GCMs have traditionally experienced some difficulty in reproducing the correct characteristics of large-scale tropical intra-seasonal wave activity (e.g. Slingo et al, 1996). Of the models that actually have any kind of coherent eastward propagating signal, most tend to exhibit weak power at too high a frequency when compared to observations.

Speculation as to the reasons for this under-performance in models is wide ranging, but the role of convection is central to many theories. Convectively generated free-troposphere moisture variability has been suggested as a deficiency which may may lead to poor MJO-type variability (Grabowski, 2003), and may be a consequence of the weak entrainment formulation of mass-flux convection schemes.

Using an alternative novel approach to the mass-flux convective formulation results from a 4-layer tropical model are analyzed to investigate intra-seasonal wave behavior. The formulation of the scheme involves the concept of successive, highly entraining convective plumes, whereby a particular plume is able to exploit the moisture signal of a previous plume. Such a framework more closely resembles observed convective cloud anatomy and exhibits a higher degree of sensitivity to tropospheric moisture than traditional weakly entraining plume schemes. Results of the general model behavior are presented and the potential for realistic intra-seasonal wave activity explored.