Formation and maintenance mechanisms of the Arctic stratus cloud studied by observations and models, and its impact to the Arctic climate system

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In 2002 and 2004, we had conducted specially designed observations of the Arctic stratus cloud (ASC) and precipitation cloud systems in the Arctic Ocean. Major findings are as follows : Comparing the theoretical and observed intensity of solar radiation, we found that about 30 % of solar radiation was absorbed or scattered by aerosols and gases. When the depth of ASC was larger than 500 m, 60 % of solar radiation was absorbed by ASC. Number density of aerosols above the low level cloud was much larger than that below it. Usually sensible and latent heat fluxes near the sea surface were very small. However, both upward turbulent heat fluxes and downward CO2 flux increased very large when the cold air outbreak occurred. From the C-band Doppler radar data, which is the most unique data sets, we studied mesoscale features of precipitation cloud systems and vertical distribution of precipitation over the Arctic Ocean. We classified synoptic disturbances into two types, that is, (1) Polar low and (2) North pacific. The height of former type was less than 5 km, and that of latter type was higher than 8 km. Most of precipitation was found to be brought by the former system. The Arctic stratus cloud also showed two types, that is, (1) Cellular convection and (2) Stratiform types. Their horizontal structures are consistent with simulations done by a high-resolution numerical model. From the huge data sets of high sensitive cloud-radars, we succeeded in making statistics of multi-layer stratification of the Arctic stratus cloud. We also succeeded in retrieving size distribution of cloud droplets of the Arctic Stratus cloud by combining Lidar-Radar data. Both cloud layering and size of cloud droplets are indispensable for study the climate impact of the Arctic stratus cloud. In addition to this, we discussed seasonal variation of moisture transport in polar regions and the relation with annular modes by using ECMWF data.