Abstract
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 (CALIPSO lidar and CloudSat radar). CALIPSO profiles indicate cloud top levels at temperature between −24°C and −21°C. In situ measurements confirm that the cloud-top lidar attenuated backscatter signal along the aircraft trajectory is linked with the presence of liquid water, a common feature observed in Arctic mixed-phase stratocumulus clouds. A low concentration of large ice crystals is also observed up to the cloud top resulting in significant CloudSat radar echoes. Since the ratio of the extinction of liquid water droplets to ice crystals is high, broadband radiative effects near the cloud top are mostly dominated by water droplets. CloudSat observations and in situ measurements reveal high reflectivity factors (up to 15 dBZ) and precipitation rates (1 mm h−1). This feature results from efficient ice growth processes. About 25% of the theoretically available liquid water is converted into ice water with large precipitating ice crystals. Using an estimate of 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 boundary layer height variation 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 modelled.
Highlights
Clouds play a crucial role in the radiative energy budget of the Arctic atmosphere
Hypothesizing an area of 1000 km×1000 km, a precipitation cover of 5% and a mean precipitation rate of 0.05 mm/h, a considerable value of 106 m3 h−1 of fresh water could be settled over the Greenland sea pool
The results may serve to improve model predictions and satellite retrievals and can be summarized as follow: The mixed-phased cloud on 9 April 2007 exhibits a cloud top layer dominated by liquid-water in which ice precipitation was yielded. This confirms the common feature observed in Arctic mixed-phase stratocumulus clouds even for cloud top temperatures down to −25◦C during ASTAR
Summary
Clouds play a crucial role in the radiative energy budget of the Arctic atmosphere. Sensitive feedback mechanisms include interaction of clouds with the usually high surface albedo in the ice covered Arctic regions, with aerosol, radiation, cloud water content, and cloud drop size (Curry et al, 1996). Serious improvements in satellite retrievals are still hampered, mainly due to the lack of evaluation from dedicated field experiments Within this context the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) 2007 project focused on detailed in situ characterisation of microphysical and optical properties of Arctic mixed-phase clouds. This paper focusses on observations obtained from a combination of instruments installed onboard the Polar-2 aircraft operated by the Alfred Wegener Institute for Polar and Marine Research (AWI) These instruments include: a Polar Nephelometer (Gayet et al, 1997), a Cloud Particle Imager (CPI, Lawson et al, 2001) as well as standard Forward Scattering Spectrometer Probe (FSSP-100) to measure cloud particle properties in terms of scattering, morphology and size, and in-cloud partitioning of ice/water content. Observations are compared to European Centre for Medium-Range Weather Forecasts (ECMWF) analyses
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