Evaluation of aerosol direct radiative forcing in MIRAGE

A variety of measurements have been used to evaluate the treatment of aeros ol radiative properties and radiative impacts of aerosols simulated by the Model for Integrated Research on Atmospheric Global Exchange (MIRAGE). The treatment of water uptake in MIRAGE agrees with laboratory measurements, an d the growth of aerosol extinction with relative humidity in MIRAGE simulat ions agrees with field measurements. The simulated frequency of relative hu midity near 100% is about twice that of analyzed relative humidity. When th e analyzed relative humidity is used to calculate aerosol water uptake in M IRAGE, the simulated aerosol optical depth agrees with most surface measure ments after cloudy conditions are filtered out and differences between mode l and station elevations are accounted for, but simulated optical depths ar e too low over Brazil and central Canada. Simulated optical depths are most ly within a factor of 2 of satellite estimates, but are too high off the ea st coasts of the United States and China and too low off the coast of West Africa and in the Arabian Sea. The simulated single-scatter albedo is consi stent with surface measurements. MIRAGE correctly simulates a larger Angstr om exponent near regions with emissions of submicron particles and aerosol precursor gases, and a smaller exponent near regions with emissions of coar se particles. The simulated sensitivity of radiative forcing to aerosol opt ical depth is consistent with estimates from measurements. The simulated di rect forcing is within the uncertainty of estimates from measurements in th e North Atlantic.