Surface Coupling Effects on Heterogeneous Ice Formation in Arctic Mixed-Phase Clouds

Abstract:

In the Arctic summer 2017 the Polarstern cruise PS106 was performed around Svalbard, combining several remote sensing and in-situ observations, to study the change of the Arctic environment. During PS106 measurements with the multiwavelength Raman polarization lidar (light detection and ranging) PollyXT-Oceanet, a 35-GHz cloud radar (radio detection and ranging) and a microwave radiometer HATPRO of the OCEANET platform were conducted.

With this comprehensive data set we have investigated the surface-coupling dependent heterogeneous ice formation during PS106. The cloud phase was determined using the polarization capabilities of the lidar and the cloud radar. Based on the thermodynamic profiles from the 6-hourly radiosoundings we derived the coupling state of the observed cloud layers in intervals of 30 minutes. A quasi-constant potential temperature profile was used to identify coupled clouds, while an inversion indicated decoupling. The evaluation of the dataset revealed an influence of the surface-coupling on Arctic clouds: We found a striking difference in the fraction of ice containing clouds with a cloud top temperature above -10°C. Within this temperature regime, ice was found in up to 60% of the surface coupled clouds, whereas only 30% of the decoupled clouds showed signals of ice hydrometeors. Below -10°C this difference decreases and both data roughly follow the same pattern. This suggests that coupled mixed-phase clouds in the Arctic appear to produce ice heterogeneously at higher temperatures than decoupled clouds and hence an influence of surface-near aerosol in the process of heterogeneous ice formation for Arctic boundary layer clouds at low supercooling temperatures. The origin of the required INPs for the ice formation under these conditions is currently under discussion. Studies of filter based INP analysis suggest that these highly active INPs originate from biological sources. At lower temperatures, the background INP reservoir seems to be sufficient to trigger heterogeneous ice formation that is strong enough to be detected by the lidar.