Factors contributing to upscale convective growth in the Central Great Plains of the United States
During the warm season, convection is a frequent occurrence across the Great Plains of the United States. Depending on storm mode, severe convection can contain a variety of threats, including tornadoes, large hail, strong winds, and flash flooding. The latter two of these are most commonly associated with mesoscale convective systems (MCSs), which, despite general improvements in forecasting convection, remain difficult to predict. While numerical weather prediction models have improved in resolution as a result of increased computational resources, predicting MCSs and their evolution remains a pertinent challenge. Thus, a further understanding of the physical mechanisms responsible for the formation of MCSs is needed in order to better predict these storms in the future. To better predict this phenomena, this work focuses on both quasi-real-world simulations using the Weather Research and Forecasting model (WRF) and and idealized cloud model (CM1) to differentiate between synoptic-, meso-, and storm-scale characteristics of non-MCS and MCS producing events.
First, the WRF model was used to simulate a total of 30 events during the 2016 warm season, including 15 non-MCS and 15 MCS producing convective days. These 24-hour simulations utilized a 3km horizontal resolution, which is representative of the resolution of current operational convective allowing models. Each simulation was analyzed to determine the ability of the WRF to properly distinguish between non-MCS and MCS producing days using GFS analyses as its initial and boundary conditions. It was found that WRF was generally able to predict these events, though identification of more specific convective modes were not considered. Then, a variety of potential factors influencing upscale convective growth were examined in detail, including several cold pool related parameters and vertical wind shear.
Second, idealized simulations utilizing pre-convective environments of the WRF were completed in a cloud resolving model (CM1). Here, horizontal and vertical resolutions were less than 500m. The 3-d structures of the convective updrafts and cold pools were examined after modifying thermodynamic variables and wind profiles to test the sensitivity of upscale growth.
It was found that changes to the overall wind profile did little to influence overall convective evolution. The edge of the cold pool was also not the primary source of lift for parcels reaching the convective updrafts, and evidence suggests that gravity waves behind the cold pool edge may be contributing additional lift. Furthermore, in experiments with shallow and deep stable layers, surface-based convection still developed. While results from the CM1 indicate that there is likely a balance between the thermodynamic and kinematic fields that encourages upscale growth, a more thorough investigation is needed to better understand the physical processes driving convective growth in the CM1.