A Numerical Study of Three-Dimensional Gravity Waves Triggered by Deep Tropical Convection and Their Role in the Dynamics of the QBO

Abstract

A 3D mesoscale model is used to study the structure of convectively triggered gravity waves in the Tropics and their role in the dynamics of the middle atmosphere. Simulations with 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). Spectral analysis is used to link the structure of the triggered gravity waves to the dominant vertical wavelength of the latent heating within the convection. 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 earth?s 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.