Gravity waves in shear and implications for organized convection

Convectively generated gravity waves have been examined in many studies, and they have been identified for their role in suppressing or favoring the formation of new convection nearby pre-existing convection. Most studies have focused on cases with a motionless background state, but the effects of a vertical wind shear might be important for cases of anisotropic, organized convection such as convectively coupled waves.

Here the nonlinear effects of gravity waves interacting with a background wind shear are considered in a simplified model. The model captures the first two baroclinic modes and interactions between them through nonlinear advection terms.

For a background shear similar to the westerly wind burst phase of the Madden-Julian oscillation (MJO), it is shown that there is an asymmetry between eastward- and westward-propagating waves. Due to this asymmetry, the westward-propagating waves create an environment that is more favorable for the formation of new convection than the environment created by the eastward-propagating waves. This is in qualitative agreement with observations from TOGA-COARE that show new convection forming preferentially to the west of pre-existing convection in that phase of the MJO. Detailed aspects of the interaction between the gravity waves and shear are also discussed.