Identifying a new normal in extreme precipitation at a city scale under warmer climate regimes: A case study of the Tokyo metropolitan area, Japan

Identifying the change of city-scale extreme precipitation (EP) under the "new normal" of global climate is critical for various urban risk assessments. Here, we investigate the change of hourly EP caused by warmer climate regimes in a large urban agglomeration in midlatitude, Tokyo, Japan. We use pseudo-global warming dynamical downscaling approach with a convection-permitting regional climate model considering two Representative Concentration Pathways scenarios 8.5 and 4.5 to simulate the August climate for the present (2005-2014) and the future time (up to 2100). The results show a substantial enhancement of local hourly precipitation regarding both frequency and intensity. Framing the change with Clausius-Clapeyron (CC) scaling relation, which is of approximately 7% per K warming, we confirmed the paradigm of "extreme gets more extreme" in the future local hourly precipitation. The 0.99 quantile precipitation is intensified at the super CC scaling rate (3 CC); whereas the intensification of lower-quantile precipitation is at the sub-CC rate. The EP intensification is much stronger than that reported for a tropical city, highlighting geographical diversity in precipitation response to the global warming effect. The EP intensification is attributed to the global-warming caused convective inhibition (CIN) enhancement. Enhanced CIN temporarily delays weak convections to initiate, allowing it to build up further, and when the convection does trigger, it becomes intense.