Thermally Induced Compression Waves and Gravity Waves Generated by Convective Storms

Abstract

A three-dimensional, fully compressible cloud model is used to simulate a convective storm in order to investigate the properties of compression waves and gravity waves induced by latent heat release. Time series of the low-level pressure perturbations caused by the propagating waves are examined at various distances from the storm. A compression wave that is close to hydrostatic balance and can be considered to be a Lamb wave, which propagates in the horizontal plane, emerges from the storm. This latter property gives the wave a distinctly two-dimensional character that is clarified by comparison with a linear model of a two-dimensional thermally induced compression wave. This has implications for its shape and results in a decay rate with distance propagated from the source of 1/(distance)1/2. The period of the Lamb wave is determined primarily by the time it takes for the storm to develop and decay. The fast-moving Lamb wave is trailed by slower-moving thermally induced gravity waves. It is found that the amplitude of the gravity waves decay with 1/distance. Distinct gravity wave modes can be identified. The first mode propagates the fastest and results in deep subsidence warming. The second mode propagates at half the speed of the first and causes weak low-level uplift, which in some convective situations might aid the development of new convection. An analysis of the transfer of internal and gravitational potential energies showed that the net transfer by the Lamb wave was approximately equal to the net increase of total energy in the atmosphere brought about by the convective storm. This result suggests that physical interpretations of total energy transfer in the atmosphere need to take into account that it can be transferred in a wavelike manner at the speed of sound. An interesting buoyancy oscillation occurred when the downdraft air overshot its buoyant equilibrium level, which resulted in a resurgence of convection. The convection was able to obtain moderate strength by feeding on moist environmental air that had been advected over the top of the cold pool. This mechanism may be a factor contributing to the early meso-? convective cycle that has been observed in many convective systems.