Simulation Of Tropical Atmospheric Intraseasonal Oscillation And Sensitivity Study Of Parameterization Scheme

Madden-Julian Oscillation(MJO)is the most dominant mode of Intraseasonal Oscillation(ISO)and is one of the major systems affecting the tropics.Its time scale is between the synoptic and the climate time scales.Thus,MJO interacts with many weather systems around the world.The successful prediction of MJO initiation and propagation can improve the weather,intraseasonal,and seasonal scale forecasts.Although many models can represent the atmospheric oscillation signals in the intraseasonal time scale,there is much room for improvement in the simulation of the period,amplitude,and propagation speed of the MJO.The mean state and planetary boundary layer(PBL)water vapor transport are the key factors affecting MJO simulation and prediction.MJO simulation relies on the cumulus convective parameterization scheme and the PBL scheme.A series of sensitivity experiments were designed by constructing the numerical model to reveal the key processes affecting the eastward propagation of MJO precipitation events.This study uses Coupled-Ocean-Atmosphere-Wave-Sediment Transport Modeling System(COAWST)to construct a tropical channel model for the first time to simulate MJO processes during the Dynamics of the Madden-Julian Oscillation(DYNAMO)observation campaign.This model only has the northern and southern boundaries and covers the entire equatorial region.Three cumulus convective parameterization schemes and three PBL schemes are cross-combined to run eight 30-day experiments.These experiments show the influence of the selection of cumulus convective parameterization schemes and PBL schemes.This study uses the fifth-generation European Centre for Medium-Range Weather Forecasts(ECMWF)atmospheric reanalysis of the global climate(ERA5),Tropical Rainfall Measuring Mission(TRMM)satellite precipitation data,and Atmospheric Infrared Sounder(AIRS)version 7leg-3 cloud cover product to validate model results.Comparison with ERA5 reanalysis data,TRMM precipitation data,AIRS cloud cover data,and model simulations show mean state and MJO precipitation biases.These biases are from different parameterization schemes.Thus,this study investigates the influence of the cumulus convection and PBL parameterization schemes on the MJO simulation and their physical mechanism.(1)MJO convective activities are regulated by cumulus convective parameterization over the Indian Ocean,not by the PBL scheme.Tiedtke(TDK)cumulus convective parameterization scheme and the University of Washington(UW)PBL scheme are much better for simulating the mean state of the atmosphere and ocean,and MJO eastward precipitation events can be well simulated.(2)Mean state in the KF experiment shows that the warm SST bias in the equatorial Western Indian Ocean(WIO)changes the zonal SST gradient,leading to the easterly wind bias in the lower atmosphere.The change in atmospheric circulation makes water vapor transported to the WIO and increases precipitation in the WIO.It is not conducive to the initiation and propagation of MJO precipitation signals.(3)The results of the cloud radiative heating show that high clouds in the tropics are well-simulated in the experiment with TDK and UW schemes,inducing the MJO convective activity.Tropical high clouds can warm the upper troposphere through the cloud-radiation feedback effect,which provides a heating source for the convection system.In this study,we also performed three sensitivity experiments with three different PBL schemes and the same TDK cumulus convection parameterization scheme to study the influence of planetary boundary layer schemes on the eastward propagation of MJO precipitation events in the Maritime Continent(MC).The results are as follows :(1)The MSE-budget analysis shows that the horizontal water vapor advection and vertical transport of water vapor from the PBL to the free atmosphere are the main factors for the model to simulate the positive MSE tendency ahead of the MJO convective center.(2)The simulation of MJO precipitation events is highly sensitive to the PBL scheme over the MC.The model experiment using the UW PBL scheme represents the positive Moisture Static Energy(MSE)tendency east of the MJO convection center and simulates the MJO propagation over the maritime continent.(3)The comparison results show that the vertical cloud coverage on the north and south sides of the maritime continent increases the atmospheric temperature and humidity.A well-simulated cloud cover allows the right amount of shortwave radiation to warm the ocean,maintaining temperature gradients toward the equator in the upper ocean and atmospheric mean circulation field.