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W. LAWRENCE GATES

Abstract

The global distributions of the mean January surface wind stress, the net diabatic heating rate, and the net rate of moisture addition as simulated in a 30-day integration with the two-level, Mintz-Arakawa atmospheric general circulation model are presented. The latitudinal distributions of the zonal averages of these forcing fields are shown to be in reasonable agreement with the available observations. The most prominent discrepancies are evidently due to the model's simulation of excessive convective precipitation (and the associated convective latent heating) in the Tropics, especially in the Northern (winter) Hemisphere. The zone of simulated tropical precipitation extends some 15° poleward of the observed position and results in a corresponding distortion of the field of evaporation-minus-precipitation (or moisture-addition rate).

In determining the monthly mean forcing fields, one must be particularly accurate in accumulating the (convective) precipitation rate during the integration; the customary use of 6-hourly fields results in a sampling error as large as 25 percent for even the zonally averaged rainfall. With the exception of a small sampling error in the mean rate of absorption of solar radiation, the components of the other forcing fields are satisfactorily determined by 6-hourly data.

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W. Lawrence Gates

Abstract

The global distribution of July climate has been simulated with a two-level atmospheric general circulation model using the surface boundary conditions of sea-surface temperature, ice-sheet topography and surface albedo assembled by CLIMAP for 18 000 years before present. These conditions respresent an approximate doubling of the ice-covered surface area of the earth, a 1°C decrease of the average sea-surface temperature, and an increase of the average surface albedo from 0.14 to 0.22. Compared with the simulation of present July conditions, the ice-age atmosphere is found to have been substantially cooler and drier, especially over the continents of the Northern Hemisphere, corresponding to an enhanced anticyclonic circulation over the major ice sheets and a general weakening of the summer monsoonal circulation. The midlatitude westerlies are strengthened and systematically displaced southward in the vicinity of the major ice sheets, along with an equatorward shift in the zones of maximum eddy activity.

On a global basis the July ice-age surface air temperature is 4.9°C lower than today's (5.8°C over the ice-free continents), while the global cloudiness and relative humidity show relatively small decreases. Ice-age precipitation in the Northern Hemisphere is about 20% below that simulated for today's July, with reduced convective rainfall over the continents accounting for most of the reduction. The intensity of the ice-age tropical Hadley circulation and the associated transports by the mean meridional circulation are reduced to about two-thirds of their present simulated values in response to reduced meridional gradients of net heating and moisture deficit. These results are in general agreement with those found from more simplified models and diagnostic calculations, and are verified at least in terms of the surface temperature by the available independent paleoclimatic data. Significant disagreement exists, however, with the temperature, pressure and circulation found in a previous study by Williams using the NCAR GCM with different ice-age boundary conditions, including in particular somewhat lower tropical sea-surface temperatures. The sensitivity of the simulated ice-age climate to such boundary condition changes needs further research, and more complete simulations are needed to establish the annual course of the ice-age climate.

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W. Lawrence Gates

Abstract

The mean global distributions of pressure, temperature, wind, moisture, cloudiness, precipitation, evaporation, and surface heat balance simulated for January by the two-level Mintz-Arakawa atmospheric general circulation model are compared with the corresponding observed fields. Although there are a number of shortcomings, in general the large-scale distribution of global climate is reasonably well portrayed by the model, in spite of its limited vertical resolution. The model simulates the semi-permanent cyclones and anticyclones of both the tropics and higher latitudes in approximately their correct positions, together with the associated large-scale temperature and circulation fields of the middle and lower troposphere. In comparison with models of greater resolution, these results suggest that with further selective improvements in the physical parameterizations, relatively coarse global models (of correspondingly lower computational demands) are useful tools in the study of many aspects of climate.

The most prominent errors of the present model simulations are in the portrayal of processes related to the transfer of moisture. The simulated cloudiness is about half that observed (in the Northern Hemisphere), and the average precipitation rate is about twice that observed and extends over too broad a zone in the tropics. The cloudiness error is evidently due to the model's production of clouds only during precipitation, and its failure to simulate nonprecipitating cloudiness at all. The precipitation error is due to an apparent simulation of excessive convective rainfall and has noticeably affected the heat and moisture balances in the tropics. Also in the tropics, the simulated January surface air temperature is higher than the specified sea-surface temperature, and has resulted in a net downward sensible heat flux at the surface between about 20N and 20S in contrast with observation. The evaporation, which occurs almost exclusively over the tropical oceans, is simulated to be about 50% too great. These errors evidently compensate each other in the net surface heat balance everywhere except at high southern latitudes, where low amounts of simulated cloudiness permit excessive surface insulation.

In the troposphere, the simulated zonally averaged January 400 mb temperature is approximately 5°C above that observed in the equatorial and tropical regions; at 800 mb the simulated temperature more closely resembles observation. The meridional gradients of geopotential height in mid-latitudes at both 400 and 800 mb are somewhat steeper than those observed, as is the meridional gradient of the 400 mb zonal mean temperature. The associated maximum zonal winds at 400 and 800 mb are about 60% stronger than the observed winds, at least in the Northern Hemisphere. At 800 mb the simulated relative humidity is approximately half that observed between about 30N and 20S, while at higher latitudes it exceeds observation by an average of about 15%.

These errors way be related to the model's tendency to simulate too great a strength for the quasi-stationary oceanic cyclones of middle and higher latitudes, and too small an intensity for the individual transient waves. The associated midlatitude Ferrel cell in the mean meridional circulation is therefore both too weak and too narrow. The subtropical oceanic anticyclones are more realistically simulated, and at least in the Northern (winter) Hemisphere, and the strength of the associated Hadley circulation resembles that derived from observation.

The simulation errors noted here furnish a guide for the continuing modification and improvement of the model, and new integrations over longer time periods with improved boundary conditions are in preparation. Such experiments, will permit the systematic determination of the characteristic or natural variability of the model's simulated climates, which is of critical importance in the model's use for the study of climatic change.

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W. Lawrence Gates

Abstract

The primitive hydrostatic equations for a rectangular homogeneous ocean with a free surface on a β-plane are integrated numerically for 60 days from an initial state of rest and undisturbed depth of 400 m. A zonal wind stress (maximum 2 dyn cm−2) and a lateral eddy viscosity (108 cm2 sec−1) are assumed. A series of transient Rossby waves of approximately 1000-2000 km in length form in the central and eastern basin, and undergo a well-marked life cycle of amplification and decay as they propagate westward at ∼1 m sec−1 relative to the zonal current. The northward boundary current in the west (∼1 m sec−1) and the counter-currents in the northwest (∼10 cm sec−1) may be identified as the first stationary members of a continuing series of waves, with subsequent transients showing characteristics of reflected Rossby waves and reaching progressively smaller maximum amplitudes. The standing wave pattern (wavelength ∼600 km) in the north-west is a characteristic nonlinear effect, and is associated with the meridional tilt displayed by the transients and the resultant (nonlinear) poleward eddy transport of zonal momentum. Near geostrophic equilibrium is maintained throughout, with the meridional Ekman flow of the order of a few centimeters per second. After a spin-up period of about 12 days, the surface potential and total kinetic energy display damped oscillations with the free period of approximately 16 days, with (long) surface gravity waves not significantly present.

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W. Lawrence Gates

Abstract

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W. Lawrence Gates

Abstract

The truncation error, stability and convergence properties of various finite-difference formulations of the one-dimensional barotropic vorticity equation are considered, and analytic solutions of the difference equations for simple harmonic initial conditions are presented. With conventional centered space differences, the schemes considered may be classified according to the method of time differencing as the forward difference case (unstable), the first-forward-then-centered difference case (conditionally stable), and the implicit difference case (unconditionally stable). The first-forward-then-centered difference scheme, corresponding to that commonly employed in meteorological numerical integration, gives rise to an oscillation phenomenon in both the amplitude and phase speed of the solution, which is most serious for a small space mesh, a large time mesh, and for the shorter wavelength disturbances. In each difference scheme considered, the truncation error leads to a cumulative phase departure of the difference solution relative to the true solution, an effect which is approximately proportional to the cube of the wavelength.

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W. Lawrence Gates

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W. Lawrence Gates

Abstract

Various measures of static stability in the atmosphere are reviewed and their uses briefly discussed. The mean vertical distribution of nine stability measures is given for 100-mb tropospheric layers and for selected stratospheric layers. The average geographical distribution over the United States is also discussed and illustrated for the measure – Tθ−1δθ/δp. The seasonal differences in stability distribution are discussed from the January and July average data for forty-five United States radiosonde stations.

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W. Lawrence Gates

Abstract

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W. Lawrence Gates

Abstract

After further experiments with a pilot case reported earlier, the results are summarized for a selected ten-day series of predictions obtained over the northern hemisphere by conventional numerical methods with the barotropic model. The overall accuracy of the forecasts is found to decay steadily with increasing forecast period up to 72 hours. In comparison with a series of barotropic forecasts prepared earlier for North America only, the hemispheric predictions are shown to be free of serious boundary-condition error in the middle-latitude regions of major synoptic activity. Outstanding among the errors remaining in the hemispheric integrations, however, are those due to the variation of the density of observational data (especially serious over the Pacific and Asiatic regions), those caused by excessive anticyclogenesis, those due to truncation error and the lack of smoothing, and those inherent in the model's neglect of baroclinic development. Research is in progress on these and other errors, in an effort to improve further the resolution of numerical prediction methods.

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