Mesoscale Vortex Development during Extreme Precipitation: Colorado, September 2013

Abstract

Abstract On 11–12 September 2013, portions of northern Colorado experienced flash flooding as a result of high rain rates accumulating over 180 mm of rain in 6 h. From 0400 to 0700 UTC 12 September a mesovortex was observed traveling northwestward toward the city of Boulder, Colorado, with enhanced upslope flow on its north side and localized deep convection. Although the mesovortex was observed in an area common for lee vortex formation, namely that associated with the Denver Cyclone, it is shown via ARW model simulations that the mesovortex intensified through the release of latent heat, similar to the processes leading to mesoscale convective vortices, rather than by dry topographic-flow dynamics. High rates of cloud water condensation at relatively low altitudes led to a strong vertical gradient in latent heating, resulting in a near-surface positive potential vorticity anomaly. Reducing the contribution of cloud water condensation to latent heating by 50% resulted in no mesovortex development in the model and a substantial decrease in precipitation. On the other hand, removing the topographical forcing in the model did not inhibit the mesovortex formation, confirming the secondary role of topography. The mesovortex enhanced upslope winds and convection, and was thus a key feature in the generation of intense precipitation over Boulder. The ability to forecast the development of these mesovortices and their subsequent environmental and hydrological effects could be critical for decision-makers and the public, given their association with high rainfall rates.