Convective-scale forecast errors over Taiwan: an ensemble analysis of error covariances and their influences

Abstract This study investigates the characteristics of ensemble-based forecast errors at the convective scale over the land of Taiwan and the adjacent sea. A case study during the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX) intensive observation period 8 (IOP 8) in 2008 is selected. Using a set of 72-member ensemble forecasts generated by the Weather Research and Forecasting (WRF) model, the structures of error variance and error correlation are examined. It is found that the coverage of the cold pool significantly influences the error variances of wind and temperature near the surface. Additionally, a comparison of the horizontal wind error variance between 9 and 3-km ensembles reveals that higher-resolution (3-km) ensemble forecasts not only better capture the uncertainties of severe weather but also display the features of multi-scale interactions within the southwesterly flow. The error variances of state variables are closely associated with the development of the weather system, including factors such as divergence and latent heating. The spatial and temporal error covariances of dynamic and thermodynamic variables under different conditions imply a potential disparity in the impact on high-resolution data assimilation. This information could provide valuable guidance for further studies on the ensemble data assimilation system over the island of Taiwan.