lyle stratus study for forecasting low stratus at randolph afb, texas

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

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FORECASTING UL F T R AT U S

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RANDOLPH F B, T E X A S

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M A J O R ICHARD . LY L E , U S A F R E S

JANUARY 19(9

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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



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



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



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



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 .



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.



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.



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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TRATUS I fUDY

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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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LYLE STRATUS STUDY f

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CASES WITHIN

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INITIAI . CE

ILING H E

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0000Z DATA CEILING EIGHT-(HUNDREDS OF EET FIG. 3 I i 1



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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y L Y L i i E STRATUS TUDY

^FEB MA R APR M A Y AWN

M INIMl EILING RAPH 1

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30 CASES VlTHIN ♦ 300FyOF MEAN

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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



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


8 3


90

FIG 5



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.

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7 ATTACHMENT



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

Oils ocument as een pproved Tor public ei.ea.se rd iaJp;

its

distribution la

nUmited.

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.

5D .',r..1473 UWCIAÜJ] Kl Kl» Spiunlv laHSifUnlion



Security Claitlflcation

i« K CV WORDS

ROLE ROLE

Meteorology

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Local brecast >tudlfts

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Stratua

lyle stratus study for forecasting low stratus at randolph afb, texas

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