The Rationale for Why Climate Models Should Adequately Resolve the Mesoscale

Summary

A review of the importance of the cyclone-frontal scale system in climate variability and the ability of present climate models to simulate them has been presented.

The analysis of three different Climate models, GISS, the NCAR community climate model CCM3, and the GFDL Finite volume AM2 (M90), have been discussed. The intention here was not to determine which one is better but rather to indicate what deficiency may be common to all of them. Evidence shows that the three models tend to be deficient in the generation of cyclone wave activity with the consequences that heat, momentum, and moisture may be deficient in the extratropical and subpolar regions. This will affect cloudiness, wind stress, and precipitation. Bauer and Del Genio (2005) have shown that the deficiency of moisture and cloudiness over the subpolar regions was due to the lack of cyclone waves to transport moisture and clouds to these regions. A discussion of complementary work done on clustering of cyclone trajectories by Gaffney et al. (2005) was also presented. Consistent with the present analysis, this study also showed that differences in trajectories between reanalysis and model simulation for each cluster of trajectories were here interpreted to be related to the lack of intense high frequency eddies of the GCM.

The previous two studies depend on the surface characteristics based on trajectories of the high frequency eddies. The present analysis on the GFDL-GCM is totally eulerian and based on the upper level eddy activities (300mb). However, a similar conclusion has been drawn from the analysis of the band pass frequency of energy and momentum for the GFDL AM2_M90 17 year runs, where it is quite clear that the momentum and energy of the very high frequency is much lower in the model simulation than in the reanalysis. The variance of meridional velocity also shows that the deficiency of the high frequency is in the latitude area where the reanalysis shows it to be positioned in the storm track: the model displaces it south of that. There is also a suggestion that to achieve the correct intensity of the high frequency baroclinic eddies, models should have enough resolution to resolve them, since this intensity depends on the lower level circulation of the frontal circulation system. The mesoscale circulation associated with cyclones could be adequately represented in models with resolution equal or superior to 1/4° resolution. It is clear that to adequately resolve the mesoscale, it is necessary to not only improve the resolution but also to improve the boundary layer and surface fluxes. Clearly, at the present low resolution of climate models, this improvement is probably unattainable. However, if the cloudiness and sea ice over the subpolar regions are important to the overall climate, this should be an attainable goal because no sophistication in the moist convection or sea-ice model could correct those deficiencies due to the unresolved dynamics.