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 Basin and 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 the literaturerely 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