Further Probing the Dynamical Influences on Regional Changes of Extreme Precipitation Intensity

Abstract:

Regional projections of extreme precipitation intensity (EPI) are strongly influenced by regional projections of “extreme ascent,” i.e. vertical velocity during extreme precipitation events. Earlier studies performed analysis suggesting that long-term changes in eddy length and vertical stability are key factors influencing extreme ascent projections. We probed this matter further with controlled experiments using the System for Atmospheric Modeling version 6.7, a cloud-resolving model (CRM).

We chose CRM domains in the subtropical South Atlantic and in the Northern extratropics, where the Canadian Earth System Model version 2 (CanESM2) projects weakening of extreme ascent. The CRM was forced with large-scale temperature, moisture and winds from CanESM2 during extreme events in the historical (1981-2000) and future (2081-2100) periods. The CRM includes a parameterization of coupling to the large-scale circulation in terms of a prescribed eddy length scale of the large-scale vertical velocity. The CRM reasonably reproduced the timing of the 20-year maximum of daily precipitation in the fully-coupled CanESM2. In most of the cases we examined, the CRM qualitatively reproduced CanESM2’s projected changes in EPI.

To gain further insight into physical mechanisms, we performed “isolation runs” in which some of the CRM’s large-scale forcings were changed to their future values, while keeping all other forcings fixed to historical values. In all cases, long-term changes in horizontal advection were the dominant driver of projected decreases in EPI. An especially interesting case arose in the subtropical South Atlantic. Here, the CRM produced an advection-driven future increase in EPI (in contrast to CanESM2’s projected decrease in EPI), but the isolation runs in which only stability and eddy length were changed produced strong decreases in EPI (consistent with CanESM2). These results suggest that, in some locations, changes in eddy length and vertical stability are important drivers of climate model–projected EPI decreases (in agreement with earlier studies), but those changes might be highly sensitive to the parameterization of convection. Changes in horizontal advection appear to be the more robust dynamical driver of projected EPI changes.