Abstract. To better understand the impacts of dust aerosols on deep convective cloud (DCC) systems reported 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. A detailed set of ice nucleation parameterizations linking ice formation with aerosol particles has been implemented in the SBM. Increasing ice nuclei (IN) concentration in the dust cases results in the formation of more numerous small ice particles in the heterogeneous nucleation regime (between − 5 and −38 • C) compared to the background ("Clean") case. Convective updrafts are invigorated by increased latent heat release due to depositional growth and riming of these more numerous particles, which results in increased overshooting and higher convective core top heights. Competition between the more numerous particles for available water vapor during diffusional growth and available smaller crystals and/or drops during collection reduces particle growth rates and shifts precipitation formation to higher altitudes in the heterogeneous nucleation regime. A greater number of large snow particles form in the dust cases, which are transported from the core into the stratiform regime and sediment out quickly. Together with reduced homogeneous ice formation, the stratiform and/or anvil cloud occurrence shifts frequency to warmer temperatures and reduces anvil cloud extents. Total surface precipitation accumulation is reduced proportionally as IN concentration is increased; though the stratiform precipitation accumulation is increased due to greater snow formation and growth, it does not counteract the reduced convective accumulation due to less efficient graupel formation. Radar reflectivity values are increased in the dust cases at temperatures below 0 • C in both the convective and stratiform regimes due to more large snow particles, and reduced in the convective core near the surface due to melt of small ice or graupel particles, consistent with case study observations.

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 10 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 15 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 3.29km in comparison with the background aerosol (Clean) case, which is consistent with observations. This is due to increased sedimentation rates resulting from earlier 20 formation of precipitation sized particles.