Asymmetries in winter cloud microphysical properties ascribed to sea ice leads in the central Arctic

Abstract. To investigate the influence of sea ice openings like leads on wintertime Arctic clouds, the air mass transport is exploited as humidity feeding mechanism which modifies cloud properties like total water content, cloud phase partitioning, cloud altitude, and thickness. Cloud microphysical properties in the Central Arctic are analyzed as a function of sea ice conditions during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019–2020. A state-of-the-art cloud classification algorithm is used to characterize the clouds based on observations by vertical pointing lidar, radar, microwave radiometer, and atmospheric thermodynamic state from the observatory on board the research vessel Polarstern. To link the sea ice conditions around the observational site with the cloud observations, the water vapor transport (WVT) being conveyed towards the Polarstern has been exploited as a mechanism to associate sea ice conditions upwind with the measured cloud properties. This novel methodology is used to classify the observed clouds as coupled or decoupled to the WVT based on the location of the maximum vertical gradient of WVT height relative to the cloud-driven mixing layer extending above and below the cloud top and base, respectively. Only a conical sub-sector of sea ice concentration (SIC) and lead fraction (LF) centered at the Polarstern location and extending up to 50 km radius and azimuth angle governed by the time-dependent wind direction measured at the maximum WVT is related to the observed clouds. We found significant asymmetries for cases when the clouds are coupled or decoupled to the WVT, and when cases are selected by LF regimes. Liquid water path of low level clouds is found to increase as a function of LF while ice water path does so only for deep precipitating systems. Clouds coupled to WVT are found to be low level clouds and are thicker than decoupled clouds. Thermodynamically, we found that for coupled cases the cloud top temperature is warmer and accompanied by a temperature inversion at cloud top, whereas the decoupled cases are found to closely be compliant with the moist adiabatic temperature lapse rate. The ice water fraction within the cloud layer has been found to present a noticeable asymmetry when comparing coupled versus decoupled cases. This novel approach of coupling sea ice to cloud properties via the WVT mechanism unfolds a new tool to study Arctic surface-atmosphere processes. With this formulation long-term observations can be analyzed to enforce the statistical significance of the asymmetries. Our results serve as an opportunity to better understand the dynamic linkage between clouds and sea ice and to evaluate its representation in numerical climate models for the Arctic system.