Aerosol-cloud-radiation interaction during Saharan dust episodes: the dusty cirrus puzzle

Dusty cirrus clouds are extended optically thick cirrocumulus decks that occur during strong mineral dust events. So far they have mostly beendocumented over Europe associated with dust-infused baroclinic storms. Since today's global numerical weather prediction models neither predictmineral dust distributions nor consider the interaction of dust with cloud microphysics, they cannot simulate this phenomenon. We postulate thatthe dusty cirrus forms through a mixing instability of moist clean air with drier dusty air. A corresponding sub-grid parameterization is suggestedand tested in the ICOsahedral Nonhydrostatic model with Aerosol and Reactive Tracegases (ICON-ART). Only with the help of this parameterization is ICON-ART able to simulate the formation of the dusty cirrus, whichleads to substantial improvements in cloud cover and radiative fluxes compared to simulations without this parameterization. A statisticalevaluation over six Saharan dust events with and without observed dusty cirrus shows robust improvements in cloud and radiation scores. The abilityto simulate dusty cirrus formation removes the linear dependency on mineral dust aerosol optical depth from the bias of the radiative fluxes. Forthe six Saharan dust episodes investigated in this study, the formation of dusty cirrus clouds is the dominant aerosol-cloud-radiation effect ofmineral dust over Europe.