Seasonal and spatial variability of heterogeneous ice formation in stratiform clouds and its possible impact on precipitation formation

Abstract:

Lidar observations of stratiform mid-level clouds were used to investigate the efficiency of heterogeneous ice nucleation as a function of cloud top temperature. The long-term lidar-based cloud datasets were collected in Germany (51°N,12°E), in southeastern China (22°N,112°E), Cape Verde (15°N,24°W), the Amazon Basin (1°N,60°W), South Africa (34°S,19°E), and southern Chile (53°S,71°W). They thus cover a variety of northern- and southern latitudinal belts from the midlatitudes to the tropics. Observations of the depolarization ratio were used to categorize the observed cloud layers into either ice-free (no depolarized signals observed) or ice-containing clouds (signals depolarized by scattering at ice crystals).

Strong hemispheric and regional differences were observed in the heterogeneous ice formation efficiency at the different sites, especially in the high-temperature range between -20 and 0 °C. The fraction of ice containing clouds in this temperature range is highest at the northern-latitudinal sites of Germany and southeastern China. Over Leipzig, 50% of all clouds contain ice at -10 °C. In contrast, over southern Chile virtually no ice-containing clouds were observed between -20 and 0 °C. Seasonal differences in the ice-cloud fraction were found over Germany and the Amazon Basin. The observed regional, hemispheric and seasonal contrasts can be explained by differences in the aerosol concentration at cloud level above the different sites. Cloud vertical motion (observed with Doppler lidar), which also determine the microphysical cloud evolution, were found to be similar for all cloud layers.

From combined observations of cloud radar and lidar at Leipzig it was in addition found that ice water contents of below approx. 10^-6kg/m³ cannot be detected with lidar. Clouds classified as pure liquid from the lidar-only observations thus could contain ice water contents of below that threshold.

Considering the hemispheric differences in heterogeneous freezing efficiencies, the formation of precipitation via the ice phase (cold rain) is possibly affected as well. This may especially be the case for the approximately 50% of precipitation formed at T > -30°C, as it was found for the Leipzig dataset.