A COMPARISON OF INERT TRACE CONSTITUENT TRANSPORT BETWEEN THE UNIVERSITY-OF-WISCONSIN ISENTROPIC-SIGMA MODEL AND THE NCAR COMMUNITY CLIMATEMODEL

Five- and 10-day inert trace constituent distributions prognostically simulated with the University of Wisconsin (UW) hybrid isentropic-sigm a (0-sigma) model, the nominally identical UW sigma (sigma) model, and the National Center for Atmospheric Research Community Climate Model 2 (CCM2) are analyzed and compared in this study. The UW theta-sigma a nd sigma gridpoint models utilize the flux form of the primitive equat ions, while CCM2 is based on the spectral representation and uses semi -Lagrangian transport (SLT) for trace constitutents. Results are also compared against a version of the CCM that uses spectral transport for the trace constituent. These comparisons 1) contrast the spatial and temporal evolution of the filamentary transport of inert trace constit utents simulated with the UW theta-sigma and sigma models against a '' state of the art'' GCM under both isentropic and nonisentropic conditi ons and 2) examine the ability of the models to conserve the initial t race constituent maximum value during 10-day integrations. Results sho w that the spatial distributions of trace constituent evolve in a simi lar manner, regardless of the transport scheme or model type. However, when compared to the UW theta-sigma model's ability to simulate filam entary structure and conserve the initial trace constituent maximum va lue, results from the other models in this study indicate substantial spurious dispersion. The more accurate conservation demonstrated with the UW theta-sigma model is especially noticeable within extratropical amplifying baroclinic waves, and it stems from the dominance of two-d imensional, quasi-horizontal isentropic exchange processes in a strati fied baroclinic atmosphere. This condition, which largely precludes sp urious numerical dispersion associated with vertical advection, is uni que to isentropic coordinates. conservation of trace constituent maxim a in sigma coordinates suffers from the complexity of, and inherent ne ed for, resolving three-dimensional transport in the presence of verti cal wind shear during baroclinic amplification, a condition leading to spurious vertical dispersion. The experiments of this study also indi cate that the shape-preserving SLT scheme used in CCM2 further reduces conservation of the initial maximum value when compared to the spectr al transport of trace constituents, although the patterns are more coh erent and the Gibbs phenomenon is eliminated.