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Dusty cirrus clouds are extended optically thick cirrocumulus decks that occur during strong mineral dust events. So far they have been mostly documented over Europe associated with dust-infused baroclinic storms. Since today's numerical weather prediction models neither predict mineral dust distributions nor consider the interaction of dust with cloud microphysics, they cannot simulate this phenomenon. We postulate that the dusty cirrus forms through a mixing instability of moist clean air with drier dusty air. A corresponding sub-grid parameterization is suggested and tested in the ICON-ART model. Only 5 with help of this parameterization ICON-ART is able to simulate the formation of the dusty cirrus, which leads to substantial improvements in cloud cover and radiative fluxes compared to simulations without this parameterization. A statistical evaluation over six Saharan dust events with and without observed dusty cirrus shows robust improvements in cloud and radiation scores. The ability to simulate dusty cirrus formation removes the linear dependency on mineral dust aerosol optical depth from the bias of the radiative fluxes. This suggests that the formation of dusty cirrus clouds is the dominant aerosol-cloud-radiation 10 effect of mineral dust over Europe. [ABSTRACT FROM AUTHOR]

The evolution and the properties of a Saharan dust plume were studied near the city of Karlsruhe in south-west Germany (8.4298 °E, 49.0953 °N) from April 7 to 9, 2018 combining a scanning lidar (90°, 30°), a vertically pointing lidar (90°), a sun photometer, and the transport model ICOsahedral Nonhydrostatic model - Aerosols and Reactive Trace gases (ICON-ART). The lidar measurements show that the dust particles had backscatter coefficients of 0.86 ± 0.14 Mm-1 sr -1, an extinction coefficient of 40 ± 0.8 Mm-1, a lidar ratio of 46 ± 5 sr, and a particle depolarization ratio of 0.33 ± 0.07. These values are in good agreement with those obtained in previous studies of Saharan dust plumes in Western Europe. Compared to the remote sensing measurements, the model simulation predicts the plume arrival time, its layer height, and structure very well but overestimates the backscatter coefficient. In this manuscript, we discuss the complementarity and advantages of the different measurement methods as well as model simulations to predict Saharan dust plumes. Main conclusions are that the ICON-ART model can predict the structure of Saharan dust plumes very well but overestimates the backscatter coefficients by a factor of 2.2 ± 0.16 at 355 nm and underestimates the aerosol optical depth (AOD) by a factor of 1.5 ± 0.11 at 340 nm for this Saharan dust plume event. Employing a scanning aerosol lidar allows determining backscatter coefficient, particle depolarization ratio and especially lidar ratio of Saharan dust both for daytime and nighttime independently. Combining lidar with sun photometer data allows constraining aerosol optical depth in different ways and determining column integrated lidar ratios. These comprehensive datasets allow for a better understanding of Saharan dust plumes in Western Europe. [ABSTRACT FROM AUTHOR]

Aeolian dust, the most dominant atmospheric aerosol by mass, influences the weather and climate directly through absorbing and scattering the radiation. Particle size distribution (PSD) controls the magnitude of these impacts through modulating the dust residence time in the atmosphere and optical properties. Although the dust PSD varies during the atmospheric transport, current models usually neglect the effect of these variations on dust optical properties. This study investigates the impact of variable PSD on dust optical properties and radiative impacts using the next-generation atmospheric modeling system ICON-ART (ICOsahedral Nonhydrostatic with Aerosols and Reactive Trace gases). Two sets of numerical experiments are conducted assuming fixed and variable PSD. A parameterization is developed to account for the effect of variable PSD on dust optical properties in the model. This parameterization is then implemented in ICON-ART to simulate a period of 1 month on a global grid with 80 km horizontal resolution. Results show that the consideration of a variable PSD increases the dust AOD. However, the magnitude of the changes depends on the dominant size modes. Although the optical properties of the fine mode show the highest sensitivity to the variable PSD, the median diameter of this mode hardly changes during transport. Thus, the new parametrization does not have a significant impact on AOD where fine mode is dominant. In contrast, when coarse mode is prevailing, the increase of AOD is more pronounced. This makes the surface slightly warmer (+0.2 K) in the source regions. [ABSTRACT FROM AUTHOR]