Changes in precipitation intensity in Mexico City: Urban Heat island effect or the impact of aerosol pollution?

It has been know for many years that Mexico City experiences the effect known as “Urban Heat Island” (UHI), whereby the temperature in the city is higher than in surrounding areas and more importantly, the nocturnal minimum temperature is several degrees elevated with respect to the surroundings. Studies in the past have also tried to relate the UHI to changes in precipitation patterns in diverse cities, with mixed results. It is still a matter of discussion what is the real effect of the UHI on precipitation.

In the context of a recently proposed hypothesis that increased aerosol pollution may lead to changes in the intensity and timing of precipitation, we have analyzed an extensive precipitation dataset available for Mexico City. The network consists of about 60 automatic stations that register precipitation as it occurs in the different sectors of the city. The data is available since 1993, so a 15-year climatology was constructed. We have determined that significant changes have occurred since 1993 in the number of intense precipitation events (when precipitation rate is higher than 20 mm per hour), that are now more frequent after 7pm. Correlations with aerosol pollutants cannot totally account for the observed changes, suggesting that perhaps other factors also influence the change in precipitation patterns.