A simple dynamical model with features of convective momentum transport

Convective momentum transport (CMT) plays a central role in interactions across multiple space and time scales. However, due to the multiscale nature of CMT, quantifying and parameterizing its effects is often a challenge. Here a simple dynamic model with features of CMT is systematically derived and studied. The model includes interactions between a large scale zonal mean flow and convectively coupled gravity waves, and convection is parameterized using a multicloud model.

The moist convective wave--mean flow interactions shown here have several interesting features that distinguish them from other classical wave--mean flow settings. First an intraseasonal oscillation of the mean flow and convectively coupled waves (CCWs) is described. The mean flow oscillates due to both upscale and downscale CMT, and the CCWs weaken, change their propagation direction, and strengthen as the mean flow oscillates. The basic mechanisms of this oscillation are corroborated by linear stability theory with different mean flow background states.

Another case is set up to imitate the westerly wind burst phase of the Madden--Julian Oscillation (MJO) in the simplified dynamic model. In this case, CMT first accelerates the zonal jet with strongest westerly wind aloft, and then there is deceleration of the winds due to CMT; this occurs on an intraseasonal time scale and is in qualitative agreement with actual observations of the MJO. Also, in this case, a multiscale envelope of convection propagates westward with smaller scale convection propagating eastward within the envelope. The simplified dynamic model is able to produce this variety of behavior even though it has only a single horizontal direction and no Coriolis effect.