CLIMATOLOGY OF THE SKYHI TROPOSPHERE-STRATOSPHERE-MESOSPHERE GENERAL-CIRCULATION MODEL

The long-term mean climatology obtained from integrations conducted wi th different resolutions of the GFDL ''SKYHI'' finite-difference gener al circulation model is examined. A number of improvements that have b een made recently in the model are also described. The versions consid ered have 3-degrees x 3.6-degrees, 2-degrees x 2.4-degrees, and 1-degr ees x 1.2-degrees latitude - longitude resolution, and in each case th e model is run with 40 levels from the ground to 0.0096 mb. The integr ations all employ a fixed climatological cycle of sea surface temperat ure. Over 25 years of integration with the 3-degrees model and shorter integrations with the higher-resolution versions are analyzed. Attent ion is focused on the December-February and June-August periods. The m odel does a reasonable job of representing the atmospheric flow in the troposphere and lower stratosphere. The simulated tropospheric climat ology has an interesting sensitivity to horizontal resolution. In comm on with several spectral GCMs that have been examined earlier, the sur face zonal-mean westerlies in the SKYHI extratropics become stronger w ith increasing horizontal resolution. However, this ''zonalization'' o f the flow with resolution is not as prominent in the upper tropospher e of SKYHI as it is in some spectral models. It is noteworthy that-wit hout parameterized gravity wave drag-the SKYHI model at all three reso lutions can simulate a realistic separation of the subtropical and pol ar night jet streams and a fairly realistic strength of the lower-stra tospheric winter polar vortex. The geographical distribution of the an nual-mean and seasonal precipitation are reasonably well simulated. Wh en compared against observations in an objective manner, the SKYHI glo bal precipitation simulation is found to be as good or better than tha t obtained by other state-of-the-art general circulation models. Howev er, some significant shortcomings remain, most notably in the summer e xtratropical land areas and in the tropical summer monsoon regions. Th e time-mean precipitation simulation is remarkably insensitive to the horizontal model resolution employed. The other tropospheric feature e xamined in detail is the tropopause temperature. The whole troposphere suffers from a cold bias of the order of a few degrees Celcius, but i n the 3-degrees SKYHI model this grows to about 6-degrees-C at 100 mb. Interestingly, the upper-tropospheric bias is reduced with increasing horizontal resolution, despite that the cloud parameters in the radia tion code are specified identically in each version. The simulated pol ar vortex in the Northern Hemisphere winter in the upper stratosphere is unrealistically confined to high latitudes, although the maximum zo nal-mean zonal wind is close to observed values. Near the stratopause the June-August mean temperatures at the South Pole are colder than ob servations by approximately 65-degrees-C, 50-degrees-C, and 30-degrees -C in the 3-degrees, 2-degrees, and 1-degree simulations, respectively . The corresponding zonal-mean zonal wind patterns display an unrealis tically strong polar vortex. The extratropical stratospheric stationar y wave field in the Northern Hemisphere winter is examined in some det ail using the multiyear averages available from the 3-degrees SKYHI in tegration. Comparison with comparable long-term mean observations sugg ests that the model captures the amplitude and phase of the stationary waves rather well. The SKYHI model simulates the reversed equator-pol e temperature gradient near the summer mesopause. The simulated summer polar mesopause temperatures decrease with increasing horizontal reso lution, although even at 1-degree resolution the predicted temperature s are still warmer than observed. The increasing resolution is accompa nied by increased westerly driving of the mean flow in the summer meso sphere by dissipating gravity waves. The present results suggest that the SKYHI model does explicitly resolve a significant component of the gravity waves required to produce the observed summer mesopause struc ture. The semiannual oscillation near the tropical stratopause is reas onably well simulated in the 3-degrees version. The main deficiency is in the westerly phase, which is not as strong as observed. There is a lso a second peak in the amplitude of the semiannual wind oscillation at the top model level (0.0096 mb) corresponding to the observed mesop ause semiannual oscillation. This simulated mesopause oscillation is w eaker (by a factor of approximately 3) than that observed. The simulat ion in the tropical stratopause and mesosphere changes quite significa ntly with increasing resolution, however. In the tropical lower strato sphere of the 3-degrees model the zonal-mean zonal wind displays a ver y weak (approximately 3 m s-1 peak to peak) interannual variation, whi ch-while rather irregular-does display a roughly biennial period and t he downward phase propagation that is characteristic of the observed q uasi-biennial oscillation.