Precipitation trends in Mexico City and their relation to microphysical and surface processes

The last 60 years of recorded hourly precipitation in Mexico City metropolitan area (MCMA) have shown distinct changes in the frequency and timing of intense events (>20mm/hr). The frequency has increased five-fold from 1950 to 1990 in the East and Southeast sectors of the MCMA, where intense urbanization has occurred, while the West has seen slight changes for the last 20 years. On the other hand, in the 40s, 70% of intense events occurred between 7pm and midnight while in the 60s this number decreased to only 10%. Notably, by 2009, 77% of intense events occurred between 7pm and midnight reverting back to conditions only seen in the 40s. In the past, it has been common (not only in the case of the MCMA but in other urban sites as well) to attribute the observed precipitation changes to the thermal impact of the Urban Heat Island (UHI), particularly at the city center where convection is expected to be greater. In this study, two alternative hypotheses are considered, namely, i) that observed changes in precipitation patterns and intensity are the result of different emission rates of aerosol pollutants present in different periods in the past and ii) that urbanization-induced land surface changes have been a major player in the observed changes. To that end, the regional atmospheric model WRF has been used. Preliminary results indicate that a combination of those two factors result in precipitation changes consistent with the observations.