The Dynamics of Nocturnal Mesoscale Convective Systems over the Great Plains and Their Potential Role in Forecast Failures in Global Numerical Weather Prediction Systems

This study investigates the dynamics of nocturnal mesoscale convective systems (MCS) over the Great Plains of North American including research based on the large multi-agency Plains Elevated Convection at Night Experiment (PECAN) field campaign. PECAN took place from 1 June to 15 July 2015 and was designed to increase our understanding of the envelopes of deep convection that begin over the higher terrain west of the Great Plains, grow up-scale, reach the Great Plains during night, and sometimes continue to propagate to the east during the following day. These propagating MCS help explain the nocturnal maximum in thunderstorm activity found over the Great Plains during the summer. One finding from PECAN is that as the nocturnal boundary layer stabilizes, convectively-generated cold pools trigger bores. that produce nearly 1 km of net lifting in the lower troposphere. Such lifting can destabilize large areas surrounding the storms, often initiating new convection or maintaining the MCS.

The change of the mechanism maintaining MCSs from cold pool lifting to destabilization from non-hydrostatic waves is a likely challenge for parameterizing convection in weather and climate models. This hypothesis is investigated given that several significant forecast failures in the ECMWF model took place during PECAN. Our research shows that the modification and/or generation of new Rossby wave packets due to the interaction between long-lived, propagating MCS east of the Rockies and the jet stream is a critical process that must be accurately represented in these global simulations. In addition, jet streams that act as wave guides that favor the triggering/amplification of Rossby wave packets over North American and subsequent wave breaking over the North Atlantic are favorable conditions for forecast failures. These results have implications for predicting regime changes, such as the NAO, across the middle latitudes and the Arctic.