A Numerical Study of Stratospheric Gravity Waves Triggered by Squall Lines Observed during the TOGA COARE and COPT-81 Experiments

Abstract

A 3D mesoscale model is used to study the structure and intensity of stratospheric gravity waves generated by tropical convection. Two prototypical cases are examined: a squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE) over the Pacific warm pool and a West African squall line observed during the Convection Profonde Tropicale 1981 (COPT-81) experiment. Gravity waves generated by these two squall lines are compared with those generated by a previously investigated event over northern Australia. Although the individual squall lines vary in intensity, the stratospheric gravity waves generated by the various storms have surprisingly similar amplitudes. The similarity in the wave amplitudes arises because within each storm there is a positive correlation between the updraft intensity and the height of the level of neutral buoyancy. When the level of neutral buoyancy is relatively high, the atmospheric density in the region of wave generation is relatively low, and this reduction in density tends to weaken the convectively triggered waves thereby compensating for the stronger updraft velocities in the more intense storms. The sensitivity of the azimuthal distribution of the convectively generated gravity waves to the upper-tropospheric and lower-stratospheric wind profile is also examined. In the absence of critical-level absorption, the motion of the storm relative to the stratospheric winds appears to be the single most important factor determining the azimuthal distribution of the waves. Stationary storm-relative waves, similar to mountain waves, may also be generated when there is a strong storm-relative stratospheric wind. The interaction of the gravity waves with shear layers representative of the eastward and westward phases of the quasi-biennial oscillation (QBO) is also examined. An analysis of the domain-averaged momentum budget supports the claim that the drag exerted by critical-level absorption of convectively generated gravity waves plays a nontrivial role in the downward propagation of the QBO.