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- Author or Editor: W. Lawrence Gates x
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Abstract
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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.
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.
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.
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.
Abstract
By the use of two separate mesh sizes instead of one in computing finite differences, an extrapolation to effective “zero” mesh size may be made in order to reduce the truncation error, as originally suggested by Richardson. This technique of “difference extrapolation” is applied to the estimation of individual differentials in the barotropic vorticity equation, and here corresponds to the use of “second-order” finite differences. The truncation-induced phase speed lag of the difference solution relative to the true solution is shown to be systematically reduced, especially for the shorter waves. Next, the extrapolation is applied with two separate solutions of the barotropic difference equation, with the result that the phase speeds are further improved, but at the expense of an amplitude distortion of about 10 percent. This amplitude distortion may be removed for a particular wavelength, and a small further phase speed improvement obtained, but the amplitude distortion remains for other wavelengths. These methods of “solution extrapolation” are therefore felt to be unsuitable for routine use. The method of “difference extrapolation,” however, preserves the solution's amplitude, and if used in conjunction with a suitable smoothing procedure should result in a net error reduction for those waves resolved by the mesh and retained by the smoothing.
Abstract
By the use of two separate mesh sizes instead of one in computing finite differences, an extrapolation to effective “zero” mesh size may be made in order to reduce the truncation error, as originally suggested by Richardson. This technique of “difference extrapolation” is applied to the estimation of individual differentials in the barotropic vorticity equation, and here corresponds to the use of “second-order” finite differences. The truncation-induced phase speed lag of the difference solution relative to the true solution is shown to be systematically reduced, especially for the shorter waves. Next, the extrapolation is applied with two separate solutions of the barotropic difference equation, with the result that the phase speeds are further improved, but at the expense of an amplitude distortion of about 10 percent. This amplitude distortion may be removed for a particular wavelength, and a small further phase speed improvement obtained, but the amplitude distortion remains for other wavelengths. These methods of “solution extrapolation” are therefore felt to be unsuitable for routine use. The method of “difference extrapolation,” however, preserves the solution's amplitude, and if used in conjunction with a suitable smoothing procedure should result in a net error reduction for those waves resolved by the mesh and retained by the smoothing.
Abstract
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Abstract
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The Atmospheric Model Intercomparison Project (AMIP) is an international effort to determine the systematic climate errors of atmospheric models under realistic conditions, and calls for the simulation of the climate of the decade 1979–1988 using the observed monthly averaged distributions of sea surface temperature and sea ice as boundary conditions. Organized by the Working Group on Numerical Experimentation as a contribution to the World Climate Research Programme, AMIP involves the international atmospheric modeling community in a major test and intercomparison of model performance; in addition to an agreed-to set of monthly averaged output variables, each of the participating models will generate a daily history of state. These data will be stored and made available in standard format by the Program for Climate Model Diagnosis and Intercomparison at the Lawrence Livermore National Laboratory. Following completion of the computational phase of AMIP in 1993, emphasis will shift to a series of diagnostic subprojects, now being planned, for the detailed examination of model performance and the simulation of specific physical processes and phenomena. AMIP offers an unprecedented opportunity for the comprehensive evaluation and validation of current atmospheric models, and is expected to provide valuable information for model improvement.
The Atmospheric Model Intercomparison Project (AMIP) is an international effort to determine the systematic climate errors of atmospheric models under realistic conditions, and calls for the simulation of the climate of the decade 1979–1988 using the observed monthly averaged distributions of sea surface temperature and sea ice as boundary conditions. Organized by the Working Group on Numerical Experimentation as a contribution to the World Climate Research Programme, AMIP involves the international atmospheric modeling community in a major test and intercomparison of model performance; in addition to an agreed-to set of monthly averaged output variables, each of the participating models will generate a daily history of state. These data will be stored and made available in standard format by the Program for Climate Model Diagnosis and Intercomparison at the Lawrence Livermore National Laboratory. Following completion of the computational phase of AMIP in 1993, emphasis will shift to a series of diagnostic subprojects, now being planned, for the detailed examination of model performance and the simulation of specific physical processes and phenomena. AMIP offers an unprecedented opportunity for the comprehensive evaluation and validation of current atmospheric models, and is expected to provide valuable information for model improvement.
Abstract
No abstract available.
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No abstract available.
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.
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.
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.
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.
Abstract
By juxtaposition of a series of one-dimensional solutions at adjacent latitudes, numerical forecasts may easily and inexpensively be incorporated into forecasting routine. Forecasts for 140 degrees of longitude, for the region 30 to 70°N, can be prepared in 60 to 90 minutes by one person. It is suggested that, for some purposes of forecasting and synoptic research, these calculations may be used to approximate the solutions obtained with the aid of electronic computing equipment, with a considerable saving of time and expense. In application to the equivalent-barotropic model of Charney and Eliassen, the accuracy of the method is comparable to that of synoptic forecasts prepared in the conventional manner. The latter are presented for comparison, in a set of four 24-hour forecasts.
Abstract
By juxtaposition of a series of one-dimensional solutions at adjacent latitudes, numerical forecasts may easily and inexpensively be incorporated into forecasting routine. Forecasts for 140 degrees of longitude, for the region 30 to 70°N, can be prepared in 60 to 90 minutes by one person. It is suggested that, for some purposes of forecasting and synoptic research, these calculations may be used to approximate the solutions obtained with the aid of electronic computing equipment, with a considerable saving of time and expense. In application to the equivalent-barotropic model of Charney and Eliassen, the accuracy of the method is comparable to that of synoptic forecasts prepared in the conventional manner. The latter are presented for comparison, in a set of four 24-hour forecasts.
