Biases in tropical precipitation such as the double Intertropical Convergence Zone (ITCZ) are a long-standing problem in many global climate and Earth system models, including DOE’s Exascale Energy Earth System Model (E3SM). The E3SM has large positive precipitation biases across the tropical Pacific south of the equator and an excessive equatorial cold tongue compared to observations. In this project, the team aims to improve the simulation of tropical precipitation and associated sea surface temperature (SST) in E3SM by addressing the following questions: (1) How and to what extent does convective parameterization contribute to tropical precipitation biases? (2) How do coupled atmosphere-ocean feedback processes affect the double ITCZ developed in E3SM? (3) How can tropical precipitation and SST simulations in E3SM be improved?

The team will perform multi-year-long AMIP-style simulations with and without nudging the model dynamic and thermodynamic states toward the ECMWF reanalysis product. They will use both the default version and a revised version of the Zhang-McFarlane (ZM) scheme in E3SM for model integration. The revised ZM scheme has demonstrated its ability to reduce the double ITCZ bias in other models. In addition, they will change the convection trigger function, updraft model, and closure in the revised ZM scheme one at a time and run them concurrently for diagnostic purposes only. The differences of the nudging tendencies, precipitation and other fields between simulations with and without nudging will be analyzed to determine the effect of feedbacks on model simulation. 

One of the reasons tropical precipitation biases, including double ITCZ, are difficult to get rid of is that they involve coupled atmosphere-ocean feedbacks. To identify and understand the atmosphere-ocean feedback processes that contribute to double ITCZ development, they will carry out atmosphere-ocean coupled simulations and analyze the feedbacks in both the atmosphere and the ocean. They will also analyze the ocean mixed layer heat budget in the tropics and examine how coupling between the atmosphere and the ocean affects the feedbacks in the atmosphere among convection, atmospheric circulation, clouds and atmospheric radiation. This will help pave the way for improving the tropical precipitation simulation in E3SM. 

Based on the above simulations and analyses, the ZM scheme will be modified accordingly. For each simulation with the modifications, it will be determined why and how a particular modification leads to changes of the simulated ITCZ. They will also utilize the Cloud-Associated Parameterizations Testbed (CAPT) simulation tool and run ensemble hindcasts for the coupled model. These hindcasts will be analyzed to understand how the biases emerge and grow in the initial stage of the double ITCZ development. These results will then be used to further refine the revised ZM scheme for use in E3SM.

This project will yield new insights into how convective parameterization and coupled atmosphere-ocean feedbacks contribute to double ITCZ development in E3SM. It will lead to an improved simulation of tropical precipitation, and thus will have a significant impact on E3SM model development. The knowledge gained from this work will also likely benefit a boarder group of General Circulation Models that suffer from the double ITCZ bias.