Mesoscale gravity waves and their environment in the central United Statesduring STORM-FEST

In an effort to better understand mesoscale gravity waves in winter storms in the central United States-their frequency of occurrence, wave characteri stics, the general conditions under which they occur, and their effects upo n the weather-mesoscale surface and rawinsonde data as well as radar and sa tellite imagery collected during the Storm-scale Operational and Research M eteorology-Fronts and Experimental System Test are analyzed. In addition, f actors 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 p eriods of 1-6 h were identified in the surface pressure data during the 6 w eeks of the project, involving 34% of the total hours investigated. A varie ty of wave types occurred, including wavelets, wave trains, and singular wa ves. The three largest amplitude events were analyzed in detail using autos pectral analysis and a Barnes time-to-space conversion objective analysis o f bandpass-filtered mesonet data. All three events displayed high perturbat ion pressure-wind covariances (<(p'u*')over bar>), consistent with a gravit y wave explanation for the disturbances (u* is the wind component in the di rection of wave propagation). The <(p'u*')over bar> values were closely rel ated to the strength of the wave amplitudes. The waves found in these event s 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 throug hout their lifetimes, suggesting that a codependency existed. Some of the w aves were evident before the rainbands formed, indicating that the precipit ation developed in response to the waves, though this was not true for all of the waves. Values of <(p'u*')over bar> 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 pr imarily 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 distanc e into the warm sector in the presence of a statically stable lower troposp here, suggesting wave ducting was operative. The results indicate that this conceptual model for the wave environment should prove useful as a tool fo r forecasting the most significant mesoscale gravity wave events.