Meteorological Variability and the Observed Aerosol First Indirect Effect over the Southern Great Plains

Aerosol-cloud interactions remain a source of large uncertainty in future climate projections. Observations of aerosol indirect effects can improve our understanding of the role of aerosols in cloud formation processes. They can also be a window in local and regional variability in cloud response to aerosol forcing that is likely affected by local atmospheric conditions. Here, we present MODIS observations of the cloud responses to aerosols in the Southern Great Plains region of the United States during the spring (April-June) of 2008-2013. We define maximum and minimum aerosol events using the surface-based IMPROVE network, and examine the relationship between high aerosol events and MODIS cloud properties. We find a statistically significant decrease in average effective radius at each of site on high aerosol days as expected from the known first indirect effect mechanism. However, cloud water path, a function of both aerosol forcing and synoptic and mesoscale conditions, determines the optical thickness of the observed clouds. We use NARR-derived profiles of temperature, wind speeds and humidity to define the meteorological context and explain differences in cloud water path response under high and low aerosol conditions. Finally, we separate observed water path responses between shallow and deep convective clouds using measurements of cloud top temperature. Overall, we find a distinct reduction in effective radius under high aerosol conditions but a variable response in the cloud water path depending on meteorological conditions. This provides observational evidence that the strength of the aerosol indirect effect is modulated by local meteorology.