Effect of mineral dust on cloud microphysics and precipitation of mesoscale convective systems

Among all recognized climate forcing mechanisms, aerosol indirect effects are currently the greatest source of uncertainty in model forecasts of climate change. In recent years vigorous research has ensued focusing on key unanswered questions that persist due to limited laboratory and field observations of aerosol-cloud-precipitation interactions. As one of the four major terrestrial sources of atmospheric aerosols (desert dust, biomass burning, biogenic and anthropogenic air pollution), mineral dust is responsible for significant climate forcing through direct effect on solar and thermal radiation as well as indirect effect on clouds and precipitation processes. Terra satellite has strong capabilities in determining dust aerosol loadings and cloud distributions and can potentially be used to reduce the uncertainties of the aerosol effects on climate. This study uses sensors on Terra and other satellites to investigate the impact of mineral dust on cloud microphysical and precipitation processes in mesoscale convective systems. A trans-Atlantic dust outbreak of Saharan origin occurring in early March 2004 is considered. For the observed mesoscale cloud systems under a given convective strength, small hydrometeors were found more prevalent in the stratiform rain regions with dust than in those regions that were dust free. Evidence of abundant cloud ice particles in the dust regions, particularly at altitudes where heterogeneous nucleation of mineral dust prevails, further supports the observed changes of precipitation. The consequences of the microphysical effects of the dust aerosols were to shift the size spectrum of precipitation-sized hydrometeors from heavy precipitation to light precipitation and ultimately to suppress precipitation and increase the lifecycle of cloud systems.