~BER~- 1992 EDDINGTON ET AL 2881 ~=~j ~, "'? ~ti ~; i Numerical Simulation of TopographicallyForced Mesoscale Variability in a Well-Mixed Marine Layer
LEE W. EDDINGTON,. J. J. O'BRIEN, AND D. W. STUART Department of Meteorology, Florida State University, Tallahassee,Florida (Manuscript received30 July 1991, in final form 23 March 1992)
ABSTRACT A simple nonlinear numerical model of a welI-mixed marine layer is used to study topographically fort:ed mesoscalevariability off coastalCalifornia. The mOOdis usedto simulate a persistentwind maximum oiRrved near Point Con<:eptionduring northwesterlywindS. The model also demonstratesthe developmentof a coastaUy trapped Kelvin wave and a marine-layer eddy when the large-scaleforcing is suddenly ~uced. The model is a one-layer, two-dimensional, gridpoint mOOd with idealized coastal topography. The model assumesthat potential temperature and wind are constant with height in the layer and that the layer is cappedby an inversion. Effectsof diamtic heating,water vapor, entrainment, and spatial variationsof ~tiaI temperatwc are neglected in order to focus on topographic effects.The model solvesfor the two horizontal components of the marine- layer wind and the marine-layer height. A comparison of the model results with observationstaken near Point Conception during the 1983 OPUS (Organization of PersistentUpwelling Structures) project shows that the model simulatesthe generalfeatures of the observedmesoscale wind maximum. The successis due to the very fine grid size of 3.5 km. The model wind perturbation and along-trajectory accelerationshow the effect of the prominent Algucllo headlandon the marine-layer wind. The northwesterly flow is blocked by the headland on the upwind side, and this causesthe marine-layer height to rise there. On the downwind side the northwesterly flow removesmass from the region, and the marine-layer height decreases.This perturbation in the marine-layer height createsa local pressure- gradient force that is responsiblefor the existenceof the wind maximum. The model simulation of the marine- layer height is found to be in agreementwith observationsin the region. The model also simulates a solitary atmospheric Kelvin wave crest in the marine layer north oftbe Alguello headland and a marine layer eddy to the south of the headland when the l~e forcing is sharply reduced. Model simulation of these phenomena supports the hypothesis that they are coastalJytrapped marine-layer responsesto changesin synoptic-Scaleforcing.
It. Introduction , From the aircraft wind analysesa persistent wind In the spring of 1981 and 1983, an observational maximum was found to exist approximately 40 km ;ofieldstudy named OPUS (Organization of Persistent south of Point Conception when the area ~ under }Ypwelling Structures) took place off the coast of Cal- the influence of northwesterly flow. It is hypothesized ";~ornia near Point Conception (Brink et at. 1984; At- that this wind maximum fonDS as the flow is forced ",ljnson et at. 1986). The purpose of the meteorological around the regional coastal topography. This hypoth- -;~ of the OPUS study was to measurethe structure esis is tested using a simple numerical model of a well- 19f the atmosphere and ocean in an area of persistent mixed marine layer with highly idealized topography. ~astal upwelling. The data include horizontal wind The model results are used to describe the physical processes responsible for the wind maximum. The :~yses- taken from aircraft observations at 152 m ~ve sea level. Figure 1 shows the location of the model simulation of the marine-layer height and its ~PPUS region and its relationship to the California relationship to the wind maximum are compared with !'COast.Figure 2 shows the location: of the OPUS project observations. };Frface observation Stations and aircraft flight legs. The model is also used to test the hypothesis that changes in synoptic-scale forcing can trigger coastally trapped marine-layer phenomena such as Kelvin waves and eddies. Simulation of these phenomena by this .t. C~enl affiliation.. Geophysics Division, Pacific Missile Test simple mechanism supports the conclusions of Mass ~ter, Point Mugu, California. et ale( 1986) and Mass and Albright ( 1987, 1989) that they are coastally trapped responsesto changesin syn-
:~ CO"espondingauthor address:Lee Eddington, Naval Air Warfare optic-scale forcing. ~ter, Geophysics Division, Code 3253, Point Mugu, CA 93042- The majority of the results in this paper were pre- ~. sented severalyears ago as a mastersthesis by Edding- 2882 MONTHLY WEATHER REVIEW
SANTA MARIA ~{t~ VALLEY TOPOGRAPHICAL PT. SAL AND' CARMALIA 0HILLS ~
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P1: ~ ~ J'j. ~ ~('::=51~'10 34°30'N ..-'~&~ ~I;':J~~ -v PT. GAVIOTA ~. GOVERNMENT PASS POINT ---',~~~~
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SANTA BARBARA CHANNEL ARENA
SAN MIGUEL ~N FRANCISCO SANTA ~NTA CRUZ ~ ROSA " ~ - -r
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FIG. Location of OPUS project study area in relation to the California coast.
observations;section 5 is devoted observedand simulatedcoastaIIy trapped phenomena;and section6 providesa conclusions.
2. The model In the absenceof synoptic-scalecyclones, . . of a large,semipermanent west.Under theseconditions a well-mixed boundary layer (potential temperature height) is found to existunder a strong, sidenceinversion (Neiburgeret aI. 1961). mixing due to upward heat flux, cloud-top DECEMBER 1992 EDDINGTON ET AL 2883
121°100W HI NOAA MET BUOY PT. SAL ANEMOMETERS + VAFB 0 OTHERS OPUS
' AIRCRAFT CONTOURS ABOVE 500 FT I
SOUNDING SHADED I , PU~I~~A eI,- LAND co.1OIMS* 100~ PUT " 0I. . . , 5I 10I 15.. / oft' NAUTlCAL-.£5 . 01.".1 5 10I 15I mI a. AIRCRAFT L TRACKS KI~__-
PT. ARGUELLO
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( "" \ , / ~ " / , " / '/ / / I / I / /' I I I -./) '. .. I I
FIG. 2. ~tion of Surfaceobservation stations and aircraftflight lep during the 1983OPUS project.
and vertical shear is responsible for the well-mixed entrainment, and horizontal variations in potential character of the layer (Lilly 1968). temperature. Under theseassumptions, the mixed-layer In an effort to numerically simulate topographically model becomes a shallow-water model. Becausethe forced mesoscalevariability in the marine layer, a sim- ple mixed-layer model including the effects of topog- raphy is created. Many investigators have used mixed- layer models to simulate mesoscale features in the planetary boundary layer (Lavoie 1972; Lavoie 1974; Keyser and Anthes 1977; Overland et at. 1979; Anthes et at 1980; Ruscher and Deardorff 1982; Han et aI. 1982; Deardorff et aI. 1984; Wilczak and Glendening 1988; Glendening 1990; Wilczak et aI. 1991). The well-mixed character of this model makes it well suited to the spring-summer conditions found off the coast of California. The model vertical potential-temperature profile is shown in Fig. 3. The only layer explicitly modeled is between Zsand h. The model assumesthat potential temperature and wind are constant with height in this layer and that fluctuations in h are not felt at H. In order to focus on topographic effects,the model is sim- plified following Ruscher and Deardorff ( 1982). This FIG. 3. Model vertical potential-temperatureprofile simplification neglects diabatic heating, water vapor, (after Lavoie 1972).
~ 2884 MONTHLY WEATHER REVIEW c TABLE 1. List of model pal3lneter values. YOL~_12 co.,. . this region. ~ rules o~t the p'<>ssibilityofthe~~. : Parameter Value layer fI~w gOl1.1g.ov~r this terrain no ~ttethow~ it may me. This IiDllts the model's appIicabiIitytO~ CD 3.0 X 10-2 (JaDd) of shallow mixed layers with strong inverSions~h~ 1.2 X 10-3(~) f 8.37 X 10-' S-I observations indicate ~at the ~ority oftheft9~c~ g 9.8 m S-2 deflected around the high terrain. -:~:-;c~~::1 K,. 1.0 X 104m2 I-i Theinitial valuesofu, v, andh areu = O~$7J~~
V. 340°, 11.1 m S-I = 0 m s -I, and h = 300 m. The initial vaIuefoi;h"~ 80 28SK 48 based on the averageof the marine-layer heigh!(9r:~ 10K the OPUS aircraft flights with northwesterlYJ1oW..6~ servations show a )arge-scaIeupward slo~irillie~ scale of motion modeled is the mesoscale,the model rine-layer height to the west of coastalCaIif~(N~' is on an f plane. The governing equations for the model burger et al. 1961). This large-scaleslo~iS;not':;~"'i"i;
are plicitIy included in the model. Negiecting~eeffec:tS"~ "c-"",.; av of nonIinearities, the model results can beooDS1~: as perturbations added to a large-scale sloPing~'e1~ - = -V.VV- fk X (V-V g) at layer. ,.
The model is spun up from rest using an
CDIV/v forcing (V g) that only accelerates the u -g'Vh- -- + KmVly (h - z,,) 1 the unperturbed flow. The tenn ah to the = -V. [V(h - z.)], telTain. This unperturbed at (2) homogeneous flow in a marine
The geostrophic wind V g is chosen so
a steady-state northwest wind of 10m S-I
. . ing northwesterlyflow. During spinup the v component of the flow is not allowed to accelerate in inertial oscillations and excessivebuildup rine-layer height along the closed boundary. thespinup procedure are - growth of the u componentof the with time is shownin FIg. 5. The model boundariesaway from the ' - open. During the model spinup the open condition is that the boundary value is neighbor'svalue toward the interior; thati~ mal gradient is zero. After the forcing i~ order to initiate boundary condition is changed to the new' tation of Orlan ski's method O'Brien ( 1980) in order to allow waves to out of the domain. Radiative - not usedduring the modelspinup because ,fc ,'I;'
"t-~ ;;":DECEMBER 1992 EDDINGTON ET AL. 2885 ~.- "
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"0 by using a wind-componentvalue at the grid point 3. WiIMi maximum inside the closedboundary equalto minus the wind- a. Meteorological situation componentvalue just outsidethe boundary (the lo- cation wherethe new wind-componentvalue is being During the spring and summer months, the pre- computed). Evidencethat this boundarycondition is vailing marine-layer wind along the central and south- negligibleis shownin Fig. 12d.This figure showsthe ern California coast is from the northwest. This pre- along-trajectoryacceleration due the eddy-diffusivity vailing northwesterly wind blows roughly parcIllel to term (the only term where this boundary condition the central California coast from Monterey to San Luis : hasany effect). Except in two small regionsof sharp Obispo. From San Luis Obispo to Point Arguello the : gradientsof wind speed,this tenD is negligiblewithin coastline runs north-south and the marine-layer flow . the completedomain including the closedboundaries. is obstructed by the coastal terrain and must go over 2886 MONTHLY WEATHER REVIEW ¥~j~~C~.'1~~fI ~ c.c:':;;~ .~ UITI During the 1983 OPUS proj~ a netWo;kEf21~ s~ace o~~on statio~s and high-~lution~~ p~ of the ~d m the ~e layer by a)~pr;9Yi~ r N a high-quality representation of the flowaro~4ibe! N Arguello headland. FIgUre 6 shows the mean ~dv';":;k~ ~ tors at the surface observation stations for6:3t.~ 19~3. The wind was strongest .and more~o~~Y:.I P Pomt Arguello, there was a turning of the WInd atPoltit'i E / R Conception, and winds were light along tlien~~~~ 5 E of .the Santa Barbara Chan~eL Caldwell et~.(Ai\§~ C . ~ng rotary Spectral analySIs, found muc~ 1~~~~1 N 0 mthe one-cycle-per-day frequency (the sea-~:;$q quency) at the Point Arguello station ~~t~y::~r1 ~ . , , , , , , I " , I the other stations in the region. Consi~~'~ 0 2 It . I 10 12 I" 16 II 20 n station had the highest average wind ~ biSfi~~ TIlE IN NIU"S suggest that channeling was the dominanteffectg~-~l . d th . '."~;-;~~ FIG. S.Growth oftbe II COmJX)Dentof the unper'tUrbedmodel WIn ere. ""';O,~ wind with time during model $pinup. Documentation of the wind maximum 'sQu~;{i>f Point Conception is found in the wind aria1~'~~~ ,~.-c, or around it. The most significantbarrier to this flow from aircraft observations at 152 m abovesea]eveL:J is the westernend of the SantaY nez Mountains,oth- During the 1983 OPUS project, 20 flights W~~2~] erwiseknown as the Arguelloheadland. m. a one-mon th pen..00 Of th ese 20 ~~41~fth 15,w«e]-o.-{;, ~~
------'-'I£;i~ ... ~-_i, ' :~~i i MEAN WIND VECTORS ~, 6-31 MAY 1983 ~",:;:I ~ , f4- ;;; ~ -Q ~ ~ . ~ - :~~ - "' ~ ~ ,..: I -- . ~~ L~ . :;~~, 'vt!.~(~ ~ :~~~:~ C 1 I--~
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FI ~~. ~ons of a persistent wind maximum downwind now in the region of the wind maximum. ~i of Point Arena along the northern California coast. The processesresponsible for the simulated wind ~;.. The wind maximum was observed during northwest- maximum are shown by calculations of the along-tra- :;~"erly winds and appearedto be the result of a local pres- jectory acceleration. This is done for each term in the J} sure-gradientforce associatedwith gradientsin the momentum equation by computing its component ~ marine-layer height. Winant et al. (1988) presented along the wind direction at each grid point. The wind : additional documentation of the Point Arena wind direction is considered the trajectory direction because ~; maximum and described it in terms of supercritical the model bas reached a near-steadystate. By super- 'f-, channel flow in a two-layer system. imposing trajectories over the contours, the contribu- ;: tion of each term in acceleratingthe air parcels along ~ their trajectories is seen. ~':; b. Model results Figure 128 shows the small contrIbution due to the ~, 1£" tennfk X (V - V.). Contour labels inthisandsjmilar ii1 In order to numerically simulate the wind !Da-~um figures are in units of 10-3 m 5-2. Acceleration due to ~! ~uth of Point Conception, the mOOeIis integrated in the tenn - g'V h, shown in Fig. 12b, has the largest ~ time to 24 h, the unperturbed flow reaches 10m S-I value of all the tenns and reveals that the pressure- ~' from the northwest, and a near-steadystate is reached. gradient force set up by the gradient in the marine- ~ Figure 10 shows the model simulation of the wind in layer height is the dominant processin the formation ~: the area of interest at this time, Comparison of the and maintenance of the wind maximum. This term ~ resultswith the wind analysesshows general agreement also deceleratesthe now beyond the minimum in the :~, in the location and structure of the wind maximum. " marine-layer height. Figure 12cshows the effectsof the ili: The effectof the hp$(iJandon the flow is shownby bottom friction term -CDIV IV(h - zs)-'.1bis term ~ the perturbed part of the wind (F 18-II). The pertu~ is small and negative everywhere,with the largestmag- ~: part of the wind is simply the simulated wind minus nitude where the marine layer is shallow. The contri- :;;: the unperturbed wind (a northwest wind of 10m s -I ). bution from the eddy~ffusivity term KmV2v, shown ; The blocking of the flow is seen both upwind and in Fig. 12d, is also ~mall throupout. except in the high 2888 MONTHLY WEATHER REVIEW V~J: 825. !'eso. ..~ 87S. f DJ. 925. , T Td I 950. "J.rI 975. .~~:'~,', ~. I' I(XX). a TE.-.[RA'~ It 1 825. .( -f 850. ... ~ V\ 87S . '#. A. ~~ ~ ~ 925. ) SPEED / 950. I ~ECTION 9'15. J(XX). b f J025'n . ~~'.S~ s. JO.I~ '~15. a..~' 15.".'.';';' , 100 270. ~ '8) 90 IS)~. SPEED ,~) DlRECTI," . 'DEC. the strongesteffect where the marine In the lee of the headland. beyond the the . the local pressure-gradientforce slopeof the marine layer. 4. Marine-layer height In the previous section, the gradient in layer height was shown to be the . -- imum. This leadsto the question modelsimulation' 13shows the simulationof the marinelayer: the completemodel domain. Robertseta!. (1970)m~~ th~",'2..;n- ~ 2890 MONTHLY WEATHER REVIEW ~ 7 ~ i "'\ I '?tj ,C)I - ~ ~ ~I "'1.0- - I . - :' - -~~~~'-~. """'-"""""""""";'."""""'.'II'~.' I """"."'.""' 1 ~ ,.l \.1 ~ o ~ ~I ~ ~'"'~~~ ~ - ...... I. " ...... -"'.-;--; ...... "1.;' "" ~--"- ~:iR ~~- ~ '.---- -.. . '. ( ~~~;!"""""', ..~, ". ..-".. )---'-""'-::::'-.-,- FIG. 12. (a) Along-flow acceleration due to -It Solid lines represent positive values; dashed lines ~~ I{ Contour interval is 0.4 X 10-3 m 5-2. -' Contourinterva1is 1.0X 10-3m 5-2. (c) As in (a) but for I' '.'. -.5 .- - Zs)-I. Contour inte1val is 0.4 X 10-3 m 5 ... _:~~~--"', : KmV2y. Contour interval is 1.0 X 10-3 m s . all terms combined. Contour interval is 0.5 X 10-3 . / ~ " ~'# California in May 1982.The propagationof the wave Evidencethat Kelvin - was inferred by the northward movement of stratus was first presentedby Gill (1977), Anh along the coast, by an increasein the marine-layer (1981), and Bannon(1981). A . height, and by southerlywinds blowing oppositethe low-watergravity wavelong enoughto northerly synoptic-scalewind. Two stratus eddies rotation due to its period being longer formed off the Point Arena and Cape Mendocino inertial period. In the presenceof a vertical headlandswhere the stratuspropagation ceased. effectof the earth'srotation is to ~ DEaMBER I 992 EDDINGTON ET AL 2891 He. too, felt that modification of the flow by the to- pography was important in initiating the eddy. Mass and Albright (1989) explained the Catalina Eddy as a coastally trapped marine-layer responseto mesoscalepressure gradients fon:ed by cha~~ on the synoptic scale.They suggestedthat the mesoscalepres- sure gradientswere produced by strong northerly winds interacting with the regional toJK)8raphy.Lee troughing and marine-layer height variations were both consid- ered important in producing the pressure variations. The basic synoptic scenario for Catalina Eddy for- FKi. 13. The model simulation of the mariDe-layerheight for the mation was found to be increasednorth to northwest complete model domain at t - 24 h (steady state). Contour interval winds ~ted with low pressuremoving inland in is 50 m. the PacifIc Northwest and expanding southward, and high pressureincreasing offshore. The amplitude of the wave is largest near the wall and Dark and Dembek (1991) presented an observa- j decreasesby the factor exp ( - y / a ), wherey is the dis- tional study of a Catalina Eddy event in July 1987. ;' tance from the wall and a is the Rossby radius of de- Their findings agreedwith those of Mass and Albright :c,formation. The Rossby radius of deformation for a ( 1989) that the eddy was initiated as the low-level ] well-mixed marine layer capped by an inversion is c / northerly winds strengthenedfollowing the pSi-5-sageof :;: J, where c is the wave speed(g'h) 1"12.In the Northern a low pressuretrough. They also concluded that mod- ~ Hemisphere the wave propagates with the wall on the ification of the low-level flow by the Santa Ynez ~t right. The wind direction is in the direction of the wave Mountains was responsiblefor the mesoscalelow pres- '~. propagation under a crest and in the opposite direction sure area south of the mountains that expanded into ~ under a trough. Although a Kelvin wave is usually the eddy over time. )' thought of as being representedby a sinusoidal wave Much smaller marine-layer eddieswithin the Santa "i~ fonn, it can be any nonunifonn along-wall variation BaIbara Ch~nnel have also beenobserved and modeled :~; large enough to be influenced by the earth's rotation. ( Hanna et at. 1991; Douglas and Kessler 1991;Kessler {.. For a completetheoretical treatment on Kelvin waves, and Douglas 1991; and Wilczak et at. 1991). These ~; the reader is referred to Gill ( 1982). eddies were shown to be ~ted with thennally ~ Mass et al. (1986) presented detailed analyses of driven local flows interacting with the lalge-scaleflow. :f summer marine-layer surgesalong the coast of the Pa- The forcing for the Santa Barbara Channel eddiesap- ~ cific Northwest and concluded that they could be ex- pears to be quite different from the coastally trapped , t~ plained as the coastally trapped mesoscaleresponse of marine-layer height adjustments associatedwith the ,~;":a two-layer systemto changesin synoptic-scaleforcing. larger eddies. ~; Dorman (1987) and Mass and Alb~t (1987) ~th ~".,. suggestedthat a coastally trapped graVIty current m a b. Model results 1"7two-layer system explained many cases of observed ~'::!;northward marine-layer surges. A coastally trapped In order to initiate coastally trapped marine-layer ~ gravity cun-ent is similar to a Kelvin wave except that phenomena using the mixed-layer model, the previous ~;" the interface betWeenthe fluids reachesthe surface at steady-state model solution is subjected to a sudden ~ the leading edgeof the surgeand slopessteadily upward decreasein the large-scale forcing. The forcing is re- duced to a value that sustainsa 5 m s -1 northwesterly i toward a deep reservoir of fluid. ~' Dorman (1985) also suggested that northward unperturbed wind, and the model is integrated out an Ii: propagation of stratus along the southern California additional 12 h. Any amount of relaxation in the large- ~ coastprior to the formation of a Catalina Eddy in June scale forcing would prodqce an adjustment of the per- ~: 1981 was due to a marine-layer Kelvin wave propa- turbed marine-layer height. The amount of relaxation chosenhere (from 10to 5 m s-1 ) is arbitrary, but con- .I gating up the coast from Baja California. Satellite im- ~L: agery from Rosenthal ( 1968) showssimilar northward sidered realistic. i, propagation of stratus prior to a Catalina Eddy event Relaxation of the forcing is accomplished by de- : ti in May 1968. Catalina Eddy is a tenn used to describe creasing the value of Vgsuddenly. The value of Ugis 1; a large mesoscalemarine-layer eddy that fonns off the decreasedslowly, so that it is equal to the unperturbed ,) COastof southern California. Bosart ( 1983) feh that U component at each time step. This allows a sharp :. lee troughing due to the low-level flow going over the decreasein the forcing without exciting inertial oscil- - '" Coastal mountains was important in initiating the Cat- lations, thus maintaining a straight northwesterly di- ~~'.,alina Eddy he studied. Wakimoto (1987) presented rection in the unperturbed flow. This is similar to the - ~; observationsof a Catalina Eddy where the sb'ongwinds technique used in the model spinup. Its application .~;, wentaround rather than over the coastal mountains. here is also discussedin the Appendix. I: " ~ EDDINGTON ET AL 2893 'I .0 ~O "D . IDDD IIJOO 10'0 J.fD lIDO .~. "" 0 . ,.. ':::::> ItJD ~ Clark and Dembek ( 1991) suggestedthat the results Mass and Albright 1988). The model simulation sim- presented here (which were available to them in Ed- ply supports the idea that either of these coastally dington 1985) represented the formation of an eddy trapped phenomena could be initiated by adjustments by the mechanism presented in Smo1arkiewiczand of marine-layer height gradients to changesin the syn- Rotunno (1989; hereafter SR). Their suggestion is optic-scaleforcing. . based on the results of SR that vortices form in an Simulation of a marine-layer eddy south of the Ar- inviscid, stratified fluid downstream of a topographic guello headland is also the result of marine-layer height obstaclewhen the Froude number (Fr = U I Nh) is less gradientsadjusting to changesin the large-scaleforcing. than 0.5 ( U is the mean wind speed,N is the buoyancy Figure 17 shows the model wind simulation 12 h after frequency, and h is the obstacle height); that is, when the initiation of the relaxation. 2894 MONTHLY WEATHER REVIEW .. VOWl(P..~ , - The model results suggestthat blocking of the~ ,~ : layer by the tenain is sufficient to produce the~: scalepressure gradients neededto induce an eddy.pro;~ vided there is a decreasein the large-scaleforcing. ieei: ~ troughing above the marine layer (which is not::m:~ CJ cluded in the model) would add to the eddy-~u~'-'~ :/ pressure-gradientforce. -::,f"Z:; ~4.o..- ':'---~ The model eddy is much smaller than a typici1cat~:.~ ~ ~~-~ aIina Eddy. A detailed simulation of a Cat3lina~'Eddy~ ~ ~ - .":.'..'" is not expected :.:.' """'f-\=--~--~~:-"""'/ \. ~ H .. "" : !~' 9.6 '. ometry. More recent simulations (EddingtOn : ", ." "'. - , -. ~ - - . ,.. on the samescale as the CatalinaEddy ~ thiSsamec~: r.~! ~~.. ~"""~\-' " .: ".. "', mechanism(Fig. 18). Realisticspatial and teIri~~;::~ . variability of the synoptic-scaieforcing plus the eff~~ '.""""""'~"~.i-I --~ of diurnal heatingand coolingcomplicate any fu~~ comparisonof the modelsimulation F'KJ. 17. As in na. 10M forl- 36 b (12 h after fcxciDgis relaxed). 6. Sum_ry and couclusioDS A - --~ ~--- . oJ - - the flow is primarily diverted around the obstaclerather of diabatic heating, water vapor, than over it. The SR eddies are associated with the horizontal. -" ~ '-' ':' -~- -- -- tilting of horizontal vorticity (produced upstream of to study topographicallyforced mesoscale the obstacle by horizontal buoyancy gradients) into in a well-mixedmarine layer - the vertical as the mean flow encounters the obstacle. model simulated the basic structure of an Direct comparison of the SR eddy mechanism and the wind maximum near Point Conception, eddy simulated here is complicated by the use of the structure of the marine-layerheight, an mixed-layer assumption, no terrain overflow, and fric- Kelvin wavethat propagated or tion. It should be noted, however, that the eddy simu- coast,and a marine-layereddy off the lated here is not produced as the mean flow encounters fornia coast.The model was usedto the obstacle(Fig. 10). Rather, the eddy simulated here physicalprocesses responsible for. is produced by the coastally trapped adjustment of the atmosphericphenomena. marine-layer height gradients to changes in the large- The model reasonablyreproduced scaleforcing. relativemagnitude, and generalflow T- 48 H 18 t1/S ~;?$~, ,., Eti-' . ~ I I I I I I . " a N 8 KIt 58 N 1 ~ ~ . . .. .c ~ .c c [t~ 4 . .c ~ ~ c ...:2- of . ~ ~ ~ ~ c 4 - ,. ,.~ ~ ~ .c c ~ ~~" . ,. A 4 ~ -c ... ,. ".:tJ" ... ~ c c, '4 . ., ,. c -c- ..~ .J ," '... ., ,., ",." ~..."... -c c . , ~ a .. ., " ,. ,. ... A ... .. " .. a. ~ - " ,.A" ... ~ ~.. " »0 ? . .. a. ".a. 3AI.. .. ,..., ~ - 4 ~ ~ ~ .. " a.. to ... to ~ ~.. ... ~ ~ ...... ~ '. ~ 4..'of . .. ~..y .. y -4 ...... »0 . .. »0...... " .'. ~ ~ " 4 ~ ...... iR;. ..'- - - -. ~.. ,,-4 d ~ ------...-. -... -- ..------'.-'-. -. -. -..-. -..-"- -.. -..- - -..------. -.-.. -.. -. -.-.. -.. -.. -.. -.. - -..-.. ... - -+------..- ...- ...- -..~~ -.. -.. -. -. ~-..~ -.. -.. -. ... -++ -+ -+ -+ -+ - - ~~~~~~-+-+ FIG. 18. Numerical simulation ora marine-layer eddy similar in scaJeto the Catalina E«kIy(from ~MjDltOD 1988). ~ DEaMBER 1992 EDDINGTON ET AL 2895 servedwind maximum. The small grid spacing of 3.5 the flow. To eliminate these problems, the model is kIn is a key model parameter. The perturbed part of spun up with only the u component ( the northwesterly the wind showed the blocking effects of the Arguello component) of the unperturbed flow being accelerated. headland on the prevailing northwesterly flow. Cal- In order to do this, proper valuesfor the external forcing culations of the along-trajectory acceleration due to (Ug and Vg)must be calculated. the various terms in the momentum equation showed The value of vgis calculateddirectly from the steady- that a local pressure-gradientforce induced by the gra- state u-momentum equation. All u's are set equal to dient in the marine-layer height was responsiblefor the the predetermined steady-statevalue for the U com- existenceof the wind maximum. This gradient in the ponent of the unperturbed How (Uus), all v's are set marine layer height was the result of the large-scale equal to zero, and h and Zs are set constant at their northwesterly flow causing the marine-layer height to unperturbed values.The steady-stateu value represents increase (decrease) upwind (downwind) of the Ar- a frictionally balancedwind of 10m s-1 from the guello headland. The model simulation of the marine- northwest. All horizontal derivatives drop out because layer height was found to be in general agreementwith the unperturbed flow is horizontally homogeneous.As observations in the region. a result, the external forcing in the u-momentum When a sudden reduction in the large-scaleforcing equation becomes was applied, the model simulated a solitary atmo- Vg = -CD(Uus)2£f(h - zs)]-I. (AI) spheric Kelvin wave crest north of the Arguello head- land and a marine-layer eddy to the south. The model It shouldbe notedthat Vg is not a functionof either results were discussedin terms of describing an initi- spaceor time. ating mechanism for observedmarine-layer surgesand In oider for the v component of the unperturbed eddies. The model simulation of these phenomena flow to remain unaccelerated, Ug must equal the u supports the hypothesis that they are coasta11ytrapped component of the unperturbed flow at each time step. marine-layer responses triggered by changes in syn- The u component of the unperturbed flow at eachtime optic-scaleforcing. step (u:) is calculated by plugging (AI) into the u- Acknowledgments.We would like to expressour ap- momentum equation and dropping all horizontal de- preciation to the National Aeronautics and SpaceAd- rivatives except V2u. Here V2u is not dropped because ministration for funding the majority of this research the finite-difference scheme used to approximate it is under the NASA Trainee Program in Physical Ocean- implicit and is not equal to zero unless a steady state ographyand Meteorology at Florida State University. is reached. The growth of Ugwith time is shown in Funding was also provided by the National Science Fig.5. Foundation Division of Ocean Sciencesunder Grant In the simulation of the coastally trapped phenom- 0CE-8213872 and the Physical Oceanography Divi- ena, a new value for Vgis computed from (AI) using "us = 5 m s -I. The value for Ugat each time step is sion of the Office of Naval Research under Grant N eddy development over the Santa Barbara ~ Meteor.. 30, 633-651. Keyser,D., and R. A Anthes,1977: layer modelof the planetaryboundary layer casting.Mon. Wea.Rev.. lOS, 1351-1371. Lavoie, R. L, 1972:A mesoscalemodel Atmos.Sci.. 29, 1025-1040. , 1974: A numerical model of tIade wind Mon. Wea. Rev.. 102, 630-637. Lee. T. F., J. Rosenthal, and R. A. Helvey. phenomena~ by GO~ sateUite. 281. Lilly, D. K., 1968: Models of cloud-topped strong inversion. Quart. J. Roy. Meteor. Soc, Mass,C. F., and M. D. Albright, shore surgesof the west coast of North Mon. Wea.Rev.,lis, 1707-1738. -. and -, 1988: Reply. Mon. Wea. Rev., -and , 117,2406-2436. -, -, and D. J. Brees,1986: Mon. Wea. 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