THE ROLE OF GRAVITY-WAVES IN SLOWLY VARYING IN TIME MESOSCALE MOTIONS

Pressure oscillations with amplitudes of the deviations from the horiz ontal mean and periods considerably less than those for the large-scal e case have been observed in a number of summer and winter storms. How ever, there is conflicting evidence about the role of these waves in m esoscale storms. In the case of mesoscale heating that is a prescribed function of the independent variables, it has been proven that the do minant component of the corresponding slowly varying in time solution is accurately described by a simple dynamical (reduced) system in whic h gravity waves play no role. This paper proves that large spatial-sca le gravity waves with amplitudes and periods of the pressure perturbat ions the same as the reduced system component of the solution can be g enerated by mesoscale storms. Because the amplitudes and the periods o f the pressure perturbations for the two components of the solution ar e similar, it is difficult to distinguish between them using temporal plots of the pressure at a single location, and this is the source of a large part of the confusion about these waves. This problem, in conj unction with the fact that the vertical velocity of the gravity waves is an order of magnitude smaller than the maximum vertical velocity in the dominant component of the solution (and therefore in the noise ra nge of current wind profilers), makes observation of gravity waves ver y difficult. In numerical simulations, if both components of the mesos cale solution are required, the lateral extent of the domain of soluti on must be considerably larger than the lateral extent of the mesoscal e heating in order that the large-scale gravity waves be correct. In t his case, it is shown that the multiscale system for meteorology devel oped earlier by Browning and Kreiss accurately describes both componen ts of the solution.