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

Data collected in a measurement campaign in February and March 1997 showed that the Mexico Basin (also called the Valley of Mexico), located atop the Mexican plateau, fails to develop the strong nocturnal inversions usually a ssociated with basins and does not exhibit diurnally reversing valley wind systems. Data analyses, two- and three-dimensional numerical simulations wi th the Regional Atmospheric Modeling System (RAMS), and a Lagrangian partic le dispersion model are used to interpret these observations and to examine the effects of topography and regional diurnal circulations on boundary la yer evolution over the Mexico Basin and its surroundings during fair weathe r periods in the winter dry season. We show that the boundary layer evoluti on in and above the basin is driven primarily by regional diurnal circulati ons that develop between the air above the Mexican Plateau and the generall y cooler surrounding coastal areas. A convective boundary layer (CBL) grows explosively over the plateau in the late morning to reach elevations of 22 50 m agl (4500 m msl) by noon, and a strong baroclinic zone forms on the ed ges of the plateau separating the warm CBL air from its cooler surroundings . In early afternoon the rates of heating and CBL growth are slowed as cool air leaks onto the plateau and into the basin through passes and over low- lying plateau edges. The flow onto the plateau is retarded, however, by the strongly rising branch of a plain-plateau circulation at the plateau edges , especially where mountains or steep slopes are present. An unusually rapi d and deep cooling of the air above the plateau begins in late afternoon an d early evening when the surface energy budget reverses, the CBL decays, an d air accelerates onto the plateau through the baroclinic zone. Flow conver gence near the basin floor and the associated rising motions over the basin and plateau produce cooling in 3 hours that is equivalent to half the dayt ime heating. While the air that converges onto the plateau comes from eleva tions at and above the plateau, it is air that was modified earlier in the day by a cool, moist coastal inflow carried up the plateau slopes by the pl ain-plateau circulation.