Evaluating and understanding the role of convective processes in general circulation model simulations of the Madden-Julian oscillation

Weak temporal variability in tropical climate such as the Madden-Julian Oscillation (MJO) is one of the outstanding deficiencies in numerical weather prediction (NWP) models and coupled general circulation models (GCMs), which have beleaguered the model developer and user communities for years. The representation of convection, cloud and radiation processes has long been recognized as one of the major problems responsible for these deficiencies in climate models. Recently, with the improvement made to the convection scheme, the Iowa State University (ISU) GCM, which is based on the NCAR Community Climate Model version 3 (CCM3), is able to simulate many features of the MJO as revealed by the observational studies. It provides a unique opportunity to investigating the mechanisms and physical processes through which convection affects the MJO. In this study, four 10-year (1979-88) simulations by the ISUGCM with observed sea surface temperatures (SSTs) are used for the analysis. The control simulation (CTL) is conducted with the standard NCAR CCM3. The ISUGCM simulation with the revised closure, convection trigger condition and convective momentum transport (CMT) is labeled by ISUCCM3. Two sensitivity simulations, NOCMT and NOTRI, are performed to investigate the impact of convection trigger condition and CMT on the MJO.;The modifications made in convection schemes improve the simulations of MJO in the amplitude, spatial distribution, eastward propagation, and horizontal and vertical structures, especially for the coherent feature of the MJO-related eastward propagating convection and the precursor sign of the convective center. The revised convection closure plays a key role v in the improvement of eastward propagation of MJO. The convection trigger helps produce less frequent but more vigorous moist convection and enhance the amplitude of the MJO signal. The inclusion of CMT results in more coherent structure for the MJO-related deep convective center and its corresponding atmospheric variances. The kinetic energy budget is conducted to analyze the physical processes responsible for the improved MJO simulated by ISUGCM with the modified convection scheme. The increased equatorial (10°S-10°N) MJO-related perturbation kinetic energy (PKE) is presented in the upper troposphere due to the three modifications, and leads to more robust and coherent eastward propagating MJO signal. In the MJO source region-the Indian Ocean (45°E-120°E), the upper-tropospheric MJO PKE is maintained by the convergence of vertical wave energy flux and the barotropic conversion through the horizontal shear of mean flow. In the convectively active region-the western Pacific (120°E-180°), the upper-tropospheric MJO PKE is supported by the convergence of horizontal and vertical wave energy fluxes. Over the central-eastern Pacific (180°-120°W), where convection is suppressed, the upper-tropospheric MJO PKE is mainly due to the convergence of horizontal wave energy flux. The deep convection trigger condition produces stronger convective heating which enhances the upward wave energy fluxes, and leads to the increased MJO PKE over the Indian Ocean and western Pacific. The revised convection closure affects the response of mean zonal wind shear to the convective heating over the Indian Ocean and leads to the enhanced upper-tropospheric MJO PKE through the barotropic conversion. The stronger eastward wave energy flux due to the increase of convective heating over the Indian Ocean and western Pacific by the revised closure is favorable to the eastward propagation of MJO and the convergence of horizontal wave energy flux over the central-eastern pacific. The convection-induced momentum tendency tends to decelerate the upper-tropospheric wind which results in a negative work to the PKE budget in the upper troposphere. However, the convection momentum tendency accelerates the westerly wind below 850 hPa over the western Pacific, which is partially responsible for the improved MJO simulation.;With the composite analyses of MJO events, the different phase relationships between MJO 850-hPa zonal wind, precipitation and surface latent heat flux are simulated over the Indian Ocean and western Pacific in ISUGCM, which is greatly influenced by the convection closure, trigger and CMT. With these modifications in the convection scheme, ISUGCM produces better MJO recharge-discharge process of moist static energy than the original GCM. The convection trigger condition for deep convection contributes to the striking difference between ISUCCM3 and CTL and plays the major role for this improved MJO simulation through the horizontal and vertical advections of moist static energy. The inclusion of the revised closure helps build up the precondition of the MJO convection with the redistribution of moisture through the positive contributions of the horizontal and vertical advection of moist static energy before the onset of MJO convection. The impact of CMT through the interaction between horizontal and vertical advection of moist static energy helps the more coherent atmospheric structure over the Indian Ocean and western Pacific. The budget analysis for ISUGCM with the modifications shows the increase of moist static energy is in phase with the horizontal advection of moist static energy over the western Pacific, but in phase with the vertical advection of moist static energy over the Indian Ocean.