Improvement of summer precipitation simulation with indirect assimilation of spring soil moisture over the Tibetan Plateau

Soil moisture can be an important preceding signal in seasonal precipitation prediction because of its persistence and influence on the energy and water balance between land and atmosphere. The thawing process of frozen ground on the Tibetan Plateau (TP) during spring makes it difficult to accurately simulate soil variables (e.g., soil moisture and temperature) due to defective parameterizations in numerical models during spring. In this study, we investigate the effectiveness of soil moisture correction using an indirect nudging scheme to simulate the coupling between spring soil moisture and the subsequent precipitation with an advanced research version of the weather research and forecasting (WRF) model. The results without assimilation show that extreme cold and dry land surface states during spring cause a large bias in the spatial pattern of summer precipitation over almost the entire TP. However, the experiments with indirect assimilation in spring show that the spatial pattern of summer precipitation improved significantly. This comparison emphasizes the importance of correcting the spring soil moisture during the freeze-thaw period to significantly adjust the heat and water vapour exchange between the land and atmosphere. Additionally, changes in the circulation of the subtropical-tropical jet, vertical ascent, and region-related moisture recycling can support an active convection environment over the eastern TP, thereby increasing summer precipitation. Furthermore, the pattern shift in water vapour convergence in summer highlights the regional horizontal transportation of water vapour. The differences between experiments with and without deep-soil temperature nudging illustrate the significance of soil temperature in soil moisture simulation in the freeze-thaw process. More so, the deep-soil temperature nudging scheme requires in-depth studies to correct its variation and quantify its degree of impact on soil moisture.

Automated summary

This study investigates the effectiveness of soil moisture correction on seasonal precipitation prediction by using an indirect nudging scheme. The results show that correcting spring soil moisture significantly improves the spatial pattern of summer precipitation on the Tibetan Plateau. The study also highlights the importance of soil temperature and water vapor exchange in soil moisture simulation during the freeze-thaw process, as well as the role of circulation patterns and water vapor transport in summer precipitation.