Observations of Extreme Short-Term Precipitation Associated with Supercells and Mesovortices

In several prominent extreme precipitation and flash flood events, radar and rain-gauge observations have suggested that the heaviest short-term rainfall accumulations (exceeding 75 mm/h) were associated with supercells or mesovortices embedded within larger convective systems (e.g., multicell clusters or MCSs).  In this research, we aim to explore how often extreme short-term rain rates in the US are associated with mesovortices, with the ultimate goal being to better understand the dynamical processes supporting very heavy precipitation.  The first part of this research involves building and analyzing an observationally based database of such events.

            First, hourly precipitation accumulation METAR data was obtained through the Iowa Environmental Mesonet from 2013-2015 and filtered for precipitation accumulations over 75 mm. This data was then manually culled by regional radar analysis to remove gauge observations that are clearly reporting false data (e.g., no precipitation is visible on radar) and to filter out winter weather events. Local radar data, including equivalent reflectivity, radial velocity, and differential reflectivity (Z_dr), was then obtained for the remaining events for the hour leading up to the METAR observation. Additionally, a similar method was performed on Stage-IV precipitation analysis data to supplement the dataset.

            The distribution of the valid METAR and Stage-IV short-term, locally extreme events shows the majority of the events located along the Atlantic and Gulf of Mexico coastlines with additional events occurring in the central plains and into the Midwest. While some of these events are associated with more traditional “training” precipitation cases, many others are associated with low-level rotation in high-precipitation supercells and/or mesoscale vortices embedded in more organized storm modes. The characteristics of the observational dataset are given and specific storm characteristics discussed. The characteristics of the observational dataset are given and specific storm characteristics discussed, and connections will be drawn to recent research on concurrent, collocated tornadoes and flash floods.