The dependence on convection parameterization of the tropical intraseasonal oscillation simulated by the UIUC 11-layer atmospheric GCM

Numerical experiments are conducted using the University of Illinois, Urbana-Champaign (UIUC), Ii-layer atmospheric general circulation model (GCM) to investigate the dependence of the simulated tropical intraseasonal oscillation (TIO) on convection parameterization. Three convection parameterizarions have been tested: 1) the UIUC GCM's original cumulus-convection parameterization, which includes a modified version of the Arakawa-Schubert penetrative-convection parameterization and a middle-level convection parameterization, 2) the parameterization of Kuo, and (3) the moist convective adjustment parameterization of Manabe et al. For each parameterization a relative humidity criterion (RH,) for convection or convective heating to occur is used, as in many GCMs. Perpetual-March simulations with these convection parameterizations have been performed for different values of RH,. It is found that the simulated TIO is highly dependent on RH,. As RH, increases, the oscillation in the simulations becomes stronger for ail three parameterizations. This dependence of the amplitude of the simulated oscillation on RH, appears to explain the differences in the TIO among previous simulations by different GCMs, The analysis of the simulations suggests that a certain degree of nonlinear dependence of the condensational heating on large-scale moisture convergence is required to give a reasonable simulation of the TIG. When large values of RH, are used, the triggering of convective activity requires the moist static energy in the lower troposphere to be accumulated to a certain amount through moisture convergence. This requirement of accumulation of the moist static energy to trigger convection leads to the weakening of the interaction between the circulation and the heating for perturbations of small amplitudes and small scales, and allows the initiation of the TIO to occur at lower frequencies. In the simulations that produce relatively strong intraseasonal oscillations, the frictional wave-CISK (conditional instability of the second kind) appears to contribute to the amplification of the TIG.