THE IMADA-AVER BOUNDARY-LAYER EXPERIMENT IN THE MEXICO-CITY AREA

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 we re instrumented. At four of the sites, 915-MHz radar wind profilers we re deployed and radiosondes were released five times per day. Two of t hese 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 la sting for several hours. Significant differences between the mixed-lay er temperatures in the basin and outside the basin were observed. Thre e thermally and topographically driven flow patterns were observed tha t are consistent with previously hypothesized topographical and therma l forcing mechanisms. Despite these features, the circulation patterns in the basin important for the transport and diffusion of air polluta nts show less day-to-day regularity than had been anticipated on the b asis of Mexico City's tropical location, high altitude and strong inso lation, and topographical setting.