Disentangling the contributions of orographic waves, boundary-layer coupling, and aerosol to the occurrence of ice in mixed-phase clouds

We will present a study on the impacts of orographic waves, surface coupling, and aerosol load on the frequency of heterogeneous ice formation in stratiform clouds using ground-based remote-sensing observations. Disentangling the convoluted effects of vertical motions and aerosols is critical for the understanding of heterogeneous ice formation and requires comprehensive observations. For the study, multi-year datasets from Punta Arenas (53.1°S 70.9°W, Chile, >2 years) and the northern hemispheric sites of Leipzig (51.4°N 12.4°E, Germany, 2.6 years) and Limassol (34.7°N 33.0°E, Cyprus, 1.5 years) were obtained by the same set of ground-based instruments (35-GHz cloud radar, Raman polarization lidar, 14-channel microwave radiometer, Doppler lidar, and disdrometer). The datasets at Limassol and Punta Arenas resemble the first multi-year ground-based remote-sensing datasets in the Eastern Mediterranean and in the western part of the Southern Ocean, respectively.

The cloud properties were extracted from the synergistic dataset and the following key results on the efficiency of heterogeneous ice formation emerged:
The apparent lack of ice forming clouds at Punta Arenas below -15 °C can be related to orographic gravity waves, which allow persistent liquid saturation. These clouds could be identified by the autocorrelation function of the in-cloud vertical air velocity. Additionally, a correlation between the surface-coupling of a cloud and the likelihood of ice formation was found for Punta Arenas and Leipzig. At T>-10°C clouds coupled to the aerosol-rich boundary layer, were found to contain ice more frequently. Taking both effects into account, free-tropospheric, fully turbulent clouds at Punta Arenas form ice less frequently than their northern-hemispheric counterparts. This difference is linked to the lower abundance of INP in the free troposphere over the Southern Ocean.