Global occurrence of aerosol layers over low- and mid-level clouds

Since the source of atmospheric aerosol is mostly at the surface, it is not surprising that the number concentration of aerosol drops off sharply with increasing altitude. However, in several locations, lofted aerosol are transported over large distances and sometimes appear lying over cloud layers. This simultaneous occurrence is a persistent feature in some regions during particular seasons. This situation results in a radiative forcing whose sign depends on the absorbing properties of the aerosol and whose magnitude depends on aerosol optical depth. The forcing, in turn, has the potential of feeding back to the cloud formation process. We have analyzed regional and seasonal occurrence of aerosol over cloud from CALIOP layer products and compared the climatology of these events with simulations made by a chemistry transport model, GOCART, which uses analyzed meteorological fields and daily emissions as input to drive the aerosol field. The success of this comparison will indicate whether interactive global climate models that include aerosol generation and transport will be able to simulate this potentially important radiative forcing.