Uncertaintines in modelling contribution of water to aerosol direct effect

Abstract:

Modelled direct radiative forcing of aerosol in global aerosol models varies between -0.06 to -0.49 W m^-2 making it currently one of the most uncertain factor in climate change. The variance in aerosol direct radiative forcing is regionally even much larger indicating significantly different spatial difference in aerosol load. This can be seen in global maps of modeled aerosol optical depths (AOD). A significant fraction of aerosol optical depth comes from aerosol water which also varies significantly between different models. In this study, we investigate the reasons for these differences in the models. We investigate how differences in modeled aerosol composition, relative humidity, and size distribution contribute to differences in AOD and optical depth of aerosol water. We compared aerosol-climate model data for aerosol optical depth against those observed by Moderate-Resolution Imaging Spectroradiometer (MODIS) instrument on board of Terra and Aqua satellites. Modeled relative humidities were compared against observations from the Atmospheric Infrared Sounder (AIRS) flying on board NASA's Aqua satellite and the National Centers for Environmental Prediction (NCEP) reanalysis data. According to preliminary results, modeled transport of aerosol has significant implications on zonal distribution of aerosol optical depth. Models that underestimate transport of aerosol towards poles also have lower AOD over high latitudes. The contribution of water to the AOD can be significantly higher in models with low poleward transport. However, it is not be sufficient to compensate low total aerosol optical depth.