Attribution of Wintertime Extreme Rainfall in South China Pearl River Delta Region to Anthropogenic Influences.

Abstract:

Precipitation extremes are becoming more and more severe in many locations under the current warming climate. In this study, we focus on a record-breaking rainstorm caused by a strong upper-level cold air intrusion to south China during 14-17 December, 2013, since it contributed to the Pearl River Delta (PRD) region’s largest wintertime cumulative precipitation of above 100 mm for the 1998-2018 but so far received little attention on its attribution analysis. Multi-physics ensemble integration using the Weather Research and Forecasting (WRF) model comprising totally six combinations of cumulus and microphysics schemes, at 2 km x 2 km horizontal resolution covering the entire PRD area, was carried out to reproduce this extreme rainfall event. The attribution analysis was performed by forcing the model with different initial and lateral boundary conditions: (1) the original ERA-Interim reanalysis (denoted as CTR), and counterfactual conditions with (2) human influence on both temperature and humidity removed (DTQ), and (3) human influence on wind circulation removed on the top of DTQ (DTQW) from ERA-Interim. These human influences are derived from the difference between historical and natural runs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble mean taken from seven models, for the 1976-2005. As inferred from the model results, human-induced low-level tropospheric warming has reached 1.1 degree Celsius in December over the PRD region. By comparing the multi-physics ensembles of CTR and DTQ runs, human-induced climatic warming contributes to both 95^th and 99^th percentile of daily precipitation in PRD by ~14%, which is nearly double of the increment expected from CC scaling of ~7%/K. This super-CC sensitivity of extreme precipitation indicates the role of human-related dynamic contributions associated with stronger vertical motion. Furthermore, larger reduction of daily precipitation is found in DTQW than in DTQ, which is probably related to the increasing ascent, stronger horizontal temperature gradient and vertical wind shear, and also enhanced inland moisture transport due to anthropogenic influences. Overall, the attribution results suggest that human impacts on both thermodynamic and dynamic changes have significant positive contributions to this extreme precipitation event.