D.Touma , A.M. Michalak, D.L. Swain and N.S. Diffenbaugh
Knowing the spatial extent of extreme precipitation events can give insights into the potential flood risk from such events, among other impacts. Increases in the frequency and intensity of extreme precipitation events has led researchers to study how the spatial extent of precipitation events is changing in response to increases in temperature, but there has not been a robust assessment of historical spatial extents. In this study, we use weather station data to understand the historical spatial extent of extreme daily precipitation events in the contiguous United States. We found that spatial extent varies both regionally and seasonally. For example, the northwest U.S. had little seasonal variation, while the eastern U.S. had strong seasonal variations with summer extreme precipitation events having half the spatial extent of winter events. Understanding how historical extreme precipitation events vary will help us understand how precipitation events are changing and how they may change in the future.
Figure: The spatial extent (km) of extreme precipitation events for each season, for daily extreme rainfall events centered at weather station locations from 1965–2014.
The spatial extent of an extreme precipitation event can be important for a basin’s hydrologic response and subsequent flood risk, and may yield insights into underlying atmospheric processes. Using a relaxed moving-neighborhood approach, we develop indicator semivariograms based on precipitation records from the Global Historical Climatology Network–Daily (GHCN-D) station network to directly quantify the climatological length scales of extreme daily precipitation over the United States during 1965–2014. We find that the length scales of extreme (90th percentile) daily precipitation events vary both regionally and seasonally. Over the eastern half of the United States, daily extreme precipitation length scales reach 400 km during the winter months, but are approximately half as large during the summer months. The Northwest region, on the other hand, exhibits little seasonal variation, with extreme precipitation length scales of approximately 150 km throughout the year. By leveraging in situ station measurements, our study avoids some of the uncertainties associated with satellite or interpolated precipitation data, and provides the longest climatological assessment of length scales of extreme daily precipitation over the United States to date. Although the length scales that we calculate can be sensitive to station density, neighborhood size, and neighborhood relaxation, we find that the interregional and interseasonal differences in length scales are relatively robust. Our method could be extended to quantify changes in the spatial extent of extreme daily precipitation in the recent past, and to investigate the underlying causes of any changes that are detected.
Touma D., A.M. Michalak , D.L. Swain, N.S. Diffenbaugh (2018) "Characterizing the Spatial Scales of Extreme Daily Precipitation in the United States," Journal of Climate, 31, 8023-8037, doi:10.1175/JCLI-D-18-0019.1.