Angular momentum budget in general circulation models of superrotating atmospheres: A critical diagnostic

International audienceTo help understand the large disparity in the results of circulation modeling for theatmospheres of Titan and Venus, where the whole atmosphere rotates faster than thesurface (superrotation), the atmospheric angular momentum budget is detailed for twoGeneral Circulation Models (GCMs). The LMD GCM is tested for both Venus (withsimplified and with more realistic physical forcings) and Titan (realistic physical forcings).The Community Atmosphere Model is tested for both Earth and Venus with simplifiedphysical forcings. These analyses demonstrate that errors related to atmospheric angularmomentum conservation are significant, especially for Venus when the physical forcingsare simplified. Unphysical residuals that have to be balanced by surface friction andmountain torques therefore affect the overall circulation. The presence of topographyincreases exchanges of angular momentum between surface and atmosphere, reducing theimpact of these numerical errors. The behavior of GCM dynamical cores with regard toangular momentum conservation under Venus conditions provides an explanation of whyrecent GCMs predict dissimilar results despite identical thermal forcing. The present studyillustrates the need for careful and detailed analysis of the angular momentum budget forany GCM used to simulate superrotating atmospheres