A long‐term regional simulation and observations of the hydroclimate in China

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This paper provides detailed analyses of observations and a 20‐year regional climate simulation to improve our understanding of the diverse hydroclimate regimes in China and examine model skill in simulating the seasonal, intraseasonal, and interannual hydroclimatic variations. Eastern China is greatly influenced by the East Asian monsoon, leading to heavy summer rainfall that exhibits large variations at the intraseasonal and interannual timescales. The Hai River and Huai River basins feature the largest interannual variations in summer rainfall followed by the middle and lower Yangtze River and Pearl River basins. In the wettest regions of the middle and lower Yangtze River basin (YRB) and Southeast Coast, the land surface plays a relatively passive role as evaporation is not limited by soil moisture. In the semiarid northern and western China, the land surface plays a more active role in the water cycle. This is supported by the strong correlations between soil moisture and sensible heat flux, evaporative fraction, and mixed‐layer depth, indicating a strong influence of soil wetness on atmospheric stability and precipitation processes. The overall spatial distribution, seasonal variations, and extreme precipitation are well simulated by the regional climate model (RCM). The model also captured the large‐scale spatial variations of annual mean soil moisture and runoff. However, the simulation produces more rainfall associated with the lower rain rates in central and southern China. Errors in the rain‐rate distributions can affect the simulation of surface hydrological processes. In addition, compared with the global reanalysis that provides the large‐scale boundary conditions, the simulation yielded lower anomaly correlations of interannual summer rainfall with observations. This suggests that simulating the interannual variability of rainfall at the river‐basin scale during the East Asian summer monsoon remains a significant challenge and deserves further studies to understand the causes for the model errors.