Origin, Evolution, and Finescale Structure of the St. Valentine’s Day Mesoscale Gravity Wave Observed during STORM-FEST. Part II: Finescale Structure

Abstract

This paper presents observations of the finescale three-dimensional kinematic and thermodynamic structure of a long-lived mesoscale gravity wave that occurred on 14?15 February 1992 during the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST). In Part I of this series of papers, it was shown that the wave was generated just behind the leading edge of an advancing dry air mass that originated as a foehnlike downslope flow over the Rocky Mountains of southern Colorado and New Mexico. Surface pressure signatures of wave motion began as the dry air mass ascended a warm front east of a lee cyclone and a rainband developed along its leading edge. The wave and rainband intensified, remaining near the leading edge of the dry air mass as the dry air mass advanced northeastward over the warm frontal inversion. After 7 h of evolution, the leading edge of the dry air mass passed over the dual-Doppler network in northeast Kansas. It was at this point in the evolution that the relationships among the mesoscale gravity wave, the leading edge of the dry air mass, and the rainband were determined using dual-Doppler kinematic and thermodynamic retrieval analyses. These analyses demonstrate temporal and spatial consistency, and were verified to the degree possible with independent datasets. Over the dual-Doppler network, the leading edge of the dry air mass was characterized by a strong gradient in horizontal momentum. Convection occurred at the leading edge of the dry air mass. Within the dry air mass, a four-quadrant region of divergence and convergence was observed. Associated with these patterns of convergence and divergence were downdrafts and updrafts. The downdraft contributed to the depression of the warm frontal inversion, creating the wave signature in the surface barograms. The downdraft branch of the circulation, which acted to depress the inversion height, was associated with net cooling due to evaporation of precipitation above the inversion and with circulations associated with the deceleration of air at the leading edge of the advancing dry air mass. The wavelength of the wave over the Doppler domain was determined by the scale of the internal circulations within the dry air and the scale of the convection. Pressure, p?, and virtual potential temperature, ???, perturbations were generated by the advancing dry air mass and associated convection. Strong horizontal p? gradients were associated with both the abrupt deceleration of air at the leading edge of the dry air mass and with the divergent flow at the top of the updraft. Vertical p? gradients were associated with convective updrafts and downdrafts. The total p? distribution was the sum of these effects. Because the convection was weak, the vertical accelerations were small, and the retrieved distribution of ??? was such that the buoyancy force exactly balanced the vertical perturbation pressure gradient force. A conceptual model for the generation and maintenance of this wave is proposed based on the analysis of the observations and the larger-scale measurements presented in Part I.