A numerical study of stratospheric gravity waves triggered by squall linesobserved during the TOGA COARE and COPT-81 experiments

A 3D mesoscale model is used to study the structure and intensity of strato spheric gravity waves generated by tropical convection. Two prototypical ca ses are examined: a squall line observed during the Tropical Ocean Global A tmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) over th e Pacific warm pool and a West African squall line observed during the Conv ection Profonde Tropicale 1981 (COPT-81) experiment. Gravity waves generate d by these two squall lines are compared with those generated by a previous ly investigated event over northern Australia. Although the individual squall lines vary in intensity, the stratospheric g ravity waves generated by the various storms have surprisingly similar ampl itudes. The similarity in the wave amplitudes arises because within each st orm there is a positive correlation between the updraft intensity and the h eight of the level of neutral buoyancy. When the level of neutral buoyancy is relatively high, the atmospheric density in the region of wave generatio n is relatively low, and this reduction in density tends to weaken the conv ectively triggered waves thereby compensating for the stronger updraft velo cities 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 profi le is also examined. In the absence of critical-level absorption, the motio n 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 ge nerated when there is a strong storm-relative stratospheric wind. The interaction of the gravity waves with shear layers representative of th e eastward and westward phases of the quasi-biennial oscillation (QBO) is a lso examined. An analysis of the domain-averaged momentum budget supports t he claim that the drag exerted by critical-level absorption of convectively generated gravity waves plays a nontrivial role in the downward propagatio n of the QBO.