MJO and convectively coupled waves in a coarse resolution GCM with a simple multicloud parametrization

Despite recent research efforts and progress in computation power, current-operational atmospheric general circulation models (GCMs) still fail to represent adequately the dominant intra-seasonal and synoptic scale variability in the tropics, characterized by the so-called Madden-Julian oscillation (MJO) and convectively coupled waves. Here we use the next generation NCAR GCM--HOMME, which is based on a spectral element disctretization of the cubed sphere, coupled to a simple multicloud parametrization through a judicious choice of heating vertical profiles for the three cloud types, congestus, deep, and stratiform, that characterize organized tropical convection, based on the vertical structure functions of Kassahara and Puri. Numerical simulations were carried out on an all ocean planet (a.k.a aquaplanet) with convection restricted to the tropical band between 45 degrees south and 45 degrees north. The background temperature is from the GATE mission, as used by Grabowski and others. In this talk, we present two simulations for the HOMME-multicloud model with different background moisture profiles and stratiform heating fractions. The first simulation uses a moisture profile inferred from GATE data and a stratiform fraction that is half of that of deep convection. It yields trains of convectively coupled Kelvin and Rossby waves and embedded westward inertio-gravity waves with periods of two days that carry most of the deep-convective power. The second simulation uses a doubled background moisture profile and a smaller stratiform fraction and results in an intra-seasonal oscillation of zonal wavenumber two, moving eastward at a constant speed of roughly 5 m/s. Both the synoptic scale waves and the intra-seasonal oscillation have phase speeds and zonal and vertical structures that are in excellent qualitative agreement with those of the observed convectively coupled waves and the MJO, respectively. The MJO in particular has strong westerly winds trialing easterlies at the surface and has a baroclinic vertical structure while the off-equatorial flow is characterized by a quadrupole vortex. The active phase of the MJO has standing spikes of convection following the large scale propagating envelope, featuring the zero group velocity that characterizes the MJO. Congestus heating dominates the inactive phase on the off-equatorial flanks and as such it helps precondition and restore the moisture level after the passage of strong MJO event(s).