JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D8, PAGES 10,081-10,102, APRIL 27, 2000

Boundary layer evolution and regional-scalediurnal circulations over the Mexico Basin and Mexican plateau

C. D. Whiteman, S. Zhong, X. Bian, J. D. Fast, and J. C. Doran PacificNorthwest National Laboratory,Richland, Washington

Abstract. Data collectedin a measurementcampaign in Februaryand March 1997 showed that the Mexico Basin (alsocalled the Valley of Mexico), locatedatop the Mexican plateau, failsto developthe strongnocturnal inversions usually associated with basinsand doesnot exhibitdiurnally reversing valley wind systems.Data analyses,two- andthree-dimensional numericalsimulations with the RegionalAtmospheric Modeling System(RAMS), and a Lagrangianparticle dispersion model are usedto interpretthese observations and to examine the effectsof topographyand regional diurnal circulations on boundarylayer evolutionover the Mexico Basinand its surroundingsduring fair weatherperiods in the winter dry season. We showthat the boundarylayer evolutionin and abovethe basinis drivenprimarily by regionaldiurnal circulations that developbetween the air abovethe MexicanPlateau and the generallycooler surrounding coastal areas. A convectiveboundary layer (CBL) grows explosivelyover the plateauin the late morningto reachelevations of 2250 m agl (4500 rn rnsl)by noon,and a strongbaroclinic zone formson the edgesof the plateauseparating the warm CBL air from its coolersurroundings. In early afternoonthe ratesof heatingand CBL growthare slowedas cool air leaksonto the plateauand into the basinthrough passes and over low-lying plateauedges. The flow ontothe plateauis retarded,however, by the strongly risingbranch of a plain-plateaucirculation at the plateauedges, especially where mountains or steepslopes are present. An unusuallyrapid and deepcooling of the air abovethe plateau beginsin late afternoonand early eveningwhen the surfaceenergy budget reverses, the CBL decays,and air acceleratesonto the plateauthrough the barocliniczone. Flow convergence nearthe basinfloor and the associatedrising motions over the basinand plateau produce coolingin 3 hoursthat is equivalentto half the daytimeheating. While the air that converges ontothe plateaucomes from elevationsat and abovethe plateau,it is air that was modified earlierin the day by a cool, moistcoastal inflow carriedup the plateauslopes by the plain- plateaucirculation.

1. Introduction have attemptedto describethe meteorologyof this elevated basin,and those that are available in theliterature rely heavily The geographicalsetting of basin topography,high alti- on surfaceobservations. In a seriesof studiesusing data from tude, and tropical latitude, combinedwith high population surfacemeteorological stations in urbanand rural areasof the density and multiple emissionsources, makes Mexico City basin, Jauregui [1973, 1988, 1993, 1997] documentedthe andits immediatesurroundings one of the mostpolluted areas urbanclimatology of Mexico City, especiallythe heat island of the world. The basin settinginhibits pollution dispersion, developmentand its associatedlocal circulations. Oke et al. and intenseyear-around sunshine at this latitudeand elevation [ 1992, 1999] reportedinvestigations of surfaceenergy budget promotesatmospheric photochemical reactions that form sec- componentsin heavily built-up areas of Mexico City and ondarypollutants such as ozone. As a result,ozone values up comparedthem to componentsat similar sites in temperate to 400 ppb have been measuredin the basin, and the Mexico cities. This was one of the first such studiesin a tropical air quality standardsare frequentlyexceeded at many loca- urban area. tionsin andaround the urbancomplex [Garfias and Gonzalez, In recentyears, two major researchprograms have carried 1992; Collinsand Scott, 1993]. out field campaignsin Mexico City and its surroundingsto While many air quality studieshave been carried out in the provide badly neededupper air meteorologicalobservations. Mexico City area using surfacemeasurements from an auto- The first of thesewas the Mexico City Air Quality Research matedmonitoring network [Raga and Le Moyne, 1996;Miller Initiative (MARl) conductedin the winters of 1990-1993 as a et al., 1994; Garfias and Gonzc•lez,1992; Bravo et al., 1996; cooperativestudy between Los Alamos National Laboratory Bian et al., 1998], upperair observationsfrom aircraftflights and the Mexico Petroleum Institute [Guzmdn and Streit, [Nickerson et al., 1992; Diaz-Franc•s et al., 1994], and 1993]. Williamset al. [ 1995] usedthe MARl datato perform photochemicalmodels [Varela, 1994;Fast, 1999], few studies mesoscalemodel simulationsof local circulationsand disper- sion in the Mexico City area for severalselected cases and to evaluatesources of uncertaintyby usingmodel-measurement Copyright2000 by the AmericanGeophysical Union. comparisons. Bossert [1997] used a mesoscalemodel to Papernumber 2000JD900039. simulate the observedwind and temperaturestructures on 0148-0227/00/2000JD900039509.00 three days during the MARl field experimentin February 10,081 10,082 WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

1991 to examinehow they affect observedpollutant concen- peratureand wind structureevolution in the elevatedbasin trations. He concludedthat both regional- and synoptic-scale usingdata from a specialnetwork of radarprofilers and radio- flows have a strongeffect on the meteorologyand air pollu- sonde stationsoperated during the IMADA-AVER experi- tion concentrations within the Mexico Basin. The second mental period and from the Mexican rawinsondenetwork researchprogram, the Investigationsobre Materia Particulada stationswithin and surroundingthe elevatedbasin. Data from y Deterioro Atmosferico-Aerosoland Visibility Research thesesources were composited to elucidatethe regular diurnal (IMADA-AVER), was conducted during the period patterns of boundary layer evolution. Numerical model February23 throughMarch 22, 1997, and was fundedjointly simulationswere used to interpret the observationsand to by Petr61eosMexicanos through the Instituto Mexicano determinethe effectof topographyand regional-scale diurnal del Petr61eoand the U.S. Departmentof Energy[Doran et al., circulationson boundarylayer structureevolution within the 1998]. The IMADA programobtained improved data on the Mexico Basin. spatialpatterns of aerosolsin Mexico City [Edgertonet al. Section2 describesthe topographicsetting of the basin. 1999] and improved spatial informationon the evolutionof Section3 providesinformation on the experimentalsites and verticalwind and temperaturestructure in the Mexico Basin. data, as well as the synopticweather conditions. Section4 Fast and Zhong[1998] useddata from this experimentto link presentsanalyses of the observedboundary layer structure boundarylayer circulationpatterns to the observedspatial and evolution. Model simulationsare presentedin section5. ozonedistribution patterns in the basinby studyingseven pol- This is followedby discussionsin section6 and conclusions lution episodesduring the IMADA-AVER experimentusing a in section 7. high-resolution mesoscale meteorological model and a Lagrangian particle dispersionmodel. Their simulations 2. Mexico Basin showedthat the day-to-daydistribution of pollutantswithin the Mexico Basin was rather sensitive to small changesin The topography(Figure 1) surroundingMexico City has upperlevel wind speedand directionthat affectedthe diurnal beendescribed variously as that of a basin,a valley,and a cycle of winds in the basin. plateau.The Mexico City metropolitan area (latitude 19øN) is Althoughthe abovementioned studies have provided valu- foundin the southwesternportion of thebroad Mexico Basin. able new information on meteorologicalprocesses in the Thenearly fiat floorof thebasin (elevation 2250 m meansea Mexico City area,they have focusedprimarily on the linkage level(msl)) is confinedon three sides by mountainridges but betweenmeteorology and air quality within the basinand are with a broadopening to the northand a narrowergap or pass thereforelimited in spatialextent and to selectedcase studies to the south-southeast.The surroundingridges vary in eleva- of high-pollutionepisodes. The presentpaper complements tion, but the basincan be consideredroughly 800-1000 m thesestudies by focusingon the characteristicsof meantern- deep. Two high snow-coveredvolcanoes (Popocat6petl,

Figure 1. Topographyof Mexico. (a) Map of the Mexico Basin,showing the locationsof the Investigation sobreMateria Particuladay DeterioroAtmosferico-Aerosol and Visibility Research(IMADA-AVER) radar profiler/radiosondesites (squares),the Mexico City airport rawinsondesite (dot), and the Mexico City metropolitanarea (shaded). The topographiccontour interval is 200 m. (b) Three coastalMexican rawinsondesites (dots), a GPS rawinsondesite (square),the threenested Regional Atmospheric Modeling System(RAMS) simulationgrids, and the cross-sectionA-A' throughthe isthmusused in two-dimensional model simulations. The topographiccontour interval is 500 m. Site abbreviationsare as follows: ACA, Acapulco;CHA, Chalco; CUA, Cuautitlan;MAN, Manzanillo; MEX, Mexico City InternationalAirport; PAC, Pachuca;TEO, Teotihuacan;TRM, Tres Mafias; UNA, UNAM; VER, Veracruz. WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,083

5452 m, and Iztaccihuatl,5286 m) are found on the mountain Table2. February-March50 kPaGeopotential Heights, ridge southeastof the basin. The gapto the south-southeastof Temperatures,Dew Points, and Wind Speeds, as the basin,when combinedwith the openingto the north, has AveragedOver a 1O-year Period (1988-1997) led to descriptionsof the topographyas being that of a broad and for 1997 valley (Valle de M6xico). The basin and its surrounding. 1O-year February- 1997 February- mountainsare locatedat the southend of a large plateau(the March Average March Average Mexican plateau) which occupiesthe center of the Mexican 0000 1200 0000 1200 isthmus. The outer slopesof this plateauto the west fall to UTC UTC UTC UTC the Pacific Ocean, while those to the east descend to the Geopotential 5859 5854 5861 5859 height (m) Atlantic Ocean. The unusual multiscaletopographic com- Temperature -8.8 -7.9 -8.7 -8.1 plexityof this regionraises the questionwhether the boundary (oc) layer developmentof this area will be primarily like that of a Dewpoint -22.1 -29.5 -22.8 -31.0 valley, a basin,or a plateau. temperature (oc) Windspeed 8.6 10.6 8.0 10.7 3. Sites, Data, and Synoptic Overview (m/s) The air quality and meteorologicaldata collected during the IMADA-AVER field campaignwere describedby Doran et al. [1998]. Only the surfaceand upper air meteorological Network soundings were made from the Mexico City data used in this paper are summarizedhere. Hourly wind InternationalAirport inside the basin and at three coastalsites profiles from four 915 MHz radar wind profilers located outside the basin (Figure lb), one south of the basin at within the basin at Teotihuacan(TEO), Cuautitlan (CUA), Acapulco,one east of the basin at Veracruz, and one west of Universidad Nacional Aut6noma (UNA) de M•xico the basinat Manzanillo. There was no suitablerepresentative (UNAM), and Chalco (Figure l a) were used to sample the rawinsonde station north of the basin. Table 1 summarizes above-basinwind field. The profilerswere operatedin both informationon the sitesand the typesof data collected. low- and high-rangemodes with verticalresolutions of 60 and The experimentalperiod was near the end of the winter dry 100 m and covered a height range of approximately100 - season. The synopticconditions during this period were char- 2000 m and 150 - 4000 m above groundlevel (agl), respec- acterizedby high pressurewith light upper level winds and tively. The temperaturestructure evolution in the basin was near-cloudlessskies. Partially cloudyskies and someprecipi- determinedfrom radiosondesoundings launched 5 times per tation occurredon a few days at the beginningof the experi- day (0800, 1100, 1330, 1630, 1930 local standard time ment when a deep trough was found over New Mexico and (LST)), excepton Sundays,from thesefour profiler sites. Texas with its influence extendingsouthward to northcentral Temperatureand humidity profiles to the north and southof Mexico and toward the end of the experimentwhen a cutoff the basinwere determined from radiosonde soundings at two low formed off the Pacific coast of central Mexico. additionalsites: Pachuca,located approximately 70 km To determinethe representativenessof the generalsynoptic northeastof Mexico City, and Tres Marias, located on the conditions occurring during the experimental period, southernflank of the mountainsforming the southern rawinsondedata from the Mexico City InternationalAirport boundaryof the basin. Soundingswere launchedat 1330, were used to determinemean values of geopotentialheight, 1630, and 1930 LST at Pachuca and at 1330 and 1630 LST at temperature,dew point temperature,and wind speedat 70 and Tres Marias. Twice-daily soundingslaunched at 0600 and 50 kPa for February and March for the years 1988-1997. A 1800 LST from the Mexican national rawinsonde network comparisonof these climatological mean values with the were usedto provide informationon regional-scaleatmos- same variables averagedover February and March 1997 is phericconditions within and surroundingthe Mexico Basin. shown in Table 2. Because no significant differencesoc- curred between the 1997 mean values and those averaged over 10 years, the upper air conditions during the 1997 IMADA-AVER field campaignare consideredtypical for the Table 1. Station Locationsand Data Types winter dry seasonin this region. Lat Long Elev Station ID (øN) (øW) (m) Data Chalco CHA 19.25 98.91 2248 R, P 4. Observed Characteristics of Boundary Layer Cuautitlan CUA 19.69 99.19 2252 R, P Evolution Teotihuacan TEO 19.68 98.85 2275 R, P UNAM UNA 19.32 99.19 2274 R, P 4.1. Mean Temperature and Humidity Structure Acapulco ACA 16.05 99.93 3 R/S Manzanillo MAN 19.07 104.33 3 R/S Radiosondetemperature data were compositedfrom all Veracruz VER 19.17 96.12 13 R/S days of the experimental period (February 23 through MexicoCity MEX 19.43 99.07 2234 R/S March 22, 1997) for three of the four basin sites(TEO, CUA, Int'l Airport Tres Mafias TRE 19.05 99.45 2810 R1 and UNA) to obtain mean temperatureprofiles at the five Pachuca PAC 20.08 98.74 2425 R2 daily soundingtimes. Missing data from the Chalcosite pre- cludeda similar treatmentthere. Figure 2 showsthe diurnal R, radiosondeslaunched at 0800, 1100, 1330, 1630, and variation of the mean temperaturestructure at Teotihuacan, 1930LST; R1, radiosondeslaunched at 1330 and 1630 LST; R2, GlobalPositioning System rawinsondes launched at 1430,1630, and UNAM, and Chalco. Becauseof missingsoundings, the data 1930 LST; R/S, rawinsondeslaunched at 0600 and 1800 LST; P, compositesat the three sites use data from different experi- 915 MHz radarwind profiler (hour-averagedata). mental days. Becauseof this the time evolution of potential 10,084 WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

a) TEO b) UNA 5 5 5 • 0•-'•'L-•"• 'l •• 1100LST •/ ...... 1330 LST •-• -• 1630LST //•':

<3- ...... 1930LST E E E -

• - 0 _ .o_ .9 .... '•2- ß 2•

1 _ 1

0 1,1,1 0 o I 'I I I I I I I J I I I I 300 310 320 330 0 2 4 3oo 310 320 33O 0 2 4 Potentialtemperature (K) S.D. (K) ß ,, Potentialtemperature (K) S.D. (K)

ß

c) CHA 5 '1

<3- E E

0 _ .9 ,._ (o2-- 2•> Q) -

1

o /.... I ' ,u' I ,,,, I'1'1 " 3oo 310 320 330 0 2 4 Potentialtemperature (K) $.D. (K) Figure2. Meanradiosonde soundings at(a) Teotihuacan, (b)UNAM, and (c) Chalco. Each sounding isa compositeof all good-qualitysoundings (typically 19-23) in the February23 to March22, 1997, experimentalperiod for the time(local standard time (LST)) indicated. The compositeprofiles are constructedfrom individualsoundings whose temperatures are averagedover 100-mvertical segments. Thestandard deviations (s.d.) of meanpotential temperatures asaveraged over 100-m vertical segments in theindividual soundings are shown in figureparts for the times indicated.

temperatureat a site is expectedto be moreaccurate than rate of boundarylayer warmingslow significantly. Most comparisonsbetween sites. While the atmosphericboundary peculiarly,a very sudden cooling occurs throughout the entire layer in the Mexico Basin evolves normally in several boundarylayer to an elevationof 2250 m agl (4500m msl) respects,it also exhibitssome unusual features, which will be betweenthe 1630 and 1930 LST soundings.The coolingdur- discussedfor the Teotihuacansite (Figure 2a). The mixed ingthis 3-hour period is equivalentin valueto morethan half layer at Teotihuacangoes through a rapid growth period of thetotal daytime heating! The mainfeatures of boundary between 1100 and 1330 LST, attaininga depththat exceeds layer developmentat the rural Teotihuacansite were also 2000 m agl (4250 m msl) by 1330 LST. Between 1330 and presentat the urbanUNAM (Figure2b) andrural Chalco 1630 LST the rate of growth of boundarylayer depthand the (Figure2c) sites. At the Chalcosite, almostno warming WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,085

T observation

600 W rn-2 500 W rn-2 400W rn-2 300W m '2

200W rn'2

6 8 10 12 14 16 18 20 Time (LST) Figure3. Mean(dots) and standard deviations (error bars) of observedconvective boundary layer (CBL) depthsat Teotihuacan.Also shownare CBL depthsestimated with a simpleencroachment model given sinusoidalsurface sensible heat fluxes with the indicatedamplitudes using the mean0800 LST soundingas an initial condition.

occurredbetween 1330 and 1630 LST, the cooling between similarto that predictedby the encroachmentmodel with a 1630 and 1930 LST was reduced relative to the other sites, maximummid day flux of 400Wm -2. and a shallow ground-basedcold air layer was presentin the Regulardiurnal circulationsform in areasof complex 0800 LST sounding. The standarddeviations of sounding terrain when horizontal air temperaturedifferences occur temperaturesfor each of the soundingtimes are shown in between locations within the mountainous area or between the Figure 2 to quantify the day-to-dayvariability in boundary air overthe topography and the air overthe surroundingplain. layertemperatures at the individualsites. Standarddeviations These temperaturedifferences lead to horizontal pressure were generallyless than 2 K above2000 m agl, exceptfor the gradients that drive diurnal mountain wind systems 1000 LST soundingsat Chalco. The standarddeviations in- [Whiteman,1990]. Thereforeas a first stepin investigating creasedbelow this level, exceeding4 K at the ground for diurnal wind systems, analyses of horizontal temperature severalof the afternoonand eveningsoundings. differenceswere performedon differentscales. The daytime boundary layer growth in the Mexico Basin The spatial variation of mixed layer temperatureswithin can be seen more clearly in Figure 3. Here the means and the basin, as determinedfrom the four basin sitesduring the standarddeviations of boundarylayer heightsare plotted as a experimentalperiod, is usuallysmall and variablefrom day to function of time for Teotihuacan. The means and standard day, so no consistentpatterns were detectedexcept for a deviations were determined subjectively from individual tendencyfor slightly cooler(0.5-1 K) afternoontemperatures potentialtemperature soundings by recordingthe heightof the at Chalco. Some differences, however, exist between the baseof the cappinginversion at the top of the unstableor con- mixed layer temperaturesand the boundarylayer structure vectiveboundary layer (CBL). Also plotted in Figure 3 are inside the basin and those south and north of the basin. At the hypotheticalrates of boundary layer growth calculated Pachucathe 3 times per day soundingsare much like the from an encroachmentmodel using the mean 0800 LST soundingsfrom the four stationsin the MexicoBasin (not Teotihuacansounding as the initial soundingand assuming shown). Boundarylayer depthsand temperaturesare similar sinusoidallyvarying surface sensibleheat fluxes between and they also show the suppressedboundary layer develop- sunriseand sunsetwith maximum valuesat solarnoon rang- ment between 1330 to 1630 LST and the rapid decay and ing from200 to 600 W m-2. The encroachmentmodel coolingbetween 1630 and 1930 LST. South of the basin at neglectsadvection, subsidence, and turbulententrainment but the higheraltitude site of Tres Mafias, the twice-dailysound- is nonethelessgenerally considered a usefultool in estimating ings at 1330 and 1630 LST exhibit shallowermixed layers boundarylayer depthsover homogeneous terrain and has been and coolertemperatures than at the othersites. Figure4 illus- shown to explain 80-90% of the observed variations in tratesthese differencesby plotting the mean potentialtem- boundarylayer depth [Stull, 1988]. The plot showsa rapid peraturesoundings at 1630 LST at UNAM, Pachuca,and Tres increase of boundary layer depth between 1100 and Mafias. The afternoon temperaturesare generally warmer 1330 LST, with an averagegrowth rate exceeding600 m per inside the basin, with the UNAM-Tres Marias temperature hour, much larger than the rate suggestedby the encroach- differencelarger than the UNAM-Pachucatemperature differ- ment model. After 1330 LST the growthrate fell off steeply ence. The warmer temperatureswithin the basin are con- in comparisonto its earlier rate of growth, attaining a rate sistentwith the frequentlyobserved afternoon flows that enter 10,086 WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

in the Mexico Basin are exacerbatedby strong temperature inversions that form within the elevated basin. The mean potential temperaturesounding at the Mexico City airport, however, shows no temperature inversion (a temperature inversionwould require dO/dz > 0.0098K m-•). In fact,the mean morning low-level stability is only marginally greater than in the free atmospheresurrounding the Mexican plateau at the same altitude. While the atmosphereover the basin is stable at night and actual temperature inversions formed occasionallyduring the experimental period, the Mexico Basin has lower mean nighttime stability than most midlatitudeRocky Mountain valleys andbasins [Whiteman et TRE al., 1999b]. In these valleys, mean potential temperature gradientsin the surface-basedinversion near sunrisefollow- ingclear nights usually exceed 0.015 K m-•. In thissense, the Mexico Basin doesnot exhibit one of the chief meteorological characteristicsof basins, the formation of strong nighttime temperatureinversions. The mixing ratio profiles at 0600 LST (Figure 5c) at the UNA Mexico City airport, Veracruz, and Acapulco were nearly identicalabove the altitude of the Mexican plateau,while the PAC profile aboveManzanillo was'drier. Mixing ratiosat sealevel in the coastalareas on either side of the plateauwere nearly 310 315 320 325 330 double those on the Mexican plateau. The highestmixing Potential temperature (K) ratios were confinedto an 800-m-deep layer on the Pacific side of the Isthmus at Acapulco, whereasthe mixing ratio Figure 4. Mean potentialtemperature soundings at 1630 LST decreasedlinearly with elevation above Veracruz on the at UNAM, Pachuca,and Tres Mafias. Soundingsare aver- Atlantic side. agedfrom all availablesoundings (12 at TRE, 20 at PAC, and At 1800 LST a deep CBL (Figure 5b) extendsto 4600 m 22 at UNA) at the 1630 observation time during the msl above the basin, as determinedfrom soundingsat the February23 to March 22, 1997, experimentalperiod. Mexico City airport. At the other sites, however, the soundingshave much the same appearanceas at 0600 LST, although they are somewhatwarmer (by 0-4 K) than the the basin from the south-southeastat Chalco and from the morningsoundings in the lowest 4000 m. A slight destabili- north-northeastat Teotihuacan and Cuautitlan, as will be zation in the lowest 200-400 m at Acapulco,Manzanillo, and discussed later. Veracruzis associatedwith the developmentof shallowCBLs A comparisonwas also made betweenMexico Basin there. The CBL above the elevated Mexico Basin is much temperatureand mixing ratio profiles and profiles obtained deeper(and warmer)than CBLs in coastalareas on eitherside from the coastalregions surrounding the Mexicanplateau. of the isthmus. The temperaturedifference between the basin Mean potentialtemperature and mixingratio soundingsfor site and the other sites decreaseslinearly from an excessof the periodfrom February1 to March 31, 1997, were con- 8-10 K at the basin floor to a deficit of 1.5 to 3.5 K at 4700 m structedfor 0600 (Figures 5a and 5c) and 1800 LST msl. Hence the basin atmosphereabove the 4300 m msl is (Figures5b and5d) for eachof thefour Mexican rawinsonde colder than its surroundings.The overheatingof the atmos- stations(see Figure lb for locations). phere within and above the basin relative to the coastalenvi- At 0600 LST, after a full night's cooling,the basinfloor ronmentis clearlyproduced by the daytimegrowth of a deep experiencesa slight potential temperature deficit relative to CBL abovethe basin floor. The strongdaytime overheating the surroundingcoastal stations (Figure 5a). The deficitnear associatedwith CBL growth over the basin and plateauis in the basin floor is about 3 K relative to air at the sameheight contrastto the weak nighttime overcoolingof the basin aboveManzanillo and Acapulco but onlyabout 1 K relativeto atmosphere.While the radiosondeobservations lack temporal Veracruz. The temperaturedeficit decreaseswith altitude resolution, it is clear from the Teotihuacan observationsthat above the basin floor. The mean vertical potential tempera- the large daytime overheatingdissipates very rapidly in the turegradient above the plateau level at all rawinsondesites is late afternoonand early evening,leaving only a weak over- about0.0043 K m-•, slightly larger than that of the midlatitude cooling in the basinby the 0600 LST soundings. StandardAtmosphere (0.0035 K m'•). Belowthe altitude of The mixing ratio profiles (Figure 5d) at 1800 LST above the plateau,soundings on the Atlanticside of the Isthmus the elevation of the Mexican plateau changelittle from the wereup to 4 K colderthan on the Pacificside. The small morningprofiles, except at the MEX site, wherethe humidity temperaturedifference between the atmosphereinside and is mixed throughthe deep CBL abovethe plateau,with sig- outside the basin at altitudes between 2250 and 3250 m msl is nificant moisteningat altitudesbetween 3800 and 6000 m not consistentwith observationsin other basinsand valleys, msl. At the altitudeof the basinfloor, the humidityis slightly whichare generally characterized by coldair confinementat lower than at the same altitude over Veracruz but slightly nightand large temperature deficits relative to theirsurround- higher than in the mixedlayer above. The meanmixing ratio ings[ Whitemanet al., 1996, 1999a,1999b]. Others[e.g., abovethe plateaubetween the surfaceand 7300 m msl, the Collinsand Scott, 1993] have stated that air qualityproblems altitude where day/night differencesbecome negligible, is WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,087

a) 0600 LST b) 1800 LST 'ß ...... MAN ,....•? ...... MEX -6 .... VER

Z? Basinfloor _ / /./..'...'..' // _ / ,..' / ./....' oO•,. /....? o ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' 29o 300 310 320 330 290 300 310 320 330 Potentialtemperature (K) Potentialtemperature (K)

c) 0600 LST d) 1800 LST

_ -6

-5

-4

Basin floor

-2 \

0 4 8 12 16 0 4 8 12 16 Mixingratio (g kg'1) Mixingratio (g kg'1) Figure 5. Mean potentialtemperature profiles at (a) 0600 and (b) 1800 LST and mixing ratio profilesat (c) 0600 and (d) 1800 LST at Manzanillo, Mexico City InternationalAirport, Veracruz, and Acapulco,as averagedover the periodFebruary 1 throughMarch 31, 1997.

2.8g kg-• at 0600 LST and 3.1 g kg-• at 1800LST. Thusa net In summary,the daytimeboundary layer evolutionabove increasein humidity occursabove the plateau during the the Mexico Basin is characterizedby a rapid heating and daytimegrowth of the mixed layer. Sincethere are no large growthstage between 1100 and 1330 LST, followedby a moisturesources on the plateau itself and the growing CBL quenchingstage between 1330 and 1630 LST in whichthe would entrain lower humidity air into the mixed layer, the rate of heatingand growthslow, and an unusuallyrapid and observationssuggest that advection from the surrounding deep coolingstage from 1630 to 1930 LST, with 3-hour coastalregions is responsiblefor this moistening. Someof coolingamounting to morethan half of thetotal daytime heat- the additionalmoisture is, no doubt,brought up from the more ing. The air abovethe basinis significantlywarmer in day- humid coastal regions by upslope flows that form during time relativeto the large-scaleenvironment as a resultof CBL daytimeon the outerslopes of the plateau. growthabove the elevatedbasin floor. In contrast,the night- 10,088 WHITEMANET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU timetemperature deficit in thebasin is small,so the potential rate of heat storagefrom the net radiation curve in the after- temperatureprofile in the basinin the earlymorning differs noon, indicatedby hatchedshading, could be interpretedas a little fromprofiles over the coastalstations on eitherside of quenchingof boundarylayer growth causedby the leakageof the isthmus. cold air into the warm basin atmospherefrom the colder surroundings. The rate of heat loss from 1630 to 1930 LST 4.2. Energeticsof the Mexico BasinAtmosphere reaches450 W m-2. Thisrate is muchlarger than typical valley or basinturbulent sensible heat flux divergencesat this Changesin basinboundary layer temperature and depth time of day. Whitemanet al. [1996], for example,showed canbe investigatedby calculatingthe rateof changeof heat measuredand calculated(from an atmosphericheat budget) storagein thebasin atmosphere from the equation eveningand nighttime sensibleheat flux divergencesfrom a Japanesebasin, a Coloradovalley, and a Colorado/Utahbasin H=cpZmf /oAOdzAt [Wm -2] (1) which were equivalentto surfacesensible heat flux lossesat 0 ratesgenerally less than 40 W m-2. Coldair advection(i.e., divergenceof potentialtemperature flux by the mean wind) wherecp isthe specific heat of airat constant pressure, p is must be invoked to explain the extremelyhigh rates of heat air density,0 is potentialtemperature, t is time, and z is loss over the Mexico Basin. The overnightcooling rate be- height. Increasesin atmosphericheat storage, as calculated tween 1930 and 0800 LST, althoughmuch smallerthan dur- fromequation (1), areproduced by convergenceof turbulent ing the afternoon and evening period, occurs at a rate sensibleand radiative heat fluxes and by advection [e.g., comparableto what would be producedif all the net radiative Whiteman,1990]. In clear atmospheresover flat terrain loss at the surface were converted to sensible heat flux duringdaytime, the contribution of radiativeflux convergencedivergence in the column. A further discussionof these to the gain of atmosphericheat storageis typicallysmall results in terms of mechanismsthat could explain the high comparedto turbulentsensible heat flux convergence.During rate of heat storagegain in the morning and the high rate of nighttime,radiative and turbulentsensible heat flux diver- heat storageloss in the late afternoonand early evening is gencesmay both play important roles leading to atmosphericfound in section 6. heat loss [e.g., Stull, 1988]. Large discrepanciesbetween measuredsurface sensibleheat fluxes and rates of changein 4.3. Mean Wind Fields in the Mexico Basin atmosphericheat storageare usuallycaused by advection associatedwith mean winds. In fact, in midlatitudes,advec- Duringthe winter dry season,the Mexico Basinis nor- tion associatedwith traveling extratropicalcyclones causes mallyunder the influence of anticyclonicweather with light largeday4o-day variations in calculatedheat storagerates. winds abovethe basin and nearly cloudlessskies, which is Becausethe Mexico Basinis in a quiescenttropical synoptic- favorablefor the developmentof thermallydriven local and scaleenvironment, day-to-day variations in heatstorage rate regionalcirculations. due to advection are expected to be small relative to Figures7a-7d show mean daily wind soundings at thefour midlatitude environments. Further, becauseair temperatures basinsites, as compositedfrom the low-rangemode radar above the Mexico Plateaubecome more or less equilibrated profilerwind data for the entire period of thefield experiment withtemperatures surrounding the plateau at night (Figure 5), exceptfor thedays of February27 and28 andMarch 1, 5, 6, large-scaleadvection is notexpected to be importantduring and 13 whensynoptic winds became unusually strong in the muchof thenight. During the aRemoon when the atmosphere upper altitudes of the basin. The four figureparts are ar- overthe plateau becomes warmer than its surroundings,any rangedso thatthe stationscorrespond to theirrelative geo- advectionis expectedto be coldair advection,which would graphicalpositions in thebasin. The individual wind vectors decreasethe atmospheric heat storage over the plateau. in theseplots are vectoraverages of all the 1-hour-average Equation(1) wasevaluated from potential temperature dif- wind vectorsat the indicatedtime and range gate for this ferences over 100-m height intervals from pairs of period.The shading in thefigure parts represents the day-to- consecutivepotential temperature soundings. The integration daywind persistence, the ratio of thevector mean wind speed wasperformed to Zm=3000m aglin orderto extendabove to scalarmean wind speedat the indicatedrange gates and the daytimeCBLs. The results,averaged over the 4-week hoursover the periodof record.If the wind alwaysblows experimentperiod, which was composed mostly of cleardays fromthe samedirection, persistence is 1; if it is equallylikely or cleardays with scatteredcloudiness in thelate aRemoon, from all directions,or if it blows half the time from one areplotted in Figure6 at themidpoint times between sound- directionand half the time from the opposite,it is zero. High ings. Alsoplotted in Figure6, for comparisonpurposes, are valuesof persistenceindicate that wind directionchanges extraterrestrialsolar radiation from a solar model [Whiteman little fromday to day,while low valuesindicate great day-to- andAllwine, 1986] and the measured total incoming solar and dayvariability. The figure shows that the mean winds within net all-wave radiation at UNAM on March 1, 1997. The the basinbelow a heightof 1000-1500m areextremely weak meanrates of heatstorage at the foursites in thebasin differ and variablein directionduring much of the day (exceptions by severalhundred watts per meter squared during the period will be discussedin the following paragraph)and that frommidaftemoon through early evening. Nonetheless, the diurnallyreversing, low-level, along-valleywinds, a key ratesof changeof heatstorage at all four sitesshow a con- meteorologicalcharacteristic of valleys, are not readily sistentdiurnal pattern, represented by the dashedcurve in apparentin the data. Anotherunusual characteristic of the Figure6. Thecurve rises rapidly in themoming, following Mexico Basin wind field is the tendencyfor the strongest the sinusoidalnet radiationcurve closely between 0800 and winds within the basin to occur in the near-surfacelayer 1330LST, attaining 550 W m-2 in midday.In theaftemoon, during the afternoonand evening as seen,for example,in the however,it falls morerapidly than the net radiationcurve, radarprofiler wind dataat Cuautitlanand Chalco (Figure 7). exhibitinga distinctdiumal asymmetry. The falloffof the It was also noted in Doppler sodardata (which has range WHITEMANET AL.' BOUNDARYLAYER EVOLUTION OVER MEXICAN PLATEAU 10,089

o CUA [] TEO Kext •, CHA

lOOO _ ,O UNA

5OO

.:1 zx1• ' H

-500

-1000 ..... ,, .... 00 06 12 18 24 Time (LST)

Figure 6. Instantaneousfluxes in the Mexico Basinas a functionof time of day. The extraterrestrialsolar flux (Kext)is computedfrom a solarmodel; the incomingsolar radiation (Kdn) and net radiation(Q*) were measuredat UNAM on the clearday of March 1. Shownfor comparisonis Oke et al.'s [1992] 25-day compositesensible heat flux Qh measurementsobtained over the periodfrom February3 to March 31, 1985,in a heavilybuilt-up part of MexicoCity. Theplotted data points indicated by symbolsand fit by eye with a dashedline are meanrates of changeof atmosphericheat storage(H) calculatedfrom radiosondes using equation (1) for Cuautitlan (circles), Teotihuacan(squares), Chalco (triangles), and UNAM (diamonds).The datapoints are plotted at themidpoints between the soundingtimes. Soundingtimes are indicatedby arrows. The hatchedarea emphasizes afternoon differences between the Q* andthe H curves.

gatescloser to the ground)and in rawinsondedata from the (1630 to 1930 LST), and then decreasein speedand depth MexicoCity Airport (not shown). The relativelyhigh near- until 0200 LST. The graduallyincreasing CBL depthabove surfacewind speedsmay be attributedto the fact that the Cuautitlanis markedby a stronggradient in persistenceat its strongestafternoon horizontal pressure gradients between the top. At Teotihuacan a wind shift between weak afternoon basinand its coolersurroundings occur at thebase of thedeep westerlywinds and strongerevening easterlies occurs during CBL. the rapid coolingperiod. Strongereasterlies come in at the The highestday-to-day wind persistence(Figure 7) occurs endof therapid cooling period and then decrease gradually in at altitudesabove 2800 m agl (about5050 m msl) where speeduntil midnight. At UNAM, windsare very weak during southerlyor southwesterlywinds prevail above Cuautitlan, muchof theday, except for northwesterliesthat occur through Teotihuacan,and UNAM and northerlyor northwesterly the whole basin depth after 2200 and continueuntil 0200 windsprevail above Chalco. Weak southerlywinds prevail LST. Winds during the rapid coolingperiod are weak and abovethe basinin the residuallayer (the elevatedremnant of fromthe southwest.At Chalcoa layerof strong,cool, south- the daytimemixed layer) at all four basin sites usually southeasterlywinds up to 1000 m deep entersthe basin beginningin lateevening and lasting until sunrise. Some per- throughthe mountaingap to the southeastof the profilersite sistentwinds also occurwithin the basin (i.e., below about after 1300 and lastsuntil 2000 LST. The high persistence 800-1000 m agl), but the characteristicsof these winds differ valuesassociated with thisflow indicatesits day-to-dayregu- from site to site. Above Cuautitlan,persistent northeast or larity. Low-levelwinds at Chalcoare light duringmuch of northwinds begin as early as 1000 LST. Theseweak winds the remainderof the day. increasein speedgradually after noon, gradually increase in Mean wind structure evolution in the Mexico Basin is depthto encompassthe entirebasin depth by midafternoon, consistentwith the observedtemperature structure evolution. strengthensignificantly during the strong cooling period At Chalcothe strongsouth-southeasterly flow representsthe 10,090 WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

a) CUA b) TEO

iI,,I,,I, •1 •,1,,I,,I,,I,,! 4

ß , II /

ß

?

c) UNA d) CHA 4 4

ß. --•(4%4.•;,•:.:...-:::.,.:.....:.: ...... • .•..•:.:..::::...:.-.:½-?:..... •;; ,•.,.?:.}•.-: .:,.•.:.-::: •-:•.:.:..(--•- . ,,

0 i , ,'i..i"'*:c•;?;:-:. i ',,:?.?i? :;?'*;-x•i½;':',ii•i,, ;•:;;::i,,':?i.;,*.: '•"'"'"''''.•'••••;::;::,'.:;;:"'"''•- -'i , , i ,...... ,.,:::::;-..¾•;, i,, -'."':-'-•: ..;!iti!i ,, I 0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24 Time (LST) Time (LST)

Figure7. Meandaily vectorwind patternsat (a) Cuautitlan,(b) Teotihuacan,(c) UNAM, and(d) Chalco. Persistenceof the day-to-daywinds is indicatedwith gray shades(see legend). Diagonaldotted lines indicatemissing data. The dottedrectangles emphasize wind regimes at differentlevels of theatmosphere duringthe approximatetimes of the rapidcooling period. Seetext. WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,091 channeledadvection of cool air into the basin through the Although it is evident from the observationsthat these mountaingap to the southeastof the basin. This cool air leak- circulationsare thermallydriven, their exactnature is unclear. age starts,on the average,just after noon, and on some days Bosseft [1997] proposedthat the northeasterlyflow that even before noon, when the air inside the basin becomes influencesthe Mexico Basin duringthe late afternoonand warmer than the air outside, and it ends after the period of eveningis affectedby severalfactors including the plain-to- rapid cooling (1630-1930 LST) when the air temperatures plateauheating differential, the persistentsurface high pres- within the basin begin to equilibratewith those of the sur- sure over the Gulf of Mexico, and the sea breeze circulation roundings(Figure 5). The leakageis probablyresponsible for alongthe coastallowlands. He suggestedthat this large-scale the lack of heatingof the boundarylayer at Chalcobetween northeasterlyflow is similarto themountain-plain circulation 1330 and 1630 LST as shown in Figure 2c and the slight foundalong the ColoradoFront Range [Bosseft and Cotton, stabilizationof the temperatureprofiles within the convective 1994a,b]. Further,he arguedthat althoughthe mountain- boundarylayer at Chalco relative to those at Teotihuacan plain circulation in the Front Range is a summertime (Figure2a) and UNAM (Figure 2b). This gap flow occurred phenomenon,the tropicallatitude of centralMexico ensures nearly every day during the 4-week experimental period sufficientinsolation even in the wintertimeto producean regardlessof the synoptic conditions,suggesting that it is analogouscirculation between the lowlandsand the high drivenby mesoscalepressure gradients produced by the over- Mexican plateau. An alternate possibility is that the heatingof the basin atmosphererelative to its surroundings. circulations in the Mexico Basin are more local in nature and Furtherdetails on this gapwind systemand its drivingmecha- develop in responseto the overheatingof the basin nismshave been discussed by Zhongand Doran [ 1998]. atmosphere relative to its immediate environs. In this The northerlyafternoon flow at Cuautitlanalso represents scenario,air is drawn into the basin over the adjacent cold-air advection or leakage into the warm basin. This mountainsby horizontaltemperature differences that develop advectionis confinedbelow the ridgetops,begins in the early betweenthe basin atmosphere and its surroundings. Although afternoon when the basin becomes warmer than its surround- openingsin theMexico Basin make the topography somewhat ings, and is strongestnear the groundwhere the horizontal different from the closed circular basin used in the potential temperature gradients (and horizontal pressure simulationsof de Wekkeret al. [1998] and Kirnura and gradients)are strongest. Kuwagata [1993] and from the single-openingbasin simu- The basic characteristics of the cold-air intrusions are con- lated by Kirnura and Kuwagata [1993], such circulations sistentwith the diurnal evolutionof temperaturestructure and wouldbe similarto the idealizedplain-to-basin flows studied heat storagein the Mexico Basin atmosphere. The cold-air by them. Notethat in thiscase, the plain would be partof the leakagesuppresses the heating and deepeningof the basin elevatedMexican Plateau and not the coastalplain of boundarylayer between1330 and 1630 LST and causesthe Bossert's[ 1997] analysis. very pronouncedafternoon falloff of the heat storagecurve in Measurementsof wind and temperaturestructure on the Figure6. The strengtheningof cold-airadvection in the even- slopeof the Mexican plateauwhich are neededto distinguish ing, the convergenceof the cold low-level flows over the betweenthese two possibilitiesare unavailable,so we turned basin floor and consequentrising motionsabove the conver- to a numericalmodel to provide additionalinformation. Spe- gence zone undoubtedlycontributes to the rapid cooling cifically, we wished to extend Bossert's[1997] analysisto above the basin between 1630 and 1930 LST. A low-level determinewhether the coastal plain-to-plateaucirculations convergenceand upper level divergencepattern is clearly simulatedin that studywere sufficientto explainthe available seenin the profilerdata of Figure7 duringthe 1630 to 1930 observationsor whether the featuresof the Mexico Basin sig- LST period. Thesepatterns are indicatedby opposingwind nificantly modified those circulations. We also wanted to directionsbetween northern and southernparts of the basin make use of the much more extensive set of observations than and betweenlower and upper levels of the boundarylayer was availableto Bossertin his earlier studyto examinesome (comparewind directionsin the dashed-lineboxes that cor- featuresof the Mexico Basin boundarylayer. respondroughly to the rapid coolingperiod in Figure 7). A The Regional Atmospheric Modeling System (RAMS) verticaladvection explanation for the cooling of the basin [Pielke et al., 1992] was used for the model simulations. atmosphereis appealing,because the horizontal flow RAMS is a primitive equation,nonhydrostatic model based strengthsare not consistentbetween sites and are weak at both on a terrain-followingcoordinate system. Sub-ghd-scaletur- UNAM and Teotihuacanduring the rapid coolingperiod, and bulent diffusion is parameterizedusing a level 2.5 scheme because the cold-air advection is confined below the [Mellor and Yarnada, 1982] with a prognosticturbulent ridgetops,while the cooling extendsabove the ridgetops. kinetic energy equation and a diagnostic length scale. Rising motions in a neutral atmospheredo not cause Turbulent sensible and latent heat fluxes and momentum advectivecooling, but as coolingoccurs in the near-ground fluxes in the surface layer are evaluatedbased on Louis's layerdue to horizontaladvection and downward sensible heat [1979] formulation. A multilayer soil model developedby flux in the late afternoonand evening,rising motionscaused Trembackand Kessler[ 1985] is usedto predictdiumal varia- by convergencecan advect this cooling higher into the tions in soil temperatureand moisture. RAMS alsocontains a residuallayer. cloud microphysicspackage and a cumulusparameterization scheme, neither of which were activated for this study. 5. Model Simulations and Physical Mechanisms Clouds, however, were allowed to form in areas of super- saturationon the basisof a diagnosticscheme, and the impact The observationsclearly point to a linkage between the of clouds on radiation were included in the RAMS shortwave regional wind circulationsand the mesoscaletemperature andlongwave radiation schemes [Chen and Cotton, 1983]. structureand heat storageof the Mexico Basin atmosphere. Model simulationswere carded out in two stages. In the 10,092 WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

a) plateau-basin

Pacific Ocean Gulf of Mexico

6 b) plateau

c) flat terrain

0 200 400 600 800 1000 Horizontal distance (km)

Figure 8. Terraincross sections used in the (a) plateau-basinsimulation (line A-A' in Figure1), (b) plateau simulation,and (c) flat terrain simulation. The locationof the Mexico Basin on the crosssection is indicatedby MB and arrows. Dashedvertical lines indicatethe locationsof temperatureprofiles in Figure 11.

first stagewe used two-dimensionalmodel simulationswith 5.1. Two-Dimensional Simulations severaldifferent terrain crosssections to investigatethe nature of the circulations and the role of the terrain in their The modelingdomain for the threetwo-dimensional simu- development. Three simulationswere carried out. The first lationsdescribed above was 1000 km wide and 16 km deep. or plateau-basinsimulation used a NE-SW vertical terrain The horizontalgrid spacingwas 5 km and the verticalgrid cross section (Figure 8a) extending from the Pacific to the stretchedfrom 20 m just abovethe surfaceto 1000 m at and Atlantic Oceans acrossthe isthmus and passingthrough the abovethe 10-km level. A total of 60 verticalgrid pointswere center of the Mexico Basin (line A-A' in Figure lb). The used with 35 of them in the lowest 4 km to resolve the boun- second, or plateau simulation, modified the terrain cross dary layer structure. This model configurationallowed good sectionby eliminatingthe basin on top of the Mexican Plateau resolutionof the terrain, including the Mexico Basin and the (Figure 8b). The third, or flat terrain simulation,used a flat slopesof the Mexican plateau. A uniform soil type (sandy homogeneoussurface at the elevation of the Mexico Basin clay loam), vegetationtype (semidesert),and soil moisture floor to provide a simulation of boundary layer evolution contentwere specifiedthroughout the domain. A constantsea without modificationsby plateau or basin topography(Figure surfacetemperature, 295 K, was specifiedfor the oceanareas. 8c). In the secondstage we used three-dimensionalsimula- All simulations were initialized at 0600 LST with calm back- tions in conjunctionwith the data from the 1997 measurement ground winds using the mean temperature and moisture campaignto provide a more detailedpicture of the flows in soundingsfrom Acapulco (Figure 5). Simulationswere run the Mexico Basin itself, to relate these flows to the heat for 24 hours. budget in the basin, and to investigatethe interaction and The simulatedpotential temperatureand wind vectorpro- dependenceof these flows on the larger-scalecirculation files at 1600, 1800, 2000, and 2200 LST are shown in patternsidentified in the two-dimensionalsimulations. Figure 9 for the plateau-basinsimulation. At 1600 LST, WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,093 6i a)16LST I i..i i 'I .i i i I i iß i '. ! i i i I i , i • 4 316 '' .-.- 316-

.-- c• 2 ...... 3•s ...... 308.-

1 3o4 ---

6 ß . b) 18LST . ' .-' ___;320 ...:ß:: _:' ,-.-- ...... -.-,'•:...... --: '-;-' i '- ...... •.3.o•. - • • 3o8.- 304 '304--

• 5 .' ;.••. '.- ''' 320" • 6t. c)20 LST3-20 I.____•• .. .••..•I . '-...!, , "ßi I ß ,. , , I :• 4 if6'' ' ''' .-- ..... '- --'i- ' . ' ...... 316---

F:'" 3 ...... 312 '''-' '-.... ' - 312--

308 --

.-- C•2

ß . • .304

6jd)22 LS•26'...: -' .•..:..__... :• '• 09• 5 .•...... _- -...- .- ._...... '•' .... 320 316-'-'' -• 4 -.;: ...... :..:---.."'...... t:: 312 -- '" 3 312 .c: ' •'-'"- 308 • C•2 .-i- 304 ß ß

0 1O0 200 300 400 500 600 SW Horizontaldistance (km) NE

0.5 m s'1 L. 8ms '1 Figure 9. Isentropesand wind vectorsfor the two-dimensional,plateau-basin simulation at (a) 1600, (b) 1800,(c) 2000, and(d) 2200 LST. Thetopography (vertical cross-section A-A' in Figure1) is indicated by the shading. The wind legendshows the differentscales used for the horizontaland verticalwind componentson this crosssection.

boundarylayer developmentover the top of the plateau and conformance with radiosonde observations at the coastal sites the plateau slopes differed considerably. A mixed layer (see Figure 5). A stronghorizontal potential temperature deeperthan 2 km developedover the plateau, and the boun- gradientdeveloped between the plateau/basinatmosphere and dary layer top was relatively uniform across the plateau, the atmospheresurrounding the plateau. The overheatingof including the basin. The mixed layer over the slopeswas the basin atmosphererelative to its large-scaleenvironment much shallowerthan over the plateau. At the coast,onshore (seeFigure 5) is thereforea resultof the plateautopography advectionof cool marineair limitedthe mixed layer growthin acting as an elevatedheat sourceduring daytime. A deep 10,094 WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

6

._m 2 -r 1

0

I .....-:4•:: ',,', ,:::::::::::::::::::::::::::::::::::::::::::::::::::::: ....

• 21 •-"io', ,---o. "'",.:-;;•i• "::,:,,':•",' •:..'7.-""-'.'-...... :' .....--o-

0 100 200 300 400 500 600 SW Horizontaldistance (km) NE Figure 10. Pa•ic]e plumes(light shading)and horizontal wind speedisop]eths on the ve•ica] cross-section A-A' (see Figure 1) at (a) 1400, (b) 1600, (c) 1800, and (d) 2000 LST. •e dotsindicate locations •om whichpanic]es were re]eased.Dashed isop]eths indicate the speedsof horizontalwinds blowing •om ]e• to fight in the crosssection; solid isop]eths indicate wind speedsin the oppositedirection. Contour interval is 1 ms -].

layer of upslopeflow developedover slopeson both sidesof penetratesfarther into the interiorof the plateau. The hot the plateau, with weaker return flows aloft. The low branch columnover the plateaudisappears between 1800 and of theseplain-to-plateau circulations brings cooler, moister air 2000LST after the plateauswitches from an elevatedheat from coastalregions to the top of the plateaualong the slope sourceto an elevated heat sink near sunsetand cold air flows surfaces,while the upperbranch advects hotter, drier air away rapidlyto thefoot of themountains bordering the basin. The from the plateau. The circulationsproduce convergence at the model simulations(Figure 9c) showthat gravitywaves are edgesof the plateaubut have little impacton the basinat this excitedby the collapseof the domeat thetop of the daytime time of the day. At 1800 LST the plain-to-plateaucirculation convectiveboundary layer. Thesegravity waves propagate in WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,095 bothdirections when the heatingof the boundarylayer ceases, regions.In lateafternoon and evening (1600 and 1800 LST) contributingto the rapid equilibrationbetween the above- whenCBL heatingslowed and reversed to cooling,particle basinatmosphere and its surroundingsover the next few fronts acceleratedonto the plateauas air flowed acrossa hours,with isentropesbecoming nearly horizontal. This barocliniczone that had developedduring daytimeon the equilibrationaccounts for the observedweak horizontal peripheryof theplateau and separated the warm plateau CBL temperaturegradients between the plateau atmosphere and its from its coolersurroundings. By 2000 LST the two fronts surroundingsin morning rawinsondesoundings (see movingonto the plateau merged over the basin, and particles Figure5a). filledthe atmosphereover the plateauand basin to a height To helpillustrate the behavior of thermallydriven plain-to- well abovethe ridgetop. plateaucirculations and the differences in thedepths of the Thesimulated potential temperature profiles from the three mixedlayer over the slopesand on top of the plateau,a simulationsfor 0800, 1100, 1400, 1700, and 2000 LST are Lagrangianparticle dispersion model (LPDM) [Fast,1995] shownin Figure11. The profilesare takenfrom the grid wasrun using the meanwind and turbulence fields from the pointnear the center of thebasin on the terrain cross section plateau-basinsimulation. Nonbuoyant and nonreactive par- for the plateau-basinsimulation (Figure 8a) and from the ticleswere released continuously from the surfaceat foursites samegrid pointfor the two othersimulations. Both the in the domainshown in Figure10: two on the edgesof the plateau-basinand the plateau simulations produce a daytime plateauand two on the plateau slopes. The movements ofthe boundarylayer that evolvesin a fashionsimilar to the particleswere tracked following their release. The rate of the observedpattern (Figures 1 l a and 1lb), withrapid growth releasewas 8 particlesmin -• ateach source and a totalof over between 1100 and 1400, which slowedbetween 1400 and 30,000particles were released during the course of theLPDM 1700, and rapid and deep coolingbetween 1700 and simulation,which commenced at 0600 LST in the morning 2000 LST. In the flat terrain simulation (Figure 11c), and endedat midnight. however,significantly different boundary layer behavior is Figure10 shows the simulated particle plumes along with producedin lateafternoon; there, very little coolingoccurs thehorizontal component of thewinds on the cross section at between 1700 and 2000 LST, and the cooling is largely 1400,1600, 1800, and 2000 LST. At 1400LST, after 8 hours confinednear the surface.The CBL heightin the afternoonin of release,particles released over the slopes were mixed up to the flat terrain simulation is also lower than for the other two a 200-to 300-m-deeplayer and were carried by theupslope simulations.A closercomparison between the profiles from flowsalong the slopesurface to the edgesof the plateau; theplateau-basin simulation and those from the plateau simu- there,they were further mixed vertically, along with those lation indicatesthat somewhatmore heatingduring the day releasedfrom the plateau edges, to a muchdeeper layer. Two and more coolingin the eveninghours occurred in the particlefronts (the leading edges of theparticle plumes) were plateau-basinsimulation. This behavior isconsistent with the clearlydefined at thistime: oneon the ridgetop on the left TAF (TopographicAmplification Factor) concept which and one a shortdistance onto the plateaufrom the plateau statesthat heatingand coolingare likely to be amplifiedin a edgeon the right. Theparticle fronts tended to remainan- basinor valleyenvironment by the terrain[Whiteman, 1990]. chored over the mountainsat the edges of the plateau Nonetheless,the mostpronounced characteristic of boundary throughoutthe morning and early afternoon by upslopeflow layer developmentin the Mexico Basin,namely, the rapid convergenceand intense rising motions over the mountain- evening cooling that extendsthrough the entireafternoon tops.Well-defined return flow layers advected particles that boundarylayer, is determinedprimarily by the plateautopo- weremixed to higherelevations back toward the coastal graphyrather than the basin topography.

o) ploteou-bosin simulotion b,),,pl,o,t,e? ,u,s,i,m, u, I?!i,o,n,,, .... c!,f,l! ,t,e ,rrp,i n, .... 5[,...., , , I 0800, I , , LSTI .... I .... I .... 0800LST / 0800LST / I ...... 1100 LST ..... 1100LST ...... 11O0 LST __ __ 1400LST .// _ _ 1400LST __.__ 1700LST ___ 1700LST t .... 2000LST __.. 2000LST __..2000 LST ,/ :r/"'J ' ,';li

,, i•1 o ..,.... i., ....,.... , ..... 305 310 3•5 320 325 330 305 310 315 320 325 330 305 310 315 320 325 330 Potentioltemperoture (K) Potentioltemperoture (K) Potentiolternperoture (K)

Figure11. Potentialtemperature profiles at 0800,1100, 1400, 1700, and 2000 LST from(a) plateau-basin simulation,(b) plateausimulation, and (c) flat terrainsimulation. The location of theprofiles was shown by the dashedline in Figure 8. 10,096 WHITEMAN ET AL.' BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

5.2. Three-Dimensional Simulations Observed 4 To provide additionalinsight on how the plain-to-plateau --1-- 0800 LST circulationsaffect the boundary layer characteristicsinside the Mexico Basin, severalcases chosen from the field experi- ..... 1100 LST mentwere simulated using a three-dimensionalmodel config- uration. Resultsfrom a 2-day period(0600 LST on March3 ß 1330 LST to 0600 LST on March 5) are presentedhere. The four radar windprofilers indicated predominately north-northeasterly 630 LST windsabove ridgetops atmoderate speeds during this time •,2 period. Inside the basin, winds were generallyweak during ---q3- 1930 LST the morninghours on eachday and becamemuch stronger in .1 late afternoonand evening as thermally driven flows entered thebasin. The modeling domain consisted ofthree nested grids at horizontal resolutions of 36, 9, and 2.25 km, respectively. The innermostgrid coveredthe Mexico Basin area,while the outer grid encompassedthe entireisthmus and the surroundingocean surfaces (Figure 1). All threegrids had 0 42 vertical levels, stretchedfrom a grid spacingof 50 m near 300 305 310 315 320 325 330 the surfaceto 1500 m at the model top at 17.8 km. Twenty- six vertical nodeswere placed in the lowest 4 km to allow a detailed descriptionof the observed deep boundary layer 4 b)Modeled developmentover the basin. Eleven soil nodeswere usedto a --1-- 0800 LST depth of 1 m below the ground surface. Soil type was specifiedas sandy clay loam throughoutthe domain. The ...... • 1100 LST initial soil moisture content was set to 20% of the saturation valuefor all 11soil layers across the domain to reflect the dry ß 1330 [.ST

wintertimeconditions. Vegetation cover in thedomain was 630 LST specifiedfrom a U.S.Geological Survey 1 kmdatabase (159 I= categories)which was convertedto 18 BATS (Biosphere- 930 LST AtmosphereTransfer Scheme [Dickinson et al. ' 1986] cate- .1• gories in RAMS. Emissivity, albedo, and roughnesslength ._• valueswere set to 0.95, 0.15 , and2 m, respectively,for the Mexico City metropolitanarea. Surfacetemperatures at the 1 ocean grid points were determined by interpolation from 2-week composite sea surface temperatureanalyses at 1ø horizontal resolution. The model was initialized at 0600 LST on March 3 with objectiveanalysis fields determinedfrom the o National Centers for Environmental Prediction (NCEP) 300 305 310 315 320 325 330 aviation model and assimilation of standard U.S. and Mexico Potential temperature (K) rawinsondesoundings. The five outermostlateral boundary pointsin the largestdomain and the top boundarypoints on Figure 12. (a) Observedand (b) simulatedpotential tem- all threegrids were nudgedusing a simplifiedDavies scheme peratureprofiles on March4, 1997. The observedprofiles are [Wangand Warner, 1988] towardthe objectiveanalysis fields averagedover the four basin sitesTeotihuacan, Cuautitlan, to allow changesin large-scaleconditions to influencethe UNAM, and Chalco, while the simulated profiles are model simulations. Hourly wind profiles at the four basin averagedfrom the gridpoints closest to the foursites. siteswere also assimilatedduring the simulations. To help understandthe mechanismsresponsible for the evolution of the temperature profiles, we examined the modeledheat budget for the basin,which canbe writtenas storagein the air columnabove the basin floor is producedby horizontaladvection (I), verticaladvection (II), turbulentflux 1km 690 1km ' tY0 convergence(III), andradiative flux convergence(IV). -- •+1,' )--W• Beforeexamining the individualterms in theheat budget it Cp0I p•-dzCp 0lp{-(u , ty0 '. is importantto considerpossible sources of error in their i II (2) evaluation.We haveno independentmeans of estimatingthe sensibleand latent heat fluxes over the Mexico Basin, nor do we have soil moisture measurements that could be used to c?ø}dz initialize the surfaceenergy budget module in the model. I•I IV Similarly,there are no measurementsof aerosolor tracegas where the overbar indicates a horizontal averageover a rec- concentrationsthat can be usedto estimatethe magnitudeof tangulararea on the basinfloor boundedapproximately by the possiblecontributions from radiative flux divergence. four profiler sites(Figure 1a). Equation(2) was obtainedby Despitesuch uncertainties, the model does a crediblejob in horizontally averagingand vertically integratingthe thermo- simulatingmany of the importantfeatures of the boundary dynamicenergy equation, which statesthat the changeof heat layer structureand evolutionin the Mexico Basin. Acom- WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,097

a) CUA b) TEO

, , i , , i , , i , , i , , i , , i i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , i , , 3.0Observed "",-.- • ' ,, •. t l Simulated [Observed"z'*,"' ...... [ Simulated ß,',' "-'-"- ' -' z,/z' / " / z ;/ ,,. •'•'"'"./'-" / / /z' •. ' • / / 1 [ /•/'"""","'*-"' / / l g 7 "1/ /1 [ /•"""""•/'"' "' '/ / I Z '. I /

2.0 zz•'•'•'-'•/ / • •_•/ •, • I .•,, I'

"-*---"/ z' •,'/ Z.•,,/ ß ,, .... ., • / '•, •..•.-'.-*- / / , ! .... z I • •" '-'""- .... ,.,,, ...... ,,,,,, ,, .-.! ! :- Time(LST) C)UNA Time(LST) Time(LST) d)CHA Time(LST) 30 I , , I , , I , , I , , I , , I .... I , , I , , I , , I , , I , , I , , I , , I , , I , , I , , I , , I I , , I , , I , , I , , I , , I , , ''/ ..... '//// t•// -

g.• /4/. g ,/,/,...,.../,... "- // JJ'j '•1/ / •'-•'*'-'•"-*-/ • •

'""""'• / /'"','" / / / " "----'. , ,'- : ' ' r.'" ; [ '// /z'/ / ' ' • ',,1/1•17." ' ,,,,.--..,•,•;••,tI•,,.'-'-," •' ,• ' • ,- ,' •.o•...... "''"/'' , , ••', • ,,•:,• •';'' .... ''"'•'•i,, • •.Z.Z•;,.',,"'-',,,,,, . •xt•.,•-*-.•l,,/,'', •• I •,1•" , ' ' ,''" ' •,•,•'", ' • •,, -• , ...... , - •,, I• "" .... "------" l---.,-,•t,•.,]•,,,,'"""' ' :'-'" '"'•'•"''"• I• ', ""-'-'•'""x•.•"" ...... '•""-'"'""''"'- ....."•-' "' ' • , ' ' -' N.',•X ,, ,, ._ , ...... •, t , ' / / I 1 . •.', , : :: : :.:'xx•xx•.•X.... , ,, ooß / , , i , , i , , i , , i i. , i ..... •; t ,i ,•, ,.,•1 ,- .• ...... !! • i , , i , , i , , i , , i , , I . . ;..,:;•> ...... , i , 6 9 12 15 18 21 6 9 12 15 18 21 6 9 12 15 18 21 6 9 12 15 18 21 Time (LST) Time (LST) Time (LST) Time (LST)

=10ms 4

Figure 13. Simulated and observedhourly horizontal wind vector profiles on March 4, 1997, at (a) Cuautitlan,(b) Teotihuacan,(c) UNAM, and (d) Chalco.

parison of observed and simulated potential temperature weak, and simulatedearly evening winds in the elevation profile evolution on March 4, the secondday of the simu- rangefrom 0.5 to 2 km do not agreewell with observations. lation, is shownin Figure 12. The observedprofiles are aver- Turning to the heat budget analysis,Figure 14 showsa agesover the four rawinsondesites inside the basin and thus time series of the individual terms of equation (2). As representthe temperatureevolution of the entirebasin atmos- expected,through most of the morningand early afternoon, phere. The simulatedprofiles are averagesof the profiles at turbulentflux convergenceheats the basinatmosphere, while the four grid pointsnearest to the four observationsites. The horizontaladvection cools the basin atmosphere. The heat basinatmosphere on this day exhibitedthe threetypical stages storagepeaks around noon as the turbulent flux convergence of boundarylayer evolutioninside the basin, i.e., rapid warm- reachesits maximumand decreases rapidly in the afternoon ing and growth between 1100 and 1330 LST, a much slower with decreasingturbulent flux convergenceand increasing growth between 1330 and 1630 LST, and rapid cooling cold-airadvection. The switchfrom heat gain to heatloss througha deeplayer between 1630 and 1930 LST. The three occursaround sunset when the heat storageterm changes stagesin boundarylayer evolutionwere capturedby the simu- sign,and the rapidheat loss during the next severalhours is lation reasonablywell, but the modeled CBL heights were dominatedby horizontalcold-air advection. This behavioris slightly lower and the temperatureswithin the CBL were consistentwith the observations.After the period of rapid 1-2 K lower than observed in the afternoon. The simulated cooling,the horizontal cold-air advection becomes relatively total coolingin the eveningwas also lower. A comparisonof smalland eventually changes from cooling to warmingin the simulated winds with the profiler measurementsshows late nightand early morning when the basinatmosphere reasonablygood agreementat all four sites(Figure 13). The becomescolder than the surrounding air. Thenighttime cool- biggestdifferences between the observationsand the simula- ing of the air withinthe basin is producedby turbulentand tions are at UNAM, where the sudden shift in wind direction radiativeflux divergence,which is partlycompensated by after 1900 LST is not well resolved, and at Chalco, where vertical advection. simulated afternoon and evening low-level winds are too The role of advectionin the rapidcooling of the basin 10,098 WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

1 ooo Gulf of Mexico (Figure 16a). Theseflows were a prominent --•-- turb fluxdiv featurein the Bosseft[1997] simulations.An observational + horiz adv study by Fitzjarrald [1986] found similar flows in the ß vert adv Veracruzregion (Figure 1). The simulationshows that the 500- strongwinds were mainly found over the slopes,while wind • rad flux div speedson top of the plateauwere much smaller. Upslope E] heat storage flows also developedover the slopesof the mountainssur- rounding the basin to the south, southeastand southwest. Theseupslope flows were againlimited to the slopesand had little impacton the basinexcept near the gap in the mountains to the south-southeast,where a strongsoutherly flow started to enterthe basinjust after this time (Figure 16c). The wind throughthe gap and the generalupslope flow from the north- northeastbrought cooler air into the heatedbasin in early afternoon,slowing down the rate of heating of the basin

-1000 atmosphere.By 1800 LST the strongnortheasterly flow that 06 12 18 24 06 originatedover the northeastslope of the Mexicoplateau pre- Time (LST) vailed over the northeastpart of the plateauand had entered the basin throughthe open northernend (Figures 16b and Figure 14. Diurnal variation of the individualterms of the 16d). Southerlywinds throughthe gap also intensified. The model heat budget equation(equation (2)) as averagedhori- convergenceof the northerlyand southerlywinds within the zontally over the basin floor and integratedvertically to 1 km basinled to the generationof a strongconvergence zone in abovethe ground. the southernpart of the basin(Figure l 6d). These results from the three-dimensional(3-D) model simulationillustrate how the inflow of coolerair throughthe atmospherein late afternoonand eveningcan alsobe seenin openingsto the basinreduced the afternoonheating rate, and Figure 15, which shows vertical profiles of the model they suggestthat horizontalcold-air advection was the major temperature tendencies averaged between 1600 and causefor the early-eveningrapid cooling of the atmospherein 2000 LST. As seen from the heat storagecurve, the cooling the basin, and rising motions associatedwith low-level con- occursthroughout the entireboundary layer, with the rate of vergeneewere mostly responsiblefor the coolingabove the coolingdecreasing from approximately 2 K h-• nearthe basin basin.The results also suggest that the cold-air advection isa floorto about1 K h-• at 2 km. Thecooling is dominatedby resultof plain-to-plateaucirculations that bring air up the horizontaladvection below 1600 m and by vertical advection slopesalong the Gulf coastto the northeastand the slopesof above 1600 m. the mountainsthat form the southernboundary of the basin. As noted above,the heat loss derivedfrom the temperature Theseplain-to-plateau circulation systems, first investigated soundingsbetween 1630 and 1930 LST was larger than the simulated loss, so the heat budget terms illustrated in Figure 14 for this period couldnot accountfully for the obser- vations. The smaller simulatedheat loss in the eveningwas mostly the result of lower than observed afternoon tem- turb fluxdiv peratures in the simulation, suggesting that either the simulated cold-air advection was too strong or the sensible heat flux was too small in the afternoon. In other cases that 1.5 horiz we simulated,the model often predicts moderateto strong • adv southerly downslope flows at the UNAM site late in the afternoon. The profiler at UNAM, however, observesweak southerly or northerly flows during these times. This dis- ß vert adv crepancyresults in a further uncertaintyin the heat budget terms. At this time therefore we must conclude that the model rad flux is not able to provide a sufficientlydetailed representation of -' dN the boundarylayer structureand dynamicsto fully explainthe observed heating and cooling rates shown in Figure 6. Nevertheless,it does appearto give a qualitativelyreasonable heat descriptionof the roles of the mostimportant contributions to storage the heat budgetand it also providesuseful information on the interactionof the basin and plateauwinds. We turn our atten- tion to some of these features next. The simulatedhorizontal wind fields at 250 m agl in the -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 afternoonand evening in the area of the basin (grid 3) and Tendencies(K hr'1) over the plateau(grid 2) are shownin Figure 16. Early in the afternoonat 1400 LST, strongeasterly to northeasterlyflows Figure 15. Vertical distribution of individual terms of the developedover the slopesto the northeastwhich separatethe modelheat budget equation as averagedhorizontally over the elevated Mexico plateau from the coastalplains along the basinfloor andtemporally between 1600 and 2000 LST. WHITEMAN ET AL ßBOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU 10,099

Grid 2, 1400 LST b) Grid 2, 1800 LST

200 ., ,t -

oo

0 100 200 500 o lOO 2oo 5oo E-w distance (km) E-w distance (km)

C) Grid 5, 1400 LST d) Grid 5, 1800 LST

z

• o m/s

Figure 16. Simulatedhorizontal vector wind fields (arrows)at 250 m agl on March 4 on grid 3 at (a) 1400 and (b) 1800 LST and on grid 2 at (c) 1400 and (d) 1800 LST. Contoursare terrain heights at 200 m intervals. The wind legendapplies to all four figure parts.

by Bossert [1997], appear to occur regularly, as is evident not be consistentwith the ratherregular time of the cool-air from the compositeobserved wind profiles in Figure 7, and arrival on the plateau. The simulations,rather, show that the they were simulated well in all the cases studied. The cool, moist air from the coastalareas is advectedup the outer strengthof thesecirculations and their influenceon the devel- slopesof the plateauin a plain-plateaucirculation, moistening opmentof the boundarylayer over the Mexico Basin varied and cooling the air surroundingthe plateau and forming a from day to day, depending on synoptic wind speed and barocliniczone on the plateauedges. In the afternoona flow direction. beginsto form acrossthe barocliniczone ontothe plateaubut The simulations show that the cool, moist air that flows is held back in locationswhere mountainsor steepslopes on rather suddenlyonto the plateauin the late afternoonor early the plateauedges anchor the risingbranch of the plain-plateau evening is not a sea breeze front that advancesup the outer circulation. The flow acceleratesonto the plateauin the late slopesof the plateauand breaksonto the plateauat this time. afternoon,however, when the energybudget reverses on the No such traveling front is apparent in the simulations. plateauand the plateauCBL startsto decay. At thistime, the Furthermore, sea breeze fronts would have different arrival energybudget has also reversedon the slopes,ending the times every day, dependingon the speed of the advancing upslopeflows on the plateauouter slopes. Thus the air flow- front and the distanceto the coast, which would vary with ing ontothe plateaudoes not representa continuousflow up synopticwind directionsand positionin the basin and would the slopesfrom the coastalareas. Rather, air flowingonto the 10,100 WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU plateaucomes from the air masssurrounding the plateauthat relative to a volume that is not limited by the sloping side- has been moistenedduring the day by the plain-plateau walls, termedthe topographicamplification factor (TAF), can circulation. reach 2 in linear v-shapedvalleys and 3 in axially symmetric basinswith v-shapedcross sections [Whiteman, 1990]. How- ever, the broadMexico Basin, with a nearly flat floor, would 6. Discussion of Energetics have an amplificationonly fractionally larger than 1, and its The energeticscalculations in section5.2 showedthat the effective TAF would be reduced further toward 1 after the rate of gain of heat storagein the boundarylayer between daytimeCBL growsabove the height of the ridgetops,as heat sunriseand noon was much higher than expectedfrom esti- would no longerbe confinedwithin the basin. mates of surface sensible heat flux. Further, the rate of Becausethe air in the coastalareas surrounding the plateau decreaseof heat storagein the late afternoonand early even- and basin is considerablycooler than the air over the basin ing couldbe explainedonly by cold-airadvection (both hori- during daytime and only slightly warmer by sunrise zontal and vertical), and the nighttimerate of loss of heat (Figures5b and 5a), advectionon this scaleis incapableof storagewas in excessof whatcould be expectedfrom sensible producingdaytime warm air advectionover the basin,except heat flux alone. Processesthat could explain the enhanced during a brief period after sunrise,and cannot explain the coolingor warmingfall into severalcategories: turbulent sen- enhanced daytime atmosphericheat storage rates, which sible heat flux divergence,potential temperature flux associ- exceed what would be obtained with a sinusoidal surface sen- ated with the mean wind (i.e., advection),and radiativeflux sibleheat flux curvethat peaksat 600 W m-2 at noon divergence.These processes are discussed,in turn,below. (Figures3 and 6). On the basin scale, however, upslope No sensible heat flux measurements were available from flows, which form over the basininner sidewallsafter sunrise, the IMADA-AVER experimentfor comparisonto the atmos- with warm rising air over the heatedslopes and compensatory phericheat storage calculations. Measurements by Okeet al. sinkingmotions producing warming over the basincenter, are [1992] during a similar time period (February3 through likely to produceenhanced heating in the basin during the March 31, 1985) were made in a heavily built-up sectionof first few hoursafter sunrise.Similarly, convergence of down- Mexico City (Tacubaya)where the sensibleheat flux was slopeflows overthe basincenter at nightwith corresponding greatlyreduced during daytime by the storageof heatin dense rising and coolingover the basin centerare likely to enhance buildingmaterials and did not becomenegative at nightbe- the nighttimecooling. causeof heat releasefrom storage(see Figure 6). Oke et al.'s Radiativeflux convergencesor divergencesmight help to 25-day-averageflux curvewas sinusoidalwith the peak flux explain the enhancedwarming and coolingrates in the basin. reachingonly 165 W m-2 at midday.T. R. Oke(personal Absorption of solarradiation by boundarylayer aerosolshas communication,1998) has suggestedthat sensibleheat flux in been proposedby earlier investigators[e.g., Zdunkowski the rural areas of the basin during daytime might reach as et al., 1976;Ackerman, 1977] to explainenhanced warming in high as 80% of the net radiation. Figure 6 shows,however, polluted atmospheres.Angevine et al. [1998] recently esti- that 100% of the measurednet radiation gain at the surface mated that direct aerosol heating accountedfor 20% of the duringdaytime and net radiationloss during nighttime would total heat flux from all sourcesin a casestudy of boundary have to be converted to sensible heat flux to explain the layer growth near Champaign-Urbana,Illinois. Direct radia- observedheating or coolingof the atmosphere,if oneassumes tive absorptionby aerosolsheats the boundarylayer without that the heat sources and sinks are localized within the basin the lossesto latentheat flux and groundheating that would and that surface sensible heat fluxes are the sole cause. occur if this conversion of solar radiation to sensible heat flux Becausethe surfaceenergy budget is drivenby net radiation occurredat the ground. If direct solar absorptionoccurs on gain or loss,and someof this energywill be partitionedinto aerosols, insolation at the surface would decrease. This groundheat flux and latent heat flux, some of it will be featurecould be testedin the future with a line of pyranom- unavailablefor heating or cooling the atmosphere.The en- etersup one of the slopesabove Mexico City. trainmentflux at the top of a growing CBL, typically adding approximately20% of the surfacesensible heat flux [Stull, 7. Conclusions 1988], cannotbe the sourceof the missingenergy because the energeticscalculations are made throughdepths well above Data analysesand numericalmodel simulations have been the CBL where the downward turbulent sensible heat fluxes used to investigatethe evolution of boundary layers and are expectedto be negligible. Two processeshave been pro- diurnal wind systemsover the Mexico Basin and Mexican posedpreviously which are capable of amplifyingthe sensible plateauduring the winterdry seasonand to determinehow the heat flux in complexterrain areasrelative to observedsen- topographyaffects this evolution. sible heat fluxes on the basin floor or observed fluxes in flat The Mexico Basin, where Mexico City is located,exhibits terrain areas. First, sensibleheat fluxes on mountain slopes few of the meteorologicalcharacteristics of a basinor valley. are expectedto be higher than measuredfluxes on a basin It doesnot have a well-developeddiurnally reversingvalley floor because of the shear and turbulence associated with wind systemas one would expectfrom a valley, and it does upslope flows [Whiteman et al., 1996, Fast et al., 1996]. not develop the strong nighttime temperatureinversions Thus measurementson the floor of a basin are not represen- expectedof a basin. Rather,the mainmeteorological features tative of the basin as a whole. Second, in an enclosedvolume seen in analysesand simulationsare associatedwith the such as that in a basin or valley, energy inputs and outputs Mexicanplateau on which the basinis located. A deepmixed produce larger air temperatureincreases and decreasesthan layer grows abovethe isolatedplateau during daytime,pro- would be producedin a flat-flooredvolume of the samedepth ducinga stronghorizontal temperature contrast between the and drainagearea [Steinacker1984]. The amplificationof the warm air over the plateauand the coolerair in the surround- daily temperaturerange in a confined topographicvolume ingsof the plateau. Regionaldiurnal wind systemsare driven WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU lo,lOl by the strongthermal contrasts,bringing cool air from the Our researchfollows Bossert's[1997] meteorologicalcase surroundingocean areas onto the plateauduring daytime and study simulationsof the Mexico Basin and has confirmed stronglyaffecting the diurnalevolution of the boundarylayer severalaspects of his work, including his basic finding that over the plateau. The phasingof thesediurnal circulationsis regional and multiscale flows have important effects on the especiallyinteresting. Mexico City area (and thus on air pollution dispersion). We The rate of changeof atmosphericheat storagein the air have added further information on boundarylayer develop- columnabove the basin was calculatedfrom the heatingor ment, energetics,and climatology. Our simulationsmake use coolingobserved between consecutive potential temperature of the samemesoscale meteorological model, and while hav- soundings.The rate of changeof heatstorage exhibits a tem- ing accessto larger quantitiesof experimentaldata, we must poralasymmetry relative to solarnoon. The atmosphericheat reiteratehis commentsregarding the lack of experimentaldata storageincreases rapidly in themorning but fallsoff gradually againstwhich to evaluate the model simulations,especially after noon. Simulations attribute the afternoon falloff in the thoseon the plateau scalebeyond the boundariesof Mexico rate of increaseof heat storageto regionaldiurnal winds that City. Further data are neededon the wind and temperature bring cool air onto the plateauand throughgaps into the structureevolution above the slopes of the basin and the basin, suppressingthe afternoonCBL developmentsome- plateau and on the surfaceand atmosphericenergy budgets what. This afternoonleakage of cold air onto the plateau over the region. The role of Mexico City aerosolson the from the surroundingsoccurs preferentially on low edgesof atmosphericheat budget, the role of gravity waves in the the plateau.Where mountains are present at the plateauedges destructionof the daytime mixed layer, and the origin, struc- or the edgesof the plateauare steep,the terraineffectively ture, and timing of air currentscoming onto the plateauand holdsback the afternoonflow of air ontothe plateau by virtue into the Mexico Basin are all topicsthat need furtherwork. of strongrising motionsthat form over the plateauedges. Our model-baseddiagnostic studies have pointedout several Theserising motions represent the risingbranch of a plain-to- areas where model improvementsare needed for complex plateaucirculation that forms over the coastalplains and terrain simulations. plateauslopes with a return flow aloft towardthe coastalareas and sinkingmotions over the oceans. This circulationpro- Acknowledgments. We thank the IMADA-AVER field study ducesan intensebaroclinic zone aroundthe peripheryof the participantsfor use of their data. We thank our colleaguesat Pacific NorthwestNational Laboratory(PNNL) for usefuldiscussions (J. M. plateauduring daytime, separatingthe warm air in the con- Hubbe and W. J. Shaw) and for valuable reviews of our manuscript vective boundarylayer over the plateau from the cooler air (W. R. Barchet). This research was supported by the U.S. surroundingthe plateau. Department of Energy, Office of Biological and Environmental The surface energy budget on the plateau and its side Research, Environmental Sciences Division as part of their AtmosphericStudies in Complex Terrain program under contract slopesreverses shortly before sunset. At this time, the strong DE-AC06-76RLO 1830 at PNNL. The U.S. Department of Energy rising motionsthat anchorthe plain-to-mountaincirculations PNNL is operatedby Battelle Memorial Institute. on the plateauedges cease and cool air convergesonto the warm plateau rather suddenly as air flows across the References barocliniczone that has built up during the day aroundthe peripheryof the plateau. The regional-scaleflow onto the Ackerman, T. P., A model of the effect of aerosols on urban climates plateau is strongestat the lowest levels of the atmosphere with particularapplications to the Los AngelesBasin, J. Atmos. Sci., 34, 531-547, 1977. where the horizontaltemperature gradients are strongest.The Angevine, W. M., A. W. Grimsdell, S. A. McKeen, and J. M. horizontalconvergence of cool air ontothe plateauand into Warnock,Entrainment results from the Flatlandboundary layer the basinleads to risingmotions that rapidly cool the atmos- experiments,J. Geophys.Res., 103, 13,689-13,701,1998. phere above the plateau and basin. Observationsabove the Bian, X., C. D. Whiteman, G. S. Iglesias,and E. W. Garcia, basindocument this very rapid coolingwhich, between 1630 Climatologicalanalyses of air pollutionin the Mexico Basin,in Preprints,10th Joint Conferenceon Applications of Air Pollution and1930 LST, counteractsmore than half of the daytimeheat Meteorologywith the Air and WasteManagement Association, pp. storagegain. Equilibrationbetween the warmboundary layer 382-386, Am. Meteorol. Soc., Boston,Mass., 1998. air overthe plateau and the coolerair surroundingthe plateau Bossert,J. E., An investigationof flow regimesaffecting the Mexico is enhancednot only by the increasedcold air inflowsat low City region,J. Appl. Meteorol., 36, 119-140, 1997. Bossert,J. E., and W. R. Cotton,Regional-scale flows in moun- levelsbut alsoby gravitywaves that propagate away from the tainousterrain, part I, A numericaland observational comparison, plateaucenter when the domedentrainment zone (Figures 9a Mon. WeatherRev., 122, 1449-1471, 1994a. and9b) at the top of the daytimeconvective boundary layer Bossert,J. E., and W. R. Cotton,Regional-scale flows in moun- dissipates(Figures 9c and 9d). The equilibrationproduces tainousterrain, part II, Simplifiednumerical experiments, Mon. WeatherRev., 122, 1472-1489, 1994b. horizontalisentropes across the Mexico isthmus by latenight, Bravo, J. L., M. T. Diaz, C. Gay, and J. Fajardo,A shortterm so by sunrise,the temperatureand stabilityof air over the predictionmodel for surfaceozone at southwestpart of Mexico plateaudiffer little from air surroundingthe plateauat the valley, Atm6sfera,9, 33-45, 1996. same elevation. The surfaceenergy budget reversal that Chen, S., and W. R. Cotton, A one-dimensionalsimulation of the stratocumulus-cappedmixed layer, BoundaryLayer Meteorol., signalsthe sudden increase of regionalscale convergence also 25, 289-321, 1983. producesshallow downslopeflows over the inner and outer Collins,C. O., andS. L. Scott,Air Pollutionin thevalley of Mexico, slopesof theplateau. Local downslope flows that develop on Geogr. Rev., 83, 119-133, 1993. the inner sidewallsof the basinadd to the regionalconver- de Wekker, S. F. J., S. Zhong,J. D. Fast,and C. D. Whiteman,A gence. These flows continuethrough the night after the numericalstudy of thethermally driven plain-to-basin wind over idealizedbasin topographies, J. Appl. Meteorol.,37, 606-622, regional-scaleflows decay, producing a marginallyenhanced 1998. stablelayer above the floor of the Mexico Basin as cold air Diaz-Franc6s,E., M. E. Ruiz, andG. Sosa,Spatial stratification and convergesover the basin floor. multivariateanalyses of Mexico City air pollutiondata, in 10,102 WHITEMAN ET AL.: BOUNDARY LAYER EVOLUTION OVER MEXICAN PLATEAU

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Box 999, MSIN K9-30, 141-148, 1994. Richland,WA 99352. (randy.x.bian•pnl.gov;christopher. Nickerson, E. C., G. Sosa, H. Hochstein, P. McCaslin, W. Luke, and doran•pnl.gov;[email protected]; dave.whiteman•pnl.gov; A. Schanot,Project AGUILA: In situ measurementsof Mexico [email protected]) City air pollution by a researchaircraft, Atmos.Environ., 26(B), 445-451, 1992. Oke, T. R., G. Zeuner,and E. Jauregui,The surfaceenergy balance in (ReceivedMay 28, 1999; revisedDecember 10, 1999; Mexico City, Atmos.Environ., 26(B), 433-444, 1992. acceptedDecember 18, 1999.)