Evaluation of High-resolution Downscaling Predictions for the July 2023 Extreme Rainstorm in the Beijing-Tianjin-Hebei Region Based on CMA-CPSv3

Integration of weather and climate forecasting is currently the frontier of numerical modeling development in China, and dynamic downscaling allows for improving the performance and resolution of global climate models to the weather scale. Focusing on the “23.7” extreme rainstorm (July 29, 00:00 - August 2, 00:00 UTC) in the Beijing-Tianjin-Hebei region (BTH), this study assesses predictions from the China Meteorological Administration Climate Prediction System version 3 (CMA-CPSv3, 45 km resolution) and 9-km dynamic downscaling hindcasts from the Weather Research and Forecasting model (WRF-9km). Unlike traditional climate anomalies approaches, direct outputs are used for evaluation, similar to weather forecasting tests. By examining, both the CMA-CPSv3 forecasts and the WRF-9km hindcasts offer a 5-day prediction window for this rainstorm. They successfully predict the rainstorms and related atmospheric circulations from July 24th onward, aligning with observed and reanalyzed data. WRF-9km, with the higher resolution and optimized physical processes, outperforms CMA-CPSv3, particularly in precipitation spatial distribution and center intensity. The WRF-9km 7/24 hindcast exhibits the most significant enhancement compared to the corresponding CMA-CPSv3 forecast. This improvement is notably reflected in the substantial increase in spatial correlation, rising from 0.68 to 0.79, as well as a reduction in the difference of center values, decreasing from -51% to -20%. Furthermore, the WRF-9km 7/24 hindcast also improves the Critical Success Index by 0.08, the Success Rate by 0.08, and the Probability of Detection by 0.29 for heavy rainfall (over 25.0 mm/d). However, improvements in large-scale circulations with WRF-9km are limited, which may restrict advancements in predictability. In conclusion, the WRF-9km can enhance the performance and resolution of CMA-CPSv3 predictions, which can serve as one route for CMA-CPSv3 to achieve weather-climate integration.