A boundary layer field experiment in the Mexico City basin during the period 24 February–22 March 1997 is described. A total of six sites were instrumented. At four of the sites, 915-MHz radar wind profilers were deployed and radiosondes were released five times per day. Two of these sites also had sodars collocated with the profilers. Radiosondes were released twice per day at a fifth site to the south of the basin, and rawinsondes were flown from another location to the northeast of the city three times per day. Mixed layers grew to depths of 2500–3500 m, with a rapid period of growth beginning shortly before noon and lasting for several hours. Significant differences between the mixed-layer temperatures in the basin and outside the basin were observed. Three thermally and topographically driven flow patterns were observed that are consistent with previously hypothesized topographical and thermal forcing mechanisms. Despite these features, the circulation patterns in the basin important for the transport and diffusion of air pollutants show less day-to-day regularity than had been anticipated on the basis of Mexico City's tropical location, high altitude and strong insolation, and topographical setting.

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*Pacific Northwest National Laboratory, Richland, Washington.

+Environmental Technology Laboratory, Boulder, Colorado.

#Los Alamos National Laboratory, Los Alamos, New Mexico.

@Desert Research Institute, Reno, Nevada.

&Argonne National Laboratory, Argonne, Illinois.

**Instituto Mexicano del Petróleo, Mexico City, Mexico.

++Universidad Nacional Autónoma de México, Mexico City, Mexico.

##Politecnico Nacional: Vocacional 10, Mexico City, Mexico.

@@U.S. Department of Energy, Germantown, Maryland.