Observed Changes in Extreme Precipitation Events over the US

Abstract:

We analyze changes in extreme daily precipitation over the conterminous US using observational (CPC unified daily gauge, ~25km grid size), reanalysis (NARR, 32km grid size) and dynamically downscaled (WRF, 4km grid size) data spanning up to 65 years in the recent past. First, we present the changes in the characteristics of extreme precipitation (95th percentile of rain-days) over six regions in the reanalysis dataset on daily, monthly and annual time scales. Our analysis shows that the spatial extent of extreme precipitation events, defined as the area of contiguous grid points of daily extreme precipitation, has been decreasing by up to 5-fold over most regions since 1979. We investigate the thermodynamic and dynamic causes of this decreasing trend in the spatial extent. Additionally, we assess if a static separation of “local” and “remote” regions is valid for the analysis of moisture fluxes feeding an extreme precipitation event given the varying spatial extents over time. Moreover, we analyze the relatively finer resolutions of observed and dynamically downscaled datasets to allow us to further understand the physical causes driving changes in extreme events. This is achieved by identifying smaller spatial extents of extreme precipitation events in these datasets and quantifying their associated finer-scale moisture dynamics. Overall, this study allows us to understand the changes in extreme precipitation characteristics in the observed past and how changing moisture dynamics have shaped these characteristics.