Low-Frequency Gravity Waves within Mesoscale Convective System Stratiform Regions

Authors

Adams-Selin, Rebecca — Atmospheric and Environmental Research, Inc.
Vagasky, Hannah Cecile — Atmospheric and Environmental Research
Blount, Dillon — University of Wisconsin–Milwaukee
Evans, Clark — University of Wisconsin–Milwaukee

Category

Convective clouds, including aerosol interactions

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.

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. Oklahoma Mesonet, ASOS, and/or field campaign instrumentation surface pressure 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. NEXRAD radar and NOAA wind profiler velocity data are also queried along with Distance Velocity-Azimuth Display (DVAD) velocity profiles from nearby radars.

Signatures of four low frequency gravity wave propagating through the stratiform regions of the MC3E MCS are identified. Two appear to be associated with upward motion in the lower levels of the troposphere, and two with downward motion. Each wave modifies the rear-to-front and front-to-rear storm-relative flow of the MCS, with perturbations opposite in magnitude occurring in the upper half of the troposphere. The wind perturbation structure and propagation speed of the waves suggest higher order wave modes (e.g., n=2, 3, or 4.) Conversely, the PECAN MCS appears to produce fewer low-frequency waves. Fewer available observations make identifying the wave mode more difficult, but the speed suggests n=1 waves.

High-resolution LES-scale simulations of these two cases are performed using the WRF model. Interestingly, spectral analysis of vertical motions within the domain suggests similar differences between the two cases: more power associated with higher-order vertical waves modes produced by the MC3E MCS, and less with the PECAN MCS. Contributions within the model to perturbations in the local heating term are examined to determine the underlying cause for this difference.