Numerical Simulation of Hail Cases over the Pearl River Estuary in 2026 Using UWIN-CM

An atmosphere–ocean–wave-coupled model, the UWIN-CM, running in real-time in the Hong Kong Observatory, has adopted radar data assimilation (DA) and been incorporated with a HAILCAST module in WRF for hail size simulation. This paper analyzes the distribution of maximum hail sizes forecast by the UWIN-CM in comparison with the Maximum Estimated Size of Hail (MESH) from radar for six hail cases reported by human or derived from radar in the Pearl River Estuary of Guangdong Province in 2026. The performance of hail forecast with and without radar DA is compared. The model with DA performed simulates hail at the locations that were consistent with the reported data and/or MESHs in five out of six cases. The timings of simulated hail aligned with reports/MESHs in three cases, and these were delayed by approximately 1 to 3 h in the remaining two. For the average hail diameters, those from simulations were smaller than MESH by around 30 to 50% in two of the five successful cases, while the other three cases had larger diameters than MESH by about 90 to 140%. Without radar DA, hails were simulated in only two out of six cases. The rate of detection was enhanced with radar DA, potentially benefiting from the improved moisture distribution and wind field of the initial conditions. The possible factors behind the differences in hail size between the simulations and MESH are discussed, including the choice of microphysics scheme and the differences in climatology between the PRE, and the contiguous US where the HAILCAST parameters were calibrated. For practical use in operation, parameter tuning of HAILCAST based on the hail climatology of the PRE would be needed. This work paves the way for providing forecasters with early alerts about the occurrence of hail in the region.