During February and March of 1997, the U.S. Department of Energy, in cooperation with the Mexican Petroleum Institute, carried out an extensive measurement program to measure the meteorology and photochemistry associated with the chronic severe air pollution that develops in the Mexico City basin. As part of this program, Pacific Northwest National Laboratory operated a boundary layer measurement site at Teotihuacan, northeast of Mexico City, that was instrumented with radiosondes, a surface meteorological mast, a sodar, and a wind profiling radar.
The Mexico City plateau is surrounded on the east, south, and west by mountains that created a partial basin of elevation 2000+ m which is open to the north. The topography drops to sea level at the Gulf of Mexico beginning approximately 100 km northeast of Teotihuacan. Boundary layer depths as measured by radiosondes typically reached 2500 m to 3500 m AGL in the afternoon. During the month-long measurement period a brisk northeast wind with a sudden onset about 1800 LST occurred on about half the measurement days. Surface meteorology and wind profiler data show that this phenomenon has a number of features in common with gravity currents in the atmosphere, including a marked drop in temperature and increase in specific humidity. In this paper we discuss the structure of this flow, its predictability according to atmospheric temperature differences derived from soundings between Veracruz on the Gulf coast and Teotihuacan in the Mexico City basin, and the potential significance of its role in the redistribution of pollutants in Mexico City.