Improvement of summer precipitation simulation in China by assimilating spring soil moisture over the Tibetan Plateau

The snow melting and frozen soil thawing processes in spring could lead to the significant anomaly in soil moisture (SM) over the Tibetan Plateau (TP), which has a strong influence on the interaction between land and atmosphere, and influencing the subsequent summer precipitation in China. However, there is distinct bias in SM in model simulation, influencing the surface energy and mass balance. In this study, the impacts of assimilating spring SM over the TP with indirect soil nudging (ISN) scheme on the simulation of summer precipitation in China are investigated. The results show that using the ISN method can reduce the spring SM bias over the TP, which further decreases the root mean square error (RMSE) of summer precipitation simulation, especially in South China, North China, northeast China, and the middle and lower Yangtze River. Compared to the experiment without assimilation, the assimilation of spring SM evidently modulates the distribution and intensity of surface sensible and latent heating over the TP, which leads to the change of Rossby wave train's (RWT) shape and propagation direction over the TP and the downstream region, affecting the general circulation over East Asia. Specifically, the upward movement over the TP gets stronger and leads to the position of 200 hPa westerly jet which shifts to west and is closer to ERA5 reanalysis; meanwhile the location of the 500 hPa western Pacific subtropical high (WPSH) and the distribution of water vapor in Eastern China are also corrected. As a result, the precipitation simulation biases are alleviated. This study suggests that decreasing SM bias caused by the snow melting and frozen soil thawing can be an effective approach to improve the seasonal prediction skills of precipitation in China.

Automated summary

This study found that assimilating spring soil moisture over the Tibetan Plateau can improve the simulation of summer precipitation in China, especially in South China, North China, northeast China, and the middle and lower Yangtze River.