Banded Convective Activity and Ducted Gravity Waves

Abstract

Convective activity is frequently organized into band-like structures with space and time scales appropriate to internal gravity waves. When the convective activity involves cumulonimbus, then latent heat release can form a significant energy source for the waves which in turn may organize the convection [as described, for example, by wave-CISK (Lindzen, 1974; Raymond, 1975)]. However, in other cases strong forcing is absent and the existence of the waves requires the existence of a duct from which very little wave energy leaks. We show that the energy cannot be contained by an inversion. Instead, we find that a stable duct adjacent to the surface must be capped by an unstable layer wherein the mean flow at some level either equals or comes close to the phase speed of the ducted waves. We also find that the wind amplitudes associated with the observed pressure amplitudes in these waves are consistent with observed squall winds. Finally, we find that the horizontal scales of mesoscale waves are closely related to the time scales of convective elements.