NUMERICAL SIMULATIONS OF CONVECTIVELY GENERATED STRATOSPHERIC GRAVITY-WAVES

The excitation and vertical propagation of gravity waves is simulated in a two-dimensional model of a mesoscale convective storm. It is shown that in a simulated squall line the gravity waves that are preferentially excited are those propagating opposite to the direction of motion of the storm. Solutions for cases with differing stratospheric mean zonal flow profiles are compared. It tums out that, in the absence of storm-relative mean winds in the stratosphere, the primary mode of excitation of gravity waves is by mechanical forcing owing to oscillatory updrafts. The stratospheric response consists of waves whose periods match the primary periods of the forcing. Owing to the tendency of the oscillating updrafts to propagate toward the rear of the storm, gravity wave propagation is limited primarily to the rearward direction, and there is a net downward momentum transport. When storm-relative mean winds are included in the model the waves excited by the oscillating updrafts are weaker, but a new class of waves, similar to topographic waves, appears in the stratosphere directly above the main updraft region. The cloud model results are compared with results from a dry model in which waves are excited by a specified compact momentum source designed to mimic the mechanical forcing caused by the regular development and rearward propagation of updraft cells. Results from this analog strongly support the notion that squall-line-generated gravity waves arise from mechanical forcing rather than thermal effects.