Aerosols and Ice Particle Size in Tropical Cumulonimbus

Abstract

A retrieval of ice crystal size near the tops of active deep cumulonimbus clouds (Cb) throughout the Tropics over a 12-yr period is presented based on radiances from the 3.7-?m channel of the Advanced Very High Resolution Radiometer (AVHRR). Effective diameters (De) are 10%?20% smaller over land than ocean. Downwind of continents, crystals are smallest when low-level, offshore transport is strongest. Other regional, seasonal, interannual, and long-term variations are also found. These are compared with variations of the Total Ozone Mapping Spectroradiometer (TOMS) retrieved tropospheric aerosol and with variations of convective intensity and amount in an effort to identify potential causes by statistical association. Ice particles prove to be smaller when aerosol amounts are greater and when convection is more intense, but appear unrelated to convective rate of occurrence. Aerosols appear to be the most important influence on seasonal and longer timescales, with a consistent ?20% decrease in Cb crystal effective diameter per unit increase in TOMS aerosol index in regions of biomass burning. In the Sahel region of Africa, where dust and burning both contribute to TOMS retrievals, this sensitivity is closer to ?10%. The variety of signals makes the possibility of accidental statistical association unlikely, although it cannot be ruled out. Based on these numbers and on geographic maps of De, open biomass burning appears to be more important than urban sources of aerosol in influencing Cb microphysics. Dust generated near burning sites may also contribute to the observed aerosol influence on De.