Evidence for the impact of aerosols on the onset and microphysical properties of rainfall from a combination of satellite observations and cloud-resolving model simulations

Satellite rainfall estimates from the Tropical Rainfall Measuring Mission's (TRMM) precipitation radar (PR) and microwave imager (TMI) exhibit large differences off the coast of China and extending east across the North Pacific storm track. These differences suggest the modification of precipitating clouds on a large scale with potentially important implications for seasonal rainfall estimates. Coincident TRMM/CloudSat observations for a case from 3 April 2007 show striking differences in both rain area and rainfall intensity from the TMI, PR, and CloudSat retrievals. Observations from the 94-GHz CloudSat radar, which is highly sensitive to the onset of rain, confirm the presence of widespread light rain/drizzle containing relatively small drops below the similar to 17 dBZ PR detection threshold. For pixels with reflectivities above the PR detection threshold, large differences are present in the satellite rain intensity estimates, which are consistent with either a decrease in the mean drop size, an increase in ratio of cloud water to rainwater, or both. To explore the potential link between aerosols and the observed changes in the observed cloud microphysics, idealized cloud-resolving model (CRM) simulations initialized for the 3 April 2007 case are performed. The model results are generally consistent with the observations indicating high aerosol concentrations leading to an overall increase in the ratio of cloud water to rainwater for developed systems, as well as a delay in the onset of warm rain. The simulations also show an initial decrease in the mean raindrop size, although larger drops develop later leading to an overall increase in the total rainfall accumulation. On the basis of the combination of observations and CRM simulations, therefore, it is hypothesized that the observed differences may be due to an increase in the ratio of cloud water to rainwater leading to an overestimate in rain intensity by the CloudSat/TMI retrievals and/or a decrease in the mean drop size leading to an underestimate by the PR retrieval.