Mesoscale Gravity Waves and Their Environment in the Central United States during STORM-FEST

Abstract

In an effort to better understand mesoscale gravity waves in winter storms in the central United States?their frequency of occurrence, wave characteristics, the general conditions under which they occur, and their effects upon the weather?mesoscale surface and rawinsonde data as well as radar and satellite imagery collected during the Storm-scale Operational and Research Meteorology?Fronts and Experimental System Test are analyzed. In addition, factors affecting the ability of objective surface map analysis to properly represent the waves are investigated. Thirteen coherent pressure pulse events with amplitudes of 0.2?4.0 mb and periods of 1?6 h were identified in the surface pressure data during the 6 weeks of the project, involving 34% of the total hours investigated. A variety of wave types occurred, including wavelets, wave trains, and singular waves. The three largest amplitude events were analyzed in detail using autospectral analysis and a Barnes time-to-space conversion objective analysis of bandpass-filtered mesonet data. All three events displayed high perturbation pressure?wind covariances (p?u*?), consistent with a gravity wave explanation for the disturbances (u* is the wind component in the direction of wave propagation). The p?u*? values were closely related to the strength of the wave amplitudes. The waves found in these events displayed mean phase velocities of 19.9?27.9 m s?1, wavelengths of 200?260 km, and periods of 2.3?3.5 h. Wave crests appeared to be closely aligned with associated rainbands throughout their lifetimes, suggesting that a codependency existed. Some of the waves were evident before the rainbands formed, indicating that the precipitation developed in response to the waves, though this was not true for all of the waves. Values of p?u*? decreased during the development stage of deep convection, but high covariance between the pressure and wind fields redeveloped as the thunderstorms and incipient gravity wave matured into a stable, coupled mesoscale convective system. Three of the four wave events displaying the largest amplitudes occurred primarily on the cool side of a stationary front in an environment in which a jet streak was approaching an inflection axis in a diffluent height field downstream from an upper-level trough. The waves also extended some distance into the warm sector in the presence of a statically stable lower troposphere, suggesting wave ducting was operative. The results indicate that this conceptual model for the wave environment should prove useful as a tool for forecasting the most significant mesoscale gravity wave events.