(006) the Structure of the Local Winds in the Los Angeles Basin

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TABLE OF CONTENTS Page Summary 1 Introduction 1 Surface Winds 2 Diurnal Wind-Regime Aloft at Uc3A 6 Average Diurnal-Wind Time-Section at u(IIA 9 Areal Pattern on an Individual By 10 Ref e rence s 11

LIST OF FIGURES Figure Title Number

1 Diurnal Changes of Surface Winds in the IDS Angeles Basin, August 1944

2 Prevailing-Wind Flow at Hour of Maximum Sea-Breeze Development (Late Afternoon), over Los Angeles Basin, August 1944

Sample Wind Time-Section, UM, 1 September 1944

4 Time Section ofwinds above KIA, 18-29 Aug- ust 1944

5 Time Section of Winds above Ua, 30August- 10 September 1944.

6 Predominant Daily Winds at UUA

7 Wind-Direction Frequency Roses-- a. 14 Aug. 17 Sep 1944 0300-1100 PWT b. n 1300-1700 PWT n C. 1900-2400 PWT

8 Wind-Direction Frequency Roses-Ua a. 18 Sep-20 Nov. 1944 0300-0900 pwf b. n 1100-1500 m n C. 1700-2400 PWT

9 Wing-Di rectiqn Frequency Roses-m a. 8-31 Jan. 1945 0500-0900 PWT b. n 1100-1500 PRT n C. 1700-,1900 PWT

10 Wind Time-Sections at Various Stations 5-6 September 1944 Oxnard, UUA, Mines Field.

11 Wind Time-Sections at Various Stations 5-6 September 1944 VanNuys, Cal. Tech, 9ntario.

12 Profiles of Land and Sea Winds at Various Hours. Los Angeles Basin, 5-6 September 1944

13 Upper-Air Charts, 5-6 September 1944. STRUCTURE OF THE LOCAL WINDS IN THE LOS ANGELES BASIN

SUMMARY Average-wind t ime-sections- are presented for one station for each of the From a large number of slow-ascent pilot- seasons: summer, fall and winter. balloon observations the three-dimensional structure in time of the sea and land breeze regime is described for the Los INTRODUCTION Angeles Basin, California. The cooperative investigation of The sea breeze is directed approxi- coastal stratus in southern California dur- mately perpendicular to the local coast ing the summer of 1944 (1) emphasized the line but follows up the thalweg of inland importance of the sea-land breeze regime as valleys. a factor influencing the local weather. The diurnal oscillation of the height of The sea breeze tends to merge in the the subsidence inversion was found to be lowest levels with valley winds, and the explained ,in large part by this coastal land breeze with mountain winds. The sea- wind pattern. During the stratus investi- land breeze regime is confined to the gation a large amount of wind data were coastal plain west of the coastal mountain collected and augmented by additional ob- range, but flows through the most westerly servations in the late fall and winter. passes and is felt by stations just east of These data constitute the basis for the these passes. present rctport. It is concerned primarily with a description of the sea-land breeze At higher levels there is a tendency regime in the Los Angeles Basin. A discus- for the sea-land breeze pattern to merge sion of some related weather phenomena and with the ocean-continent winds which are the results of attempts to forecast the controlled by the thermal low over the beginning and end of ,the sea breeze on southwqstern desert. When the therm1 low particular days will be covered in a later persists both night and day for short paper. periods of time and is strong even at levels up to 5000 feet, the ocean-con- Most of the data were collected by the tinent regime approaches the character of a Army Weather Station at the University of true monsoon. However, when the land wind California at Los Angeles (VU) by means is strong it separates the sea breeze in of s low - asc e n t p i 1o t - b a 11o ons wh ich a 11 ow- the lower layers from the westerly ocean- ed observations to be taken at vertical in-5 continent wind by a layer of easterly land tervals of about IS0 feet. The periods of winds. observations were 15 August to 20 November 1944 during which 570 ascents were observed, When a high pressure center aloft and 8 to 31 January 1945, 149 ascents. centers over the Great Basin, easterly flow Each ascent was run to 5000 feet if circum- due to the general circulation may merge stances allowed, which constituted approxi- with the land wind and confine the westerly mately 30 readings taken at intervals of sea breeze to a small depth and short either 25 or 30 seconds. In addition, duration. analyses were made of the ordinary pilot- balloon data and surface winds for sur- The combined sea-valley breeze is rounding stations. Of particular import- first noticable near the coast and makes ance is this study were the Rawin data its appearance progressively inland. It taken at Mines Field (XA) which is located deepens during the course of the afternoon nine miles SSE of UCLA. at all stations and reaches a greater depth in summer than in. winter. The description of the wind patterns necessitated some discussion of possible The combined land-mountain wind causes of observed phenomena in order to appears first at stations along the mountain set up a nomenclature to be usedo It is slopes near the coast and progressively af- not presumed that the present report is a fects stations more inland. At times it 'discussion of all factors related to a appears last at the coastal stations. It complete synoptic description of the wind deepens faster inland than at the imedi- regime. However, the presentation of basic ate coast. data in organized form is one of the pre- 8 1 liminary steps toward the complete under- typical of the summer conditions with little standing. change in the general circulation from day to day. Space limitation required the use In this report the sea-land wind of call letters to identify stations on regime is defined as a diurnal oscillation some of the figures. These are listed, of direction at any level due to the effect together with station elevations, in Table of the land-sea temperature difference. A 1 below. diurnal change in wind direction in the Los Angeles Basin is found both below and above TABLE I the temperature inversion which is a characteristic of the summer in this area. CALL LETTERS AND STATION ELEVATIONS Below the inversion the marine stratum is moist compared with the air aloft, but in the present definition, the sea breeze may call Elevation occur both in the lower moist and in the Station Letters (Ft. msl.) upper dry air. Oceanside oc 10 The valley-mountain wind regime is El Tor0 (Marine Corps 332 similarly defined as a diurnal oscillation Air Station) of direction at any level due to temperature Orange County Army Air Dsw 52 differences between the air over the valley Field and air over the lower plain. The%up- Long Beach Army Air HB 40 valley wind is termed simply the valley Fie Id wind while the down-valley wind is called San Pedro (Naval Air Ncx 10 the mountainwind. In general, slope or Station) drainage winds induced by temperature dif- Lomita Flight Strip 106 ferences between the free air and the sur- US Angeles Airpoit 96 face of the mountain slopes are considered (Mines Field) to be part of the valley or mountain wind. University of Cali f ornia 505 at Los Angeles As will be shown, most stations in the Glendde (Grand Central DGX 46 2

Los Angeles Basin experience winds which Terminal) 9 consist of a merger of the sea and valley Van Mys (Metropolitan DVN 7 97 breezes or of the land and mountain breezes. Airport) Surface winds of this type, as well as Newha 11 1190 cases when the true classification cannot Oxnard Flight Strip 82 be determined, will be termed the "sea- Pasadena (CIT) 758 valley" and "land-mountain" winds, respect- Oltario Army Air Field 9 20 lively. Since the studies of the winds San Bemardin0 Army Air 1098 aloft largely involved only stations near Field the coast, where the land and sea breezes Riverside (March Field) 1528 are the primary effects, the term "sea Beaumont 2592 breeze" when applied to winds above the sur- Palm Springs Army Air 420 face will be understood to include any FieId effects of the valley wihd. Similarly the San Nicolas Island NCB so5 term "land breeze", when applied to the (Naval Air Station) upper air will include mountain winds. San Clemente Island NSA 906 (Naval Air Station) Pacific War Time is used throughout Burbank (Lockheed Air 7 25 this report. Terminal) Los Angeles (WW.B.City 5 12 Off ice) SURFACE WINDS Wind direction frequency roses were GENERAL PICTURE prepared for 21 stations from the hourly teletype reports. From these wind roses The diurnal changes in the surface the prevailing wind direction during various winds at a number of stations in the Los portions of the day could be described. Angeles Basin were studied in detail for The surface wind regime for each station is the month of August 1944. Although only summarized in Figure 1 on which are in- 31 days were thus used in deriving the dicated the average time of beginning and averages, it is believed that this month is ending of the prevailing directions. Any

2 hours not included in the station model may ward San Gorgonio Pass in which Beaumont be considered to be times of variable wind (TUT) is located. The sea-valley wind direction, usually the transition fran one regime is felt at Palm Springs (IIPZ) east regime to another, The Beaufort forces of the San Gorgonio Pass and 80 miles in- given by the barbs on the arrows represent land frqm the coast, and slightly affects the average maximum daily speeds. the winds at San Clemente Island (NGA), 60 miles out to sea. San Nicolas Island Some explanation of the criteria used (-1, 80 miles from the coast, apparently to decide when a wind is prevailing may be is not subject to the diurnal shifts of the desirable. A certain amount of subjective land-sea breeze regime, interpretation of the wind roses was required. In general, however, if the per- The effect of the "Catalina Eddy" is centage of winds from a particular 45O apparent in the change from prevailing Nw sector was more than twice the percentage winds at San Miguel Island (not shown in from any other 45' sector (or calms) the Figure 1 but about 80 miles NW of NCB) to direction represented by the center of WNW winds at NQB and to WSW winds at %A. this 45' sector was considered a "pre- lhis appears to *be a semi-permanent feature vailing" wind at that hour. This defi- of the circulntion rather than a diurnal nition permitted the analysis of the night 2 f f ec t. breezes which usually showed rather low f requeiicies but were nevertheless apparent The northernmost streamline in Figure because of the even distribution of the 2, representing westerly flow over UCLA, other iirections. All calms indicated as turns into a stream over the Santa Monica "prevailing" in Figure 1 represented over mountains at Cahuenga Pass about 5 miles 50% of the observations for the hours given. west of the Los Angeles River. This has been verified by "trajectory balloons" The daily sea breeze was apparent at released at UCLA. These balloons were a11 stations in the basin. In addition, inflated to a very small or zero free lift, a light land or mountain breeze was ob- released, and observed with theodolites. served at most stations during the night. @ly a very few runs were made but some In marly cases, however, the effects of balloons were observed to return up the mount,iin and valley breezes are quite San Fernando Valley to the meridian of difficult to separate from the sea and land UCLA after passing over Cahuenga Pass. breeze circulation, since both tend to The strong subsidence inversion, combined move in similar directions. All surface with the negligible free lift, succeeded winds are influenced markedly by the in keeping most of the balloons to eleva- topography, even the lowest hills playing tions below 2500 feet. a noti:eable part in the summer circulation Hills lower than 1000 feet could-not be It will be noted from Figure 1 that shown in Figure 1 but their effect will be the night winds shown in dotted arrows mentioned in the discussions. For a more are present at nearly all stations as far detailed map of the topography of the Los inland as March Field (RV) and seaward as Angelcs Basin the reader is referred to far as San Clemente Island. These night Figurc: 2, taken from the report of Cali- winds are, however, weaker than the sea foknis. Stratus Investigation of 1944 (1) * breeze at all stations. 'he daytime wind regime is indicated by the solid arrows in Figure 1. Wind The details of the diurnal regimes directions characteristic of the late at various stations are more complicated afternoon are shown with heavier lines. than the general picture just described. These should represent the flow near the time of maximum sea breeze, which is also shown by the streamlines in Figure 2, It is immediately apparent from the ov SFA AND VAUEY BREEZES picture of the afternoon sea breeze the. flow of air comes onshore approximately (a) ?he sea breeze and the effect of perpendicular to the local coast line, but valley winds.- At some stations a westerly once .Inland is guided up the main valleys wind begins two or three hours after sun- toward the passes. The flow splits into rise (about 0620 in August) but comes from two miin streams, as can be clearly seen a direction slightly different from that on Figure 2.- One stream flows northeastward of the maximum sea breeze. This early wind into the San Fernando Valley where Burbank can usually be interpreted as a valley wind (BU) and Van Nuys Metropolitan Airport which coinmences earlier than the more (DW are located, thq other eastward to- general sea breeze.. 3 Evidence of the valley wind merging stations near the coast. Under such con- into a sea breeze is also provided by the ditions noticeable changes in both temper- fact that the westerly sea-valley wind ature and moisture occur simultaneously begins earliest at stations located on a with the wind shift. This effect may be n,qrrow coastal plain. Rapid heatirlg of quite startling at locations within about the mountain slopes would cause the valley a mile of the shore, when the previously breeze to start prior to the sea breeze inaudible roar of the surf strikes the ears in these cases. of the observer.

Analysis of the diurnal changes at (b) Effects of the San Fernando Orange County Army Air Field (DSW) is Valley.- The reversal of wind at Newhall typical of the reasoning which may be ap- 0 at 1100 may be explained as the change plied to many other stations. The sea- from a light northwesterly breeze blowing v:tlley breeze begins at about 0930 from the up the Santa Clara Canyon to the strong WSW and backs gradually to SW by 1430. southeasterly sea-valley breeze caning over T?is change of direction is opposite to Newhall Pass from the San Fernando Valley that which would be expected as the result (see Figure 1). It is interesting to note of the approach of a true sea breeze to that there is practically no evidence of a balanced flow. During the progress of the sea breeze reaching NH by way of the Santa day the Coriolis force would tend to pro- Clara Valley. This would be a route of dilce veering of wind at coastal stations. nearly the same length as the trajectory The WSW wind at the time of onset may be followed in passing up the San Fernando regarded, however, as the vector sum of a Valley. The reason for the predominance 9Y sea breeze perpendicular to the coast, of the latter flow probably lies in the and P W breeze blowing into the valley east broader expanse of low elevation in the San 01' Santa Ana which can be seen in Figure 2. Fernando Valley, with relatively small loss The sea breeze effect increases as the day by diversion as up-valley winds into the progresses, becoming predominant by 1430 and higher mountain ranges. cimtinuing until two hours after sun'set. In general, the speed attained by the Flow in the San Fernando Valley also sea-valley breeze appears to be nearly the undergoes peculiar changes early in the same at all stations. Since a true sea evening, a definite shift beginning at sun- breeze should diminish in speed farther in- set (about 1945 in August). The easter- hind it is again apparent that a merging of ly or southeasterly winds during the first the sea breeze into the valley wind takes half of the night, although indicated as place. Noticable increases in speed at a night winds in Figure 1, seem to be a f tw stations (e.g., IX)I, IW, RQ are pro- continuat ion of the sea breeze. b:ibly the result of channelization of the flow by topography rather than thermal At least part of the explanation of elcf e c t s . this flow up the valley for a large portion of the night must lie in the relatively It will also be noted that the onset broad expanse and gentle slope of the OF the sea-valley breeze at inland stations valley. According to theory based upon such as Newhall (NH) and March Field (RVI the German investigations (2), the time occurs only one or two hours later than at required to reverse from the valley breeze the coastal stations. 'Ihis is insufficient to a mountain (down-valley) breeze depends time for the air which crossed the coast inversely upon the inclination of the there at the onset of the sea breeze to valley floor. This is because the adiabatic hive reached the inland stations, even if cooling processes depend upon the amount tile seaabreeze front moved with the speed of verticrl motion which can take place. of the sea wind. Moisture content of the The steep inclination of the eastern side air is, therefore, not a characteristic of of the San Fernando Valley, as well as the the sea-valley breeze in this report. raqd radiational cooling of the high Fi-esh marine air will, of course, be carried mountain areas, probably accounts for the inland by the sea breeze later in the day, shift to more easterly winds at 2000. The but the change of wind direction may occur small inclination of the valley floor, how- several hours before the arrival of the new ever, tends to retard the disappearance of air at the inland stations. the valley wind and the development of the reverse flow, A similar explanation will nodoubt account f0.r the late ending of the 01 days when an off-land pressure gra- sea-valley breeze at March Field (RV). dient exists the passage of the sea-breeze "front" frequently has been observed at (c) Effect of the San Pedro Hills.-

4 The Sari Pedro Wills, which surmount the Valley a' light N wind hegins at 0100, A point of land west of San Pedro Bay, set up similar downslope wind commences at 0330 at a small wave in the sea-breeze circulation the Lockheed Air Terminal (Ru) and at about near Long Beach as can be seen hy the wave 0230 at Grand Central Air Terminal (I=). in the streamlines on Figure 2. The sea he to its position in the narrow part of bree%e at Long Beach (HB) begins from the the valley the flow at IXX reaches great- SSE at 1000, veering gradually to SSW by er speeds than at the other stations. This 1200, where it remains until 1430. At San flow joins a broader easterly flow which Pedro (r4a) the sea-valley breeze comnences crosses the coast along Santa Monica Ray. at 0800 from'the SE, veering gradually to a Cold air From the San Fernando Valley may steady '3SW by 1100. Lomita Flight Strip account for the northeasterly beginning of 0took three-hourly obscrvations only, the land breeze at Mines Field (LA), the so the times of onset and cessation of the wind veering gradually to E as the flow predominant winds are not indicated. At from the Santa Ana River basin increases. 2330 a southerly drainage-wind prevails, ?he southeasterly wind at IDng Reach Army changing to SW at 0230 and 05-30. The Air Field (HB) during the night and the strong WNW sea breeze is prqdominant from easterly flow at Mines Field (ui) seem to 1130 tcl 2030. A wave in the streamlines indicate that Saota Monica Ray is a over San Pedro Bay must thus occur between major outlet of the night winds. The SE 1000 and 1430. This wave either mows to breeze at HB (which-is actually upslope) the south or decreases considerably in no doubt results fran the guiding effect of amplitude by 1430 to permit the arrival at Signal Bill and adjacent ridges. This HB of the WMni flow from Santa Monica Bay. chain of low hills,while only 200 to 300 feet in height, extends in a NW-SEline LAM) AND MOUNTAIN BREEZES just to the south of HB (Figure 2). If the major outlet of the land breeze were San The land breeze in the Los Angeles Pedro Bay these hills should deflect the Basin is generally from the east, that is flow into a Mli rather than a SE breeze at in the opposite direction from the sea HB. breeze. The land breeze, being generally At San Bernardino (DJI,) the night wind of smaller speed, is affected even more by appears to come down the canyon of the Santa the local topography. After the sea breeze Ana River, canmencing at midnight and stop- dies down, a period of variable wind or ping soon after sunrise. March Field (RV) calm usually ensues) followed by a drainage has a light northerly wind between 0100 or mountain wind, later reinforced by a and 0300 which changes rather abruptly to a general easterly land breezer. SE breeze. The north wind mav be a mountain breeze off the ridges to the north of F3r example, at Orange County Army the field, which reach elevations of 3000 Air Field (DSW) the period of calms or feet. This is overcome at 0300 by the more variables lasts until midnight when a general dowh-valley flow from the SE. The definite E wind begins, continuing until SE breeze weake~sslowly after sunrise the sea breeze starts at 0930. The cooling until 0900, when a somewhat stronger S mountam slopes madify any tendency for a wind begins. This S wind is probably the land breeze perpendicular to the shoreline. upslope wind corresponding to the northerly It .is somewhat surprising that this E wind downslope breeze between 0100 and 0300, At continues so long after sunrise (0620 l%T 1100 the S wind is replaced by the sea- averag,: for August) while the sea-valley valley breeze, a m wind which reaches breeze apparent-ly stops within two hours speeds of 15 mph or more by 1730 and di- of sunsiet. There is a slight weakening of minishes gradually thereafter. By 2330 the Ialld-mountain breeze after 0800, with the Nw winds is no longer the predominant shifts to NE and SE, but the general direction and a short period of calms or easterly canponenit: predominates until 0930- variable wind directions prevails.

Evrdence that the nighttime surface wind is primarily determined by the cooling SPECZAL EFFECTS AT of the mountains and the topography of the THE SAN GORGONIO PASS basin, rather than by the sea-land ternper- ature difference, is shown by the stations aumqnt (WT-2592 feet), which lies to the north and west of DSW, UU(PUC), in the mouth of San Gorgonio Pass shows a Pasadena (WA)3 Ontario (Tx)l) and Lomita diurnal wind regime quite different from m) !;how mountain breezes directlyoff the the other stations. The predominant wind slopes behind the stations. At Metropoli- is not the sea-valley breeze but a lighter tan Airport (DVN) in the San Fernando wind which takes over as the sea breeze

I 5 I drminishes around 2200. This Nw wind These observations indicate that the remains very steady throughout the night nighttime layer of easterlies must thin (over 70% of observations report this rapidly toward the pass, allowing a con- direction), but at 0800 begins to back stant flow of the higher level westerlies sj.owly, reaching WSW by 1130, At this through the pass. At the eastern outlet of titme the full effect of the sea-valley the pass this westerly flow disappears breeze has presumably reached the station. rapidly by mixing into a southeasterly The sea-valley breeze does not remain steady flow over Palm Springs, where the air re- from the WSW, however, but shifts errati- mirs calm at the surface. cally between WSW and%W during the afternoon. This is doubtless an alternation The 6-hour interval between regular between flows up one and the other side of pibals is too long to provide data for the ridge north of March Field. By 2100 detailed study of the diurnal wind-changes the persistent northwesterly flow begins aloft. Special observations for this to predominate and very few southerly com- purpose will be described in the following ponents occur after that time. sections.

firing the afternoon the flow over San Q)rgonio Pass exhibits the characteristics DIURNAL WIND-REGIME ALOFT AT UCLA of what is termed the "Maloja wind" (2). Although the summit of the pass is near The diurnal wind-regime observed at Beaumont the surrounding ridges continue Uais similar to that at most stations to rise for another 14 miles to the east. in the area, consisting of a sea breeze from %lis favors the continuation of the wester- the SW during the afternoon and an easterly lit wind through the pass in spite of the off-land wind during the night. dc:scending valley floor. Narrowing of the dley toward the east and probably accourts To illustrate this regime Figure 3 for the high speeds of the wind reaching presents a time-height section of the winds Rtlm Springs (m). observed at UCLA on 1 September 1944. Though no individual day shows all the It seems evident that during the time features of the average picture, this day the light easterly surface winds are flow- demonstrates some of the prevalent charac- irg at DJL and RV, an entirely different teristics of the regime. Bring the period flow is prevailing at TUT. At 0300 about from midnight to noon the winds at the 7!;% of the surface winds at TUT were NW. surface and in the lowest 2000 feet were E The most frequent wind directions reported or SE at ssall speeds, typical of the off- at the same time by the pibal stations in land nighttime wind. At noon the surface the vicinity are given in Table 2. wind veered to S, the characteristic

TABLE 2

MAXIMUM DlRECTXON-FREQUENCIES FROM PIBALS AT 0300 PWT 1-31 AUGUST 1944

S t a t ion Height Directi.on* and Frequency (msl) Maximum Secondary Maximum

San Bernardino @&) 2500 SSE 43% Nw 18% N N 40 00 S 41% wsw 33% W n SO00 W 58% SW 42% March Field (RV) 2000 E 29% SSW 29% N n 3040 E 53% w 35% N n 4000 WNW 33% SSW 24% n W SO00 W 40% ssw 25% Palm Springs (mZ) 1200 SSE 3977 W 36$ n W 2500 SSE 59% UlNW 19% W n 5000 SE 57% DE 26%

*Direction given includes winds 22 l/Zo on each side.

6 harbinger of the sea breeze. The SW sea land wind zone all winds which show the breeze was well developed by 1330 below 2000 effect of any easterly component. It is, feet while the easterlies persisted in a however, frequently inipossible to make layer aloft. The sea breeze disappeared such a distinction and for this reason such at the surface at 2030 while still blowing 1inii t s we re considered uns atis factory. aloft. By shortly after midnight the wind Where the transition was not distinct or in the layer from 1000 to 2000 feet backed where data were insufficient to place it to SE. with accuracy dashed lines have been used.

During the entire day the winds from The sea breeze, which is ordinarily 5000 t(2 10,000 feet were steadily from the SW, often veers into a NW flow at higher NW quadrant, thus emphasizing the reality levels in the same manner that the easterly of the easterly wind in the lowest levels. winds backed through N to W. The differ- nough occurring on many days, a layer of entiation of the top of the sea breeze is, east winds persisting during the entire therefore, also arbitrary. In the present day, :is in the case of 1 September, is analyses of time sections, the solid lines by no means invariable. The time section defining the sea breeze at LJCLA include has been analyzed to show the separation winds from w to Ssw. of tht: easterly from the southwesterly winds in the lower levels, and from the In the differentiation of surface winds northwes terlies aloft. and winds in the lowest levels, topography determines the typical directions for a ?‘he delineation of a sea breeze and given station. For this reason, in the a land breeze is not always easy inasmuch separation of surface directions into land as there is a transition zone between the and sea breezes, local factors have been sea breeze, land breeze, and upper flow considered and no uniform set of criteria characterized by a gradual turning of wind adopted. with height. As an example, at 1500 PWT 1 September (Figure 3) the ESE winds at In order to demonstrate the variqty 2000 f’eet backed gradually to Nw at 8000 of patterns experienced, Figures 4 and 5 feet. It is clear that the easterly provide a time section for 24 consecutive winds represented the land breeze while days at UCLA. A relatively small number the NW or WNW winds predominating aloft of wind arrows are plotted on these throughout the day were part of the general diagrams but they have been chosen to : circulation. represent the general condition of flow at that level and time. Ch most days the “The wind direction in the transition number of pibals available for the con- layer can be considered the vector re- struction of Figures 4 and 5 was comparable sultant of an easterly component, the to that shown on Figure 3 in which all land-breeze effect, and a m-wind vector available data have been plotted. representing upper flow. ne increase of the W effect with height and the decrease If the winds in the 8000-10,000 foot of easterly component would produce the leyels have a direction different from observed backing with height. At what those below the demarcation between local level the land wind may be considered to sea-land breezes an upper flow can be end and the general circulation to begin easily seen, but when there is a gradual must be an arbitrary definition. Even merging of the two, no line of separation if a zone of transition were outlined as can be used. As in the case of 18 August a separate layer, it would be necessary (Figure 41, the top of the sea breeze to set up limits of each zone on the basis between 120U and 1800 PWT can not be of direction. separated from the general circulation aloft. The stippling indicates sea breeze ]:or purposes of the wind time-sections in the lowest layer, but the absence of in this report (e.g. Figures 4 and 5), the a full-line boundary on top indicates the division lines between land wind and upper gradual merging of the local and general flow lave been drawn to include the winds circulation. In contrast 1, 2 and 3 from SSE to NNE in the land-breeze zone, September (Figure 5) show definite bounda- and north winds in the upper flow. This ries of the sea breeze. Similarly the is th2 same as saying that the vector com- line denoting the boundary of the land wind ponent of upper flow is considered to has been broken or omitted when the zone predominate over the easterly Component of transition is absent or is indistinct. when the wind has backed tonorth. It would perhaps be preferable to include in the The patterns in Figures 4 and 5

7 demonstrate a number of facts. In some winds at other stations in the area. ne cases the sea breeze begins in the level sea breeze was evident only at stations just above the ground before it is dis- between the coast and the coast range of cernible at the surface. lhis is presumably mountains. It will be noted that above the result of friction, and has been noted 6000 feet the winds were westerly, re- in other regions (3). In practically no presenting the replacement of the low case does the land breeze begin aloft pressure in the lower layers by a high aloft before it is seen at the surface. This as the low center sloped upward to the results from the fact that a drainage north. 01these days the land breeze merged mountain-wind begins somewhat sooner than with the easterly winds caused by the thle more general land wind and merges in- thermal low. distinguishably with it. me stability of ne second pattern is exemplified by these cold layers tends to reduce the 5-6 September (see Figure 13) at which height through which the delaying effect time a strong thermal. low extended far up of friction is noticeable. the inland valleys of California giving Also, in nearly every case, the sea rise to westerlies or lW' winds at 3000- breeze persists aloft above the land breeze. 5000 feet at UCLA. In this case the sea Not as often, but in many cases, the land breeze merged with the upper flow. At wind persists aloft for a time after the 10,000 feet, however, a high was centered sea breeze begins at the surface. over the Great Basin, Wyoming and Idaho, The sea breeze in nearly all cases has and the easterly winds from 6000-10,00V g reat er speeds than does the land wind. feet are differentiated from the winds This is due in part to the fact that the between 3000 and 6000 feet. tmperature of air over land during the day r.tses farther above the temperature of air Thus it appears that there are three over sea than it falls below the latter layers each of distinct wind pattern, below during the night. Also, owing to the quasi- the general upper flow. The lowest and permanent Pacific anticyclone, the general most local is the valley-mountain wind circulation in the lower layers is from the regime, controlled by the details of N'8 over' southern California which results topography. ne next is the sea-land breeze in a normal on-shore pressure gradient. pattern, also characterized by a diurnal cycle, activated by the differential heat- Easterly winds representing the land ing of air over land and sea. This regime breeze occur on every day except in unusual is confined to the coastal plain and the instances of very strong N or hW winds. (3n ocean area immediately adj acent to the ssme days the Land wind persists aloft over coast. Just above the sea-land breeze is the sea breeze during the whole day as in the third wind pattern, controlled by the the cases of 30 August to 3 September. thermal low, and not confined to the coastal plain. It affects the flow inland The zone of transition between sea- as far as the center of the southwestern land breeze regime and upper f.low is desert. On particular days the thernial low usually marked by a gradual turning of is definitely in evidence at 5000 feet not wind with height, but not in any obvious only during the daytime but for a portion regular manner; thus no consistent regime of the night. It has a diurnal cycle of of ,advection can be described. changing intensity, but at times exists at 5000 feet during a period of more than one The merging of the sea-land wind day. The diurnal wind regime produced has regime with the general circulation aloft been called a continent-ocean wind (2). has two distinctive patterns. The first, At such times during which a low pressure characterized by persistent easterly flow area over the land persists, the on-shore above the sea breeze, is usually the flow in the layer just above the sea breeze result of the thermal low centered over approaches the character of a true monsoon. the southern Mojave desert. Bring the reriod 1-2 September, the thermal low me general circulation aloft which was strong at the surface and definitely is little affected by the local heating in evidence at 5000 feet, controlling all of the coast line or the desert thermal the winds at that level through~utthe low prevails above the monsoonal.winds. 5,outhwestern states. This low was strength- tned by a trough associated with a closed low over southern Idaho at 20,000 feet. nele is ordinarily a gradual merging 'fie easterlies at UCLA shown on the time of each of these patterns into the one section between 2000 and 6000 feet on above it, but on certain days the limits these dates fitted into the pattern of of each layer can be definitely seen. AVERAGE DIURNAL WIND TIME-SECTION AT UCLA

The average diurnal wind regime in are predominant in the lowest layers height and time has been represented in throughout the night and early morning, Figure 15 for three periods, roughly re- but aloft are present during the morning presenting summer, fall and winter seasons. hours, being replaced by a different upper These diagrams have been constructed to flow between midnight and 0300. show the predominant wind direction and force for each 500-ft level and for as many The fact that all three seasons show individual hours during the day as the data the relatively small depth of the east allowed. wind between 0000 and 0300 PW indicates that this feature isprobably characteristic ?he 45' sector containing the largest of the sea-land breeze structure in the number of cases determined the predominant area. Similarly, the great depth of easter- direction which is represented by the arrow. lies at 0900 and 1100 PWT during all three Each wind arrow islabeled to show the total seasons suggests that this is also charac- number af observations at that time and t e r i s t ice level and the number of observations in which the wind was from the direction Inasmuch as the average height sf the shown by the arrow. Men another direction base of the temperature, inversion at IJCLA also contained a large number of obser- was about 1500 feet, the average time- vations, a secondary dotted arrow is shown sections corroborate the findings of the and labeled with the number of cases. The stratus investigation (1) that the height wind force is indicated by barbs in the of the diurnal wind fluctuation is not Beaufort scale and is a rough average limited by the inversion. It has been determined by visual inspection of the suggested (2) that "an inversion of temper- tally. ature and a decrease of relative humidity characterize the vertical extent of the This method of representing the land breeze." In the Los Angeles Basin, dominant wind direction does not give the the diurnal wind changes attributed to the completc: distribution as would a wind rose sea-land breeze regime definitely occur but has the advantage of facilitating a considerably above the prominent subsidence visual summary of the data. The complete inversion. wind direction frequency roses at most of the same levels and times are presented in The maximum velocities of the sea Figures 7 to 9. The roses will allow the breeze occurred at 1000 to 1500 feet nisl reader to check for himself the predominant at UCLA, the elevation of which is 500 directions summarized in the average feet. . diurnal time-sections. All data used in these averages were obtained at the AAF Similar to the time sections for Rather Station, UClA. i-ndividual- days, the averages show that the land breeze is still blowing aloft The average time sections have been at the time the sea breeze begins in the analyztzd in the same manner described lower layers. The persistence of the sea previously. On the basis of the analysis breeze above the initial land wind is, comparisons between seasons may be drawn. however, much less distinct. The averages for all three seasons have the following characteri-stics in It will be noted that in the average common. In the lowest 2000 feet there is picture the sea breeze veers slightly with a cyclic change from SW in the afternoon height while the land breeze has a tendency to easterly in the evening and early to back with height. As previously mention- morning. The SW sea breeze had the ed inspection of individual days did not longest duration in the summer and the reveal any consistent pattern of turning shortest in the winter. with height. The turning cannot be explained by advection of the trough of the The total height of the sea breeze temperature inversion because it occurs apparently exceeds 5000 feet. in summer, in January when the inversion is absent appears to be'about 5000 feet in the fall as well as in summer when it is present. and 3500 feet in the winter, The easter- The observed turning is satisfactorily lies ccnstitute a greater duration of time explained by the consideration of the in the low'er layers than does the sea addition of vectors representing the breeze and apparently extend to greater decreased effect of the land-sea breeze height than the sea breeze. 'he easterlies regime with height and the merging with

9 t:he upper flow as explained previously in slow ascent pibals at time intervals of the discussion of the wind time-section 1 1/2 hours for a 24 hour period. Thc for 1 September 1944. time sections of wind for all stations are shown in Figures 10 and 11. , An attempt was made to relati: the turning of wind with height to temperature The day chosen for the observations was advection making use of wind and raob data at the end of a period of stratus and the plotted on constant-level charts for each beginning of a foehn condition caused by 1000-foot level. Even with the.close the development of a high-pressure center network of radiosondes in the IDS hgeles over the Great Basin, :Bash during the summer of 1944, satis- factory analyses could not be made because The time-height pattern of the sea Ithe temperature and pressure differences and land breeze regime at various stations between stations were no larger than the is given by the solid,or dashed lines in probable error in the values measured by Figures 10 and 11. Since the flow between the radiosonde. 2000 and 5000 feet was generally westerly throughout the 24 hours, the pattern of At the time the sea breeze begins in the easterly land wind in the lowest layers the lowest levels the easterlies are still is well delineated. blowing aloft and assume the nature of a countercurrent to the lower winds. The At 1500 m, 5 September, the thermal same thing happens to a lesser degree when low over the Mojave Desert was well marked the land wind begins near the surface, at 5000 feet and still present at that 'Ihe increase in depth of the sea breeze at level at 2100 m, as can be seen on Figure the expense of the countercurrent aloft 13. At 10,000 feet at this time winds fits the theory of the sea-land regime over the area were controlled by the high most generally accepted (I), (3). The center in Idaho. There were no fronts countercurrent aloft lasts longer at Batavia in the western United States during the (3) than in the Los Angeles Basin, but at period. Felixstow, England the average depth of the sea breeze is 1500 feet and there is no Inspection of the UUA time-section of indication of a countercurrent, at least up this date (Figure 10) shows that the to 5500 feet (4). The Batavia study showed westerlies in the level 2000-5000 feet that the effect of the countercurrent persisted from 1500 on the 5th to noon of reached as high as 12, 000 feet, while at the 6th. The coastal sea breeze in the Los Angeles the easterlies usually merge lower layers apparently merged into the into an upper flow at levels not exceed- westerlies controlled by the thermal low. ing 6000 feet. However, if the prevailing Circulation around the thermal low can wind aloft were weak as in the case at be seen at 5000 feet at 0300 PWT on 6 Batavia, the land-sea winds prpbably would September (Figure 13). By 0900, however, be shown at levels higher than 6000 feet. the winds at 5000 feet over most Californi As prefiously explained, the turning of stations were easterly, now controlled b wind with height indicates that some coni- the Great Basin high. ponent of the land-sea regime is often discernible above the lines on the time To provide an easier synthesis o sections delineating the easterlies and the the geographic picture of the develop+ southwesterly winds. ment of the local-wind pattern, the cross section of Figure 12 is presented. I Having now obtained some knowledge of represents a profile from the coast inland the average diurnal wind changes in the each station shown approximately at per Los hgeles area, a detailed examination pendicular distance from the coast. Va of the changes on one particular day proves Nuys (DVN) lies in the San Fernando Valle interesting. which experiences a sea breeze flowing i from the passes to the southeast as ex plained in the discussion of Figure 1. Ihe AREAL PATTERN ON AN DAY distance along the trajectory of the sea INDIVTDUAL breeze was used to place DVN and IW on the profile. To make a three-dimensional picture of the sea breeze on an individual day, the At 1800 PWT on the 5th all stations cooperation of certain AAF weather stations shown on the profile were experiencing was enlisted. On 5-6 September 1944, six westerly sea breeze, The upper portion o stations in the Los Angeles area observed Figure 12 shows the development of th 10 1 land wind from 1900 to 0100. At 1900 the LA and UCZA at 1500 feet, and at 3000 feet wind shows only as a shallow layer of at DVN, while a considerable thickness of easterly winds near the surface at UCLA, easterlies is still present at (2000 to WN and CUA. At 2100 easterlies occur from 4200 feet) and at DO1 (3500 to above 5000 the surFace up to 1500 feet at UUA, DVN feet). The fact that the sea breeze at and IJUA, but westerlies are still occurring 1300 PWT is thicker at DVN than at IXJA is at L4 and at Dol. The development eastward probably explained by the fact that the of the land wind at the surface as shown two stations are not as close together by the profiles was checked with the sur- geographically as the profile indicates. face winds at stations laying near the cross-section line. REFERENCES ?he lower set of profiles in Figure 12 shows a uniformly developing layer of fie Cali fornia Stratus Investigation easterly wind represented by the area below of 1944 by Neiburger, Beer, and the profile lines of 0300 and 0600. It Leopold; published by U.S. Dept. of will be noted that the land wind began near Commerce, Weather Bureau, April 1945 the coast and worked inland, increasing in for the Joint Meteorological @nunittee. depth fastest over the inland stations. The first pibal showing any land wind Local Winds; Weather Division Report above L4 was 0300 as can be seen on Figure #982, Hq Army Air Forces, February 10. This late beginning of the east winds 1945 (Restricted) at LA is typical of the average surface-

I wind regimes shown in Figure 1. Tropical Land and Sea Breezes; (British) Naval Meteorological Branch, At 0900 the land wind is developed at hlemo 126, 1944 (Restricted). all sta.tions but LA shows a sea breeze up to llQO feet and a layer of easterlies Sea Breezes and Land Breezes; between 1100 and 2700 feet. By 1300 Meteorological Office, (British) Air westerly sea wind occurs at all stations Ministry, Synoptic Division Technical except the most inland, TUT. .4 thin layer Memo #SS, 1943. (Reprinted U.S. Navy, of east: winds can be seen at this hour at NAVAER 50-IR-$2, August 1944)

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