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8/13/2019 Lyle Stratus Study for Forecasting Low Stratus at Randolph AFB, Texas
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B J* . /Irs. tA 7s*r LY L E STRATUS T U D Y
FO R
FORECASTING UL F T R AT U S
A T
RANDOLPH F B, T E X A S
B Y
M A J O R ICHARD . LY L E , U S A F R E S
JANUARY 19(9
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8/13/2019 Lyle Stratus Study for Forecasting Low Stratus at Randolph AFB, Texas
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LYLE STRATUS STUDY
TABLE OF CONTENTS
History of Stratus Study
age
Definition of Gulf Stratus
age
Wind Flow a s a Function of Pressure Gradient
age
Selection of Graphs for Stratus Formation
ag e
Occurrence/NonOccurrence Graph
ag e
Time of Occurrence Graph
ag e
Initial Ceiling Height Graph
ag e 0
Minimum Ceiling Graph
ag e 2
Verification of 1 9 6 8 Data
age 4
Guidelines for Using the Graphs
age 6
Worksheet (Attachment 1 ag e 7
LIST O F ILLUSTRATIONS
Figure 1 . Occurrence/NonOccurrence Graph ag e
Figure 2 . ime of Occurrence Graph ag e
Figure 3 . Initial Ceiling Height Graph
ag e 1
Figure 4 . inimum Ceiling Graph ag e 3
Figure 5 . erification Contingency Table for 1 9 6 8
ag e 5
i
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LYLE STRATUS STUDY FOR
FORECASTINGGULF STRATUS AT RANDOLPH AFB, TEXAS
HISTORY OF STRATUS STUDY
Problem ; Forecasting the occurrence an d dissipation of stratus In
the San Antonio area ha s been a problem since aircraft operations began.
Many studies have been made since 1 9 3 0 t o determine an accurate method
of forecasting the formation of stratus. Some of those that have
worked on the problem have Included J . J . George, Wayne Leach, Zahn and
others no t as well known. os t of the studies have been concerned
with the formation of stratus rather than dissipation although some
concluded that dissipation time varied with the cloud thickness. The
parameters used by most of the studies In the past have Included moisture
and wind flow.
Action: ince the terrain rises from sea level along the Texas Gulf
coast to about seven hundred to eight hundred feet in the San Antonio
area, a nearly saturated ai r mass over the Gulf of Mexico would cool
adiabatically to saturation i f it were moved upslope into the San
Antonio area. further cooling due to nocturnal radlatlonal heat loss
acts to raise the relative humidity and bring an unsaturated ai r mass
toward saturation. he downward sloping terrain toward th e East an d
South makes the favored wind direction fo r stratus formation to be
from 90 ° to 1 8 0 ° an d other directions generally unfavorable since they
would const i tute downs lope wind conditions an d consequent adiabatlc
warming an d drying. Most of th e stratus studies have recognized the
importance of the flow direction and have been largely based upon wind
flow and moisture content. Th e study by Captain Wayne Leach of Kelly
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in 9 54 nd 9 55 sed the radient wind aken ro m San Antonio 500CST
upper winds. The tudy was ery productive ntil th e eather ureau
changed th e wind bservation times o 200CST nd 1800CST nd inally
discontinued he an Antonio winds ri avor f Victoria, exas ounding
in 966. Various tudies ave sed s moisture arameter either elative
humidity or emperature ewpoint pread.
Definition of ulf tratus : Gulf tratus s efined s tratus loud
layer formed y nocturnally cooled gulf ir low. The os t mportant
parameter s measure f he ast h ru outh or ulf low of M ari time
Tropical ir pslope nto he outh entral exas rea. Stratus lso
occurs n h e an Antonio rea ue o rapping f oist ir below
frontal nversion nd ue o verrunning rontal urface n which
moist ir s orced upward y the omponent f ir low normal o he
frontal lope. T he orecast f hese rontally nduced tratus onditions
is largely orecast f the rontal position nd pper ind low, hile
gulf tratus s largely orecast f he ow evel low below ,000
feet.
T he irst bjective work one b y he uthor on stratus orecasting
was onducted t andolph AFB n 961 because f n operational roblem
occurring when student training ould not e egun uring h e id nd
late ooming hours ecause f he ersistence f tratus eiling below
2500 eet. N o eliable ethod f orecasting he time the tratus eiling
would lift bove 500 eet r ecome cattered w as then nown. The
assumption was ade that the tratus breakup time as unction f the
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cloud thickness hrough which the olar ea m must ass to each he
earth s urface. Cloud hickness as etermined y oting he ifference
In eight etween he inimum elling t andolph nd the loud ops
determined y the top f the oisture evel n the an ntonio AOB.
Late reakup as ccasionally oted nd easoned o e caused y strong ow evel ind low ringing dditional oisture rom he ulf
of Mexico.
Further ork was onducted to etermine he ffect f the trength
of he ind low n reakup nd as found o e ore ffective than loud
thickness. Because he ow evel low ould ot e etermined ccurately
enough, pressure radient as hosen s representative f ind low.
T he ow evel ind low s ften ffected y errain, obstructions,
thunderstorms, and ther local eatures nd hereby ften ot epresen-
tative f et low ver arge rea. T he ariability f peed nd
direction f the ocal inds ith time ake etermining ea n low very
difficult. Winds taken t om e ltitude bove he urface that inimizes
local onditions re ore eliable s orecast redictors than he
surface flow onditions ut re taken y ery parse etwork nd
only very ix hours. T he pper ir tations n se nclude rownsville,
Victoria, Del io, and ort Worth, none f hich re ithin 100 iles f
San ntonio. Changes ccurring etween the bservation times re ot
detected for s uch s ix ours. Upper winds re ften ot vailable
at the times eeded r re ot representative y reason f location.
Wind low s Functio n f ressurt.' dra dient ; B y iwlng urf.ico rCMiire
gradient as the ind flow .irnmctor the problen f H mfnj' , of roporls
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Is olved since ourly eports re available from all h e eather ureau
and Flight ervice tations n h e rea. The luctuations f ressure
are lso uch less than those n wind bservations nd h e ressure a n
e bserved ith uch igher degree f ccuracy than h e winds. The
mean wind low s ssumed o b e proportional o he urface ressure
gradient ver the rea. Gradients elected or he tratus issipation
were ouston-Laredo sea evel ressure dded o he ouston-Brownsville
SL P nd plotted ersus time fter unrise f tratus reakup. T he um
of he wo radients esulted n 0 f the ases ndicating breakup
within plus r inus inety inutes. Because f he uccess f the
pressure radient arameter or orecasting tratus reakup, the ame
system was thought ossible or tratus formation.
Data was requested ro m Asheville nd when he ata years f 1951
thru 955 were eceived, San Antonio urface bservations or he ime
period were ollected nd tabulated ccording o tratus occurrence.
Work n he basic graphs as ompleted n anuary 1966.
M a n y raphs ere lotted sing various arameters aken t arious
times rom 800CST o idnight. The tempera t ur e-dewpoint pread t
various oastal tations w as sed as ne arameter lotted gainst ressure
gradient r ombinations f ressure radients. An ptimum spacing or
pressure radient as ound to b e around tw o undred nd ifty miles.
A pacing very uch horter permitted he fluctuations nd ocal ffects
to ask he real pressure lfferenct's with the ariations n ime, at
one tation, often riMter h a n lu' resuuro if f erfncc etween the tatlunH .
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A uch larger pacing ermitted mportant ressure features to e lost
between he tations with mesoscale trough nd idges sometimes oing
undetected nd hereby iving n rroneous icture f he Gulf air
trajectory.
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Selection of Graphs for Stratus Form a t i on
After the graphs were completed In January 1966 using San Antonio
humidity and stratus data, work sheets were devised an d the study was
nspd as a forecast tool at Randolph A F B during the balance of the
year. In January 1967 , the graphs were recomputed using Randolph
data for 1966. Only slight modifications were made in the basic
curves.
Note: The Graphs will ot e alid f ;
1.
andolph s orecast o e nder rontal r quall line
influence.
2.
ver-running s ccurring or orecast.
3. tratus r ain s lready occurring.
Th e Occurrence/Nonoccurrence raph (FIG ) ; Th e raphs lotted generally
indicated better elationship with espect o he radient arameters
than he moisture arameters. The asic radient ersus gradient raph
w as hen rawn nd ound o be ore ccurate han ny ther ombination
of parameters sed t hat time. Th e radients hosen were Houston ea
Level Pressure Brownsville LP) add>d o Houston LP Del io LP)
plotted ersus College Station LP Cotulla LP) . Accuracy f round
8 9 an be xpected rom his asic hart sing only he 1800CST ata.
The ormation f Gulf Stratus s relatively ndependent f moisture with
stratus ccurring ven with ow nitial umidities f trong enough
gradient s resent.
From es n the ccurrence/nonoccurrence raph roceed to he
time f ccurrence raph.
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Time f ccurrence raph (FIG ) ; The im e f occurrence f ulf tratus a s
thought o e elated o he umidity nd pressure radient. A actor
of 0 f he elative humidity was riginally hosen to bring he
humidity term nto he ame rder r magnitude as the radient erm.
A raph was later rawn sing of he umidity with etter erification
than the 1 0 actor. Seventy ercent f h e ases erified within
on e ou r nd ifteen inutes nd eventy-seven percent ithin ne nd
one-half hours f th e rediction sing he actor. Graphs er e
eventually rawn sing relative umidity actors f % , 5 1 0 nd
20%. The raph was ignificantly etter h an he thers when th e
best urve was rawn hrough the lotted ata. F or hose wh o re isturbed
b y the ixed nits f h e ime ndex, consider he radient erms to b e
multiplied y onstant erm of mb-1, and h e ndex herefore o b e
a nitless uantity.
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24 L
\
\2H |63% r S CASES ITHIN ILYLE
TRATUS I fUDY
2 3
22
1 A
j \ : • -M O F C AN T IM E OF CCURRENCE GRAPH
^\ 25 or
C
\
1 86 CASES FORMED ATER THAN MS AFTER MEAN
5 d 8
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CASES MORI r
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EARLI ER H AN MEAN
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SED ON DATA YEARS 1951,52 1 » 4 » ♦
53,54 l6 ( K
o- 0000Z A T A TIME Cl
0
G FO RM S AFTE R L NSE1 (Hrs & Mir S l( 2
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Initial elling Height raph (FIG ); The eight f th e elling t h e
time f ccurrence was thought to ary nversely with th e umidity actor.
The reasoning, although ot ompletely alid ssumption, a s that fairly onstant ixing atio xisted n he ropical low evel flow
and hat he eight f th e aturated ayer would herefore be Inversely
proportional to th e urface umidity nd echanical turbulent ixing.
Since empera t u re-dewpoint pread would b e n ntegrated measure f
humidity nd ixing, It was sed n onjunction with the radient actor.
Ten ercent of th e temperature-dewpoint pread was taken o ring h e
moisture actor nto h e ame rder f agnitude s the radient factor.
F or he 9 6 6 ata, factors f nd 0 or h e emperature-dewpoint
spread were lso alculated but were ot s ccurate s he reviously
used 0 of temperature-dewpoint pread dded o he um of he ressure
gradients. Sixty wo percent f he ases ell nto he + 00 eet urve
a s ompared o he 3 or he an Antonio eather ureau ata. O ne
must gain onsider h e ndex s nitless uantity.
10
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• ; »
u
21
20
LYLE STRATUS STUDY f
21 2 62% or
CASES WITHIN
•
INITIAI . CE
ILING H E
G H T
G R A P H
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h- — 5ED ON D - ATA YEAF S 19 51,52 ',53, 54,'€ 6
2 4 6 8 10 12 1 4 16 I? 20 22 24 26
0000Z DATA CEILING EIGHT-(HUNDREDS OF EET FIG. 3 I i 1
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Minimum Cel l ing Graph (F IG 4 ) T he f inal char t In t he series Is the minimum
celling graph. ind speeds In t he lower two t housand feet of atmosphere
have long been used t o determine the s t ra tus cloud base . I have used
pressure gradient as a measure of the net wind speed due to I t s avai labi l i ty
and representa t iveness . ew curves were drawn for t he Randolph A FB da ta
from 1 9 6 6 which agree wi th t he f requent ly lower ceilings at Randolph
as compared with San Anton io and Kelly AFB. Insufficient cases were
available for the curves to be seasona l ly divided as accurately as desired.
1 2
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.,
i
y L Y L i i E STRATUS TUDY
^FEB MA R APR M A Y AWN
M INIMl EILING RAPH 1
/ te 6JÖL
30 CASES VlTHIN ♦ 300FyOF MEAN
^
A U6
i CASES A P
CT
)
THIN
i WITHIN - 300 F T OF
A MEAN
I7^ASES Wl
t
t 300 F r F MEAN
f
j
>
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i
r
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BASED Or \ 4 *
g Ai >
'A ROM. 1966 \ 1 \ l( ; i : P
1 Ml NIMU M C F ILIN( (H I JNDRI EDS F EET) F IG 4
\
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Verification f 968 Data ; Independent ata rom 968 were ecorded on the worksheet hown n age 17 nd were sed to erify he forecast
study. Throughout 968 he tratus tudy w as sed s ally orecast
tool at Randolph F B nd w as erified or all he forecast arameters.
Results ndicated lightly better verification than he riginal graphs
called or n Occurrence/Nonoccurrence. Height f Ceiling at Formation
an d inimum Ceiling Height an d oorer han xpected n he im e f Occurrence .
Stratus ccurrence s defined s tratus eiling below 500 eet
Initially ccurring after unset but before A0OZ the ollowing morning.
Results f the first ear f ndependent ata 1968) are hown n
FIG .
U
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0
B
S
E
R
V
E
D
Y E S
N O
TOTAL
Y E S
125
17
142
F O R E C A S T
NO
8
87
95
TO TA L
133
104
237
Percent orrect orecasts
9.5
Percent orrect ccurrence orecasts
8.0
Percent orrect on-occurrence orecasts
1.6
Percent orrect ccurrences forecast orrectly 4.0
Percent orrect on-occurrences forecast orrectly 83.7
Heldke kill core
78
Appleman kill core
76
Forecast im e
of
Onset
Within rs
of orecast im e
85 %
Within 1/2 rs f
Forecast im e
58%
Within 1 hr of Forecast Time
4 4
Within 0 in f
Forecast Time
35%
15
T O T A L ASES 125
Forecast eiling
at
Time of nset
Within 300'
of orecast t
74%
Within 00' of
Forecast t
82%
Within 5 0 0 ' of Forecast Ht
8 5
Forecast Minimum Ceiling
Within 300 ' of Forecast Ht
7 4
Within 4 0 0 ' of Forecast Ht
8 3
Within 5 0 0 ' of Forecast Ht
90
FIG 5
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Guidelines or sing he raphs : The raphs hould ot b e sed when
the mechanism of loud ormation s ther h an octurnally ooled
maritime ir low r when th e ecessary arameters rv on-representative.
Soi - . i e f h e ases f on-verification have een ue to erroneous ea
level ressures r apidly hanging ressure atterns. Stratus
formadon n the irst ay f eturn flow from he ulf fter old
frontal assage s usually th e ardest to forecast nd will ften e
later n formation than the time f ccurrence raph ormally ndicates.
About n ighteen our verwater rajectory s ecessary for ufficient
moisture o b e icked p to form stratus. Expect fog n all nd Winter
when ewpoints re igh nd urface r ow evel winds re outhwesterly
and ess h an ight nots. This s egitimate ulf tratus ith he
very weak radient flow odified b y thermal nd errain eatures.
1 6
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7 ATTACHMENT
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Jjy CLASSIFIED ^omT Uisificalion
D O C U M E N T CONTROL D T D (Srcurtly clmnlllcation ol Uli», ody ol mbntmrl and ndtMlnj mnolatlon miiHl r mnftrd hi<n hr yrtmll rpntl n lamUlfd)
I ORGN* TINC ACTIVITY Corpurml* mulhorj
Dst
0,
4th
Weather
quadron
RandDLph AFB, TX blkti
MREPONT SCCURITV C L Altl FIC A TION
UKLASSZRBD
»b . now
N/A 1 Ht ORT TITLt
LYLR TRATUt; TUDY O H ORECASTING GUi.F TRATUM T Al^DniiPH FB , TV XA S
4 DEtCRIRTivC NOTtt Typ» ol rmporl m nd nclunivr dalti)
Final
9 AU THORltl Flril nam», mlddl» nlllml, mtl nmm»)
Major Richard lyle, USAFRKS
« REPORT OATC
January y t
N/A
N/A
B/A
7a. TOTAL NO . OF PACES
17 7b. NO OF REFf
•a. CONTRACT OR GRANT NO.
b. PROJCC T NO.
•a. ORIGINATOR' REPORT UMBERI*)
N/A
*b. OTHER REPORT CI») Any olh»t numbott tml may » m»tl0t»d Ihl» nporl)
N/A
I' DISTRIBUTION STATEMENT
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II. SUPPLEMENTARY NOTES
H/A
12. SPONSORING MILITARY ACTIVITY
Hq 7 th Weather Wing Scott AFB, IL 0 2 2 2 5
Gulf stratus In Texas is defined as a stratus cloud layer formed by a nocturnally cooled gulf air flow. arm moist air from the Texas Gulf coastal area Is cooled to saturation by nocturnal radiation and by adiabatic cooling a s it moves upslope from the coast t o the five to nine hundred foot elevation of central Texas. Forecasting methods have concentrated primarily on th e forecasting of low level moisture an d
wind flow. ressure gradient is used rather than wind flow for this study because pressures were thought to be more representative and timely than wind parameters. The 0 0 Z pressure data gave 8 9Z accuracy fo r occurrence and nonoccurrence of a stratus ceiling at Randolph AFB. Formation of stratus ceilings occurred within one hour an d fifteen minutes in 63 of the cases and formed within 30 0 f t of forecast height in 62 of the cases. Minimum ceilings within 30 0 ft of forecast occurred in about 65 cf the cases.
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8/13/2019 Lyle Stratus Study for Forecasting Low Stratus at Randolph AFB, Texas
http://slidepdf.com/reader/full/lyle-stratus-study-for-forecasting-low-stratus-at-randolph-afb-texas 21/21
Security Claitlflcation
i« K CV WORDS
ROLE ROLE
Meteorology
H undolph F B T X
Local brecast >tudlfts
Gulf tratus
Stratua