Numerical simulations of a gravity wave event over CCOPE .1. The role of geostrophic adjustment in mesoscale jetlet formation

Mesoscale model simulations are performed in order to provide insight into the complex role of jet streak adjustments in establishing an environment favorable to the generation of gravity waves on 11-12 July 1981. This wave event was observed in unprecedented detail downstream of the Rocky Mountains in Montana during the Cooperative Convective Precipitation Experiment. The high-resolution model simulations employ a variety of terrain treatments in the absence of the complicating effects of precipitation physics in order to examine the complex interactions between orography and adiabatic geostrophic adjustment processes. Results indicate that prior to gravity wave formation, a four-stage geostrophic adjustment process modified the structure of the mid- to upper-tropospheric jet streak by creating secondary mesoscale jet streaks (jetlets) to the southeast of the polar jet streak in proximity to the gravity wave generation region (WGR). During stage I, a strong rightward-directed ageostrophic flow in the right exit region of the polar jet streak (J(1)) developed over west-central Montana. This thermally indirect transverse secondary circulation resulted from inertial-advective adjustments wherein momentum was transported downstream and to the right of J(1) as air parcels decelerated through the exit region. During stage II, a highly unbalanced jetlet (J(2)) formed just northwest of the WGR in response to the inertial-advective forcing accompanying the ageostrophic circulation associated with J(1). The mass field adjusted to this ageostrophic wind held. An adiabatic cooling and warming dipole resulting from this thermally indirect secondary circulation was the cause for frontogenesis and a rightward shift in the midtropospheric pressure gradients. Since this secondary circulation associated with J(2) occurred above a dramatic vertical variation in the thermal wind, the vertical transport of potentially colder air from below was larger ahead of and to the right of J(1), thus shifting the new jetlet (J(2)) well away from J(1) as the mass field adjusted to the new wind field. Stage III was established when the new mass field, which developed in association with J(2) during stage II, set up a dynamically unbalanced circulation oriented primarily across the stream, and directly over the WGR. This new leftward-directed ageostrophic cross-stream flow (A) formed between jetlet J(2) and the original exit region of the polar jet streak J(1). Finally, a midlevel mesoscale jetlet (J(3)) is simulated to have developed in stage IV over the WGR in response to the integrated mass flux divergence associated with both the stage II and III adjustment processes. This lower-level return branch circulation to jetlet J(2) was further enhanced by velocity divergence accompanying the localized cross-stream ageostrophic wind maximum (A), which develops during stage III. The entire multistage geostrophic adjustment process required about 12 h to complete over a region encompassing approximately 400 km X 400 km.