Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics, Part 1: Ice Formation and Cloud Properties

Abstract

To better understand the impacts of dust aerosols on deep convective cloud (DCC) systems revealed by previous observational studies, a case study in the tropical eastern Atlantic was investigated using the Weather Research and Forecasting (WRF) model coupled with a Spectral Bin Microphysics (SBM) model as a two-part study. A detailed set of ice nucleation parameterizations linking ice formation with aerosol particles have been implemented in the SBM for this study. It is found that, dust, transported from the Sahara desert and acting as ice nuclei (IN), increases heterogeneous formation of ice particles at temperatures above −38 °C by approximately a factor of four per IN magnitude increase from 0.12 cm −3. Homogeneous ice formation is reduced below −38 °C by up to 79 %, due to greater conversion of liquid drops to ice at warmer temperatures. The ice particle size distribution (PSD) is shifted towards smaller sizes at heterogeneous temperatures and median sizes at colder temperatures due to increased vapor competition and crystal aggregation. Graupel sizes are reduced due to increased riming of more numerous, but smaller, ice particles. Liquid mass is reduced by up to 85 % at midlevels due to increased riming, drop freezing and Bergeron process evaporation. Despite the enhanced vertical motion in the dust cases (up to 30 %), average cloud top height was found to be lowered by up to 3.29 km in comparison with the background aerosol (Clean) case, which is consistent with observations. This is due to increased sedimentation rates resulting from earlier formation of precipitation sized particles.