Land Surface Sensitivity of Mesoscale Convective Systems
July 08, 2016
Robert Tournay
Committee: Russ Schumacher (advisor), Tom Vonder Haar (co-advisor), Sue van den Heever, Peter Nelson (Civil and Environmental Engineering)
Abstract
Mesoscale convective systems (MCSs) are important contributors to the hydrologic cycle in many regions of the world as well as major sources of severe weather. MCSs continue to challenge forecasters and researchers alike, arising from difficulties in understanding system initiation, propagation, and demise. One distinct type of MCS is that formed from individual convective cells initiated primarily by daytime heating over high terrain. This work is aimed at improving our understanding of the land surface sensitivity of this class of MCS in the contiguous United States.First, a climatology is developed of MCSs originating in the Rocky Mountains and adjacent High Plains. These MCSs are found to be most important, in terms of total warm season precipitation, in the western Great Plains. Examining MCSs by longevity reveals that longer lasting systems tend to form farther south and have a longer, more southerly track. The environment into which MCSs moved showed differences across commonly used variables in convection forecasting, with some variables, such as convective inhibition, showing more favorable conditions ahead of longer lasting MCSs, other variables, such as convective available potential energy, showed improving conditions through time for longer lasting MCSs, and some showing no difference across longevity of MCSs, such as precipitable water.
From this MCS climatology, three regions of origin were chosen based on the presence of ridgelines extending eastward from the Rocky Mountains known to be foci for convection initiation (CI) and subsequent MCS formation: Southern Wyoming, Colorado and northern New Mexico. Composite initial and boundary conditions were developed from reanalysis data, from which control runs of regional MCSs were made as well a series of experiments with idealized, imposed large-scale soil moisture (SM) conditions to study to impact to each regional MCS from SM variations in initiation region as well downstream in the Great Plains. While the distribution of SM has a major impact on parcel buoyancy and the location and magnitude of CI, also important are the differences in shear driven by the differences in large scale SM, playing a varying role depending on how the MCSs interact with these shear anomalies.
Additionally, the land surface sensitivity of CI and elevated mixed layers (EMLs) in the region was studied. A climatology of CI was developed and the sensitivity of timing in relation to SM and vegetation was explored. Also studied was the relationship of common convection forecasting variables to SM and vegetation at CI points. Important to the environment of convection and MCSs in the region, the land surface sensitivity of EMLs in the region was studied through a climatology to origin points over high terrain of the CONUS using back trajectories as well as a modelling case study.