The Role of Gravity Waves In Slowly Varying In Time Mesoscale Motions

Abstract

Pressure oscillations with amplitudes of the deviations from the horizontal mean and periods considerably less than those for the large-scale case have been observed in a number of summer and winter storms. However, there is conflicting evidence about the role of these waves in mesoscale storms. In the case of mesoscale heating that is a prescribed function of the independent variables, it has been proven that the dominant component of the corresponding slowly varying in time solution is accurately described by a simple dynamical (reduced) system in which gravity waves play no role. This paper proves that large spatial-scale gravity waves with amplitudes and periods of the pressure perturbations the same as the reduced system component of the solution can be generated by mesoscale storms. Because the amplitudes and the periods of the pressure perturbations for the two components of the solution are 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 conjunction 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 range of current wind profilers), makes observation of gravity waves very difficult. In numerical simulations, if both components of the mesoscale solution are required, the lateral extent of the domain of solution must be considerably larger than the lateral extent of the mesoscale heating in order that the large-scale gravity waves be correct. In this case, it is shown that the multiscale system for meteorology developed earlier by Browning and Kreiss accurately describes both components of the solution.