THE DYNAMIC FORCING OF THE GREAT PLAINS LOW LEVEL JET

The Great Plains low level jet (LLJ) is a persistent mesoscale feature of the nocturnal boundary layer over the central plains of the United States. The heat and moisture transported by the LLJ can vitalize and sustain isolated nocturnal thunderstorms as well as mesoscale convective complexes which are responsible for a significant fraction of rainfall that occurs over the Great Plains in summer. The LLJ also serves as an excellent testbed for the study of boundary-layer-forced dynamics over complex terrain. In summer 1996, the University of Wyoming King Air aircraft was used to document the spatial structure and temporal evolution of the LLJ. The flight pattern consisted of two perpendicular 70 kilometer legs at 940 hPa "sandwiched" between shorter "flux legs" at 930 and 950 hPa. The flight pattern was repeated four times over each case study to assess the temporal behavior of each term in the momentum equation.

The primary objective of this study was to resolve the individual forcing terms in the horizontal momentum equation to first-order closure. Estimates of the storage of momentum (inertial term), horizontal advection, pressure-gradient force (PGF), Coriolis acceleration, and vertical momentum flux divergence were obtained. Special emphasis was placed on the determination of the momentum flux and the PGF. The momentum flux was determined using both covariance and eddy-correlation methods as reported in an earlier paper. The PGF was obtained by measuring the radar altitude of the aircraft along a constant pressure surface and adding it to the terrain elevation to yield the height of the aircraft (i.e. the pressure surface) above mean sea level. Least-squares regression of the resulting trace is a direct measure of the height gradient on a constant pressure surface and, therefore, an estimate of the PGF or the geostrophic wind. With a robust estimate of the geostrophic wind, and direct measurements of the actual wind, the total ageostrophic nature of the LLJ can be resolved. Moreover, since flight-leg-redundancy was built into the study as a means of assessing temporal behavior, the total ageostrophic acceleration can be further resolved to estimate the contributions from both the inertial and isallobaric ageostrophic components, which are related to frictional decoupling and time-variations in the PGF, respectively. The results indicate that individual accelerations in the horizontal momentum equation can be measured to first-order closure directly and independently using an aircraft as an observational platform. Diagnosis of each term supports the view that the dynamics of the LLJ may be understood in terms of an interaction between the boundary-layer-induced PGF generated by differential heating over sloping terrain, frictional decoupling attending the cessation of turbulent mixing around sunset and the concurrent acceleration of the wind field, and a time- dependent PGF responsible for isallobaric effects.