|Title||Uncertainty quantification and parameter tuning in the CAM5 Zhang-McFarlane convection scheme and impact of improved convection on the global circulation and climate|
|Publication Type||Journal Article|
|Year of Publication||2013|
|Authors||Yang B, Qian Y, Lin G, Leung LR, Rasch PJ, Zhang GJ, McFarlane SA, Zhao C, Zhang YC, Wang HL, Wang MH, Liu XH|
|Journal||Journal of Geophysical Research-Atmospheres|
|Type of Article||Article|
|Keywords||community atmosphere model; cumulus cloud ensemble; energy; environment; large-scale; liquid water; microphysics scheme; microwave imager ssm/i; momentum transport; system ceres; trmm precipitation radar; water-vapor|
In this study, we applied an uncertainty quantification (UQ) technique to improve convective precipitation in the global climate model, the Community Atmosphere Model version 5 (CAM5), in which the convective and stratiform precipitation partitioning is very different from observational estimates. We examined the sensitivity of precipitation and circulation to several key parameters in the Zhang-McFarlane deep convection scheme in CAM5, using a stochastic importance-sampling algorithm that can progressively converge to optimal parameter values. The impact of improved deep convection on the global circulation and climate was subsequently evaluated. Our results show that the simulated convective precipitation is most sensitive to the parameters of the convective available potential energy consumption time scale, parcel fractional mass entrainment rate, and maximum downdraft mass flux fraction. Using the optimal parameters constrained by the observed Tropical Rainfall Measuring Mission, convective precipitation improves the simulation of convective to stratiform precipitation ratio and rain-rate spectrum remarkably. When convection is suppressed, precipitation tends to be more confined to the regions with strong atmospheric convergence. As the optimal parameters are used, positive impacts on some aspects of the atmospheric circulation and climate, including reduction of the double Intertropical Convergence Zone, improved East Asian monsoon precipitation, and improved annual cycles of the cross-equatorial jets, are found as a result of the vertical and horizontal redistribution of latent heat release from the revised parameterization. Positive impacts of the optimal parameters derived from the 2 degrees simulations are found to transfer to the 1 degrees simulations to some extent.