Microphysical and optical properties of Arctic mixed-phase clouds – the 9 April 2007 case study
1Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont-Ferrand, France
2Institute for Atmospheric Physics, DLR Oberpfaffenhofen, Wessling, Germany
3Johannes Gutenberg University, Institute for Atmospheric Physics, Mainz, Germany
4Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany
*now at: University of Leipzig, Leipzig Institute for Meteorology (LIM), Leipzig, Germany
Abstract. Airborne measurements in Arctic boundary-layer stratocumulus were carried out near Spitsbergen on 9 April 2007 during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign. A unique set of co-located observations is used to describe the cloud properties, including detailed in situ cloud microphysical and radiation measurements along with airborne and co-located spaceborne remote sensing data (Lidar on Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations [CALIPSO] and radar on CloudSat satellites). The CALIPSO profiles evidence a cloud top temperature which varies between −24°C and −21°C. The in situ cloud observations reveal that the attenuated backscatter signal from lidar along the aircraft trajectory is linked with the presence of liquid water and therefore confirms a cloud top layer dominated by liquid-water, which is a common feature observed in Arctic mixed-phase stratocumulus clouds. A low concentration of quite large ice crystals are also evidenced up to the cloud top and lead to significant CloudSat radar echo. Since the ratio of the extinction of liquid water droplets and ice crystals is high the broadband radiative effects near the cloud top are mostly dominated by water droplets. CloudSat observations as well as in situ measurements reveal high reflectivity factors (up to 15 dBZ) and precipitation rates (1 mm h−1). This feature is due to efficient ice production processes. About 25% of the theoretically available liquid water is converted into ice water with large ice crystals which precipitate. According to an estimation of the mean cloud cover, a considerable value of 106 m3 h−1 of fresh water could be settled over the Greenland sea pool. European Centre for Medium-Range Weather Forecast (ECMWF) operational analyses reproduces the variation of the boundary layer height along the flight track. However, small-scale features in the observed cloud field cannot be resolved by ECMWF analysis. Furthermore, ECMWF's diagnostic partitioning of the condensed water into ice and liquid reveals serious shortcomings for Arctic mixed-phased clouds. Too much ice is modeled.