Regional Climate Modeling of the Changes in Environmental Stability and Heavy Rainfall under Global Warming

Heavy rainfall produced by deep convection depends on environmental stability conditions such as temperature lapse rate and moisture content that would control the development and organization of deep convective clouds. From the previous studies and the past IPCC assessment reports, it has been understood that the occurrence of extreme rainfall events will increase in projected future climates under global warming. Regional climate models have been providing datasets at higher resolutions that can somewhat resolve convective-scales. Therefore, quantitative estimations on heavy rainfall have become plausible at least in a regional-scale area. At the same time, downscaling experiments with regional climate models enable to diagnose the effects of climate change on the development and evolution of specific extreme events through a pseudo-global warming assumption in which incremental amounts from the historical to a future climate are added to the analysis field of a past event. In this study, we investigated the effects of global warming on the changes in the relationship between environmental stability and heavy rainfall by conducting pseudo-global warming experiments for the heavy rainfall period in August 2014 in Japan. The Weather Research and Forecasting (WRF) model/Advanced Research WRF (ARW) with three nested computational domains (downscaled at 9, 3, and 1 km as horizontal grid spacings) was used. A long-term reanalysis dataset, JRA-55, was used as the initial and boundary conditions for reproducing the August 2014 case as the control run. Four types of the incremental data from MRI-AGCM3.2S with four SST patterns imposed under the RCP8.5 scenario were added to the analysis field from JRA-55 in order to conduct pseudo-global warming experiments. Furthermore, a “pseudo-global cooling” experiment was conducted to examine how the climate change affects the changes in the relationship between environmental stability and convective rainfall in an assumed cooled climate. The analyses were made mainly for the outputs from the 1-km-mesh domain. It was found that in the warmed conditions the monthly total rainfall over the land areas decreases and the frequency of the extreme total rainfall decreases. On the other hand, the frequency of extreme hourly rainfall increases in the warmed conditions, irrespective of the four different GCM runs. In the cooled condition, the difference between the pseudo-global cooling and the control run seemed to be smaller than that between the pseudo-global warming and the control run. Examining the environmental conditions indicated that the temperature lapse rate in the troposphere decreases in the future climate conditions while increases in the cooled climate. In response to the changes in temperature, precipitable water vapor increases (decreases) in the future (cooled) climate. The present regional modeling study for the month-long heavy-rain spell has clearly demonstrated that competing effects of the decreased lapse rate and the increased precipitable water in the future climate conditions result in the increased frequency of extreme short-term rainfall events while in the decreased frequency of the accumulated rainfall amounts.