Evaluation of Real‐Time Convection‐Permitting Precipitation Forecasts in China During the 2013–2014 Summer Season

Abstract

Forecasts at a 4 km convection‐permitting resolution over China during the summer season have been produced with the Weather Research and Forecasting model at Nanjing University since 2013. Precipitation forecasts from 2013 to 2014 are evaluated with dense rain gauge observations and compared with operational global model forecasts. Overall, the 4 km forecasts show very good agreement with observations over most parts of China, outperforming global forecasts in terms of spatial distribution, intensity, and diurnal variation. Quantitative evaluations with the Gilbert skill score further confirm the better performance of the 4 km forecasts over global forecasts for heavy precipitation, especially for the thresholds of 100 and 150 mm d−1. Besides bulk characteristics, the representations of some unique features of summer precipitation in China under the influence of the East Asian summer monsoon are further evaluated. These include the northward progression and southward retreat of the main rainband through the summer season, the diurnal variations of precipitation, and the meridional and zonal propagation of precipitation episodes associated with background synoptic flow and the embedded mesoscale convective systems. The 4 km forecast is able to faithfully reproduce most of the features while overprediction of afternoon convection near the southern China coast is found to be a main deficiency that requires further investigations.

The Controlling Role of Boundary Layer Inertial Oscillations in Meiyu Frontal Precipitation and Its Diurnal Cycles Over China

“…As the southerly monsoon flows weaken in the afternoon, cold pool and gust front forming from morning precipitation in the Meiyu rainband region can push ahead toward the south, producing propagating squall lines that eventually detach itself from the main confluence zone and decay. Such behaviors are also suggested by Hovmőller diagrams of precipitation in observations and forecasts of 2013 and 2014 summer months (Zhu et al, ).…”

“…Such precipitation is clearly due to afternoon surface heating, which tends to trigger convection more easily at the lower latitudes in southern China. Zhu et al () pointed out that the overprediction of afternoon convective precipitation was the most noticeable forecast deficiency in the WRF 4‐km real‐time forecasts used here, although the afternoon peak is definitely found in the observations also.…”

“…As discussed in Zhu et al (2018), the summer (June-August) season precipitation of 2014 was biased toward southern part of China, compared to more typical years. Because of the focus of this study is on the mean diurnal behaviors of Meiyu frontal rainband precipitation over a number of similar cases, we want to restrict our selection of cases whose rainband was located at more or less similar locations, to avoid smearing of features by the composite averaging process.…”

“…The forecasts starting from 00 and 12 UTC have roughly comparable prediction skills, and in this study only the 12 UTC forecasts are used. As shown in Zhu et al (2018), in the first 6 hr of forecast, the precipitation forecast skill is poor because of the model spin-up but after 6 hr, the predicted mean hourly precipitation shows a good match of the diurnal cycle to observations, with a slight overprediction in the amplitude, mostly in the afternoon. To avoid the initial spin-up period, 12-to 36-hr forecasts are used to produce composites of 24-hr forecasts and to investigate the diurnal cycle issues.…”

What Is the Main Cause of Diurnal Variation and Nocturnal Peak of Summer Precipitation in Sichuan Basin, China? The Key Role of Boundary Layer Low‐Level Jet Inertial Oscillations

“…The model was run at a 4‐km grid spacing during June through August since 2013 at Nanjing University, China, and the forecasts are produced twice daily, starting from NCEP Global Forecasting System 00 and 12 UTC analyses, and forced at the lateral boundaries by Global Forecasting System real‐time forecasts at 3‐hourly intervals. Zhu et al () show that the forecasts capture well the precipitation diurnal cycles in 2013 and 2014 when verified over entire domain, as well as the spatial distributions and season variations of precipitation, better than operational global forecasts from several numerical weather prediction centers. The heavy precipitation center near SB is also captured well.…”