Gravity waves in shear and implications for organized convection

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 in terms of 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. First, the model here provides predictions for the following question: In which direction will a CCW propagate in a given background wind shear? The propagation direction is determined by which side of preexisting convection is more favorable for new convection, and the model here provides predictions of this for different background wind shears. Second, it is known that upstream-propagating gravity waves can excite new convective cells within an individual MCS. The model here demonstrates some of the mechanisms behind this phenomenon, including the effects of background wind shear and the MCS's propagation speed.

These effects of background wind on gravity waves are captured in a two-dimensional model that is further simplified by including only the first two vertical baroclinic modes. Numerical results are shown with a nonlinear model, and linear theory results are in good agreement with the nonlinear model for most cases.