Enhancing the Understanding of Nocturnal Convective System Morphological Evolution

Iowa State University, Ames IA

Investigators

Abstract

Large thunderstorm clusters, known as “Mesoscale Convective Systems” (MCSs), often occur at night in the central United States, bringing damaging severe weather but also much of the rainfall needed for agriculture during the growing season in this region. Forecasting of MCSs by computer models is often not very accurate, especially when they are growing from a few intense thunderstorms, often in early evening, to a much larger region of storms and steady rain during the night, and again toward dawn when MCSs usually dissipate. The evolution of MCSs is related to the pool of cool air that forms from evaporation of the rain, and that cool pool is influenced by the temperature and humidity near the storm, and the processes that form precipitation in the cloud. Other factors influencing MCS evolution include changes in wind with height, the presence of nearby weather fronts, and even smaller-scale atmospheric features like gravity waves that interact with a current of faster wind that often develops at night over this region, known as the low-level jet. The project will study how all of these processes influence MCS evolution, and the improved understanding will improve forecasting of MCS rainfall that often causes flooding and severe weather. The research will support several graduate students, be incorporated into undergraduate courses, and be presented in outreach to the public and National Weather Service offices. Accurate prediction of nocturnal MCSs, particularly their rainfall and morphological evolution, remains elusive. A refinement of horizontal grid spacing from 3 to 1 km has been found to increase the number of linear systems simulated by WRF, agreeing better with observations, but the lines were produced in the wrong cases, at the wrong times, and usually with problems in the depiction of stratiform rain. Problems are particularly abundant during the upscale growth period, and again toward dissipation. The increase in number of lines occurs as more continuous zones of higher reflectivity develop, possibly because of better resolution of lift along cold pool boundaries. It is likely that several factors affecting the cold pools, such as near-storm evening boundary layer characteristics and microphysical processes within convection play an important role in the upscale growth morphological evolution, along with the ambient wind shear and positioning of boundaries. Gravity waves, bores, intrusion currents, and their interaction with the low-level jet also influence evolution. Factors influencing stratiform region formation may also play a role that could explain model problems in simulating mode late in the life cycle of MCSs. The project will use three models, WRF, FV3-SAR, and CM1, along with observations to improve understanding of nocturnal morphological evolution and reveal the primary problems preventing accurate simulation of some modes and transitions between them. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.