We annalyzed 6-years of satellite data (MODIS-derived cloud and aerosol propertis, TRMM-derived rainfall totals) along with AERONET in-situ observations to asses aerosol impact on cloud microphysical processes and accumulated surface rainfall.

Aerosols have two competing effects on surface rainall: One is that anthropogenic urban aerosols reduce surface insolation by up to 30 wm-2 in a normal day in large cities like New York. This weakens the surface heat island effect (UHI), and consequently may reduce dynamic forcing that may lead to enhancement of convective rainfall. On the contrary, aerosols also interact with clouds as cloud condensation nuclei and sequentially affect urban rainfall. Our hypothesis and analyses suggest that the net effect of aerosols on precipitation may depend the relative strength of large-scale synoptic forcing and urban-induced dynamic processes are.

Under typical summer conditions, large scale forcing can be minimal while convective-mesoscale forcing associated with urban surfaces can be significant. Under such circumstances, we hypothesize that mesoscale-forcing related to urban-affected thermodynamics and dynamics are of first-order significance for initiating convection while aerosol effects are more important for precipitation duration and efficiency.