Abstract

Gravity waves in shear and implications for organized convection

Sam Stechmann
University of California, Los Angeles (UCLA)

It is known that gravity waves in the troposphere can favor or suppress the formation of new convection. Here it is shown that, in the presence of wind shear or barotropic wind, the gravity waves can create a more favorable environment on one side of preexisting convection than the other side. Implications of this are discussed for two types of organized
convection: (i) convectively coupled waves (CCW) as envelopes of multiple mesoscale convective systems (MCS), and (ii) individual MCS as envelopes of multiple convective cells. For (i), the model here provides predictions for the following question: In which direction will a CCW
propagate in a given background wind shear? For (ii), the model demonstrates some of the mechanisms, including resonance, behind the upstream-propagating gravity waves that can excite new convective cells within an individual MCS. The model for these effects is derived by projecting the three-dimensional Boussinesq equations onto the first two vertical baroclinic modes, including projections of the nonlinear advection terms. The resulting equations are therefore nonlinear, and they provide a simplified setting for understanding interactions between gravity waves and shear. Numerical results are shown with the nonlinear model, and linear theory results are in good agreement with the nonlinear model for most cases.