GRAVITY-WAVES GENERATED BY A TROPICAL CYCLONE DURING THE STEP TROPICAL FIELD PROGRAM - A CASE-STUDY

Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere-Troposphere Exchange Project (ST EP) show the presence of two mesoscale phenomena: a vertically propaga ting gravity wave with a horizontal wavelength of about 110 km and a f eature with a horizontal scale comparable to that of the cyclone's ent ire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. Th e 110-km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is compar able to that of 100-to 150-km-diameter cells of low satellite brightne ss temperatures within the overall cyclone cloud shield: these cells h ave lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil sh ow that these cells correspond to regions of enhanced convection, high er cloud altitude, and upwardly displaced potential temperature surfac es. A three-dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variable s associated with the 110-km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400- 600 m, a lifetime of 4.5-6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity wave s is definitely important to the westerly phase of the stratopause sem iannual oscillation and possibly important to the easterly phase of th e quasi-biennial oscillation. There is strong evidence that some of th e mesoscale gravity waves break below 20 km as well. The effect of thi s wave breaking on the diabatic circulation below 20 km may be compara ble to that of above-cloud diabatic cooling.