Evaluating precipitation prediction skill for the Huanan pre- and post-rainy seasons in ECMWF subseasonal forecasts

Abstract The rainy seasons in South China are divided into two phases according to the seasonal progression of the East Asian summer monsoon: the Huanan pre- and post-rainy seasons. The precipitation prediction skill for the two rainy seasons are investigated using subseasonal-to-seasonal (S2S) hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) for 2001–2019. The precipitation prediction skill and biases differ in the two rainy seasons, although some similar characteristics exist in regard to circulations and their influence on precipitation. During the two rainy seasons, the prediction skill of the circulation at 850 hPa in key areas is relatively high, and the circulation’s influence on precipitation is well captured; additionally, the relationship between the circulation in key areas at 500 hPa and precipitation is less accurately constrained. Moreover, the precipitation prediction skill in the prerainy season is higher than that in the post-rainy season. The main bias is that the 200 hPa westerly provides favorable divergence conditions for prerainy season precipitation (pre-precipitation), while the post-rainy season precipitation (post-precipitation) displays almost no correlation with the circulation in the reanalysis product; however, the simulated circulation at 200 hPa is both tightly connected to the precipitation in the two rainy seasons, so the lower prediction skill in the post-rainy season is likely associated with overestimation of the complex physical mechanism of the upper-level circulation in the model.