Origin, evolution, and finescale structure of the St. Valentine's day mesoscale gravity wave observed during STORM-FEST. Part I : Origin and evolution

On 14-15 February 1992 a long-lived mesoscale gravity wave was observed over the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test observational network. A precipitation band formed and intensified as the wave moved across Kansas and Missouri. The disturbance was tracked for 14 h. Surface, dual-Doppler radar, and wind profiler analyses, as well as isentropic analyses derived from a special rawinsonde network, are used to examine the origin and evolution of this wave. The wave originated at the leading edge of a dry air mass associated with downslope flow in the lee of the Rockies. The earliest surface pressure signatures of wave motion began as a dry air mass, associated with the downslope flow, ascended a warm front east of a lee cyclone. A weak rainband developed simultaneously with the wave at the leading edge of the dry air mass. The mesoscale gravity wave, and convection, remained tied to the leading edge of the advancing dry air mass during the first 8-10 h of evolution, suggesting that both convection and dynamical processes near the leading edge of the dry air mass were instrumental in maintaining the wave during this phase of its evolution. These processes are investigated in Part II. The orientation of the wave front, determined from isochrone analyses of minimum pressure occurrence, and the orientation of the rainband, determined from radar analyses, corresponded closely to the leading edge of the advancing dry air mass for the first 8-10 h. In central Missouri, the convection and wave decoupled from the leading edge of the dry air mass. Barograms downstream in eastern Missouri and Illinois suggest a change in wave structure to a wave of elevation occurred during this latter stage of evolution.