Origin, evolution, and finesclae structure of the St. Valentine's Day mesoscale gravity wave observed during STORM-FEST. Part III: Gravity wave genesis and the role of evaporation

On 14 February 1992, a long-lived moderate-amplitude mesoscale gravity wave formed in Kansas during the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST). Wave formation was evident in correlated surface pressure and wind fields. The wave of depression, accompanied by a weak rainband, tracked across the state. A wealth of data was collected on the mature wave as it passed over the STORM-FEST dual-Doppler domain. However, the mechanism of genesis remained difficult to ascertain, since wave formation occurred in a region of less comprehensive observations. The genesis of the STORM-FEST gravity wave is successfully simulated using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (Penn State-NCAR) Mesoscale Model (MM5), which was run at 6-km grid spacing in the innermost domain. The Ice cyclone movement, dry airmass development, and gravity wave formation over Kansas were successfully captured by the model. Results presented here indicate that evaporative processes associated with a rainband resulted in subsidence warming and depression of the underlying warm-frontal inversion. The reduced inversion height produced surface pressure falls, the surface manifestation of a developing gravity wave. Numerical experiments with and without evaporative processes have isolated the key importance of evaporatively driven downdrafts in wave genesis. A conceptual model of the development and evolution of the wave is presented that is consistent with both observations and the findings of the numerical experiments.