| Soil moisture is an important variable in the climate system, which links water and energy cycles over land. It plays a significant role in the climate change by changing surface albedo, heat capacity, land sensible heat flux and latent heat flux and its importance in climate system is just next to sea surface temperature. High quality soil moisture data will help further research and understand the importance of soil moisture in the climate system and weather forecasting. However, soil moisture is not a conventional meteorological factor and the absence of observation data is a problem. Based on the latest soil moisture product developed by European Space Agency from microwave observations of multiple satellites, this paper firstly analyzes the seasonal variation of soil moisture and compares it with the station-based observations from National Meteorological Administration. On this basis, we applies the ERA-interim reanalysis data from European Centre, using empirical orthogonal function (EOF), singular value decomposition (SVD) and other methods to study temporal and spatial variation of soil moisture in China and the possible relationship between soil moisture and precipitation. The main conclusions are as follows:(1) Based on the latest soil moisture product from microwave observations of multiple satellites, this paper analyzes the seasonal variation of soil moisture and compares it with the station-based observations. Results show that the seasonal variation of soil moisture using the passive satellite data is significantly different from the active satellite data in the summer monsoon region in East China, where the passive data appears a dry summer and a wet winter, which is consistent with in-situ observations. However, the spatial difference of seasonal variation exists for the active data. The seasonal variation of soil moisture from active data is similar with that in the observations in the northern China, but shows opposite phase, i.e., wet summer and dry winter, in the southern China. Their inconsistency of seasonal variations reveals that it is not proper to produce a multi-decadal soil moisture data in the summer monsoon region in East China by merging the passive and the active microwave observations.(2) The climatic distribution of soil moisture in China presents the characteristics that the soil of Xinjiang, Inner Mongolia is drier than in the northeast and southeast China; in the aspect of the seasonal changes, soil in China shows the characteristics of dry summer and wet winter; in the aspect of the trend analysis, it almost rendered globally consistent drought situation in warmer seasons and wetter in colder seasons of East China in the past nearly 26 years(1988-2013), the results are consistent with the conclusions by EOF analysis.(3)We found the links between soil moisture in late spring and rainfall in summer in East China by using the SVD method. When soil moisture in late spring in the Yangtze River to north China is relatively dry, the corresponding surface evapotranspiration decrease, which increases the surface temperature, thereby increases the thermal difference between sea and land and the East Asian summer monsoon enhances. The stronger East Asian summer monsoon is also beneficial to make the position of the western Pacific subtropical high eastward and northward, which brings the rain belt northward. The north and south China have more rain while the Yangtze River Basin has less rain due to the control of the western Pacific subtropical high. Although these results are obtained by diagnostic analysis, it is significate to explore the relationship between soil moisture and seasonal prediction. In the future, we can take advantage of numerical simulation for further study. |
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