Abstract
Reasonably modeling the magnitude, south–north gradient and seasonal propagation of precipitation associated with the East Asian summer monsoon (EASM) is a challenging task in the climate community. In this study we calibrate five key parameters in the Kain–Fritsch convection scheme in the WRF model using an efficient importance-sampling algorithm to improve the EASM simulation. We also examine the impacts of the improved EASM precipitation on other physical process. Our results suggest similar model sensitivity and values of optimized parameters across years with different EASM intensities. By applying the optimal parameters, the simulated precipitation and surface energy features are generally improved. The parameters related to downdraft, entrainment coefficients and CAPE consumption time (CCT) can most sensitively affect the precipitation and atmospheric features. Larger downdraft coefficient or CCT decrease the heavy rainfall frequency, while larger entrainment coefficient delays the convection development but build up more potential for heavy rainfall events, causing a possible northward shift of rainfall distribution. The CCT is the most sensitive parameter over wet region and the downdraft parameter plays more important roles over drier northern region. Long-term simulations confirm that by using the optimized parameters the precipitation distributions are better simulated in both weak and strong EASM years. Due to more reasonable simulated precipitation condensational heating, the monsoon circulations are also improved. By using the optimized parameters the biases in the retreating (beginning) of Mei-yu (northern China rainfall) simulated by the standard WRF model are evidently reduced and the seasonal and sub-seasonal variations of the monsoon precipitation are remarkably improved.
Highlights
During warm seasons, abundant rainfall over most areas of China is brought by the East Asian summer monsoon (EASM), which is a hybrid of tropical and subtropical monsoon with variations at different time-scales (Tao and Chen 1987; Ye and Huang 1996; Ding 1992; Ding and Chan 2005)
The parameters related to downdraft, entrainment coefficients and CAPE consumption time (CCT) can most sensitively affect the precipitation and atmospheric features
When using the standard Weather Research and Forecasting (WRF) with the default KF parameters, the precipitation are all overestimated during the selected years, which is consistent with our previous studies over the South Great Plain (SGP) region (Yang et al 2012; Yan et al 2014)
Summary
Abundant rainfall over most areas of China is brought by the East Asian summer monsoon (EASM), which is a hybrid of tropical and subtropical monsoon with variations at different time-scales (Tao and Chen 1987; Ye and Huang 1996; Ding 1992; Ding and Chan 2005). Yang et al (2012) applied an importance-sampling algorithm, i.e. Multiple Very Fast Simulated Annealing (MVFSA, Ingber 1989; Jackson et al 2004) to quantify the uncertainties of five key parameters in the Kain–Fritsch (KF) convection scheme in the WRF RCM over the US South Great Plain (SGP) region Their results showed that the model performance was most sensitive to a few parameters in the KF scheme. To address the above questions, in this study we apply the MVFSA approach to investigate the impacts of several key parameters in the KF convection scheme in the WRF RCM (Skamarock et al 2008; Yang et al 2012) on the simulated EASM precipitation, especially the Mei-yu precipitation over the YRB region, in three years with strong, normal and weak EASM intensities, respectively.
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