A numerical study of three dimensional gravity waves triggered by deep tropical convection and their role in the quasi biennial oscillation
A three dimensional mesoscale model is used to examine the structure of gravity waves triggered by tropical convection and their role in the dynamics of the lower tropical stratosphere. Three distinct cases of tropical convection are considered. The first case is thought to be representative of a seasonal thunderstorm storm, named Hector, occurring over North West Australia during the northern hemisphere winter. The second case is a squall line observed during the TOGA-COARE experiment over the Pacific Warm Pool. The last case is a squall line that occurred during the COPT 81 experiment over northern Ivory Coast (West Africa).The dependence of the characteristics of gravity waves on the features of the generating storm, such as updraft intensity and latent heating distribution, and on features of the tropospheric background flow, such as wind profile, is examined. For this purpose a variety of spectral analysis techniques are used.In all the simulations, the domain spans the entire troposphere and the lower stratosphere, from the ground up to 42km in the vertical, well above the QBO shear regions. For each of the convective events, three stratospheric background zonal wind cases are examined. In the first case the background wind profile is constant, the other two are representative of the easterly and westerly phases of the quasi-biennial oscillation (QBO).In the QBO-wind-shear cases, upward propagating gravity waves are damped as they approach their critical layer. The signature of critical-layer absorption is clearly visible in the profiles of vertical momentum-flux divergence. In the simulations with open boundary conditions, the response to vertical momentum-flux divergence takes the form of large dynamic pressure differences between the east and west boundaries together with accelerations in the mean zonal wind. To capture the mean flow accelerations that occur in response to vertical momentum-flux divergence in a horizontally periodic domain such as the earths atmosphere, the simulations were repeated in a domain with periodic lateral boundaries. In these simulations, the mean flow acceleration is almost entirely balanced by gravity-wave momentum-flux divergence while all other terms are virtually null.Quantitative analysis of the simulated stratospheric response to gravity-wave momentum-flux divergence is used, together with statistics of mesoscale convective systems, to estimate the average forcing caused by convectively generated gravity waves in the lower stratosphere and their role in the dynamics of the QBO.
- Atmospheric sciences