Convectively Coupled Waves Propagating along an Equatorial ITCZ

The dynamics of convectively coupled gravity waves traveling over a precipitating region is analyzed in an idealized model for the large scale atmospheric circulation. In particular, the model is composed of a shallow water system coupled to an advection equation for moisture through the convection term (utilizing a quasi-equilibrium relaxation to moisture closure).  Because convection acts to remove the convective instability, this model can be viewed as an extension of the strict quasi-equilibrium (SQE) theory for the tropical atmosphere.  In SQE theory, convection must be active everywhere in the atmosphere, whereas in this model it is possible to capture the dynamical effect of the waves when convection is inactive in some portion of the atmosphere.  

The  interface between active/inactive convection, referred here to as precipitation front, is characterized mathematically by a discontinuity in the precipitation rate and in the vertical velocity.  Assuming the limit of infinitely fast relaxation, the system decouples into an equation for the precipitating region and non-precipitating regions, where the solutions match at the interface.  

This framework is applied to study the behavior of a disturbance propagating along a narrow precipitation band, similar to the InterTropical Converegence Zone (ITCZ).  First, we obtain stationary solutions with convection is active in some limited portion of the domain ( the "ITCZ"). We then analyze the propagation of a small perturbation of this basic state, in effect, analyzing the effects of the width and location of the ITCZ on the propagation of  Kelvin, Yanai and Rossby waves.

We begin with a background state consisting of a stationary symmetric Hadley circulation and a precipitating region centered at the Equator.  In this problem, the width of the precipitating region is controlled by the saturation mixing ratio at the surface.  Next, we obtain the equations for a small perturbation  of the equilibrium state and derive the relationship  between the width of the ITCZ and the speed of propagation of the wave. These analytic solutions are then compared to numerical simulations.  We find that for an ITCZ width of the order of the equatorial Rossby radius, Kelvin waves propagate at the moist gravity wave speed (about 15 m/s), whereas for a narrow ITCZ, the propagation speed is comparable to the dry gravity wave (about 50 m/s). Similar results are found in the case of the ITCZ off the equator. Moreover, a meridional circulation, present even for Kelvin waves, plays an important role in redistributing energy between the dry and moist regions. At the surface and where the zonal wind converges, the meridional wind flows towards the ITCZ at the interface, it ascends inside the precipitating region and then flows towards the poles. We propose some interpretation of the mechanisms involved in this process.