Multiscale Interactions of Initial Condition Perturbations for Convection-Permitting Ensemble Forecasting over South China during the Rainy Season

Abstract The multiscale interactions of initial condition (IC) perturbations for 12-h convection-permitting ensemble forecasting were explored in this study. The heavy rainfall events occurring in South China in the rainy season during 2013–20 were focused on and classified into three types: weak-forcing, strong-forcing, and tropical cyclone (TC) cases. The impacts of both large- and small-scale IC perturbations on the multiscale characteristics of forecast perturbations and the forecast performance were investigated. Both the upscale growth of small-scale IC perturbations and the downscale propagation of large-scale IC perturbations favored perturbation growth, with damping impacts of both processes beyond the first few hours. Compared with upscale growth, downscale propagation showed greater impacts on forecast perturbations in both magnitude and location and more evident variable-dependent impacts. The small-scale IC perturbations spread the high-probability rainfall downstream, while the large-scale IC perturbations spread the high-probability rainfall in all directions to the low-probability areas. Both of these two types of rainfall spreading led to increasing locational perturbations. Probabilistic forecasts of heavy short-duration rainfall benefited from both the small- and large-scale IC perturbations, leading to performance improvements of multiscale over single-scale IC perturbations. In particular, the small- and large-scale IC perturbations more benefited the forecasts of weak-forcing heavy rainfall with larger spatial forecast errors and at earlier lead times, respectively. The flow regimes with large forecast uncertainties, which were determined by the complicated interactions between synoptic-scale forcing and moist convection, directly boosted both upscale growth and downscale propagation. Significance Statement Currently, forecasts of heavy rainfall over South China in the rainy season are still characterized by large uncertainties, which are expected to be addressed by the convection-permitting ensemble forecasting. The studies related to the interactions of initial condition (IC) perturbations at different scales can provide guidance for designs of convection-permitting ensembles but remain limited. This study investigates the multiscale interactions of IC perturbations for convection-permitting ensemble forecasting over South China in the rainy season. There are greater impacts of the downscale propagation of large-scale IC perturbations than the upscale growth of small-scale IC perturbations on forecast-perturbation evolution. Probabilistic forecasts of heavy rainfall benefit from both the large- and small-scale IC perturbations, leading to performance improvements of multiscale over single-scale IC perturbations. Both upscale growth and downscale propagation are more evident under flow regimes with larger forecast uncertainties.