A Continental United States Climatology of Precipitation Whiplash Using a New Event-Based Definition

Abstract

Precipitation whiplash events have been increasingly studied in the scientific literature over the recent decade. However, no studies have considered the spatial coherence of grid points in their examination of whiplash events. Therefore, we developed an algorithm that defines spatially coherent precipitation whiplash events on the subseasonal-to-seasonal timescale between 1915 and 2020. We define precipitation whiplash events to occur when the 30-day Standardised Precipitation Index (SPI) moves from at or above +1 to at or below −1, or vice versa. Once grid points were identified as having undergone a whiplash occurrence, Kernel Density Estimation was used to create event polygons, taking into consideration the continuity of grid points. Events were then clustered into geographical regions using k-means clustering, allowing for the examination of their climatology and characteristics across the continental United States (CONUS). The largest drought-to-pluvial and pluvial-to-drought events were found to be increasing in both frequency and total area impacted across the CONUS, primarily driven by increasing trends across the Southeast and Northeast. Additionally, regions that experience drought conditions for a longer period before a drought-to-pluvial event exhibit a greater change in SPI, whereas areas that are in pluvial conditions for a longer period before a pluvial-to-drought event exhibit a smaller change. Overall, the developed definition is designed in such a way that it can be adapted, and the databases generated can be used in future impact and predictive studies, greatly enhancing the understanding of precipitation whiplash events across the CONUS.