14 February 2014

The Dependence of ITCZ Structure on Model Resolution and Dynamical Core in Aqua-Planet Simulations


Simulation of the Inter-tropical convergence zone (ITCZ), a belt of east-west-oriented high precipitation region over the tropics, in general circulation models is one of the most challenging aspects of modeling. Simulations of the ITCZ show two prevalent structures: a single ITCZ as observed that peaks to the north of the equator, or double ITCZs with peaks of precipitation on either side of the equator. Most climate models show a second precipitation belt over the southern hemisphere especially in the Pacific, showing a double peak structure when zonally averaged. Research led by U.S. Department of Energy scientists at Pacific Northwest National Laboratory examined the structure of the ITCZ in two sets of aqua-planet simulations produced by two different dynamical cores within the Community Atmosphere Model version 4 (CAM4): Model for Prediction Across Scales-Atmosphere (MPAS-A) and High-Order Method Modeling Environment (HOMME). The team found the structure of ITCZ is dependent on the feedbacks between convection and large-scale circulation. The dominance of anti-symmetric waves in the model is not enough to cause a double ITCZ, and the lateral extent of equatorial waves does not play an important role in determining the width of the ITCZ but rather the latter may influence the former. Despite similar model configurations, a change in one of the basic states, e.g. humidity, from differences in the dynamical cores alone can lead to differences in climate processes amplified by positive feedbacks that result in different structures of the salient circulation patterns.