18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001: 11:45 AM
The role of evaporative Processes in gravity wave genesis
Brian F. Jewett, Univ. of Illinois, Urbana, IL; and M. K. Ramamurthy and R. M. Rauber
Poster PDF (383.3 kB)
On 14 February 1992, a long-lived, moderate-amplitude mesoscale gravity wave formed in Kansas during 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 structure as it passed over the STORM-FEST dual-Doppler domain. Radar analyses have since revealed that the wave and rainband were located at the leading edge of a dry airmass and wind maximum moving atop the warm frontal inversion. The weak rainband subsequently intensified into deep convection as the dry airmass boundary, wave and precipitation moved into Missouri. The mechanisms of genesis, in particular the relationship between the wave and rainband, were less clear, as the surface wave signatures and precipitation aloft first became evident in southwest Kansas, a region of less comprehensive observations.

Our numerical simulations of this case have revealed a new mechanism responsible for wave genesis. The Penn. State/NCAR MM5 mesoscale model, run at 6 km resolution, successfully captured the lee cyclone movement, dry airmass development and gravity wave formation over Kansas. Simulations with key physics omitted have revealed that evaporation of precipitation from the weak rainband at the leading edge of the dry airmass was essential for wave genesis. Evaporation-induced subsidence above the frontal inversion led to surface pressure falls and gravity wave initiation. In the model, as in observations, a long-lived wave of depression developed, remaining tied to the dry air boundary and the rainband. MM5 90s time series data revealed a region of highly (>0.9) correlated surface wind and pressure perturbations accompanying the wave. A transition to wave of elevation occurred as deep convection developed in the model, and significant precipitation reached the surface. The evolution of the dry air mass, cyclone, speed maximum, rainband and gravity wave will be discussed in the hours leading up to and following wave genesis.

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