Tropical North Atlantic Dust Increases the Prevalence of Deep Convective Clouds: Diurnal Patterns Offer Clues as to Why

Deep convective clouds (DCCs) supply a majority of the precipitation to the tropical North Atlantic. Dust, an effective ice nucleator, is thought to impact DCC microphysical development. However, there is a lack of consensus on how important dust is to DCC freezing and lifetime, due to confounding meteorological co-variation, the co-occurrence of direct and semi-direct effects, and other model and data uncertainties. Here, we estimate regional-scale dust effects on cloud properties using a new method that provides substantially more confident conclusions than previously possible. The method is broadly applicable to other aerosol-cloud interaction-type studies as well, and is based on combined satellite cloud and meteorological observations and modeled aerosol levels, validated from ground data. We account for moisture and stability differences under dusty and clean conditions by stratifying by a variety of meteorological variables. Upon stratification, a substantially stronger association between DCC variability and the presence of dust is revealed. Dust is associated with a ~75% increase in DCC frequency compared to clean conditions. Studies that do not account for co-varying meteorology are likely to underestimate these effects. Comparisons between night- and day-time results offer clues into the relative importance of microphysics, which appears to be secondary to direct/semi-direct effects. Differences in dust effects in the presence and absence of combustion aerosols will also be discussed.