Climate Warming Impacts on Strongly and Weakly-Forced Synoptic Precipitation Event in the Southern United States

We investigate the question of how strongly and weakly-forced precipitation events could change in a warmer climate. Previous studies have found that in a warmer climate, extreme precipitation events could increase in intensity while light precipitation event decrease in frequency. The water vapor content of the atmosphere is expected to increase with increasing temperatures, potentially leading to more intense heavy rainfall along with an increased risk of flooding. For this study, we focus on the synoptically forced extreme precipitation events over the southern states during the cool season and spring season. We classify extreme events as those in which three or more inches of precipitation occurred within 24 h at three or more stations located at least 10 miles apart. A composite of these events is generated using North American Regional Analysis (NARR) data for a strongly and weakly-forced precipitation event.

We use the Weather Research and Forecasting (WRF) model to simulate this composite event. The purpose of using WRF for this simulation is to enable the application of thermodynamic changes derived from the IPCC AR4 GCM data. We then compare the current to the future WRF simulated composite events to examine changes in intensity and structure. By using this method, we determine how present-day strongly and weakly-forced precipitation events in this region could change in a warmer climate, as well as identify environmental parameters that have a significant influence on the observed change. We find that the weakly-forced precipitation event decreases in intensity in the warmer climate, while the strongly-forced precipitation event increases in intensity. In the strongly-forced precipitation event, the low level jet and surface low pressure system play a large role and the increase shows a super Clausius-Clapeyron relationship. One of the main findings in this study is that the future precipitation is correlated to the strength of upper-level forcing.