Sensitivity of the inertia- gravity waves generation to the degree of baroclinicty

The generation of inertia–gravity waves (IGWs) in the idealized simulation of vorticalflows is investigated using the isentropic two-layer, primitive-equation model on thesphere. The contour-advective semi-Lagrangian (CASL) algorithm is used to solve theprimitive equations in potential vorticity, velocity divergence, and accelerationdivergence representation. The CASL algorithm consists of both Lagrangian and Eulerianparts. The Lagrangian part addresses the potential vorticity equation, which is solved bycontour advection. The Eulerian part addresses the remainder of the model including theprognostic and diagnostic equations for the grid-based variables of velocity divergenceacceleration divergence and the depth. The Eulerian part is solved by the spectraltransform in longitude, the forth-order compact differencing in latitude, and a three-timelevelsemi-implicit scheme in time. The power of CASL rests in its ability to represent thefine-scale structures in potential vorticity. Therefore, using CASL, it is possible todetermine more precisely the generation and propagation of the IGWs generated byvortical flows.The initial state of the numerical experiments is comprised of a balanced, zonal jetwith a very small perturbation added to trigger instability. With regard to the balancedinitial conditions used, the IGWs are generated mainly through spontaneous-adjustmentemission. To determine the sensitivity of the IGWs generated to the degree ofbaroclinicty, four experiments were carried out in which the upper-layer potentialtemperature was set to 310, 315, 320, and 325K from the first to the fourth experiment,respectively. The lower-layer potential temperature was set to 280K in all of theexperiments. As a result of increasing the upper-layer potential temperature, the staticstability increased and thus the baroclinicity decreased from the first to the fourthexperiment. To identify the IGWs accurately, the Bolin–Charney potential vorticityinversion was used to decompose the flow into a balanced part representing vortical flowand an unbalanced part representing free IGWs.