SATELLITE & MESC#METEQR()LO(;VRESEARCH PROJECT

D~epaIrtmnt~i of the(~oh.sia %i~

The~ I niver.4v ] Iactg-

MESOSCALE ASPECTS OF OROGRAPHIC INFLUENCES

ON FLOW AND PRECIPITATION PATTERNS

by

Tet~suya Fujita

- --7ý

SMRIP Research Paper

NUMBER 67

June 1967

MLi~sohm rFOROLOGY PROJECT .. RESEARCH PAPERS

1. Report on O te Cicag Tornail. of Matrkh 4. Itifil Roidgr A. Blrown and Tetauya, Fujira

2,0 indet to the NSSP Sulfate Netw.,rk f't-i~tiys lujit

3. a Outline of a Techniqx. for Preci.e Reteoi.. ,i atwn,f sit. hulie Cioudi Phi.*.igraphus Teti-iuya PujitA

4.0 Iorimontal Structure oft Mountain Aind.. Ml.,ry A litr.,ii

S. * An Investigation of I)vvirlopmio-wal Pri~ ik f - . the Wali tx-litts..iin Throiugh Fitccas, Pressure Analysis of Nocturnal Showers

Joseph L.. Goldman

6. * Precipitatiti. in the 14N)i lI~iibz.ilt M... -rti- ... K~i ~w.K4h nneti A. Styber

7.00 On a Method of Single and D~ial lImAWv Phii. wr~atimetrv of Panorami,ý Aerial Photographs - Tetituya Pujut&

1. A Review of RCwsrchi's -in Anatlyticil )dicnit iv--roh,g lutsuii l~ujita

I. Meteorohloicail Interrret~atimii mt Corivq ct'.. Nqth..y..tvin Aiis'.urinig in% TIROS Cloud Photograg*, Tejsuys Fupta, ToshnimitsiiUahijima. Wilham A. t14%, and Grtorgv T. Dvhlert. Jr.

10. Study of the Developmunta of Prefrontal Squ ysytti : ivo .' r~n N~s Ntwork Data -Joseph L. fkoldman

1I. Analysisi oftit Seleced Aircraft Dit~i front NSSP (lpt-ration. l~h.2 Tetbuya Fujita

12. Study of a Long Condent~ation rrail Photoigraphed by TIROS I T~ilshimii..u Usthijma

13 A Technique for Precise Analyvi% if Satellite Data, Volume I Ph,)ugramimetry (published at, MSL Report No. 14) - Tetsuya Fujita

14. Investigation oif a Summer Jet Stireami Uoing riiiRs and Aerologii~al Data - Kozo Ninomlys

15. Outline of a 'Theory and Examplvh for Precise Analy%li. of Satellite Radiation Dotsa Tetsuys Fujita

16. Preliminary Res.ult of Analyst., of the Cuniulonimbini. Cloud tof April 21. 1961 -Tetpuya Fujita and James Arnold

17. A Technique fur Precim- Analv~iý if s~iwilitt. rlwiTigaph.ý Telstuya Fujita

is. Evaluation of Limb Darkening trim TIROS III Radiation Data -S. H.H. Larsen, Tetpuya Fujita. and W.L. Fletcher

19. Synoptic Interpretation of TIROiS Ill Meva~urement- oft Infrared Radiation -Finn Pedersen and Tetauya Fujita

30. TIROS 113 Measuremaents. of Terreitrial Radiation .anti Reflected and Scattered Solar Radiation -S.II.I. Larsen. Tetauya Pulits.

and W. L. Fletcher

21. On the Low-level Struuknjre of a ,qull Lint- henry A. llrown

22. Tiunderrtotirmi and the Low-level Jet -Wilhijiin D. Blitwnr

23. The Mestianalymi, of an Organized Coiw-tveive Symiem - Henry A. hi~rown

34. Preliminary Radar and l'hotogramnuivtric Study of the- Illin-uis Tornadoesi of April 17 and 22. 1963 -Joseph L. Goldman and Tetmpy FuJita

23. Use of TIROS Pit turr s for Studies tif the Ioternal Structure oft Tri-pical %torms -Tetsuya Fupita with Recttfted Picrilres from TIOS IOrbit 125. R/O 128 - Tiiuhir~tnisi UhIilimi

26. An E~xperiment in the Ll)~ernhiiattion of ýis~e~irphit. andi Isalloliaric Windi' from NSSP Prespure Data - William 11onnetdi

2.Proposed Mechanism of Hook Fiahui Forn iaon -Teiosuva Fupits with a Preliminary Metiosynoptic Analysis of Tornado Cyclone Case ofMay 26. 1963 -Teltsuya Fujita andl Roiati Sruhnier

28. The Decaying Stage titf hurricanc Anna .'f July l~fil aý Portraveil lv TIROS Cloud P'hotographs and Imiated Radiation from die Top of tie

Storm - Tetpuya Pujita and James Arniold

29. A Technique for P'recisec Ansly-i. ofi SWtelitr Data. Volume It1 Radiation Analyids. Section 6. Fixed-Position Scanning - Tetaujis Pujita

30, Evaluation of Errors in thi. Graphic.al Rectification of1 Sale~ait. rapha - Tetmaya Pujita

3I. Tables of Scan Nadir and Horizintal Angles William D, tne

32. A Simplified Grid Technique for I1.termining Scan Lines 'Wisrted bty i doiine elr- James Hf. Arnold

33. A Study of Cumulus Clouid% tov' tir. rlagittaff Research petork with the 1.111"t &ph* - Viiiotlhy L. brildary andTetuiuya Fujita 1

14. lixe Scanning Printer and It% Application to Dt-tailed AnirtIt5Al6#4 ite Radiation Data etmiy Puita

85. Synoptic Study of Cold Air outbreak over the Mediterraneanl usilg Satelt Ph gaps a Radiation Data Aaamumd Rtabbe and

Triouve Pujita

16. Accurate Calibration of [ivprler Winds (fr their use in fthe .C anowtats&.M~hAaaweWt Plolda -Tot lit ujiws

37. Proposed Operation of Intrumentedf Aircraft for Reserh i Moistu FrePont .1. =UPreasions -Tetaiy Fuialta and DorothY

5L. Braduixry4, 116L - 1

$6, Statiaticil and Kinematical Properties ouf the Low -level Jet Sj :eft¶n Wt~llafiI. 11owier

39 The Dhlinou.. Tornailtr oiil 17 andt 22 April 1Q6r3 -joe~uph L, Iodtnan al1

40. Resolutioni of the Nimbus Hfigh Resolution Infrared Ihdiomilter Tlet nuya Pujita and William Rt. Dlandreft

41. Otn the Datormindtion of the Pitchanife Coeufficientst in Cytrhciv Clouds, - Rodp'r A. Broiwn

* Oil of Print . ,iz 100To he puttliiheit (Continued on iist~ tovw?) G ~O K~~

MESOMFTEOROLOGY PROJECT - - - RESEARCH PAPERS

(Continued from front cover)

42. A Study of Factors Contributing to Dissipation of Energy in a Developing Cumulonimbus -Rodger A. Brown and Tetsuya Fujita

43. A Program for Computer Gridding of Satellite Photograph, for Mesoscale Research -William D. Bonner

44. Comparison of Grassland Surface Temperatures Measured by TIROS VII andAirborne Radiometers under Clear Sky and Cirriform Cloud Condition! - RonaldM. Reap

45. Death Valley Temperature Analysis Utilizing Nimbus I Infrared Data and Ground-Based Measurements - Ronald N1. Reap and Tetsuya Fujita

46. On the "Thunderstorm-lligh Controversy - Rodger A. Brown

47. Application of Precise Fujita Method on Nimbus I Photo Gridding - Lt. Cmd.Ruben Nasta

48. A Proposed Method of Estimating Cloud-top Temperature. Cloud Cover. andEmissivity and Whiteness of Clouds from Short- and Long-wave Radiation DataObtained by TIROS Scanning Radiometers - T. Fujita and H. Grandoso

49. Aerial Survey of the Palm Sunday Tornadoes of April 11, 1965 - Tetsuya Fujita

50. Early Stage of Tornado Development as Revealed by Satellite Photographs -Tetsuya Fujita

51. Features and Motions of Radar Echoes on Palm Sunday. 1965 - D. L. Bradburyand Tetsuya Fujita

52. Stability and Differential Advection Associated with Tornado Development -Tetsuya Fujita and Dorothy L. Bradbury

53. Estimated Wind Speeds of the Palm Sunday Tornadoes - Tetsuya Fu~ita

54. On the Determination of Exchange Coefficients: Part II - Rotating and NonrotatingConvective Currents - Rodger A. Brown

.55. Satellite Meteorological Study of Evaporation and Cloud Formation over theWestern Pacific under the Influence of the Winter Monsoon - K. Tsuchiya andT. Fujita

56. A Proposed Mechanism of Snowstorm Mesojet over Japan under the Influence ofthe Winter Monsoon - T. Fujita and K. Tsuchiya

57. Some Effects of Lake Michigan upon Squall Lines and Summertime Convcction -Walter A. Lyons

58. Angular Dependence of Reflection from Stratiform Clouds as Measured by TIROS IVScanning Radiometers, - A. Rabbe

59. Use of W\et-beam Doppler Winds in the Determination of the Vertical Vclocitvof Raindrops inside Hurricane Rainbands - T. Fulita. P. Black and A. Loesch

60. A Model of Typhoon- Accompanied by Inner and Ouier Rainhands - TetsuyaFu ita, Tatsuo Izawa, Kazuo Watanabe, and Ichi ro Inat

MESOMETEOROLOGY PROJECT - - - RESEARCH PAPERS LU • C/)

(Continued from inside back cover) LU c:>

61. Three-Dimensional Growth Characteristics of an Orographic ThunderstormSystem - Rodger A, Brown.

62. Split of a Thunderstorm into Anticyclonic and Cyclonic Storms and their Motionas Determined from Numerical Model Experiments - Tetsuya Fujita and HectorGrandoso.

63. Preliminary Investigation of Peripheral Subsidence Associated with HurricaneOutflow - Ronald M. Reap.

64. The Time Change of Cloud Features in Hurricane Anna, 1961, from the EasterlyWave Stage to Hurricane Dissipation - James E. Arnold.

65. Easterly Wave Activity oxer Africa and in the Atlantic with a Note on the Inter-tropical Convergence Zone during Early July 1961 - James F. Arnold.

66. Nle.uscale Motions in Oceanic Stratu.s as Revealed by Satellite Data - Walter A.Lyon-, and Tetsuya Fujita.

SATELLITF AND MESOMETEOROLOGY RESEARCH PROJECT

Department of the Geophysical Sciences

The University of Chicago

MESOSCALE ASPECTS OF OROGRAPHIC INFLUENCES

ON FLOW AND PRECIPITATION PATTERNS

by

Tetsuya Fujita

SMRP Research Paper No. 67

June 1967

The research reported in this paper has been sponsored by the Meteorologic,lSatellite Laboratory, ESSA, under Grant Cwb WBG-34 Research. The collectionof precipitation data from the Flagstaff mesometeorological network in 11460and 1961 was supported by the U.S. Air Force Office of Aerospace Re-earch(GRD) under Contract AF 19(604)-7259.

%M[ %11\SPFCTS 01: OR GRAPI IC INFLUTINCFITS

iNI LOW AND PRECIPITATION PATTERNS

Tet~tuya Fji Ia

Departme2nt of the Geophv'sic~al Sciencvý-

T1he Univer'itv of Chcag

Cýhicag- Illinois

ABSTRACT

Since horizontalI dimlens ions of orogranhv Iin rol aon

to cb)oid form atiton and development are miostly Iin the we so -

-cale, we usually ohberve niesoscale nephsystemý In a read

with topographic winluences. Ini addition to their barrier effects',

mountains during the daytime act as effective high-level heat

~-oiirces, or as cloud gene rator.s. At night, however, theyV

iipesthe. Cloud torwationl and act as icloud isptr

kWhet; the',L (A fectJ are- Lom bined with the height of the cove;(c -

tive. cloud ha se-, which could be either higher oir lower than that

of the mountains, the patterns, of orographic nephsyvstemsý and

precipitition arc qu~ite complicated. By using actual cases of

clou'id and prec~ipitation) mclasurenieiit., - detailed cl ima tological

and mecsosy noptic patterns of clouds and precipitation are dis-

1.Orographic Clouds ind Precipitation in Relation toJ the Base of Convective Cloud!-

It is well-kniowni that the worldwide rainfall patterns are closely related to

the large - scale flow patterns and( the orography which blocks the flow in v'arious

wavs. F ye(-n though the role of the mountains upon natural stimulation of precipitation

is not full\, understood, the difference beetween the height of the convective cloud base

and thait of moutnta in tops resultsý in significant variation in the rainfall patterns on

and around the mountains.

When the condIensationr level or cIloud base is, considera bly lower than the

n iointa in tops, CIloud growth anrd prec ipi tat ion taike place oil the upwi Id side of the

The re search reported in this paper hat- beeti spomn-i red by thre MeteorologicalýsatcllIite Labi ralory, FSSA , wider Grant CM) WIG .34 Riare.The collection(i preccipita tion dlata from the Fla gstaff niesonieteotrilogical network in 19(0 andl~ol was supported by the U.S. Air Force Office of Aerospace Research (GRI))

under Contract AF 11)(6f4)-72.59.

mountains. The effcct of solar radiation becomes insignificant as soon as the

upwind slopes are covered with thick convective clouds with high albedo.

The mechanical lifting of moist air is the most important mechanismn for the

release of latent heat of condensation when the stratification is conditionally unstable.

Elliott and Schaffer (1962) showed that the ratio of amounts of mountain and flat-

ground precipitation decreases appreciably with increasing air-mass stability,

suggesting that the orographic stimulation of precipitation does not occur when

the air mass is stable.

Even small mountains blocking a rapid flow of moisture in the trupics,

such as that accompanying hurricanes and typhoons, result in a significant amount

of precipitation on the upwind side of the mountains. Figure 1 shows an example

of orographic rainfall over Japan under the influence ot Typhoon Bess in August,

1963. Winds plotted in the standard form in the lower figure represent velocities

of the radar echoes computed by Fujita et. al. (1967). The height of the cumulus

base on the upwind side varies between 1000 and 2000 ft while the mountains are as

high as 5000 ft. It is of interest to see in the upper chart that along the Pacific

coast the one-hour rainfall ending at 0500Z is only several millimeters. Heavy

rainfall, up to 18 mm, is seen only on the upwind slope of the mountains. No more

than a trace of rainfall is reported from the downwind regions, thus showing the

effective blocking of precipitation by such relatively low mountains.

When the condensation level is located near or above the mountain tops,

convective clouds forming on the peak or over the leeward side of the mountain

produce precipitation as they drift away from the ridge line. As seen in Fig. 2,

such cloud formations are quite common over dry regions. Note that clouds over

Baja California extend northeast from the ridge line of tho Sierra de Juimrez and

that similar cloud formations in Nevada, Utah, and Arizona have the same relative

location with respect to orography.

In such cases, the heating of the mountain slopes plays an important role

upon the acquisition of buoyancy by orographic cumuli. Especially when the low-

level wind is southerly, the orographic clouds forming and developing above the

northern slopes of mountains do not effectively shield the solar radiation reaching

the southern slopes along which the heated air travels upward.

Braham and Draginis (19hO), and Silverman (1960), using data obtained from

an instrumented Air Force aircraft, made detailed analyses of the moisture and

temperature pattern, abo,'e the Santa Catalina Mountains north of Tucson, Arizona.

They revealed that thl ha 111oi st ail-, beCan se of the lifting effect oJI the c rrli-r

and the high-level heating it c! of the mountain, reached 300(0() to 5()()() ft ahove the

mountain-top level aftc' the ýlope w hl(atedl hV theW l11(.1-1i1g sun.

The growth of cumul clouds after their formation above nmouitain-top level

wa- investigated by Glas- and Carlson (1963), who made detailed photogram metric

anialysves of cumulus development over the San Francisco Mountains north of Flag-

staff, Arizona. Their results iutdicite that initial clouds with their base,, at 17, 51 1(1 -

20),0(0(0 it formed a fcw tenth, of a mile downwind from the 12,680 ft peak. After

the formation, each clul drifted away from the ,solrce region while increasing its

volume as much as 1(0 times. They studied mostly small cumuli which were eroded

away after about 10 minutes. These clouds consisted mostly of up- slope thermals

which were mixed with environmnntal air several thousand feet above the moumtain

tops. It is very unlikely that these small clouds would receive a continuous supply

of moist air from the lower levels on the downwind side of tho mountain. If one of

these cumuli developed into a huge cunn.ulonimbbu', it would maintain its circulation

practically independent of the or ,graphy which gave rise to the development of the

initial cloud. Thus, a large storm could move away from the mountains for a great

distance downwind.

2. Distribution of Rainfall Patterns around Isolated Mountains

In an attempt to inves.tigate pressure disturbances associated with the orographic

precipitation around the San Francisco Mountain"s of Arizona, a mesometeorological

nenvork of stations was es-tablished and operated by the University of Chicago and

sponsored by the U. S. Air Force. Figure 3 shows the area doRminated by the San

Francisco Mountains- in nirthern Arizona.

Long before the attempt by the Air Force to study orographic convection

in the Flagstaff area, Indians knew and told the participating scientists that their

rainy seasoin would start as soon as the traditional annual R iTn Dance was performed

at Flagstaff early in Jly. BotV 190(0 and 1961 networks thegan to operate shortly

after the day If the R,,in Dance, which actually signalled the beginning of the moi-t

spcll.

The total precipitation aniowints for July 16-29, 100OIH, plotted and contoured

in Fig. 4 clearly s'l,%v a ma, imunm directly over the mountains, and are surrounded

by a ring- haped zo) i, -f relatively low total anmounts (see hla,,K dots circling thd .

rnouitailns). Out ide this ring were scattercd are.as of high values, up to 2. 2 inch(--.

4

In order to learn whether the 1960 pattern of orographic rainfall represents

a general distributioii or lot, a second and more extensive network was established

in 1961. The number of raingauges was increased from 15() to 220. The pattern of

the total precipitation turned out to he very similar to that obtained in 1960. The

total amount of rainfall in the 1961 period was about twice that of the previous year,

due partially to more days of operation, which were increased from 1 to 19 day.,

and to the increased conve-tive activity. Nevertheless, the low total -precipitation

ring surrounding the mountains appeared practically the same as in 196t).

To represent variation in the rainfall as a function of the distance from

the mountain, six pie-shaped areas were 'ionstructed by drawing six azimnuths,

(00 0 , 0600, 1200, 180', 2400, and 3000, radiating from the center of the mou:tain,.

The scatter diagram (Fig, 6) obtained by plotting the precipitation within each Pic-

shaped area reveals the existence of a low total-precipitation area at a distance of

about 8 1m from the mountain. A ring of maximum rainfall s also seen along the

radius of 10 to 12 nm. This evidence poses the question as to the cause of thcsc

rings of minimum and maximum precipitation.

Fujita, Styher, and Brown's (1962) study of daily precipitation during the

1960 season indicated that the over-mountain rainfall mostly occurred on the days

with low wind speeds and that the distant rainfall, 10 to 12 nm, from the mountain

center, fell quite often at night. Braham (1058) concluded in his study of cumulu-

clouds in Arizona that every one of the cloud parameter., showed marked day-by-day

variations. His statement is very important since it implies difficulties in explaining

the daily regime of orographic convection from the broad synoptic conditions.

From this we may assume that over-mountain precipitation is predominant

when straight convection take.- place under the influence of relatively low wind speeds.

A distant rainfall may occur at night when the mountains are cold, thus acting as

cloud diss-ip.-tors rather than cloud generators. If a very high cloud hase and relatively

high winds, occur simultaneously. it is likely that convective clouds will drift away

some distance from the mountains, producing precipitation far away from the mountains.

Thus, somehow, a minimum amount of precipitation is received along a ring of

about 15 mi in diameter circling the mountains.

3. In fluenIce of Airflow and Moisture

It iý the comrnon; beIcllt ii: northern Arizona that rainfall occurs when nioitst

air, nio-tly from tht southeast, blows against the plateaus and mountains. To

.5

obtain a quantitativL' rL IUtioto-hip betwe.,en the precipitation, the airflow, and thc

humidity, the winds dolt and relative humidities measured at Flagstaff are presented

in Figs-.. 7 and •, respectively. At the upper part of each figure is the variation

of mean daily precipitation obtained by adding total daily amounts from all network

stations and then dividing the total by the number of stations. The mean daily pre-

cipitation, therefore, gives approximately the mean precipitation which fell c;ch

day over the network area.

An overall inspection of these figures does not lead to a conclusion as to

C cause and effect relationship. Therefore, the daily means of wind speed anld rehtive

humidity between the surface and the 10,000 ft level were computed from upper-

air data taken early each morning before the onset of daily orographic convection.

Thes-e valueý are entered in Figs. 7 and 8 below the line representing the station

elevation.

The standard presentation of 3-parameter correlati'n shown in Fig. )

requires more observed values in order to find a conclusive relationship. Nnnethe-

less, the scatter diagram of rainfall amounts vs. relative humidity permits us, to

draw a crude but reasonahl- straight line, indicating that there would be little rain

when the mean relative humidity is less than 10 to 15ý7,. In case the humidity exceed>

80`11 it is likely there will be mere than 0.2 inches (if rai 1 no I tter how weak t(1o

wind speed. Due to the fact thnt there were no cases of high wind speed when the

humidity was 607, or more, it is not feasible to extrapolate the precipitation which

would occur under high-wind and high-humidity conditions. For low-humidity cases,

however, the pre:ipitation tends to increase with increasing wind speed.

The most reasonable empirical lormula for estimating mean precipitation, R,

from V, the wind speeu, and H, the relative humidity would be

R=±(H-O.I)(I+O.OIV)

and RO= when H < 0.1

whero. R is cxpressed in inches and V in knots. The line of a specific value of

in thi> i,,rniula is a rectangular hyperbola with V and H axes translated to

V = -100 kt and H = (). 1, respectively.

4. Split of a ThUnder torm in the Wake Flowv of the San Francisco Mountains

Due to th2 harrier -ind heating effects of the isolated peaks one would expeCt

some unusual characteristics of thunderstorms in the wake region of the mountains.

To monitor such thunderstorms, a time-lapse lb-mm camera was placed on the

rim of Meteor Crater pointing toward the San Francisco Mountains, about 40 mi

away. irspection of a large number of the movies thus obtained during 1901 operatlons

revealed the fact that a precipitation area observed beneath a cumulonimbus cloud

travelling downwind generally split into two separate precipitation areas.

A series of pictures enlarged from the film taken on July 19, 1961, is shown

in Fig. 10. Although the exposures were made for every 15 sec, frames presentedi

in the figure were selected at about 15-min intervlas. The first cumulus over the

mountain appeared at 0845 MST. After repeating the processes as described by

Glass and Carson (1963), a towering cumulus stage was reached around 1000 MST.

A few minutes later, both anvil and virga extended from the cloud.

Shortly after 1030 MST the rain area in the movie widened rapidly until it

split into two parts. Then the one on the left moved almost toward the camera.

A close inspection showed that the left part moved back Alightly toward the direction

of the peaks when viewed from the movie camera. The right part, on the other hand,

moved to the right, very quickly moving out of sight shortly after 1130 MST.

A MRI SX0 radar with 3. 2 cm wave length constructed and operated by

Meteorological Research Incorporated, Altadena, California, was not turned on

until 1127 MST, because the thunderstorm in question was previously hidden bh'hind

Elden Mountain when viewed from Flagstaff Airport, the site of the radar.

When the radar picture taken at 1130 MST and an enlarged cloud picture

from Meteor Crater are combined precisely as shown in Fig. 11, it is found that

the shape of the echoes from 'ie split rain showers displayed rotational charactcr-',tics.

This was e-peciallv true for he one on the right, viewed from the movie camera,

which showed an anticyclonic rotation lasting for over 30 nln after the radar was,

turned on. The other showed a slight cyclonic curvature. The estimated path:§

of the echo before and after the split revealed a remarkable resemblance with the

echo-ýplit phenomenon studies by Fujita and Grandoso (1966). They obtained a

numerical model which included a pair of thunderstorms which rotated in oppý,itc

directions. If their concept of split-echo phenomenon is applied to this observationil

evidence, it may be concluded that an area of wakc flow behind the Sail Francisco

Mouitains extends more than 30) mi downwipd.

A schematic -plan view of an estimated wake flow is shown in Fig. 12. Note

that a zone of anticyclonic vorticity along the northern boundary of the wake and

7

its counterpart .,' ni , ý vcl iic vorticity along the southern boundary are produced

a> a result of the tiarr nr cfect. When a developing thunderstorm moves over this

area >t]cking up the aillr trm the wake region, the left-hand portion (facing the direction

of motion) and the righ,-hand part will quickly accumulate negative and positive

\'orticities, resp-ctively. Thus, the storm rapidly splits into or," with cyclonic,

and the other with anticyclo: ic rotation.

A Flag.-taff soundhnlg taken at 1235 MST, July 19, 1901, is shown in Fig. 12.

It i.- seen that the lay•er- up to 21, 000 ft, slightly above the cloud base, were practically

adiahatic with the moistur-e decreasing slightly with increasing height. This layer

iý- topped b.,, a thick layer of dry air moving from west to northwest at 12 to 15 kt.

The temperature curve for the morning sounding, released at 0840, showed little

differencc from that of the noon sounding. Therefore, the flow probably remained

unchanged during the morning hour-s, acting in favor of the establishment of a steady-

state flow.

5. S•ummlary and Concluion

Thu importance of the height of the convective cloud base in relation to that

of the orography hlocking the flow has been discussed, based m several examples.

In interpreting ,atellhte photographs it is always necessary to keep such regimes of

convection in mind u•bcaus.e the locations of clouds relative to the mountains vary

according t,, the IRIc.,h[t Of the cloud hase.

Thc r ,,graphic precipitation around the San Francisco Mountains was studied

in detail, aind ni rel.a ted ring of light precipitation surrounding the mountains wa's

iitnlJd t01 i'xi-t duiring I th the 1 9•0) and 1961 operations. li order to explain >uch a

di.tribution, the group of mountains is assumed to act as a cloud generator in day-

time and aý a cloud di>> ipator at night. Finally, the wake effect of the mountains,

which give., rise to the echu split phenomenon as proposed by Fujita and Grandoso

(10ino), wa> diLscu..ssd. A large number of satellite pictures over Arizona taken in

July and Aug;i> ere examined in an attempt to find patterns of clouds associated

.vi th the wake 11( miw niouintains. However, no conclusive evidence of wake flow

'v.- ltujlnd. The regionm of wake flow in satellite pictures as described by Hubert and

Lehr (l107) arc con-iderahlv m-re extensive than the island which blocks the flow.

Th( -.e 1-11a1d, >usually isnd ablove the top of a shallow stable layer, but the mountains

i Ar i zrna block only hiv lowcr portion of an adiabatic layer over 10,000 ft thick.

When the nature of such wake flows is clarified in general, it will become feasible

to use satellite pictures for the establishment of both barrier and heating effects' of

topography upon the mesoscale orographic convection.

9

RFFFRENCES

Braham, R. R. , 195S: Cumulus cloud precipitation as revealed by radar- - -Arizoina

l05S. J.. Meteor:., 15, 75-S.3.

'and M. Dragini., , 1960: Roots of orographic cumnuli. . Meteor. , 17,

214- 22t.

Flliott, R. D., and R. W. Shaffcr, 19t2: The development of quantitative relation-

ships between orographic precipitation and air-mass parameters for use in

forecasting and cloud seeding evaluation._J. Appl. Meteor., 1, 218-22h.

Fujita, T. , K. A. Styber, and R. A. Brown, 1962: On the mesometeorological

field studies near Flagstaff, Arizona. J. Appl. Meteor., 1, 20-42.

and H. Grando.s(, I 9on: SIOt of a thunderstorm into anticyclonic and

cvclonic stormin as determined from numerical model experiments. SMIRP

Rc_. Paper N). 62, 4:3 pp. (Available from SMRP).

, T. Izawa. K. Watanabe, and I. Imai, 1967: A model of typhoons accom-

pan eId by inner and outer rainbands. J. Appl. Meteor. , (), 3-19.

Gla-.-, M. , and T. N. Carlson, 1963: The growth characteristics of small cumulus-

clouds. _. Atmos. Sci. . 20, 397-406.

Hubert, L. F. , and P. E. Lehr, 1967: Weather Satellite-,. Blaisdell Publishing

Company, Waltham, Mass., 120 pp.

Silverman, B. A. , l9t)0: The effect of a mountain on convection. Cumulus Dynamics.

Pergamon Press, New York, 4-27.

04-05 Z

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Fig.~~~~~~~~ I nea peo rorpi rcpt -rnwl h ovtsV u 50-ochh pe orvpr prt(i i rmIn- i(( ,iiS iIN

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Fig 4 Daily mean precipitation in ). l0 -inch units plottOd on relative humidity v's. wind speed co-ordinates. General tr(nds,

oi mean preipitation are contoured at 0i, I-inch intervals from the empirical formula in the text. S)lid antd open circle, in the

R if - rainfall (right) and the wind speed Vs. rainfall (upper) diagram represent the values with R H. helow and ahove Sif

re-pe tive-v

17

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