsnd Roob Stulion Rouin Station

0 Rlmdole 0

Victorvilla

Weather Ship

San Clemente -1 Is. 118 L NZY Son Diego I Arms Abavtl S,~l,oOo' I20 Elavotion L

FIGURE I

LOCATION MAP

G RAOB AND RAWlN STATIONS

3

The portion of the California coast concerned in t h i s study is that extending from 35"N. l a t i tude t o the Mexicen border. accompanying map (Figure l ) , the coastal p la in here forma an irregular crescent bounded by high mountain r q e e . The crescent l e widest at b e Angeles w i t h horn8 pointing w e s t at Pt. Arguello, 34-1/2 degrees IV., and south at San Diego. A t Pt. Arguello the shore makes an abrupt bend frum east-nest t o south-north, with the plain becoming broader again t o the north i n the v ic in i ty of Santa Maria. Within the arch formed by t h i s coastal plain me re la t ive ly shallow coas ta l waters w i t h 8 nmber of mountainoua islands, including the Santa Barbara or Channel Islands, Santa Catalina, San C l e m e n t e and San Nicalae Islands. The detailed topography in the vicini ty of b e Angeles is ehown in the relief model photograph (Figure 2).

As shown in the

The abrupt turning of the coast at Pt. Arguello, the preeence of the isl&nds, and the extended region of shallow water influencing the water temperature dis t r ibut ion aXo= the coaat, a l l contribute t o diff erent ia t iag the behavior of etratue M thie region frm chat farther nerth. Wllhweag in Morthern California the s t r a tus mby be treated primari phenomenon, dynamic effects are predominant In South study of round-the-clmk Sounding6 during three short periods in 1943 tl*l showed that dynamic heating and cooling due t o ve r t i ca l motion could explain changes in the temperature structure shown i n eucceesive soundwe during the occurrence and dissipation of etratue which advectAon could not

an advmtive

eXphin.

To understand the nature of the strat- problem i n t h i s region, one should keep i n m i n d the average circulation pattern over the eastern North Pacific Ocean. During the summer the surface wlnds are predaalnan t h t o northweet. (1) Together w i t h f r i c t iona l outflow from the high preseure region and the acquisition of anticyclonic curvature, the southward motion cawee the air Co~umns t o eubside and thus be subjected t o dynamic heating. motion across warmer water causes some heating from below and the establishment of M adiabatic lapee rate, with accompanying dietribution of the mter evaporated from the sea throughout the adiabatic layer. The eubsi- dence produces an inversion of conelderable magnitude which, except f o r the heating below, would extend down t o the sea surface. below is emall close t o the c o a t , because due t o upwelling the sea surface Isotherms are oriented north-south, with a w a t e r temperature minimum

t t l e north of Sen Pbancieco (Figure 3, averqp sea surface isotherme eastern Pacific during July, August and September). Thus the northerly

current C O n S i S t 8 of a cool moist layer w i t h nearly adiabatic lapse rate, ted by a lqyer of lower m 0 1 6 t W ~ 9 content i n which the temperature

This movement of air from higher la t i tudes ha6 two

(2 ) The

The heating from

cremes rap,idly w i t h height {Figure 4). The adiabatic layer is usually although the inVersion layer is irequentlg also

lght of the inversion is meant height of the base Bi

it Numbered references appear a t the end.of the a r t ic le .

5

a cn

c a 3 0 3 a

h 3 -a

-

u- 0

6

6 8 IO Mixing Ratio - Gms/Kg 20 Temperature - OC

DI AG R A MM AT I C TYPIC A L SOUND IN G

,

FIGURE 4

When t h e winds dxle eaet of' north, the air comes down off the mountain- ous continent, and is i n i t i a l l y w a r m e r than the sea surface, so that the subsidence Inversion is augmented by contact cooling and the marine stratum is absent. I n t h i s case the inversion height is zero.

Due to t he strong land-sea horizontal temperature gradient the wind changes with height from northerly t o southerly. a t upper levels the l a t i t u d i n a l e f f ec t tends to produce horizontal convergence ra ther than divergence, and the r e s u l t is t h a t t he subsidence decreases with height and a t su f f i c i en t ly high levels a s teep lapse rate obtains. interested in the Inversion top and the layers above only i n so f a r as they yield clues t o the behavior of the m r i n e layer and the inversion base. Briefly, it may be stated t h a t s t r a t u s w i l l occur whenever the temperature a t the inversion base is su f f i c i en t ly low that a parcel having the mixing r a t i o charac te r i s t ic of t he marine stratum w i l l be saturated. Such a Pow temperature w i l l occur when (1) the temperature of the en t i r e marine stratum is low, o r (2 ) the Inversion base I s high enough so tha t the temperature decreases w i t h height t o the necessary value.

In the southerly current

As t he s t r a tus occurs below the inversion base, we shall be

The 1943 soundings re fer red t o above showed t h a t the thickness of both the marine stratum and the inversion layer exhibi ts rapid f luctuat ions w i t h period of the order of one d as w e l l ae longer period trends. It w i l l be appreciated from the statements of the previoue paragraph t h a t these f luctuat ions are of grea t importance i n the occurrence of stratus. "he study of the 1943 eoundinge indicated that the. f luctuat ions m i g h t be due t o t h e convergence and divergence of the sea-and-land breeze.

In view of the above diecussion It is c l ea r that knowledge of d iurna l behavior of t he inversion baee is fundamental f o r forecasting t h e occurrence of s t ra tus . It waa t o f ind the character of t h i s diurnal variation, and t o discover t h e mechanisma which control it, t h a t the 1944 investigation was undertaken. With the ul t imate objective of deriving methods of forecasting s t ra tus , t h e immediate purpoees were (1) t o observe the d iurna l changes i n t he three-dimensional temperature f i e l d over the coas ta l area; ( 2 ) t o check the r e l a t i v e ro les of t he various fac tors which contr ibute t o those changes; (3) t o examine the re lat ionehip between changes in temperature s t ruc ture and the occurrence of s t r a tus , and (4) t o invest igate methods of forecasting the changes in temperature s t ructure .

To accomplish these purposes, radiosonde observations were taken a t eeveral points, varying i n distance from the coast and at varying posit ions along it. The period of detailed observations was 17 Ju ly t o 30 September 1944. personnel as w e l l (18 t he question of cost limited the number of radiosonde s ta t ions t o f i v e regular ly par t ic ipat ing ones, two of which s t a r t ed obeerva- t ione late i n the period. In addition t o the radiosonde data, airplane observations were taken at a f e w s ta t ions , including some very valuable reconnaieeance f l lghte, and captive balloon soundings were made 'by the Navy Radio and Sound Zsborstory from a sh ip a few miles off the coast near Oceanside. observations e

Prac t i ca l d i f f i c u l t i e s such RE the ava i l ab i l i t y of equQment and

Figure 1 shows t h e location of s t a t ions taking upper air

8

Besides the temperature soundings, upper air wind observations were considered of paramaunt importance. Rawln s t a t ions were act ivated and maintained at Santa Maria, San Diego, and U s Angeles Municipal Airport, fo r most of the duration of the proJect. The regular p i l o t balloon observations in t he area contributed 8 0 m informatian, and a special series of slow ament p i l o t balloon observations a t frequent in te rva ls were r& by the AAF Weather Stat ion, University of Cal i fornia a t Ins Angeles, giving d e t a i l s regarding t he s t ruc ture of t he sea breeze.

Table 1 gives a summary of the upper air observations made In connec- t i o n w i t h the program, or avai lable t o it.

The great mms of data accumulated during t h i s br ie f period over t h i s r e l a t ive ly small area w i l l be appreciated by hot ic ing tha t the number of temperature soundings alone exceeded 2000, approximately ten times those col lected In the Meteor expedition [2]. checked, had t o be represented in ways which would permit t h e i r ready interpretat ion. of the var iables were too small t o give much value t o local constant leve l charts. I n addition, technical d i f f i c u l t l e s prevented simultaneous obeervations at a l l s t a t ions much of the time, so t h a t cross sections could be drawn f o r only a f r ac t ion of the synoptic times, and contour maps of the inversion base f o r even fewer t i m e s .

A l l these data, a f t e r being

It was found tha t f o r the most par t the horizontal gradients

The pr inc ipa l forma of representation of the temperature sounding data, in addition t o the adiabatic char t s of t he individual soundings were (1) temperature time sections f o r each s t a t ion showing the var ia t ion of Inversion height, temperature, and ce i l i ng height by Isopleths with time as abscissa and height as ordinate (see Figures 5-9); (2) humidity time sect ions showing mixing r a t i o l ines and inversion base and top in the sme way aa temperature wae shown in (1); (3) superimposed sounding eurves e i the r f o r a single s t a t ion a t various t i m e s o r f o r two more more s ta t ions at one time, f o r comparative purposes; (4) thermograms f o r various levels for each s t a t ion ; (5) ~ c a t t e r diagrams t o test the correlat lon of various feature8 at one s ta t ion , o r of t h e same fea ture at d i f f e ren t s t a t ions ; (6) averages f o r s t r a t u s and nonebetratus periods for each synoptic time. In addition t o these, cross-sections along and normal t o the coast were dram f o r a l l synoptic times when da ta sufficed and lnvereion contour mape ware drawn f o r t he few times of complete coverage.

The local upper air w i n d s were represented in time section, and ale0 represented on those cross-sections f o r which they w e r e available. The prevailing d iurna l wind behavior wae summarized. t i v e effecte and the convergence i n the wind f i e l d were attempted.

Studies of the advec-

Three-hourly sec t iona l surface m a p were drawn throughout t h e period of the observations, and t he stratus d is t r ibu t ion shown by them campared and correlated with the wind f i e l d and the upper l eve l temperature etructure.

E

TABU I o LIST OF UPPER A I R OBSERVATIONS , ----- - ---- 9

RADIOSONDE OBSERVATIONS

C . I. T I , Pasadena- Santa Maria Santa Ana (began 14 August) San Clemente Is land (began 24 A u g u s t ) San Diego (twlce dal ly) Oakland (twlce da i ly ) ~ u r o c ( i r r egu la r )

CAPTIVE BALAOON SOUNDING3

NRSL Ship (near 33'N, 117" 3 O S W ) Oxnard (irregular)

AIRPLANE OBSERVATIONS

El Tor0 (one f llet dai ly) Mojave (17 J u l y t o 15 Aug. one d a i l y ) Oxnard (irregular) Reconnalssancs F l i g h t s Goleta

W I N D S ALOFT OBSERVATIONS

404 )Scheduled 3 7 1 ) s i x times 221 ') da i ly at 0000, 140 ) 0400,0800 etc., PWT. 152 152 69

63 (two eeven-day periods) 18

73 27

6 6

36

Regularly Scheduled Pibals (4 da i ly) .

Gasdner F ie ld Glendale March Field Burbank Ontario Mines Field (LA) S m t a Ana San Diego Oxnard San Bernardino Van Nuys Muroc Palm Springs Salinag lhng Beach U. C. La A.

t o 30 September) San Diego (23 July t o 30 September) Santa Maria (17 July to 22 August)

Slow Ascent Plbale U.C.L.A. (6-hourly d t e r 17 Aug., 90-mlnute intervals on selected days). U-Ce L A . , -C- I . T . ,- Ontario, Oxnard, and Van Nuys (90-minute in te rva ls for 24 hour6 5-6 September). (393 vbeervationd 'includkg rabals).

10

b D Temprutura 'F

I / R

12

I

i

P $ Temmrolurs 'F

1.3

Temparolure *F Elwotion In Thcusondi of Faet M.S.L.

14

Temperoiura OF Elevation In Thouaonds of Feat M.S.L

15

Tmporolurr *F

IC W n t 10 N - c z Elevolion in Thousands of Fed M.S.L.

16

Temparoture O F c W - - > ?Jzn

Elevation In Thourondr of Feat M S.L

7

6

2

Surface Dew Point ’

Surf ElOV.

905 I t 4 September 1944

FIGURE 98 SAN CLEMENTE 4 - 8 S E P T TIME SECTION

20

The larger sca le changes of c i rcu la t ion on the surface and upper leve l maps were a l so etadied t o f ind relat ionships with the var ia t ion of temperature s t ruc ture and s t r a t u s occurrence.

Not a l l these swrmarizations and etudiee have been canpleted i n time fo r inclusion In t h i s report. for analysis of the data have been suggested by s tudies already completed and much fur ther work remains t o be undertaken. ever) t o put in the hands of the forecaster a l l the information which so far has been gleaned from the data in time f o r use durlng the next s t r a t u s season, and t o make the data avai lable t o other investlgators.

Furthermore, addi t ional direct ions

It is desirable, how-

The complete repor t consiste of two parts; the f i r s t is a discussion of the malpis of t he da ta together with representations of some of them in t he form of averwes, ample time-sections, etc.; the aecond (Unpub- l ished) consis ts of the radiosonde data taken i n connection with the project w i t h a l l per t inent notes which are necessary t o render the data useful to the investigator. w i l l remain unpublished, and for t ha t reaeon no references have been made In the first pa r t t o spec i f ic data in t he second. are discussed, they are reproduced with the discussion.

It i e ant ic ipated that the second part

When such data

The organization of Part I may be gathered from the table of contents and the summary which precede this introduction. s t r a t u s and temperature and humidity s t ruc tu re is covered first; then the var ia t ion of temperature, humidity, and wind s t ruc tu re In space and time is discussed; and f i n a l l y the forecast ing implicatione of the r e s u l t s are presented

The r e l s t i o n between

In the discussion, some of the Stat10n6 axe referred t o by abbrevia- t ions ; these a re UCLA, standing f o r the University of Cal i forn ia& Los Angeles, in the extreme western pa r t of the c i ty , and Cal Tech or C.I.T., for t h e Cal l forn is I n s t i t u t e of Technology, i n Pasadena. Call letters f o r t h e s t a t ions are sometimes used i n the figures; these may be identi- fied either In the leGend or by reference t o Figure 1, Base Map ehowing Location of Stations.

All times given in Part, E of t h i s repor t a r e Pacific w a r Time (105th Meridian Time). it should be remembered t h a t local solas noon occurs near l300 FWT i n t h i s region.

In cor;lsiderlng Insolation or similar e f f ec t s

SECTION I. REIATION Bl?IWEXg STRATUS AND THE TEMPE8A!FURE -- The condensation of water vapor i n t o fog or cloud

2 1

STRUCTURE OF THE AIR --- is a l n a n due t o the

decreme of Its temperature below t h a t at whish the r e l a t i v e humidity ex- de some c r i t i c a l value, usually near sa tura t ion unless exceptionally roecopic nuc le i are present. In the c w e of stratus, t h e v e r t i c e l t a n -

perature s t ruc ture almost elwaye assumes a very charac te r ie t ic pattern, with tt l a y e r next t o the ground having an aitiabatic lapse r a t e , surmounted by an lnvertrion layer i n which t h e temperature rises and the r e l a t i v e humid- i t y decresses (Figure 4) . coolest , and it is i n t h i s cool region t h e t s t r a t u s occurs. The low t a n - perature required f o r the formation of s t r a t u s could occur e i the r b y t he establishment sf the e d i ~ ~ b a t i c l a y e r from one more steble , or by cooling of a pre-existing adiabatic layer. I n e i t h e r csse it is obvlous t h e t t he tern perRture a t t h e base of the inversion is the lowest, and therefore t h e c r i t i c a l one for t h e existence of s t ra tue . For t h i s reaaon the study O f s t r a t u s focueee on the temperature e t the base of t h e inversion, ra ther than on the s t r a t u s itself. Before s h i f t i n g our a t ten t ion t o the inversion, however, w e sha l l examine 801118 observational evidence regarding t h e position of t h e s t r a t u s with respec t t o t h e Inversion.

The upper pa r t of t he adisbat ic layer 1s t h e

A. Position of t h e S t r a tus Top. It 1s geqercllly accepted that the cloud tops in stratus very nearly

coincide with t h e base of the inverflion. (1) observations by a i r c r a f t , and ( 2 ) records of humidity i n redlosonde or other meteorograph mcente. Because of t h e i r regular upper s u r f a c e of t he cloud it is n o r m l l y d i f f i c u l t t o observe its height c loser than the nearest f i f t y feet. S i m i h r l y , 1% i n t h e temperature and hunziditg elements, together with, i n t he cme of the radiosonde, uncertaintq due t o the svi tching from one element t o another, lead t o an uncertainty of BS much 8s 200 feet i n the posit ion of the inversion base. In the following discussion it w i l l be shown that , w i t h f e w exceptions, t h e cloud top m d the inversion base a r e coincident w i t h i n the error of meaaurement.

This hypothesi& may be checked by

During t h e course of the invest igat ion R number of soundings were made through s t r a t u s .by airplane, and blimp. Table 2 presente 8. l i 8 t of suck observations of the stratw top and the inversion height. . I$ will be seen tha t t h e a.vsr8ge difference between inversion b m e 8r1d cloud top 1s less than the 200 fee t a t t r i bu tab le t o observational error , thRt i n ah08t a l l c86e8 the individual differences w e smaller than t h i s value, and t h a t i n many case8 the observations indicate them t o be zero. It should a b 0 be noticed tha t the cloud top ie usually Indicated t o be the a m e or higher than t h e inversion base, r a the r thpyi lower-

Paother check on t h e posit ion of the cloud top is t h e humidity measure- menta recorded by the rctdioeonde. While t h e Instrment, does not record temperature and humid1t.y simultaneously, the posit ion where the temperature etarte iricreasing and that where tho humidity start8 dec~eae ing mfty be closely approximted. c ident within t h e e r ror of a.pprox1mation.

3.1 prac t i ca l ly every record the Dositions were Coin-

22 TABU Zo-AIRCRAFT OBSERVATIONS OF CLOUD TOP AND IWVETJRSION HEIGET

Date Time Observed. Observed Lapse Rate C loud Top (1944) (PWT) Height of Inversion in Minus

Stra tus Top Height Stab l e Iayer Invere Ion Height ( f t . ) ( f t . ) 0 c / 1000 ' ( f t . )

Observations Taken Over Sea

5 Aug. 30 Awe

31 P.W.

2 Sept.

7 Sspt.

8 Sept. 11 Sept.

12 Sept. 16 Sept.

18 Sept.

25 Sept.

28 Sept.

1400 1300 1320 1334 1345 1250 14 15 l315 1325 1340 1400 1108 1120 1200 12 10 I240 1235 14 15 143 8 I500 1320 14 10 1435 1245 l3 LO 2355 1450 14 l5 1420 1100 1230

1000 1100 1100 I300 I500 1950 1950 900 2700 2550 2 600 350 200 200 200 250 100 1800 1850 1800 2550 2550 2600 3900 3250 3250 3250 950 950 2560 2730

1000 1100 950 1300 14 00 1900 1850 900 2600 24 00 2500 350 2 00 200 200 250 100 1800 1700 1800 2400 2500 2000 3300 3200 3200 3 150 950 950 2560 2660

-14.5 -23. 0 -15.6 -28.4 -36.7 -34.6 -29.7 - 0.4 -15.0 -13.0 -l3*0 -53.6 -21.2 -23.0 -160 3 -20.5 -16.5 -3 1.5 -10.0 - 26.5 -25.2

- 6.9 -19.5 -20.5 -37.0 -26. @ -59.2 -38.8 -59.8 -L55.5

-34.5 *

0 0

E O 0

100 50 100 0

100 150 100 0 0 0 0 0 0 0

150 0

150 50 600 600 50 50 100 0 0 0 70

Observations Taken Over Land

20 J u l y 0700 900 900 0 25 July 0945 2200 2 100 - 10.2 100 27 Ju ly 1343 1850 1850 - 5.6 0 31 July 0930 2000 1650 6.4 35 0

3 Sept. 1000 17 00 2200 -13.6 - 500 9 Sept. 0900 I500 1890 -14.3 -390 11 Sspto 1140 2400 23 80 - 8.7 20

18 Sept. 1009 43 00 4250 - 18.0 50 25 Sept. 1117 ROO 1lAO - 15.4 - 390 28 Sept. 1000 24 00 2520 -12.2 - 123

w Septo woo 2500 2200 - 1 2 e B 300 15 Sept. 1335 3000 2820 -33.1 180

23

Final ly the r e s u l t of B o r n e t r i e l s of R cloud top indicet,ing device, o r "cloud-sonde" may be ci ted. ins t rument hbora tory of the Ur:iversita of Chicago, cons is t s of B F l i gh t ly modif led rtxdiosonde trp-nsmitter, arranged so t h a t t h e aud i o freqdency 18 controlled only by R humidity s t r i p r e s i s t o r epecls l ly prepared t o g i v e uniform resis tmice up t o a c r i t i c a l high r e l a t i v e humidity;, ani! verj low reslcctance Above it. humidities below the c r i t i c s 1 velue (pre~sunably outs ide t h e cloud) 8i1d gives e higher fraqilvncy aigne.1 when t h e c r i t i c e l value is exceeded (within the cloud). i n conriection with the s t r a t u s project. The r e s u l t s fo r those made near the time when a radiosonde observakion w a s made, so t h a t the inversion height w a s known, showed very few d i s c r e p e n c i e ~ between the s t r e tue top and the invereion base when ellowance w88 made f o r t he lack of elmultansity between the observatiana of cloud top by t h e cloud-wnde and t h e determination of inversion base by radiosonde. I n f ac t , i n each of the radiosonde obser- vatlons the humidity showed a d e f i n i t e decreaee beginr:ing at the inversion base. While t h e cloud-sonde gives e more precise indicat ion of the abrupt drop i n humidity, it seems l ike ly t h a t t h e differences ae re due t o difference in time, ra ther than t o difference i n .sensi t ivi ty of t he two instruments. i n time of observation the &server repor t s "This f l i g h t wae made through a t h in a t ra tue overcast with break8 beginiiing t o appear . . . . a f t e r conclusion of the f l i g h t t he cloud diss ipated completely." In t h i s caae It seems possible t h a t very tenuous clouds vere present r i g h t up t o t h e inveraion, bu t with breaks with 8mewha.t lower huniidity in to which the balloons passed before they reached the inversion.

Th i s instrument,, developed a t the' meteorolcglcal

The r e e u l t is thAt the sudio a i g n a l motorboets" e t

Twenty-eight t r i ~ l f l i g h t s were maile e t SRII Clemente I s l a n d

In t h e only caee with a h r g e difference not explnined by difference

A few minutes

Further corroboration of t he f a c t t,het the cloud top is a t the inversion base lies In the ce i l i ng height measwement,s. t h e ce i l ing meaeurements are lower then the inverrrlon haight, and i n most c88e8 the Ceiling6 are almost t he ~ a m e a8 the invarslon height (Rbove the s t a t i o n ) j uh t after formation a i d j u s t before diss ipat ion. T h i s is shown i n the time sec t ions (Figures 5-9). reported a t a higher l eve l than the inversion bme, it was estimated ra.ther than memured. base may be a t t r i bu ted t o the methods of measnrement. both ceil ing balloons and ce i l i ng l i gh t s give ceiliw observations which are too high.

In elmost a l l cmee

Most of the times when the ce i l i ng was

The few cases of measured ce i l ings higher than the inversion I n FI tenuous cloud,

B. Relat ive Humidity i n Stretue Clouds Beceuse of the p r e s e n s of hygroscopic nuclei , clouds may be present

at somewhat lower r e l a t i v e humidity than 106. humidity frequencies measured by radiosonde observations through stra. tus clouds at t h e various s ta t ions . each sounding is used t o eliminate the e f f ec t of instrumental lag.

Table 3 ehows r e l a t i v e

The maximum r e l a t i v e humidity recorded i n

100 63 37 3 2 0 0 6 71 99 14 a 2 1 2 2 4 5

3 4 5 6! 98 15 9 5 4 7 9 1 97 23 14 4 3 3 4

96 18 11 2 1 7 9 9 11 95 6 4 5 4 7 9 6 a 94 5 3 a 6 2 3 4 5 93 9 5 7 5 8 10 4 5 92 8 4 13 10 5 6 10 12 91 1 1 9 7 6 7 6 8

89 88 R7

1 1 11 R 3 0 0 12 9 5 1 1 10 8 3

4 1 1 6 3 4 4 2' 3

86 9 7 6 7 1 1 85 2 2 3 4 1 1 84 4 3 0 0 0 0 83 7 5 0 0 0 0 82 1 1 1 1 3 4 81 3 2 2 2 0 0 80 0 0 1 1 1 1 79 1 1 1 1 78 1 1 0 0 77 1 1 0 0 76 or less 1 1 1 1

Figure 10 these data are e h m graphically. The curves f o r Santa W i a UCLA were moothed, s h o e i r regular i t lee eeeaaed primarily due t o observer

there were fewer soundings throwh stratus , and smooth curvss could not eference for certain valuea in reading off the data.

Line8 connecting the point8 were drawn ra ther than block hieto-

A t CIT and Santa

dram. in order that a11 stations could be represented on ~ n a diagram.

It w i l l be ~ e e n that at Santa Maria 106 i a the most frequent humidlty s t ra tus w i t h practically a l l observatlone showing values greater than

On the other hand, both UCLA and CJT ehow maximum irequenclee at 90$ w i t h values as l ow aa 78q6, while Santa Ana appeare to have an

As a cheek, the surfsce hwaiditiae measused by the wet-bulb psychro- meter during perioder of heavy fog at UCLA were t sbuh ted and are repreeanted

That heavy fog frequently ir s t ra tus which reaches the ground It will be 88831. that a e m t h

8 5 5 .2

intermediate maxbm frequency Qf about 94$.

Figwre 11, own in the time eections, P"lgure8 5-9.

ancy at l0Ogm urve t o fit the data l e similar t o the one fo r Santa Maria, wlth maximum

The few meaeuranente which have been msde by wet-bulb hrmeter in stratue cloude in t h i s area were taken by hand peychraneter aero-psychrograph d u r i n g the six a i r p h e aecante through etratus over-

10 t o 60 miles off the cosrmt. Theee aata ab0 show maximum frequency 0 0 4 0 1

at lo@ w i t h the lowest observed value be- 91s.

While theee weS-bulb pepbrameter memuremente caet some doubt on the val idi ty of the radiosonde meaeuramenlx of humidity in the b e Angeles Basin, it see~m unlikely that the difference in grouping between theee and the Santa u t a rneaeuraments with the adme type inetrumente Is fortuitoue. explanation of the difference could be that the hulaidity i8 lower when the stratus forme and dl8eipates than i n the leiddle of i t a period of duration, and that Santa Maria took mope soundings during the middle of the stratus, while the stattaa6 in the U a Angelee region took most of their soundings through s t r a tus near the t l m s of formation and dieelpatian. is shown In Pigure 12, in which the f'requemcy of varioue ncaxiwZm re la t ive humidity value8 in st ratus ie given for different obeervatlm tlmea for UCIA md Santa Maria. A t Santa Maria at 2000 PWT there l e a defini te tendency f o r the observations t o group around 96s ra ther than lo$, MI- cating a tendency far the humidity at formatIan t h e to be h e r than a t later houre. WOO, In the middle of the s t ra tus period, but at eaoh obeervation t i m e the WCLA values are lower than thoee at Santa Maria, indicating a definite difference between the two etations other than the duration of stratue.

h e poeeible

A t e s t of this

A t TELA there I S a correeponding tendency for hlgher humiditlee at

A plausible explanation of the difference l i e s e f a c t that there is l i t t l e Indmtrlal ac t iv i ty in the ~ i c i n i t y of W-t Toe Angeles Bmln there are a great number of f producte and other forms of hyigrosoopic nwlsf supported by the fac t that; at Santa Ana, which of pollution than UCU and CIT, somewhat hi@er

26

p 70 L 0

100

0c

a I I

.--- Santo Ana -_*e -6 ---< C. I. T. _ _ - - - U. C. L.A.

L -+----I- - 20 30

Frequency Observed, %

RELATIVE HUMIDITY IN STRATUS 'CLOUDS

AS RECORDED BY RADIOSONDES FIGURE IO

_ _ _ - - - - - - - - - _ _

I 10 20 30

F ~ c ~ u E ~ c ~ Obssrved, %

RELATIVE HUMIDITY IN HEAVY SURFACE FOG AT U.C.L.A. From Wet and Dry Bulb Tamps.

FIGURE I I

27

From Figure 12, it appears t h a t stratus forme a t a r e l a t i v e humidity about 9@ in the Angelee Baain and 965 a t Santa Maria.

C. p i t i o n of the Cloud Baee and the Thiclmess of the S t ra tus Iayer. In general, $he lapse r a t e below the inversion is near the adiabatic

*at v e r t i c a l mixing is favored, and the mixing r a t i o tends t o be approxl- e l y constant up t o the inversion. Actually the lapse rate shows some

a1 variation, wlth maximum values in the heat of the afternoon and um values in t he early morn-. e adiabatic (marine)layer f o r UCIA, Santa Maria, and the ship.

Table 4 show the average lapse r a t e

3.0 2.4 3.5 2.2 1.7 4.3 3.0 2.9 308 2.9 7.8 5. 0 3.5 4.6

2000 3.6 2. 7 2.0

-6. 8 -7.7 77.6 -6.4 . -5.4 -5.9

-3.9 -3.7 -7.4 -3.3 -9.7 -3.8 -6.7 -3.7 -5.4 -4.5 -6.7

More often than not, no apparent change in lapse r a t e n m obaerved i n p s i n g upward f r o m t h e c l ea r air in to the s t ra tus . t o deficiencies In the Instruments used In taking the soundings. there waa a change t o greater s t a b i l i t y i n the cloud, with leothermalcy not infrequent. somewhat with height. i n Section V. (See Figure 32.)

This may be due largely Occasionally

Because of the s l i g h t s t a b i l i t y , the mixing r a t i o actual ly decrePasee The moisture d is t r ibu t ion w i l l be discussed i n d e t a i l

With t he temperature decreasing rapidly eLnd the mixing r a t i o nearly cons tmt with height in the normal sounding, t he r e l a t i v e humidity increases with height and may reach the value a t which condensation occurs. the (nearly constant) m i x i n g r a t io , and r is the r e l a t ive humidity at which condeneation begins, then condensation w i l l occur when the temperature

If x is

eaches a value such tha t t h s corresponding saturat ion mixing r a t i o xc is It is c l ea r t h a t xc is greater than x i f condensation

It wae shown i n the previous ection t h a t condensation occurs In the Im3 Angelee area with 9@, and at anta Maria with 9616 r e l a t ive humidity. a t i o value6 t o be used for xc should be 1.1lx and 1 . 0 4 ~ respectively.

?* given $3. I C = occure when t e humidity is lese than lo@. Thus f o r these aredB the mixing

28 0 0

8 % 0 In 0

0 c 0

I!

In Q,

0 m

In 03

0 0

10 0 9

0 (D

0 In 0 NUMBER OF

J

v - 5- a

.e)

c .-

d- 0

0 0 0 0

a L 3 0 X

29

On them assumptions, the level of the cloud base w i l l be determined on a thermodynamic diagram by the in te rsec t ion of the lapse rate curve w t t h the saturat ion miring r a t i o line having the value xc. Thus In Figure 13 if x 18 the appropriate sa tura t ion mixing r a t i o line, the cloud base w i l l be at point B if ABC is the sounding curvet a t polnt R i f FCC is the soundlng curve, add at point S i f HJg I s t he soundlng curver value of mixing r a t io , the baas of the cloud becomes progressively Lower a8 the adiabatic r;aSer becomes colder,

Given a par t icu lar

Since the cloud top was shown t o be ne the inversion baee, the cloud thlckneee thue depends on three f m t o r a ; (1) e temperature (or po ten t i a l temperature) of t h e marine layer, (2) the mixing ratio In the marine lapr, and (3) the height of the inverq$on base. t h e mashe layer or dmreaee of the m i x i n g r a t i o or inversion height would tend t o reduce the thickness or d ias ipa te %he s t r a t u s , would tend t o increase the thickness or cause the str

An increase in the teunper&x.re of

e opposite ch

From the t h e of formation, when the cloud base is n e w u a t the Ilnversioa, the cloud thickness normally Increases both by the l i f t i n g QP t he @ion and by cooling of t h e adiabat ic layer. Tabla 5 shows the average percentage of the marine layer occupied by a t ra tua at the varioufs obeervaticm hours. and lmge In the early morn- when the nocturnal cooling has lowered the temperature of t he h e r l a y e r . high also on the average at these hours.

The percentage i e low at time of formation and diss ipa t ion

It will be ahom that the inversion baee is

TABU 5. A-E PERCENTAGE OF MARINE LAYER OCCUPIED BY STRATUS

Santa Maria UCLA CIT Approximate

0000 39 67 0400 49 79 0800 48 6 1 1200 1 40 16 00 0 0 2000 32 55

31 46 40 65 34 54 15 45 4 4 1

20 4 1

18 48 24 65 2 1 54 8 44 2 51

4 44

c loud c r a f t

When the inversion baee is high and the m a r i n e stratum is cold, t he thiclmees may be very large. aecent during the period of observations wae 2500 %est.

The th ickes t layer subetantiated by alr- Occasionally

the cloud is present In t w f l l gh t of 13x5

increased egain be

30

I f , as l e approximately true, the lapse ra te is adiab ing r a t i o is constant i n the marine stratum, t h e height of (ce i l ing) will be a function almost en t i r e ly of t he dew poi because the triangles formed by the adiabatic l inee, the and t h e isobars on an adiabatic diagram are p rac t i ca l ly have a l t i t udes approximately proportional t o the bases,

ions, obtained frm observations a t Smta Maria and UCLA. It t h a t a t Santa M a r but at UCLA it ho deviation fo r h ig f o m a t relative h m i d i t i e s lower than lo@.

Temperhtura Dewpoint Difference

0 1 2 3 4 5 6 7 8 9 10 11 1% 7 12

71 103 17 7 55 24 30

8 11 5 2 0 0 1 2

40 1s 0 350 600 670 1000 1050 1200 1650 1550

- -

3 600 2400

24 23 70 115 49 17 11

3 5 5 4 6 4

8

6 60 8 6 10 870 6 60 1000 1100 83 0 7 00

D. Temperature Chariges a t Inversion Base. O f the three fac tors l i s t e d above which coi?trol the presence end thick-

The sir t h a t a r r ives a t the c m s t normally has t rave l led a long nee8 of t h e s t r a t u s , changes i n mixing r a t i o may be considered t h e leeat s ignif icant . distance over water, and at ta ined a mixing r a t i o v a l u e which is nearly the same for most t r a j ec to r i e s it c m fo1l.m. Changes in e i ther of t h e other fac tors , temperature of the adiabatic layer and height of the inversion, produce changes i n t he tenpera twe a t t h e Inversion besc.

31

The processes by which the temperature a t t h e inversion base may be changed are i l l u s t r a t e d i n Figure 13. t o represent an i n i t i R l t yp ica l sounding. layer without change of the InverRion layer, changing the curve t o FGC, could occur If the ground surface cooled an amount AF, with su f f i c i en t turbulence t o maintain tho adiabatic lapse rate. I f x represents t he condensation mixing r a t i o value of the marine layer, t h e process would produce cloud, and rad ia t ion from the cloud top could tend t o maintain the adiabatic lapse rate .

The heavy line ABC may be taken Cooling of the en t i r e adlebatic

A change from ABC t o HJX could occur by horizontal advection of colder air a t all levels. In t h i s case, If the condensation mixing r a t i o were t he same in the a i r being brought in, t he cloud would already be present i n it. The process would represent a case of advection of s t r a tus .

The reveree effects , heat ingnat the ground tlnd warm air advection, me shown by changes from ABC to DEC and LMN, respectively. caees the tomperatwe a t and below the inversion is increased, removing the poes ib i l i ty of s t ra tus .

In both

The e f f ec t of v e r t i c a l expansion due t o convergence i n t h e marine stratum is i l l u s t r a t ed by t he change from AEIC t o A X . The mesine layer is assumed t o expand Fer t ica l ly , so t h a t t h e column which w a s represented by AB now has a greater v e r t i c a l extent, and the par t a t B i s l i f t e d t o P, every pa r t of the column above A undergoing a proportional r i s e and cooling. This process produces a lower temperature a t t he new position of the inversion base, and a region of condensation. A t each leve l belolr t h e position of the or ig ina l inversion base B, however, there is no cooling and no cloud forma.

Tn t he example shown it is assumed t h a t there is E compensating horizontal divergence i n the Inversion layer, so t h a t point C remains a t the same pressure. This i s not neces8arily. true. I n individual cases ' the convergence frequently extends up throwh theinversion l a y e r , SO tha t the inversion top rises more than the inversion base.

The m m e change would be produced by horizontal advection i f the inversion t i l t e d up t o windward, so t h a t a deeper layer of marine a i r having the same poten t ia l temperature w a s carr ied in t o re9lace the a i r over the s ta t ion . In t h i s case of advection, ~193, the cloud would be present i n the a i r before it wa.8 car r ied in.

The reverse process of hori?ontal divergence and v0rticR.l shrinking The tsmperetlwe a t the inversion would produce ~i change from AEC t o AOC.

base is increased md stratus is dissipated. This change a l s o could be produced by Rdvection, T i th a shallover marine s t r a t u m replncing the one p- e8 en t .

32

Temperature OC

ILLUSTRATION OF PROCESSES CHANGING OF INVERSION BASE

FIGURE I3

33

A l l theoe processes, heating advection of warmer or colder air, normally occur e imultaneous ly, and

or cooling at the ground or cloud tope, and v e r t i c a l expansion or shrinking, the obeerved var ia t ions of temperature

a re due t o t h e resu l tan t combination of them.

Another process, turbulent mixing, can produce some change by stirring p a r t of t he inversion layer i n to the adiabat ic layer. required t o overcome the s t a b i l i t y of the inversion layer is great , the winds are almost never strong enough for ‘this e f fec t t o be considered. marine layer is not adiabatic, however, an increase of wind may produce enough turbulence t o es tabl ish an adiabatic lapse r a t e , w i t h consequent lowering of the temperature at the inversion baae.

Since the energy

I f the

The f i n a l poseible cause of temperature chmges is rad ia t iona l ex- change between the moist marhe stratum and the r e l a t ive ly dry air a b y e . It w a s thought previously t h a t because water vapor is the pr incipal radia-

would be r e l a t i v e l y transparent t o the long wave radiat ion f r o m the marine stratum, re su l t i ng i n cooling of the marine air. However, it has been demonetrated [l] tha t the temperature of the dry air l e so much higher than tha t of the marine stratum t h a t the downward radiat ion from it exceeds the upward radiation, and the e f fec t is a s l i g h t warming ra ther than a cooling, unlees the.humidity i n the dry air is of the order of l$= low humidities were not observed.

t i6n substance In the atmosphere, the dry air above the inverelon baee

Such

In the preceding discrussion we have seen t h a t one of the importans fac tors determining whether stratus w i l l or w i l l not occur is the height of the Inversion. poeition of the inversion, and its normal diurnal variation.

In the next two sact ions we s h a l l consider the average

34

SECTION 11. DISTRIBUTION OF INvERSIm Bv SPACE - --

The existence of the masked dry-type inversion alon the Pac i f ic Coast i n summer has been known f o r many years. Blake [3f ca l led atten- t i on t o its presence i n the ear ly a i rplane observations taken b - the Navy at Sen Dlego, and pointed out its re l a t ion t o s t ra tua . Bowie c341 discussed the formation .of sumnertime s t r a t u s i n the coas t a l valleys of t he San em- c isco Bay region on the bas i s of rad ia t iona l e f f ec t s a s s o c i ~ t e d with the inversion in t h a t mea. Previous t o free air soundings the existence of t he inversion was demonstrated by comparison of temperatures a t Mount Tamalpais and San Francisco [s].

1

The d is t r ibu t ion of the inversion i n space is less well known, and i n f ac t , obeervations have not been avai lable t o show how far t o sea it extends over t h e Pacific and how it afariee i n height i n t h a t direction. The obsep YatlOIle takgn 1J;r GOIUeGtiQA %ltb $hi@ prOj0Gt hGlUd0 those f r a two s t a t ions off the coast, the NRSL sh ip about 10 m i l e s out, and San Clmente Island about 60 miles from shore. Thus, while data f a r the r t o sea are lacking, t he behavior i n the immediate v i c i n i t y of the coast has been determined.

The data col lected by the NFSL sh ip const i tuted an indispensable part of the baeic material necessary for the construction of cross-sections and prof i les of the inversion base. The soundings taken by the sh ip usually showed an adiabatic or super-adiabatic marine stratum, with an extremely . s t ab le inversion layer. perature from 18 "C at the Inversion baee t o 35 "C at the inversion top, and a lapse rate of -9.6 Co per 1000 feet. While the magnitude of the inversion i n therim sh ip observartione waa t he la rges t observed i n the project , much g3peater s t a b i l i t y of the inversion layer ~ ~ 1 8 recorded over t he sea by air- pLme and blimp soundings, with the Inversion sometimes approaching a mathem- a t i c a l discontinuity.

Table 4,

A number of soundings showed an increase of tem-

These lapse rates are recorded in Table 2. The aver- e8 by hours for the inversion layer a t the ship a r e shown i n

-coast cross-section of the inversion averaged fo r the L sh ip tooKeoundings is shown i n Figure 14. I n t h i s m e sounding s t a t ions have been arranged according t o

e coast, independently of t h e i r position along shore. Be- mht bigher Ia t i tude than the other s ta t ions , the

aria. were not dram for . Ths data from San Clemente Island during the escond -part of the ship period but have been w e r e taken

adjusted fo r the e n t i r e pax3&, E% w i l l b e seen that the inversion is

show that %hi@ ehmacter i8 t fc is not t3 pecul ia r i ty of the period choeen, t he avsra$e curve far the ent i re month of September (the only calendar month throwhout which 8 3 . 1 land s t a t ions took observations) is shorn.

Imest at the coaat, nloping M w d both towar6 the sea and inland. To

35

Although t h e sh ip data f o r t he e n t i r e month were not available, the s t a t ions nearest the coast showed the lowest Inversion. The curve f o r the l a t t e r half of August is similar over land, but there were no da ta over sea t o show the pat tern there.

The r ise of the inversion toward the sea is not unexpected because over the Atlant ic Ocean a s i m i l a r phenomenon .has been observed. observatione over much of the t rop ica l Atlantic have been made, primarily during an expedition of t he German sh ip Meteor i n 1925-7, and these show that there is an inversion throughout the t rade wind area a l l the way across the Atlant ic i n both the northern and the southern hemispheres. Von Ficker s6J hae arr ived a t a t en t a t ive char t of the topography of t he inversion by averaging the observations by f i v e degree squares and emooth- ing these averages. It shows tha t except near t he equator, t hedve r s ion elopes upward from the coast of Africa where it is approximately 500 meters, t o about 1500 meters some 1000 m i l e s from t he co-t and r emine n e a r b comtant in height Pram there westward. Atlantic is associated w i t h the c i rcu la t ion in the subtropical anticyclones there, and t h a t the inversion o m the Pacific should bear an analogous relat ionship t o the Pacif ic anticyclones. The inversion should thus be expected t o have a elope upwmd from the w e s t coast of America i n a fashion similar t o that off Africa; namely, with a slope of 1:1600.

Upper a i r

This char t is reproduced in Figure 15.

It is reaeonable t o suppose that the inversion over the

The average slope toward sea from the sh ip t o San Clemente Island is about 1:800 in Figure 14, and thus is about twice that given by Von

Ficker f o r the Atlantic. There is reaeon t o suppose, however, that h i s data were inadepuate t o determine th i s slope accurately. I n f ac t , more recent d a t a c7] on the inversion height on Tenerlfe Island In the Canaries, 175 miles fram the coast of Africa, gives a mean of 1300 meters f o r the summer months (and higher' i n winter) compared w i t h the 700 meter value given t h a t region by Von Ficker. than t h a t observed off Southern California during the ship period.

This value would give an even larger slope

An a c t u a l v i sua l observation- of the slope of the inversion which deserves mention w a s t h a t found on a reconnaieance airplane f l i g h t , 6 September, 1000-1200 FWT. In t h i s case the plane w a s flown i n a l i n e more or lees normal t o the coast from a point 30 m i l e s south-southwest of Santa Cruz Island, a t the l e v e l of the top of the s t r a tus . The s t r a t u s top descended from 1200 feet a t the start t o 800 f e e t at Santa Cruz, a elope cf 1:400. because of the €Lee effect, and from the coast sloped upward Inland.

Shoreward of the is land the s t r a t u s disappeared f o r a distance,

Von Ficker 's char t does not give data on the distributim of the inversion baae a t l a t i tudes higher than khat of Ins Angeles. obtain some was prepared. of Awust, t he month of September, and the Bhip period, at near-cocustal s ta t ions from sobth t o north. The values at San Clemente Island and C I T a r e shown but not drawn for. Los Angeles Basin, indicated by the UCLA values. t o the onshore convergence of wind caused by the "Catalina Eddy".

In order t o

It shows the average inversion heQht during t h e last half idea of the var ia t ion with l a t i t ude along the shore, r i p e 16

In a l l the GUTVBB there le a bulge over the This bulge may be due

This

36

$ 1 .- I Leqend

-0 September - Period of Ship Observation -----O AuquSt 16-31

OA Ooklond 6 NZY Son Oloqo

Ship Coos? I; NGA

AVERAGE CROSS SECTION OF INVERSION BASE NORMAL TO COAST

Stovions Are Arronged Accordinq To Distonce From Shore Independently Of Position Along It

FIGURE 14

TOPOGRAPHY OF INVERSION BASE OVER THE ATLANTIC ( After Von Ficker)

Heights )n Meters FIGURE 15

37

0 In m 8 v, cu Height Hundreds Feet M. S-L.

I- u)

0 0

W z

a

2 a

W u) a m z 0 - m a a- W > z g 3

k g z 0 I- V W u)

u) v) 0

0

W W

W >

-

a

a

a

39

eddy is discussed in Section IV of t h i s report .

The inversion we18 w e l l defined a t Oakland and Eureka, California throughout the euIIpIler. The northward l i m i t of it 1% unknorm. Oaklctnd shows an average inversion height (based on two sounding8 per day) about equal to t h a t observed a t Sen Diego i n August, and during the sh ip period, which include8 the first week of September. a t Oakland is much lower. There is, therefore, no general meridional slope indicated, except l a t e i n the seaeon. concentrated J u s t north of Lo8 Angeles.

is almost 88 1 i n September as a t QakPand. In t h e other periods it is lower,

The average f o r September soundings

Even i n September the slope seem Both a t S m t a Barbara (with an

e baeed on once-a-day airplane obsertations f o r about half' the month, dusted f o r the critire period) and at Santo Maria, the inversion baas

Pi lo ts f l y ing the route t o H a w a i i r epor t thrtt t he height of the e t r a t m top fncreaaeo from the coaet for a dfetance of 500 m l l ~ or mom. become thicker and aseume a stratocumulus character. Near the Eawaiian group, the clouds are usua l ly cumuIus, reaching a height of 5000 fee t .

The cloud8

From these da ta i t seemsplausible t o atmume t h a t (1) the inversion con- t inues t o s lope upward t o sea of4 California, reaching a height of 5000 fast some 500 t o 1000 miles off shore an& extends westward from there a t an approximate ly constant elevation, 88 is %he case over the Atlant ic ; ( 2 ) t he Inversion off northwest Africa dlpn downward toward t h e coast from 4300 feet a t Tenerife Island t o about 1600 feet a t the shore, as is the case off Calif ornia. .

In Figure 17 the da ta have been put together Into m average inversion The contour map fo r t h e period of the ehip obeervations and for September.

contour liries have been d r a m on the hypotheeis t h a t t he slope normal t o the coast w a s roughly constant along it, and t h a t the. trough along the coast pereieted when the s h i p observations were absent. one of a trough j u s t off shore, with the inversion sloping upward onshore againet the coastal mountaina and a l so upnard t o sea. see Figures 1 and 2) . ponds also t o t h a t over t he Atlantic, it seem l ike ly t h a t the m e a n condi- t i on i n the summer over t h i s mea is repreeented by it.

The p ic ture is thue

(For land topography Since t h i s pat tern f i t 8 the observatiozls and corres-

Individual cases vary g rea t ly from t h i s pattern. F i r e t , there h ~ v e been tfmee i n the past season during which t h e inversion base was observed a t the ground at most of t h e s t a t ions within the region concerned. it is frequently the caee t h a t the posit ion of the minimum height of the invereion does not coincide with the coast l ine. the number of ~ 8 8 8 8 In which t he inversion m l n l m m occurred a t each s t a t i o n in 76 cross-sections normal t o the coast .

Second,

The following table show

Time (pm

0000 0400 0800 1200 1500 2 coo

S m C lemen t e

2 2 2 . 6

0 14

--

P) c

Ship _y-

0

6 2 8 5

23

,.. L

-

Smi t a Ann

1 2 1 4 7 1 16

--

-I_

On ground B t 2 or more s ta t ions --

1 2 3 2 2 4 14

--

TotRl s --

5 13 14 15 19 10 78

--

Yne inveivion minimum thus is most frequent new the ohip, but occurs n o t infrequently elsewhere. There is some suggestion i n t h i s tab le of d d i u r n a l period i n the position of the inversion minimum, with t he greatest frequency off shore dur ing the day and inlmcl i n the early morning. The d i u r n a l vmia t ions 3f the inversion w i l l be studied i n the next section.

P

\

41

SECTIOTJ XICI. DIURNAL VARIATION - OF IIWE3SION

The d iurna l behavior of the inversion is shown i n Figure 18, i n which the monthly and seaaonal average heights of base a n d top are plot ted f o r each s t a t i o n AS a function o f time of day, separate averages havin,g been evaluated f o r days wlth s t r a t u s mcl days with no s t ra tus . In o rde r - to compnre the d iurna l f luctuat ions at various l a n d s t a t ions with those a t the ship, the nverages fo r a l l s t a t ions during the periods 17-23 August and 31 August- 6 September, when the s h i p took observations, are ehown in Figure 19. A8 San Clemente was not in operation during t h e first of these two periods, i t a averages i n t h i s f i gu re a r e bhsed en t i r e ly on the second, m d m e probably somewhat higher than they would be f o r the en t i r e period. In t h i s "shfp pex-iod" s t r a t u s w m reported a t the s h i p a n d m o s t h d s ta t ione every day, but 8om atatlons had no s t ra tua on individual days Pvithin the per.3 od

33-1 Table 8, a s m a r y of the cha rac t e r i s t i c s of t h e inversion b w e height var ia t ion i s abstracted from these f igures , and TRble 9 preeents a e i m i l a r ~ u m n : t ~ y fo r the inversion top. The s ta t ione a r e arranged i n order of increasing dis tance i n l m d .

42

1

S t r a t u s

Clear s 4

3

2

I

i i it

v)

UI P

0 UI 3 0 c 1 Stta t u s I- 4 I

PUG 12 4 WA 12

7'ZM 8

5 ' I

L a g e n d I Z M anta Maria 229'

PUC U.C.L.A. DSW Sanla Ana 588' EHJA 8.I.T. 7 s :

~ V ~ ~ A G ~ HEIGHTS OF INVERSION BASE AND TOP F I G ~ R E M A

SJT IIW

9s W Z l

Joaru old on

0

I

2

€

P

r 2

B 3 r h r

0 c

3

I

2

E

P

E

c Msa

€ 3M

e Wm

€ Msa I 31M

e m a

44

TABLE 8m-DIUIWAL VARIATION OF HEIGHT I OF INVERSION BASE - Time of Maximum Heinht

Ssn Santa UCLA Santa C I T C lemente Ship An8 Maria

Clear Days 1300 1200 1300 1200 1430 Stra tus Days Ship Period

Clear Days

S t r a t u s Day8

Ship Period

Clear Bye Stratus Days Ship Period

0800 0900 1130 1330 1430 0730 0730 0900 1200 1330 15 00

Time of Flnimum Heicrht

0000- 0000 0400 0000 04 00 0800 2 900- 2000 1600 2000- 04 00

123 0 14 00 2030 2300 2200 04 00 0000 0400

Maximum Height Minus Mflinimum Eeight (Feet)

65 0 850 600 500 12 50 300 775 450 400 750 460 5 00 880 66 0 3 80 1080

TABU 9e-DIURNAL VARIATION OF HEIGHT OF IMrERSIOTi - TOP - - Time of Maximum Height

S#3 Senta UCLA SantR CI!C C lemente Ana Narla

116 00 12 00 04 00 0400 2200

1200 Stratus Days OR00 0600 0800 1900 0400 &

Time of Minimum Hei&ht

Clem Days ti600 0800 1800 0000 2 000 S t r a t u s Days 16 00 16 00 1600 0000 0000

Clear Daya 1150 750 R 5 0 1150 65 0 Stratue Days 4 00 500 1000 500 250

/

45

Several features i n the f igures md tables m e orthy of note. Most conspicuous is the contrast between t h e ayerages f o r s t r a t u s bnd c lea r clays.

Sear day8 the inversion bcase and top are lower a t a l l hours than on tws daya, w i t h the inversion base prac t ica l ly a t the ground on clear

nights. The amount of var ia t ion (maximum height minue larger on Clem than on s t r a t u s da

urn inversion he ight are generally e

From these times of maximum and mini the early morning the inversion i e r i s i n g any l i f t i ng -due t o Insolational heating, and over the etea

gs we ~ e e t h

t begin6 sinking during the morning, when ineoliati o cauee it t o r i s e . Thus it 18 clear t h

e i 8 control led by fac tors other than

In t he sh ip period the contrarjt between $8 ik ing fashion. In the morning the inversion b

Inland ( ds over the 888, EJ a.lmost the from afternoon to morning it rises.

opposite phase, with the rise from sunrise t o afternoone

11 s ta t ione except .$anta Maria, the inversion top shows 8n oecil- l s t i on in s t r a t u s period8 s i m i l l s r t o that of the inversion base over the sea. Since the e f fec t of ineolatlon on the height of inversion top l e s l igh t , t h i s suggests t h a t the same fac tors operate t o cause the inversion osc i l l a t ion over sea and Zand, but over land the e f f ec t o f insolation i e added. This poss ib i l i ty w i l l be tes ted l a t e r i n t h i s section.

The number of c lear dags (days with no s t r a tue overhead from noon t o noon) w m smaller than the number of stratus dags a t most s ta t ions . There- f o re , the averages for the c lear period a re not a8 s igni f icant as those f o r the s t r a tus period. reasonable picture, with inversion base constant near the ground through t b night, and l i f t e d by insolat ional heating t o CL maximum at or shor t ly a f t e r 1200 PPPT. except CIT suggests that these was some othezf fac tor operating i n opposition t o the l i f t i n g e f fec t of ineolat ional heettfng. a t CIT was wiped out by heating ewes had t o be omitted from the averaging; i f there were some way of including them t h e e f f ec t would be t o s h i f t the mbximwm toward 1600F'WT.

Nevertheless, the pat tern of t h e m averages makes a

The descent after; 1200 PWT (before solar noon) a t a l l s t a t ions

Occe~ions l ly the inversion These t the grouna during the afternoon.

The degree of dispersion of the individual ca8es about the m e a n pattern is shown by Figure 21, the frequency of occurrence of da i ly maximum

minimum inversion height at each observation t i m e . It w i l l Be seen

approximately a t the . t there m e days with t h a t each obeervation time, but the Limes s h m by the CLV

Figure 22 shows t he at each observation t i m e

An ~ d ~ ~ t ~ o n a ~ idea e deviation of' 3-ndiv 5 t o 9, the t i m e 8ec

- 0

- IC I I 0 04 08 12 16 20 22 -

/ \ f \

\ \

Time in Hours

AVERAGE DIURNAL HEIGHT OF INVERSION BASE RESULTANT HEIGHT OF INVERSION BASE AFTER DURING SHIP PERIOD SUBTRACTION OF HEIGHT AT THE SHIP

FIGURE 19 FIGURE 20

47

Santa Maria 60

50

40

30

20

IO

- 0 C 0)

0

0)

L

a

c .-

c. 1. T U. C. L A . Santa Ana 60

50

40

30

20

IO

0

H o u r of D a y

Time of Doily Maximum Inversion Height

W

0) Santa Maria C. I. T. 60

50

40

30

20

IO

0

16 00 08

U. C. L.A. Santa Ana 60

50

40

30

20

IO

0

H o u r of D a y

Time of Daily Minimum Inversion Height

San Clemente

San Clemente

FREQUENCY OF OCCURRENCE OF MAXIMUM AND MINIMUM HEIGHTS OF INVERSION BASE AT OBSERVATION TIME

FIGURE 21

0 1 2 3 4 5 6

0400 0800

2 5 0

," 30

5 20

2 IO - 0 m Z O 1 2 3 4 5 6

50

40

30

20

IO 0

Hours 0000

50

40

30

20

IO

0 1 2 3 4 5 6

0400

0 1 2 3 4 5 6

FREQUENCY

0800

50

40

30

20

IO

0 1 2 3 4 5 6

1200 50

40

20

10

0

50

40

x)

20

IO

0 I 2 3 4 5 6

50

40

30

20

IO

0

12co

Height lnverslon Base in Thousonds Fee? M S L

OF VARIOUS HEIGHTS OF ENVERSION BASE FIGURE 22

50

40

30

20

IO

0 1 2 3 4 5 6

1600

0 1 2 3 4 5 6

1600

BY HOURS

0 1 2 3 4 5 6

2000

1 2 3 4 5 6

2000

49

a sample period. SiOn dieappears and a new, lower one takes its place. Occasionally, the change of imers ion I s In steps so t h a t no s ingle point may be ca l led the invereion baee. No individual day conforms exactly t o the average, Nevertheless, the impression one gets from looking a t the var ia t ion day after day is that of a d iurna l o sc i l l a t ion similar t o tha t shown by the average values.

In individual case6 a double inversion occurs or one lnver-

The exietence of a diurna l oeci l la t ion, Indicated by the 1943 data, is thus corroborated, and the queation arisea as t o what are t he cont ro l l l rq factors. waa due t o divergence in the sea breeze. a l i g h t onshore component of the wind due t o the isobaric configuration is augmented i n the daytime by the aea breeze near the coast , but not f a r the r out t o sear The marine air near the coast is carr ied Inland f a s t e r than it is replaced, with consequent decrease in i t e v e r t i c a l extent. breeze i 8 replaced by a light wind from the land, and the marine stratum is again inflated according to thie hypotheale.

go on due t o the uneven heating of land and 888, and the r e su l t an t 1388 breeze circulation.

The explanation proposed i n the 1943 rspor t was t h a t the osc i l l a t ion It w a ~ pointiid out that the normal

A t night the sea

The diagrams in Figure 23 i l l u e t r a t e the V a r i O U 8 proceeees which could

Figure 23A show the e f f e c t of pure advection due t o the sea breeze, i n

The result l e that a

S ta t ions t o eeaward and

whioh the trough i n the inverelon baee is car r ied f a r the r out t o sea during the night, md Inward over the land dur ing the day. s t a t i o n between the extreme posit ions of the trough would experience a semi- diurna l period of oec i l la t ion of inversion height. landward would experience a single diurnal oec i l l a t ion w i t h minimum height Inland a t the end of the sea breeze and at sea at the end of the land breeze.

The effect of divergence In the sea breeze circulat ion, disregarding the change6 due t o advection and considering the breeze of limited inland extent, l e shown i n Mgure 23B. a maxlmm at the ehore and being zero su f f i c i en t ly far out t o sea and inland, the r e s u l t is divergence and lowering of the inversion at sea during the day, the air removed serving t o i n f l a t e the marine layer over land. night the opposite e f f e c t obtains. A sea s t a t ion thus would experience minimum inversion height a t the end of the sea breeze in the evening and maximum in the morning, and a land s t a t i o n would have the timee of maximum and minimum revereed.

With the traneport normal t o the coaet having

During the

If, instead of being l imited by the mountains, t he 888 breeze merges in to the mountain breeze which carries the marine air s t i l l f a r t h e r inland, the convergence over land i n the afternoon would be replaced by divergence and the e f f ec t would be that shown i n Figure 23C. A t a l l s t a t ions within the

50

marine stratpm In t he morning the depth of murine air would be decreased by i f 6 spreading farther inland. On the other hand, s ta t ions 8 far inland not t o have m y marine air i n the morn- would be c the spreading t h in layer in the afternoon. Except f o r the stat inland, the minimum invereicm height over land would occur at the tuna of' the sea breeze, i n phaee w i t h the var ia t ion over sea.

The f i n a l e f fec t t o coneider l e that of the Increase i n height of the It can be shorn thu t when no other Inversion due t o lnsolat ional heating.

processe~l are aoting and an adiabatic lapse rate Vd is maintained belw an inversion layer of lapse r a t e (J , the increase i n depth of the adiabatic laser due t o an increase of temperature A T a t the ground is r l~ - r l For average c a n d I t l a n ~ w e may take ('Vd - V) a 7 C e / l O O O f t . Due t o t 0 e large mount of the inoident solar energy which is car r ied in to the sen by turbulence and the amount used i n evaporation, inaolat ion a f fec ts the temperature over the ocean very l i t t le . On e t ra tue day8 the lncreuee in temperature

CIT. Thus the amplitude of the insolatlonal change in Inversion height would be near zero at sea, 1000 f e e t at UCIA and Santa Ana, and 1900 f ee t a t Cnl. Tech. sion upward from sea to land i n the afternoon, aa e h m in Figure 23D. Thie slope at the coast would lead t o a new advective effect , a6 a r e s u l t of which the lower inversion over the sea is car r ied inland by the B e & breeze during the afternoon. magnitude of the Insola't.ionalrise in Inversion near the shore. The t o t a l r e s u l t of ineolat ional heating would be an oeci l la t ion with maximum height

from minimum to maximum avsragsa 7 C" a t UCLA and Santa Ana, and 13 C" a t

The insolat ional effect would produce a slope of the inver-

Advection thus would tend t o o f f s e t the

in the afternoon, and amplitude increasing with & i s t q c e inland. ,,

The average curves at the varioue s t a t ions f o r the period of ship obeer- sa t ions can ncm be examin& t o s ee which of the proceesee of Figure 23 apptsarp t o be responaibls for the observed var ia t ion in invereion height.

%?I6 carve f a r irivereion height a t the s h i p (Figure 19) Bhowe no indi- stmi-diurnal oeci l la t ion, euch as would be caused by advection ion of the Inversion minimum in its vicinity. In addition, S m

nts ItlZa31d h& mazslmum inversion height in the morning, whereas ad.vectlon due t o the 888 breeze would produce a maximum a t night.

8 e f f e c t of advection of the average position of t leaiet over water, Since ineolation produces

e a t the ship ie horizontal divergence. ion at the ship and Sem Clemente Island doe8

aha3 t no variatl. Pemins to exylaln the

@ion height over the eea, the one process t h a t It

j h l l be seen that the v 0911ow the pat tern of momin$ maximum - afternoon minimum, which would occur t sea due to the divergence of the p1 breeze in e i ther of the casee d i e - Weed above. It Seem p b W l b l e to me that this process is the Principal

factor in producing the observed changes i n the inversion height at the ship-

The shift of the maximum from morning t o late afternoon w i t h dietance Inland indlcatss that ineolation plays an lncreaelng role there. i a t o deternine the extent t o which divergence and advection contribute t o

The problem.

88 Over land.

CROSS SECTION OF IMYERSION BASE AT VARIOUS HOURS OF DAY

51

DIURNAL CHANGES OF INVERSION HEIGHT

A Advection of permanent trough

Sea Coast Inland

B Convergence in sea breeze, limited extent

I 12 18 24 00 06

Hour of Day

C Convergence in sea breoze, air rscoping through mountain posses

D Insolation

sea Hour of Day Coast' Inland

EFFECTS OF VARIOUS PROCESSES ON INVERSION HEIGHT FIGURE 23

52

2c

15

IO

5 i vi 5' c Q

$ 0 c 0

u) v E z ¶ I

c .-

E .- 0 0 I

2c

15

IO L

5

0

-. '*.

I I '

1 I NGA . Ship

2

6 !!?

b400

AVERAGE PROFILES OF INVERSION BASE FROM OCEAN INLAND Note: Dato Averaged fsr Period of Ship Observations

FIGURE 24

53

It w a s 8een that the effect of divergence of the sea breeze could be either In the same phase over land a8 over sea, or opposite, depending on whether or not the breeze waa of limited extent. For a first hypothesis, w e assume thfit the e f f ec t l e t o produce exactly the Bame osc i l l a t ion at a l l land s t a t ions a.8 is observed over the sea. If this is true, by subtract ing the sh ip var ia t ion frm the observed var ia t ion at each land stati.on w e should ellminate the e f f ec t of divergence and obtain a curve showing the eflbcts of inso la t ion and advection alone. Insolation should be expected t o be the predominant e f fec t , and thus the curves obtained by this subtraction should be In phase with the diurnal temperature var ia t ion.

Figure 20 shows the r e s u l t of this subtraction. A t each s t a t i o n the max;trmum difference occws near the time of maximum temperature. c i e s between t h e difference curves and the curves which would correspond t o the diurnal temperature osc i l l a t ion may be a t t r i bu ted t o advection. If the inversion osc i l l a t ion a t t h e ship is reversed before subtract ion from the one at C I T , the r e s u l t l e a curve w i t h maximum at m i d n i g h t and minimum a t 0800.

ould be predorminantly due t o ineolation. We m u s t conclude, therefore, that the divergence e f f ec t has the sdme phase over land OB over eea, corresponding t o the hypotheeie t h a t the mountain breeze augments the sea breeze in producing divergence in the m i n e stratum. except fo r inaolation, the f ac to r producing the inversion o s c i l l a t i o n is the same inland and off-shore w m suggested earlier by the s imi l a r i t y of the var ia t ion of ' inversion top at San Clmente and the coas ta l s t a t ions t o that a t CIT, farther Inland. of the va l id i ty of this hypothesis.

Discrepan-

This wouW be"incmpat1ble with t he concept that the reminder

The idea that,

The difference curves g ive fu r the r Indication

The difference curve8 do not have RS large an amplitude 88 heating would produce as a r s a u l t of the observed mean surface temperatures. may be a t t r i bu ted t o (1) the deviation from adiabatic lapee rate In the marine stratum at time of minimum temperature, and (2) the e f f e c t of advection.

This

The effect of advection on the Invers im height may be considerable During the a t near-coastal s t a t ions during the onset of the sea breeze.

morning the invereian is raised rapidly by Insolation over land, bu t is not ra ised over the sea, so t ha t at the coast an extremely steep slope is developed. t h i s slope in t h e inversion base I s carr ied inland, and s t a t ions c lose t o

f a r t h e r seaward. between Santa Ana and the sh ip and between UCLA and the ship, even though the surface temperature aamslns high.

As soon ae t he sea breeze reaches the l eve l of the inversion,

t he shore experience an inversion height cha rac t e r i s t i c of a posit ion "hie process explains the dwreaee a t noon of the differences

@BB@S %It t~~~~~~ I.@ B prof i let for the sound1 times are pr

The curveB were dram for an average between the two near-coastal s t a t ion , UCLA and Smta Ana* than the other s ta t ions , its data were neglected. The San Clemente da ta a r e only fo r the latter pa r t of t he period of ship observatione, and the

BCKm.Mit3 Santa Marla IEI conalderably farther north

54

seaward slope indicated is larger than it would be If the San Clerasnte record were cmplete .

From midnight t o 0800 €WT t h e inversion shcms a general rise sea corresponding t o convergence during the period of the lan een 0800 and 1200 the inversion deacenda over the ocean due

gence with the on8 of t he sea breste , but over land the insola e f f ec t is predomin and the inversion is raised. Between 1200

where the inversion had become s t eep by noon, the advective effect beomes ibmportt~~t, overbalanclag the e f f ec t of heating, and the inversion under- goa8 m abrupt drop. Out t o 8ea the inversion r iaea, pe rhap also due t o advectim. By 2000 the sea bresze has decreased to almost zero, and the inversion begins l i f t i n g over the see. Over lend, however, it appears t h a t the e f fec t of advection of the lower inversion has extended fa r ther inland, and C I T shows a decreaee in inversion heightl Frm 2000 t o mid- n ight the convergence due to the land breeze cdu8e8 a genernl rise in the inversion over both the sea and the land, except at C I T where no s igni f icant change is obeerved.

heating continues t o raise the invereion at points Inlmd. Near the shore,

The average behavior of the invereion base has been shown t o f i t in to a coherent pat tern, in which the three processes, inso la t iona l heating, advection, and convergence, all play roles of importance. The deviations of individual days from the average pattern obviously m u s t be due t o varia- t ion i n the contributions of these factors , primarily with the duration of cloudines

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OF 1944 - Earth and Planetary Scienceeps.berkeley.edu/people/lunaleopold/(008) The California... · 2012. 10. 30. · OF 1944 Morrie pelburger Assistant Professor of Meteor University