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K. MIYAKODA and L. UMSCHEID JR.

Abstract

The effect of an artificial lateral boundary (the wall) at the Equator on a simulated atmospheric circulation was studied numerically. By comparing the solutions of two 30-day integrations of a global model with and without the wall, we found that the discrepancies of the wind and temperature at the middle and high latitudes became appreciable at approximately 8 days and serious at approximately 12 days. This suggests that the wall (hemispheric) model may be applied as a forecast model for a maximum of about 12 days. The disagreement in the wind between the two cases starts just below the tropopause level at the Equator and spreads toward the higher latitudes. Eventually, the middle latitudes respond to this equatorial effect, and the disagreement is amplified to the natural variability level. Insertion of the wall considerably increases the condensation of water vapor in the Tropics for the winter hemisphere; the reverse is true for the summer hemisphere. The result is that, in the winter hemisphere, the tropical troposphere and the stratosphere are cooler and the higher latitude troposphere is warmer in the wall case than in the control case. The opposite is true for the summer hemisphere.

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C. T. Gordon, L. Umscheid Jr., and K. Miyakoda

Abstract

Numerical simulation experiments are performed with a 9-level global general circulation model to help determine how much wind data in the tropics are needed for the reconstruction of meteorological fields. Prediction runs are updated every 12 hr with hypothetical data generated from the same model.

It is found that the asymptotic root mean square (rms) wind errors in the tropics, particularly in the 11S-IIN “equatorial” latitude belt, fail to meet the GARP data requirements for the FGGE if surface pressure and temperature data alone are used for updating. The addition of tropical wind data at just two vertical levels leads to a significant, but insufficient, reduction of rrns wind errors within “tropics” (26S-26N); the largest errors remain near the equator. However, these errors become acceptably small if wind data are inserted at all 9 levels within the equatorial region. Another result is that insertion of tropical wind data at just two levels has a sizable influence upon wind errors even in the extratropics.

A critique of some implicit assumptions made in simulation experiments of the type we have performed is included.

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J. D. Mahlman, L. J. Umscheid, and J. P. Pinto

Abstract

The GFDL “SKYHI” general circulation model has been used to simulate the effect of the Antarctic “ozone hole” phenomenon on the radiative and dynamical environment of the lower stratosphere. Both the polar ozone destruction and photochemical restoration chemistries are calculated by parameterized simplifications of the still somewhat uncertain chemical processes.

The modeled total column ozone depletions are near 25% in spring over Antarctica, with 1% depletion reaching equatorial latitudes by the end of the 4½–year model experiment. In the lower stratosphere, ozone reductions of 5% reach to the equator. Large coolings of about 8 K are simulated in the lower stratosphere over Antarctica in late spring, while a general cooling of about 1–1.5 K is present throughout the Southern Hemisphere lower stratosphere. The model atmosphere experiences a long-term positive temperature-chemical feedback because significant ozone reductions carry over into the next winter.

The overall temperature response to the reduced ozone is essentially radiative in character. However, substantial dynamical changes are induced by the ozone hole effect. The Antarctic middle stratosphere in late spring warms by about 6 K over Antarctica and the lower midlatitude stratosphere warms by approximately 1 K. These warming spots are produced mainly by an increased residual circulation intensity. Also, the Antarctic vortex becomes tighter and more confined as a result of the reduced ozone. These two dynamical effects combine to steepen the meridional slope of quasi-conservative trace constituent isolines. Thus, the entire transport, radiative, and dynamical climatology of the springtime stratosphere is affected to an important degree by the ozone hole phenomenon. Over the entire year, however, these dynamical effects are considerably smaller.

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Kevin Hamilton, R. John Wilson, J. D. Mahlman, and L. J. Umscheid

Abstract

The long-term mean climatology obtained from integrations conducted with different resolutions of the GFDL “SKYHI” finite-difference general circulation model is examined. A number of improvements that have been made recently in the model are also described. The versions considered have 3° × 3.6°, 2° × 2.4°, and 1° × 1.2° latitude–longitude resolution, and in each case the model is run with 40 levels from the ground to 0.0096 mb. The integrations all employ a fixed climatological cycle of sea surface temperature. Over 25 years of integration with the 3° model and shorter integrations with the higher-resolution versions are analyzed. Attention is focused on the December–February and June–August periods.

The model does a reasonable job of representing the atmospheric flow in the troposphere and lower stratosphere. The simulated tropospheric climatology has an interesting sensitivity to horizontal resolution. In common with several spectral GCMs that have been examined earlier, the surface zonal-mean westerlies in the SKYHI extratropics become stronger with increasing horizontal resolution. However, this “zonalization” of the flow with resolution is not as prominent in the upper troposphere of SKYHI as it is in some spectral models. It is noteworthy that—without parameterized gravity wave drag—the SKYHI model at all three resolutions can simulate a realistic separation of the subtropical and polar night jet streams and a fairly realistic strength of the lower-stratospheric winter polar vortex.

The geographical distribution of the annual-mean and seasonal precipitation are reasonably well simulated. When compared against observations in an objective manner, the SKYHI global precipitation simulation is found to be as good or better than that obtained by other state-of-the-art general circulation models. However, some significant shortcomings remain, most notably in the summer extratropical land areas and in the tropical summer monsoon regions. The time-mean precipitation simulation is remarkably insensitive to the horizontal model resolution employed. The other tropospheric feature examined in detail is the tropopause temperature. The whole troposphere suffers from a cold bias of the order of a few degrees Celcius, but in the 3° SKYHI model this grows to about 6°C at 100 mb. Interestingly, the upper-tropospheric bias is reduced with increasing horizontal resolution, despite that the cloud parameters in the radiation code are specified identically in each version.

The simulated polar vortex in the Northern Hemisphere winter in the upper stratosphere is unrealistically confined to high latitudes, although the maximum zonal-mean zonal wind is close to observed values. Near the stratopause the June–August mean temperatures at the South Pole are colder than observations by ∼65°C, 50°C, and 30°C in the 3°, 2°, and 1° simulations, respectively. The corresponding zonal-mean zonal wind patterns display an unrealistically strong polar vortex. The extratropical stratosphere stationary wave field in the Northern Hemisphere winter is examined in some detail using the multiyear averages available from the 3° SKYHI integration. Comparison with comparable long-term mean observations suggests that the model captures the amplitude and phase of the stationary waves rather well.

The SKYHI model simulates the reversed equator-pole temperature gradient near the summer mesopause. The simulated summer polar mesopause temperatures decrease with increasing, horizontal resolution, although even at 1° resolution the predicted temperatures are still warmer than observed. The increasing resolution is accompanied by increased westerly driving of the mean flow in the summer mesosphere 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 structure.

The semiannual oscillation near the tropical stratopause is reasonably well simulated in the 3° version. The main deficiency is in the westerly phase, which is not as strong as observed. There is also a second peak in the amplitude of the semiannual wind oscillation at the top model level (0.0096 mb) corresponding to the observed mesopause semiannual oscillation. This simulated mesopause oscillation is weaker (by a factor of ∼3) than that observed. The simulation in the tropical stratopause and mesosphere changes quite significantly with increasing resolution, however. In the tropical lower stratosphere of the 3° model the zonal-mean zonal wind displays a very weak (∼3 m s−1 peak to peak) interannual variation, which-while rather irregular-does display a roughly biennial period and the downward phase propagation that is characteristic of the observed quasi-biennial oscillation.

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K. Miyakoda, L. Umscheid, D. H. Lee, J. Sirutis, R. Lusen, and F. Pratte

Abstract

Global upper air and surface data for the entire GATE period from 15 June to 24 September 1974, were collected by the Data Assimilation Branch of NMC and mailed to GFDL. After processing these data, a four-dimensional analysis technique was applied for the entire GATE period, using a global numerical model. For a selected period, several different versions of the data processing scheme were tested. The resulting analyses were compared with each other and with the objective analysis of NMC in Washington D.C., and ANMRC in Melbourne. Overall, the analyses for the extratropics were satisfactory for the Northern Hemisphere, and to a lesser extent, for the Southern Hemisphere, though flow patterns are somewhat excessively smoothed. The analyses for the tropics were not of the same quality as those for the extratropics, and yet they were much improved compared with those of several years ago. A noteworthy point is that tropical cyclones were successfully represented in several cases.

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E. Kalnay, M. Kanamitsu, J. Pfaendtner, J. Sela, M. Suarez, J. Stackpole, J. Tuccillo, L. Umscheid, and D. Williamson
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