Trends in the Spatial Extent of Daily Extreme Precipitation Totals

Using various gridded precipitation data sets, trends in extreme rainfall occurrence will be evaluated in terms of areal extent. Rather than relying on point rainfall amounts, polygons are defined based on 5.08 cm (and other extreme threshold) daily rainfall isohyets. Across the Northeast (and other US climate regions), areal precipitation will be characterized based on the annual and seasonal number of extreme precipitation polygons and the total area of the polygons. Preliminary results suggest that the gridded datasets are able to replicate the observed station-based rainfall. From a spatial perspective, the number of extreme precipitation polygons does not change significantly through time, but there appears to be a substantial increase in the area covered by the polygons. The results also have practical implications for flooding and also runoff design specifications. For instance, a systematic change in the spatial extent of extreme rainfall could affect existing empirical relationships relating point rainfall observations to areal rainfall amounts.