Attribution of Wintertime Extreme Rainfall over the Pearl River Delta to Anthropogenic Influences

AbstractDuring 14 to 17 December 2013, the Pearl River Delta (PRD) in South China received its largest wintertime 4-day precipitation of above 100 mm since 1998, due to strong cold air intrusion. Here we investigate the extent to which such extreme rainfall can be attributed to human activities, by carrying out Weather Research and Forecasting (WRF) model multi-physics integrations at a convection-permitting resolution. The factual WRF runs were conducted using the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA)-Interim as boundary and initial conditions, and the counterfactual runs by the same ERA-Interim forcing with human influences removed. The latter was deduced by subtracting the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical-natural from the historical run outputs. Results show that human-induced thermodynamic (TH) forcing alone accounts for ~9% increase in the 4-day mean rainfall for 1.2 K near-surface warming, whereas combined dynamic (DY) and TH (“DY+TH”) forcing contributes to ~14% increase for 2 K warming (nearly CC rates). However, TH and DY+TH forcing can intensify the 95th percentile daily rainfall by ~13% and ~19%, respectively. This suggests that human-caused DY forcing can further exacerbate the TH-driven rainfall enhancement. Indeed, under the combined TH and DY effects, there is stronger land-sea thermal contrast with anomalous low-level southerly wind and convergence in coastal South China. The frontal system and ascending motion are therefore intensified, resulting in even stronger rain rates than TH-only forcing. Moisture budget analysis reveals that the DY component accounts for most of the 95th percentile rainfall changes while the TH contribution is negligible. Our findings highlight the salient role of dynamic effects on intensifying PRD’s extreme rainfall in wintertime.