Attribution of the December 2013 extreme rainfall over the Pearl River Delta to anthropogenic influences

During 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 the 4-day mean rainfall could increase by 11% for 1.2 K near-surface warming on average under human-induced thermodynamic forcing in relation to humidity changes, whereas it increases by 17% for 2 K warming under all forcing (i.e., including dynamic forcing associated with wind circulation changes), which is nearly the Clausius–Clapeyron rate. Moreover, the former and latter forcing can intensify the 95th percentile daily rainfall by ~ 13% and ~ 19%, respectively, suggesting that human-caused dynamic forcing can further exacerbate the thermodynamic-driven rainfall enhancement in this event. Indeed, there is stronger land-sea thermal contrast with anomalous low-level southerly winds and convergence in coastal South China under all forcing. The frontal system and ascending motion are therefore intensified, resulting in even stronger rain rates than under thermodynamic forcing. Moisture budget analysis reveals that the dynamic component accounts for most of the increase in 4-day mean rainfall while the thermodynamic contribution is negligible under all forcing. Our findings highlight the salient role of dynamic effects on intensifying PRD’s extreme rainfall in wintertime.