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

2002年和2004年，我们对北极层云和北冰洋降水云系统进行了专门的观测。主要发现如下：比较太阳辐射的理论强度和观测强度，我们发现约30%的太阳辐射被气溶胶和气体吸收或散射。当ASC深度大于500 m时，60%的太阳辐射被ASC吸收。低空云层上方气溶胶的数量密度远大于低空云层下方。通常海面附近的感热通量和潜热通量都很小。而冷空气爆发时，向上的湍流热通量和向下的CO2通量都有很大的增加。利用最独特的c波段多普勒雷达资料，研究了北冰洋降水云系统的中尺度特征和降水的垂直分布。我们将天气扰动分为两种类型，即(1)极低压和(2)北太平洋。前者高度小于5 km，后者高度大于8 km。大部分降水是由前一系统带来的。北极层云也表现出两种类型，即(1)细胞对流型和(2)层状云型。它们的水平结构与高分辨率数值模型的模拟结果一致。利用高灵敏度云雷达的海量数据集，成功地对北极层云的多层分层进行了统计。结合激光雷达数据，成功地反演了北极层云的云滴大小分布。云层的分层和云滴的大小对于研究北极层云的气候影响是必不可少的。此外，利用ECMWF资料讨论了极地水汽输送的季节变化及其与环模的关系。

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.