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 associated with basins and does not exhibit diurnally reversing valley wind systems. Data analyses, twoand three-dimensional numerical simulations with the Regional Atmospheric Modeling System (RAMS), and a Lagrangian particle dispersion model are used to interpret hese observations and to examine the effects of topography and regional diurnal circulations on boundary layer evolution over the Mexico Basin and its surroundings during fair weather periods in the winter dry season. We show that the boundary layer evolution in and above the basin is driven primarily by regional diurnal circulations that develop between the air above the Mexican Plateau and the generally cooler surrounding coastal areas. A convective boundary layer (CBL) grows explosively over the plateau in the late morning to reach elevations of 2250 m agl (4500 rn rnsl) by noon, and a strong baroclinic zone forms on the edges 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 rapid and deep cooling of the air above the plateau begins in late afternoon and early evening when the surface energy budget reverses, the CBL decays, and air accelerates onto the plateau through the baroclinic zone. Flow convergence near the basin floor and the associated rising motions over the basin and plateau produce cooling in 3 hours that is equivalent o half the daytime heating. While the air that converges onto the plateau comes from elevations 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 plainplateau circulation.