Gravity Waves and Convection in Colorado during July 1983

Abstract

In order to gain insight into the complex dynamics of a convective system interacting with a gravity wave train, we have carried out an experiment in northeast Colorado during July and August, 1983, utilizing data from several program areas in NOAA. Pressure data from the PROFS mesonetwork of microbarograph stations were combined with velocity profiles from the Wave Propagation Laboratory UHF wind profiler (ST) radar at Stapleton Airport in Denver and convective cell location data from the NWS Limon weather radar. Several events were clearly visible in the microbarograph data, from which four (called Events A, B, C and D) in late July were selected for further study. These events differed from each other in fundamental ways. In each event the waves represent oscillations of a substantial depth of the troposphere and seem to appear and disappear together with the convective cells. In Events A and B the waves have a critical level and are probably unstable modes generated by wind shear in the jet stream, from which they extract energy. We suggest that the convective cells cause the selection of some modes over others in a system that is initially dynamically unstable. In Event A the wave appears to be locked together with the convective cells, which move at the same velocity as the phase velocity of the wave. The wave and the cells seem to grow and evolve synergetically. In Event B the wave and convective cells commence at about the same time, but the cell velocities are quite different from the wave phase velocity. The cell velocities vary substantially over the time of the event and appear to be controlled by the local winds. In the Events C and D, the waves move faster than the maximum wind in the jet and at least twice as fast as the convective cells. It is suggested that these are nonsingular neutral modes whose excitation depends on a number of mechanisms, such as vertical convective motions and acceleration in the jet flow.