Eliminating Double ITCZ in the NCAR CESM1 through Improvement of Convection Parameterization

Description

Several improvements in convective parameterization based on analysis of ARM observations are implemented into the NCAR CESM1 to investigate the roles of convection parameterization in the simulation of tropical precipitation. It is shown that the double intertropical convergence zone (ITCZ) biases of precipitation, sea surface temperature (SST) and wind stress in the model are eliminated in all seasons with the use of these improvements in the Zhang-McFarlane (ZM) convection scheme. Further analysis shows that the elimination of the double ITCZ bias is achieved not by reducing other potential bias contributors, such as stratus cloud bias off the west coast of South America and cloud/radiation biases over the Southern Ocean, but through a chain of interactions in the tropics associated with convection itself. This work demonstrates that the source of the double ITCZ problem is in the tropics and convection scheme is the primary contributor to the double ITCZ bias in the CESM1. The atmospheric model simulations forced by observed SST show that the original ZM convection scheme tends to produce double ITCZ bias in high SST scenario, while the improved scheme does not. The impact of changes in each core component of the convection scheme on the double ITCZ bias in atmospheric model is identified and further investigated.