Low-Frequency Gravity Waves within Mesoscale Convective System Stratiform Regions

The ability to simulate MCS stratiform regions that closely match observations remains elusive. The issue is further complicated because the characteristics of the stratiform region are closely tied to the detrainment of hydrometeors from the convective updraft as well as evaporation and sublimation induced by mid-level rear inflow. These intra- and extra-system circulations are largely products of three processes: low-frequency gravity waves, line-end vortex-induced flows, and environmental wind. While these processes ultimately control the characteristics of the stratiform region, their individual contributions to the MCS wind field have not been formally quantified.

As a first step to the quantification of these relative contributions among waves, vortices, and environment, we have identified low-frequency gravity waves within the stratiform region of two mature MCSs. These MCSs were observed during the Midlatitude Continental Convective Clouds Experiment (MC3E) and Plains Elevated Convection at Night (PECAN) field campaigns. Given the small pressure and wind perturbations typically associated with these types of waves, establishing their presence can be a difficult task. However, by identifying complementary wave signatures in multiple observational datasets wave existence can be confirmed. Oklahoma Mesonet and ASOS surface pressure station data are analyzed using a high-pass Lanczos filter before signatures of coherent propagating features are identified. These features are then compared to low-level profile changes observed by Atmospheric Emitted Radiance Interferometers (AERIs), microwave radars, and lidars at Lamont, OK, at the Southern Great Plains ARM site, and deployed during the PECAN campaign. Distance Velocity-Azimuth Display (DVAD) velocity profiles from nearby radars are also queried. Several low frequency gravity wave signatures are identified propagating through the stratiform regions of each MCS.

These observed waves are compared to similar low-frequency gravity waves identified within high-resolution LES simulations of the MCSs, and the implications of differences in wave type and frequency between models and observations are discussed.