Effect of Latent Heating on Mesoscale Vortex Development during Extreme Precipitation: Colorado, September 2013

On the evening of 11 September 2013, Boulder, CO experienced flash flooding as a result of high rain rates accumulating over 180 mm of rain in 6 hours. From 0400-0700 UTC 12 September, a mesovortex was observed to travel northwestward towards Boulder. This circulation enhanced upslope flow and was associated with localized deep convection. The mesovortex originated in an area common for lee vortex formation (i.e. the Denver Cyclone). The WRF model was used to properly represent the evolution and processes of interest during the event and test the importance of latent heating to the development of the mesovortex. The results from various latent heating experiments suggested that the mesovortex did not develop through lee vortex formation and the latent heat released just before and during the mesovortex event was important to its development. Results also showed latent heating affected the flow field, resulting in a positive feedback between the circulation, associated low-level jet, and convection leading to further upslope flow and precipitation development. Further experiments showed condensation of cloud water was the dominant microphysical process responsible for a positive vertical gradient in latent heating near the surface. This gradient led to potential vorticity generation; a similar mechanism to that of a mesoscale convective vortex, except closer to the surface. Finally, an experiment where the latent heating was reduced by 50% after 1800 UTC 11 September resulted in no mesovortex development and a substantial decrease in precipitation.