Boundary Layer Evolution and Regional Thermally Driven Circulations in the Mexico Basin and over the Surrounding Mexican Plateau.
C. D. Whiteman, S. Zhong, X. Bian, J. D. Fast and J. C. Doran
Pacific Northwest National Laboratory
Richland, WA 99352
Upper air soundings and radar profiler data are used to determine diurnal variations in the mean temperature and wind structure of the Mexico Basin atmosphere and its surroundings during the winter dry season. The heat stored in the convective boundary layer above the basin floor during daytime and the heat loss at night are much greater than can be explained by previously measured sensible heat fluxes at the basin floor. The rate of change of atmospheric heat storage during daytime exhibits an asymmetry in which the increase in heat stored in the atmosphere falls off rapidly after noon. The decay in the rate of increase of heat storage in the afternoon is attributed to a leakage of cold air into the warm basin from its surroundings through a gap to the south of the basin and a large opening to the north. In the late afternoon (1630 and 1930 LST), a very rapid cooling occurs, extending far above the basin and amounting to more than half of the daytime heat storage. This rapid cooling and a nighttime near-equilibration with the surrounding atmosphere are features of the observations that have apparently not been seen in other basins, and are attributed to the rapid decay of the convective boundary layer above the plateau as the heat flux changes sign and the plateau suddenly switches from an elevated heat source to an elevated heat sink. The regional winds respond rapidly to this change in status of the plateau as plain-to-plateau circulations converge into the plateau from the surroundings, driven by strong horizontal potential temperature gradients that are built up between the plateau and surroundings during daytime. These gradients are observed from the twice-daily soundings of the Mexican rawinsonde network. The converging cold air rises rapidly above the plateau and basin, cooling the atmosphere through a deep layer. A companion paper investigates the physical mechanisms leading to the observed boundary layer evolution over the Mexico Basin, focusing on the influence of large-scale thermally driven plain-plateau circulations.
Corresponding author: Dr. C. David Whiteman, Battelle Northwest Labs, P.O. Box 999, Richland, WA 99352, (509)372-6147, dave.whiteman@pnl.gov