JOINT DISTRIBUTIONS OF POTENTIAL VORTICITY AND INERT TRACE CONSTITUENT IN CCM2 AND UW THETA-SIGMA MODEL SIMULATIONS

This letter describes an experiment that examines the ability of the U niversity of Wisconsin (UW) hybrid isentropic-sigma (theta-sigma) and sigma (sigma) coordinate models and the NCAR Community Climate Model 2 (CCM2) to transport and conserve the joint distributions of isentropi c potential vorticity (P-theta) and a source-free inert trace constitu ent related to the initial distribution of P-theta, called proxy ozone (O-3), during 10-day isentropic integrations. Under the idealized con ditions of this experiment, the governing equations for the atmospheri c continuum require that the initial joint distribution of P-theta and O-3 be conserved, thereby establishing a test of model accuracy from statistical comparisons of paired values of O-3 and P-theta. Any decre ase in the initial correlation of unity of P-theta and O-3 after integ ration is an objective measure of a model's skill. Results show that c orrelation coefficients for the UW theta-sigma model remain higher tha n those from CCM2 and the UW sigma model, demonstrating an inherent ad vantage in the simulation of trace constituent transport relative to d ynamical processes in isentropic versus sigma coordinate models.