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"B^J*^. /Irs. J<tA. 7s*r LYLE STRATUS STUDY
FOR
FORECASTING GULF STRATUS
AT
RANDOLPH AFB, TEXAS
BY
MAJOR RICHARD W. LYLE, USAFRES
JANUARY 19(9
««Pfoduced by (he
I This documen I release and
Hi
D D C IJ. I
2^11969
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Uli-- i
t has been approved for public
sale; its distribution is unlimited
c
LYLE STRATUS STUDY
TABLE OF CONTENTS
History of Stratus Study Page 1
Definition of Gulf Stratus Page 2
Wind Flow as a Function of Pressure Gradient Page 3
Selection of Graphs for Stratus Formation Page 6
Occurrence/NonOccurrence Graph Page 6
Time of Occurrence Graph Page 8
Initial Ceiling Height Graph Page 10
Minimum Ceiling Graph Page 12
Verification of 1968 Data Page 14
Guidelines for Using the Graphs Page 16
Worksheet (Attachment 1) Page 17
LIST OF ILLUSTRATIONS
Figure 1. Occurrence/NonOccurrence Graph Page 7
Figure 2. Time of Occurrence Graph Page 9
Figure 3. Initial Ceiling Height Graph Page 11
Figure 4. Minimum Ceiling Graph Page 13
Figure 5. Verification Contingency Table for 1968 Page 15
i
LYLE STRATUS STUDY FOR
FORECASTING GULF STRATUS AT RANDOLPH AFB, TEXAS
HISTORY OF STRATUS STUDY
Problem; Forecasting the occurrence and dissipation of stratus In
the San Antonio area has been a problem since aircraft operations began.
Many studies have been made since 1930 to 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 not as well known. Most 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: Since 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 air mass over the Gulf of Mexico would cool
adiabatically to saturation if it were moved upslope into the San
Antonio area. A further cooling due to nocturnal radlatlonal heat loss
acts to raise the relative humidity and bring an unsaturated air mass
toward saturation. The downward sloping terrain toward the East and
South makes the favored wind direction for stratus formation to be
from 90° to 180° and other directions generally unfavorable since they
would constitute downs lope wind conditions and consequent adiabatlc
warming and drying. Most of the stratus studies have recognized the
importance of the flow direction and have been largely based upon wind
flow and moisture content. The study by Captain Wayne Leach of Kelly
in 1954 and 1955 used the gradient wind taken from San Antonio 1500CST
upper winds. The study was very productive until the Weather Bureau
changed the wind observation times to 1200CST and 1800CST and finally
discontinued the San Antonio winds iri favor of a Victoria, Texas sounding
in 1966. Various studies have used as a moisture parameter either relative
humidity or temperature - dewpoint spread.
Definition of Gulf Stratus: Gulf Stratus is defined as a stratus cloud
layer formed by a nocturnally cooled gulf air flow. The most important
parameter is a measure of the East thru South or Gulf flow of Maritime
Tropical Air upslope into the South Central Texas area. Stratus also
occurs in the San Antonio area due to trapping of moist air below a
frontal inversion and due to overrunning a frontal surface in which
moist air is forced upward by the component of air flow normal to the
frontal slope. The forecast of these frontally induced stratus conditions
is largely a forecast of the frontal position and upper wind flow, while
gulf stratus is largely a forecast of the low level flow below 5,000
feet.
The first objective work done by the author on stratus forecasting
was conducted at Randolph AFB in 1961 because of an operational problem
occurring when student training could not be begun during the mid and
late rooming hours because of the persistence of a stratus ceiling below
2500 feet. No reliable method of forecasting the time the stratus ceiling
would lift above 2500 feet or become scattered was then known. The
assumption was made that the stratus breakup time was a function of the
cloud thickness through which the solar beam must pass to reach the
earth's surface. Cloud thickness was determined by noting the difference
In height between the minimum celling at Randolph and the cloud tops
determined by the top of the moisture level on the San Antonio RAOB.
Late breakup was occasionally noted and reasoned to be caused by a
strong low level wind flow bringing additional moisture from the Gulf
of Mexico.
Further work was conducted to determine the effect of the strength
of the wind flow on breakup and was found to be more effective than cloud
thickness. Because the low level flow could not be determined accurately
enough, pressure gradient was chosen as representative of wind flow.
The low level wind flow Is often affected by terrain, obstructions,
thunderstorms, and other local features and thereby often not represen-
tative of net flow over a large area. The variability of speed and
direction of the local winds with time make determining mean flow very
difficult. Winds taken at some altitude above the surface that minimizes
local conditions are more reliable as forecast predictors than the
surface flow conditions but are taken by a very sparse network and
only every six hours. The upper air stations in use Include Brownsville,
Victoria, Del Rio, and Fort Worth, none of which are within 100 miles of
San Antonio. Changes occurring between the observation times are not
detected for as much as six hours. Upper winds are often not available
at the times needed or are not representative by reason of location.
Wind Flow as Function of Pressurt.' dradient; By iwlng surf.ico prCMiire
gradient as the wind flow p.irnmctor, the problen nf Hmfnj', of roporls
Is solved since hourly reports are available from all the Weather Bureau
and Flight Service Stations in the area. The fluctuations of pressure
are also much less than those in wind observations and the pressure can
be observed with a much higher degree of accuracy than the winds. The
mean wind flow is assumed to be proportional to the surface pressure
gradient over the area. Gradients selected for the stratus dissipation
were Houston-Laredo sea level pressure added to the Houston-Brownsville
SLP and plotted versus time after sunrise of stratus breakup. The sum
of the two gradients resulted in 70% of the cases indicating breakup
within plus or minus ninety minutes. Because of the success of the
pressure gradient parameter for forecasting stratus breakup, the same
system was thought possible for stratus formation.
Data was requested from Asheville and when the data years of 1951
thru 1955 were received, San Antonio surface observations for the time
period were collected and tabulated according to stratus occurrence.
Work on the basic graphs was completed in January 1966.
Many graphs were plotted using various parameters taken at various
times from 1800CST to midnight. The temperature-dewpoint spread at
various coastal stations was used as one parameter plotted against pressure
gradient or combinations of pressure gradients. An optimum spacing for
pressure gradient was found to be around two hundred and fifty miles.
A spacing very much shorter permitted the fluctuations and local effects
to mask the real pressure dlfferenct's with the variations in time, at
one station, often ^riMter than tlu' presuuro differfncc between the MtatlunH.
A much larger spacing permitted Important pressure features to be lost
between the stations with mesoscale trough and ridges sometimes going
undetected and thereby giving an erroneous picture of the Gulf air
trajectory.
Selection of Graphs for Stratus Formation
After the graphs were completed In January 1966 using San Antonio
humidity and stratus data, work sheets were devised and the study was
nspd as a forecast tool at Randolph AFB 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 not be valid if;
1. Randolph is forecast to be under frontal or squall line
influence.
2. Over-running is occurring or forecast.
3. Stratus or rain is already occurring.
The Occurrence/Nonoccurrence Graph (FIG 1); The graphs plotted generally
indicated a better relationship with respect to the gradient parameters
than the moisture parameters. The basic gradient versus gradient graph
was then drawn and found to be more accurate than any other combination
of parameters used at that time. The gradients chosen were (Houston Sea
Level Pressure - Brownsville SLP) add>d to (Houston SLP - Del Rio SLP)
plotted versus (College Station SLP -'Cotulla SLP). Accuracy of around
89% can be expected from this basic chart using only the 1800CST data.
The formation of Gulf Stratus is relatively independent of moisture with
stratus occurring even with low initial humidities if a strong enough
gradient is present.
From a yes on the occurrence/nonoccurrence graph proceed to the
time of occurrence graph.
97 %
I OCCURRENCE 33 NON-OCCURRENCES
NO
-99|0/« OCCURRENC ES
18 NON-OCCURRENCES
-6«- 3 -2
1 [ LYLE. STRATgS gTUPY
OCCURRENCE/NON-OCCURRENCES
% PROBABILITY OF GULF STRA CEILING BELOW
7)4 OCäURRENCES 26 NON-OCCURRENCES
GRA •H
2500 FEET BEFORE I400Z
209 C 14 H
94%
us
OCCURRENCES ON-OCCURRENCES
13 OCCURRENCES 36 NON-OCCUR
91 % II OCCURRENCES
RENCES
97 NON-OCCURRENCES
"99 % NO OCCURRENCES
22 NON-OCCURENCES
BASED ON DATA i
TT*
NO
YEARS 1951,52^53,54,66 i 7 i <
OOOOZ DATA (PCLL+^COT) MBS FIG. I it
>
Time of Occurrence Graph (FIG 2); The time of occurrence of Gulf Stratus was
thought to be related to the humidity and pressure gradient. A factor
of 10% of the relative humidity was originally chosen to bring the
humidity term into the same order or magnitude as the gradient term.
A graph was later drawn using 5X of the humidity with better verification
than the 10% factor. Seventy percent of the cases verified within
one hour and fifteen minutes and seventy-seven percent within one and
one-half hours of the prediction using the 5% factor. Graphs were
eventually drawn using relative humidity factors of 0%, 5%, 10% and
20%. The 5% graph was significantly better than the others when the
best curve was drawn through the plotted data. For those who are disturbed
by the mixed units of the time index, consider the gradient terms to be
multiplied by a constant term of lmb-1, and the index therefore to be
a unitless quantity.
24 L
\
\2H |63% or S CASES WITHIN
ILYLE STRATUS SI 1 1 1 1 1
fUDY 1 1 2 3
22
r 1 A i
j \ 4 : • -M .OF MC
\
AN
1 V T IME OF OCCURRENCE GRAPH
1 1 1 1 1
^ \ 25 % or
Z r
C 1
\
1 1 1 1 86 CASES FORMED LATER THAN MS AFTER MEAN
5 d 18
Q.
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CASES MORI r i THA N
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EARLI ER TH AN MEAN |
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SED ON DATA YEARS 1951,52 1 » 4 » ♦ !
, 53,54l6( 1 •>
K o-
0000Z DATA TIME Cl 0
G FO RMS AFTE :R SL INSE1 T (Hrs & Mir \S Fl( 3. 2
Initial Celling Height Graph (FIG 3); The height of the celling at the
time of occurrence was thought to vary Inversely with the humidity factor.
The reasoning, although not a completely valid assumption, was that a
fairly constant mixing ratio existed in the tropical low level flow
and that the height of the saturated layer would therefore be Inversely
proportional to the surface humidity and mechanical turbulent mixing.
Since temperatu re-dewpoint spread would be an Integrated measure of
humidity and mixing, It was used in conjunction with the gradient factor.
Ten percent of the temperature-dewpoint spread was taken to bring the
moisture factor Into the same order of magnitude as the gradient factor.
For the 1966 data, factors of 5% and 20% for the temperature-dewpoint
spread were also calculated but were not as accurate as the previously
used 10% of temperature-dewpoint spread added to the sum of the pressure
gradients. Sixty two percent of the cases fell Into the + 300 feet curve
as compared to the 63% for the San Antonio Weather Bureau data. One
must again consider the index as a unitless quantity.
10
• ■
; »
u
21
20
1 1 LYLE STRATUS STUDY /
f 212
62% or CASES WITHIN
•'
1 INITIAI
1 . CE
1 ILING HE GHT
1 GRAPH
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i30< 3 FT ( DF ME *
AN
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58 CASE S FOR MED J f / /
f LOV fER T HAN I IOOFT O/M iAN
' /
Pin / / /
f
Q.I0 l / > / 3
J[ f / / 73 CASES FORMED HIGHER
It CD
d)=(IH
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THA N 30( )FT 0 F ME/ \H
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I
h- f— H 5ED ON D- ATA YEAF \S 19 51,52 ',53, 54,'€ .6
2 4 6 8 10 12 14 16 I? 20 22 24 26
0000Z DATA CEILING HEIGHT-(HUNDREDS OF FEET) FIG. 3 I i 1
Minimum Celling Graph (FIG 4); The final chart In the series Is the minimum
celling graph. Wind speeds In the lower two thousand feet of atmosphere
have long been used to determine the stratus cloud base. I have used
pressure gradient as a measure of the net wind speed due to Its availability
and representativeness. New curves were drawn for the Randolph AFB data
from 1966 which agree with the frequently lower ceilings at Randolph
as compared with San Antonio and Kelly AFB. Insufficient cases were
available for the curves to be seasonally divided as accurately as desired.
.
12
- .,
/
i 1
y /
LYL i i i i i i E STRATUS STUDY /
^FEB MAR APR MAY AWN
■
M INIMl M CEILING GRAPH 1 i i i i
/ te % / i i /
6JÖL %
/
30 CASES VlTHIN ♦ 300FyOF MEAN
■^
/ / /
AU6
i 1 CASES
/ A EP CT
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THIN
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WITHIN -
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A MEAN
I7^ASES Wl
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'A FROM. 1966 \ 1 % \ • l( ) i i i; i i: \ 1 P.
1
Ml NIMU M CF :ILIN( 5 (HI JNDRI EDS ( )F FEET) FIG 4 \
Verification of 1968 Data; Independent data from 1968 were recorded
on the worksheet shown on page 17 and were used to verify the forecast
study. Throughout 1968 the stratus study was used as a dally forecast
tool at Randolph AFB and was verified for all the forecast parameters.
Results Indicated slightly better verification than the original graphs
called for on Occurrence/Nonoccurrence. Height of Ceiling at Formation,
and Minimum Ceiling Height, and poorer than expected on the Time of
Occurrence.
Stratus occurrence is defined as a stratus ceiling below 2500 feet
Initially occurring after sunset but before 1A0OZ the following morning.
Results of the first year of independent data (1968) are shown in
FIG 5.
U
0 B S E R V E D
YES
NO
TOTAL
YES
125
17
142
FORECAST
NO
8
87
95
TOTAL
133
104
237
Percent correct forecasts 89.5 Percent correct occurrence forecasts 88.0 Percent correct non-occurrence forecasts 91.6 Percent correct occurrences forecast correctly 94.0 Percent correct non-occurrences forecast correctly 83.7
Heldke Skill Score .78 Appleman Skill Score .76
Forecast Time of
Onset
Within 2 hrs of Forecast Time
85%
Within 1 1/2 hrs of Forecast Time
58%
Within 1 hr of Forecast Time
44%
Within 30 Min of Forecast Time
35%
15
TOTAL CASES 125
Forecast Ceiling at
Time of Onset
Within 300' of Forecast Ht
74%
Within 400' of Forecast Ht
82%
Within 500' of Forecast Ht
85%
Forecast Minimum Ceiling
Within 300' of Forecast Ht
74%
Within 400' of Forecast Ht
83%
Within 500' of Forecast Ht
90%
FIG 5
Guidelines for Using the Graphs: The graphs should not be used when
the mechanism of cloud formation is other than a nocturnally cooled
maritime air flow or when the necessary parameters arv non-representative.
Soi-.ie of the cases of non-verification have been due to erroneous sea
level pressures or rapidly changing pressure patterns. Stratus
formadon on the first day of return flow from the Gulf after a cold
frontal passage is usually the hardest to forecast and will often be
later in formation than the "time of occurrence" graph normally indicates.
About an eighteen hour overwater trajectory is necessary for sufficient
moisture to be picked up to form stratus. Expect fog in Fall and Winter
when dewpoints are high and surface or low level winds are Southwesterly
and less than eight knots. This is a legitimate Gulf stratus with the
very weak gradient flow modified by thermal and terrain features.
16
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17 ATTACHMENT 1
Jjy CLASSIFIED ^omT^Uisificalion
DOCUMENT CONTROL DATA R&D (Srcurtly clmnlllcation ol Uli», hody ol mbntmrl and IndtMlnj imnolatlon miiHl hr mnftrd whi<n Ihr nyrtmll rrpntl In rlamUlfd)
I ORIGIN* TINC ACTIVITY (Corpurml* mulhorj
Dst 10, 24th Weather Squadron RandDLph AFB, TX Jblkti
M.REPONT SCCURITV C L Altl FIC A TION
UKLASSZRBD »b. anow
N/A 1 Ht^ORT TITLt
LYLR STRATUt; STUDY POH PORECASTING GUi.F STRATUM AT HAl^DniiPH AFB, TV'XAS
4 DEtCRIRTivC NOTtt (Typ» ol rmporl mnd inclunivr dalti)
Final 9 AU THORltl (Flril nam», mlddl» Inlllml, Imtl nmm»)
Major Richard W. lyle, USAFRKS
« REPORT OATC
January ]ytj
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 NUMBERI*)
N/A
*b. OTHER REPORT NCI») (Any olh»t numbott Html may b» m»tl0t»d Ihl» nporl)
N/A
I' DISTRIBUTION STATEMENT
Oils document has been approved Tor public rei.ea.se ard fiaJp; its distribution la unUmited.
II. SUPPLEMENTARY NOTES
H/A
12. SPONSORING MILITARY ACTIVITY
Hq 7th Weather Wing Scott AFB, IL 02225
Gulf stratus In Texas is defined as a stratus cloud layer formed by a nocturnally cooled gulf air flow. Warm moist air from the Texas Gulf coastal area Is cooled to saturation by nocturnal radiation and by adiabatic cooling as it moves upslope from the coast to the five to nine hundred foot elevation of central Texas. Forecasting methods have concentrated primarily on the forecasting of low level moisture and wind flow. Pressure gradient is used rather than wind flow for this study because pressures were thought to be more representative and timely than wind parameters. The 00Z pressure data gave 89Z accuracy for occurrence and nonoccurrence of a stratus ceiling at Randolph AFB. Formation of stratus ceilings occurred within one hour and fifteen minutes in 63% of the cases and formed within 300 ft of forecast height in 62% of the cases. Minimum ceilings within 300 ft of forecast occurred in about 65% cf the cases.
/
5D .',r..1473 UWCIAÜJ] Kl Kl» Spiunlv ClaHSifUnlion
Security Claitlflcation
i« K CV WORDS
ROLE ROLE
Meteorology
Hundolph AFB, TX
Local Fbrecast i>tudlfts
Gulf Stratus
Stratua
UMCUUSIPIkD KiMurity CluNsiricmion
