The Impact of Initial Conditions on Soil Moisture Predictability in Early Summer in Eastern China

Soil moisture has an important impact on weather and climate. Some previous soil moisture initialization experiments indicate that appropriate initial soil moisture can improve the forecasting skill of the model. The development of land surface initialization method is the urgent demand for improving the forecasting ability of the land model. However, studies about the predictability of the soil moisture in model are still relatively insufficient. To investigate the impact of soil moisture initial value on the prediction of soil moisture in BCC_CSM2_MR model, two experiments are carried out using a respectively true initial value of soil moisture (OH experiment) and an ideal initial value of soil moisture climatology (CH experiment). The lack of soil moisture observations and coordination between the observation and model is a difficult problem in the development of land initialization method. The GSWP (Global Soil Wetness Project) introduces a new method to get the appropriate land initial data. The method indicates land initial data can be obtained using the accurate observed field (including atmospheric field, precipitation data, and radiation field) to force the land model. Therefore, an OF experiment is designed, which forces BCC_CSM model from 1994 to 2013 by using the NCEP atmospheric reanalysis data and China National Meteorological Information Center (NMIC) precipitation data to produce a good model initial value. The soil moisture from OF experiment can reflect the observed distribution and variation of soil moisture over eastern China as the corresponding correlation is high. Thus, it will be used as observations. Based upon soil moisture data from OF experiment, OH and CH experiments are respectively initialized on 1 May during 1994-2013, and the lead time is six months. Comparison of two hindcast experiments shows that the predictability of soil moisture in BCC_CSM model is about 3 pentads in surface layer and even exceeds 1 month in deep layer in some regions of China. At the same time, soil moisture initialization has an impact on forecast skill of soil moisture. This impact can reach 2-3 pentads in surface layer and longer in deeper layer. Variations of soil moisture in shallow layers strongly depend on variations of rainfall, and there is a 1-2 days lag between variations of soil moisture and rainfall, but about 5 days for that in the middle layers.