Warm-phase spectral-bin microphysics in ICON: reasons of sensitivity to aerosols

Abstract

The warm-phase version of the Spectral-Bin Microphysics (SBM) scheme has been implemented into the ICON model and has been tested in a supercell storm simulation. The results of two-moment (2M) bulk scheme which exists in ICON and SBM were compared in all aspects related to warm processes, especially with regards to sensitivity to aerosols.In all simulations a strong storm with maximum updrafts exceeding 25 m/s was simulated. The maximum vertical velocity in the 2M bulk scheme does not depend on aerosols. However, the maximum vertical velocity in SBM with high aerosol concentration was found substantially higher than in the case of low aerosol concentration.Despite the insensitivity of maximum vertical velocity to aerosols, the microphysics of the 2M scheme is sensitive to aerosols. Both SBM and 2M schemes showed faster rain formation in case of low aerosol concentration; both schemes indicate that first raindrops form when the mean volume radius exceeds ~15 μm. In both schemes the first raindrops form near cloud top in an undiluted core and fall along the cloud edges at the mature stage. At the decaying stage, raindrops fall through the cloud core. The zone of rainfall at the lower few kilometers coincides with the zones of strong downdrafts. A substantial effect of aerosols on intensity and spatial distribution of precipitation was found. In the case of low aerosol concentration raindrops form earlier and at lower altitudes. As a result, the area covered by rain is lower, but the maximum of the rain intensity is higher than in case of high aerosol concentration. Finally, we discuss the hindering effects of large supersaturation via mass loading on the cloud development in case of low aerosol concentration, and examine the validity of the well-known saturation adjustment procedure.