Impacts Of Convective And Associated Cloud Radiative Heating On Intraseasonal Oscillation Simulation In A Global Climate Model

Intraseasonal oscillation over the tropics(i.e.,Madden Julian Oscillation,MJO)is an essential part of the tropical climate system,bridging weather and climate because of its special time scale.Thus,its accurate simulation and prediction are critically important.Observational studies suggest that the vertical structure of diabatic heating is important to MJO development.In particular,the absence of the top-heavy heating profile is believed to have been responsible for poor MJO simulations in global climate models.In this work,we investigate the role of the vertical heating profile in MJO simulation by modifying the convective heating profile to represent different vertical distributions(from top-heavy heating to bottom cooling)and thus mimicking the mesoscale convective heating in the National Center for Atmospheric Research Community Atmosphere Model version 5.3(NCAR CAM5.3).Results suggest that incorporating a mesoscale stratiform heating structure can dramatically improve the MJO simulation.By artificially adding stratiform-like heating and cooling in the experiments,many observed features of MJO are reproduced,including a clear eastward propagation,a westward-tilt vertical structure of MJO-scale anomalies of dynamic and thermodynamic fields and a strong 20-80-day spectral power.Further analysis shows an abundance of shallow convection ahead of MJO deep convection,confirming the role of shallow convection in preconditioning the atmosphere by moistening the lower troposphere ahead of deep convection during the MJO life cycle.Additional experiments show that low-level cooling contributes more to improving the MJO simulation.All these features are absent in the control simulation,indicating the important role of the mesoscale stratiform heating,especially its low-level cooling component,in the MJO simulation.Previous observational and modeling studies have suggested that moisture plays a dominant role in MJO evolution.This study also investigates contributions to moisture variation during different MJO phases using a realistic MJO simulation.Results of a column moist static energy(MSE)budget show that zonal advection dominates the horizontal structure contributing to the eastward propagation through low-level moisture preconditioning.Vertical advection acts to build up MSE for the development of deep convection while radiative heating and surface heat flux serve to discharge MSE.A further analysis combining moisture and temperature budget equations is performed to demonstrate the effects of vertical advection and cloud processes within the column.Vertical profiles of column confined moisture tendency show that large-scale vertical advection induced by latent heat released within shallow convective clouds is responsible for the lower tropospheric moistening at initiation.This confirms the important role of shallow convection in preconditioning.Radiative heating is vital across all MJO phases and its warming effects recharge the column humidity in mature phases.None of these features are reproduced by the standard CAM5.3,confirming that realistic moisture variation is important in an MJO simulation.Also,cloud radiative feedback during MJO has been investigated.Results suggest that radiation related processes work as an energy source term for invigorating deep convection.Meanwhile,the interactive longwave cooling feedback from low clouds is also critical in triggering organized deep convection systems.There is abundant shallow convective cloud at the pre-mature MJO stage.Its longwave radiative cooling yields a secondary circulation which helps to mantian the intensitiy of shallow convection.Afterwards,more energy can be accumulated in the boundary layer for the development of deep convection at the mature stages.All of these further confirm the critical role of shallow convection in low-level troposphere at MJO initiation.