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Abstract
A calculation of the spectral and spatial energetics of the Goddard Institute for Space Studies (GISS) 9-level model of the global atmosphere is given. Results are presented from a model simulation of January 1973 and compared with previous model studies and an observational analysis for the Northern Hemisphere. The spectral analysis permits detailed study of those physical processes of scale larger than the grid spacing and allows indirect conclusions concerning unresolved subgrid-scale processes. For larger-than-grid-scale processes we conclude that: 1) the GISS model with a relatively coarse resolution of 4° latitude by 5° longitude produces significant nonlinear energy transfers; 2) baroclini conversion is large near planetary wavenumber 6, but both the GISS model and another orographic model show a less peaked structure than non-orographic models and observations; and 3) Saltzman’s previously published compilation of observed spectral energetics is based on analyses that, when normalized, are probably too large by about 25%. A linear fit to the kinetic energy spectrum yields a power law of –2.6 in the wavenumber region 8 to 15. For subgrid-scale processes, the turbulence theory result of very small energy flux cascade is obtained. For the spatial energetics we find good agreement with recently published climatological data.
Abstract
A calculation of the spectral and spatial energetics of the Goddard Institute for Space Studies (GISS) 9-level model of the global atmosphere is given. Results are presented from a model simulation of January 1973 and compared with previous model studies and an observational analysis for the Northern Hemisphere. The spectral analysis permits detailed study of those physical processes of scale larger than the grid spacing and allows indirect conclusions concerning unresolved subgrid-scale processes. For larger-than-grid-scale processes we conclude that: 1) the GISS model with a relatively coarse resolution of 4° latitude by 5° longitude produces significant nonlinear energy transfers; 2) baroclini conversion is large near planetary wavenumber 6, but both the GISS model and another orographic model show a less peaked structure than non-orographic models and observations; and 3) Saltzman’s previously published compilation of observed spectral energetics is based on analyses that, when normalized, are probably too large by about 25%. A linear fit to the kinetic energy spectrum yields a power law of –2.6 in the wavenumber region 8 to 15. For subgrid-scale processes, the turbulence theory result of very small energy flux cascade is obtained. For the spatial energetics we find good agreement with recently published climatological data.
Abstract
Relations between stratospheric wind errors, initial states and 500 mb skill are investigated using the GLAS general circulation model initialized with FGGE data. Erroneous stratospheric winds are seen in all current general circulation models, appearing also as weak shear above the subtropical jet and as cold polar stratospheres, In this study we show that the more anticyclonic large flows are correlated with large forecast stratospheric winds. In addition, we find that for North America the resulting errors are correlated with initial state jet stream accelerations while for East Asia the forecast winds are correlated with initial rate jet strength. Using 500 mb skill scores over Europe at day 5 to measure forecast performance, we find that both poor forecast skill and excessive stratospheric winds are correlated with more anticyclonic large-scale flows over North America. We hypothesize that the resulting erroneous kinetic energy contributes to the poor forecast skill, and that the problem is caused by a failure in the modeling of the stratospheric energy cycle in current general circulation models independent of vertical resolution.
Abstract
Relations between stratospheric wind errors, initial states and 500 mb skill are investigated using the GLAS general circulation model initialized with FGGE data. Erroneous stratospheric winds are seen in all current general circulation models, appearing also as weak shear above the subtropical jet and as cold polar stratospheres, In this study we show that the more anticyclonic large flows are correlated with large forecast stratospheric winds. In addition, we find that for North America the resulting errors are correlated with initial state jet stream accelerations while for East Asia the forecast winds are correlated with initial rate jet strength. Using 500 mb skill scores over Europe at day 5 to measure forecast performance, we find that both poor forecast skill and excessive stratospheric winds are correlated with more anticyclonic large-scale flows over North America. We hypothesize that the resulting erroneous kinetic energy contributes to the poor forecast skill, and that the problem is caused by a failure in the modeling of the stratospheric energy cycle in current general circulation models independent of vertical resolution.
Abstract
Integrated and spectral energetics of the Goddard Laboratory for Atmospheric Sciences (GLAS) general circulation model are compared with observations and examined when subdivided beyond hemispheric integrals. In the monthly mean zonal averages, we find qualitative improvements over previous versions of the model in eddy kinetic energy and barotropic conversions although vertical shear above the subtropical jet remains weak. We trace this problem to the erroneous growth of kinetic energy above strong jets, show that this difficulty is common to all orographic general circulation models, and suggest that this problem has significant effects on upper-level long-wave predictability. In particular, we consider whether hemispherically integrated kinetic energy or low-wavenumber spectral coefficients retain any skill as one approaches the predictability limit. In the time-dependent comparisons, the model shows significant skill in predicting the hemispherically integrated eddy kinetic energy out to two weeks for one of two cases. The low-order spectral coefficients show less skill, particularly for wavenumber 3. We examine other diagnostic properties of the spectral decomposition, and observe that both the model and observations show evidence of single wavenumber dominance in eddy kinetic energy and a positive correlation between spectral kinetic and potential energy.
Abstract
Integrated and spectral energetics of the Goddard Laboratory for Atmospheric Sciences (GLAS) general circulation model are compared with observations and examined when subdivided beyond hemispheric integrals. In the monthly mean zonal averages, we find qualitative improvements over previous versions of the model in eddy kinetic energy and barotropic conversions although vertical shear above the subtropical jet remains weak. We trace this problem to the erroneous growth of kinetic energy above strong jets, show that this difficulty is common to all orographic general circulation models, and suggest that this problem has significant effects on upper-level long-wave predictability. In particular, we consider whether hemispherically integrated kinetic energy or low-wavenumber spectral coefficients retain any skill as one approaches the predictability limit. In the time-dependent comparisons, the model shows significant skill in predicting the hemispherically integrated eddy kinetic energy out to two weeks for one of two cases. The low-order spectral coefficients show less skill, particularly for wavenumber 3. We examine other diagnostic properties of the spectral decomposition, and observe that both the model and observations show evidence of single wavenumber dominance in eddy kinetic energy and a positive correlation between spectral kinetic and potential energy.
Abstract
Excessive wind speeds are present above the subtropical jets in all current general circulation model forecasts. In this paper we study the longitude and time dependence of these errors in 14 five-day winter forecasts using the NASA Goddard Laboratory for Atmospheres (GLA) fourth-order general circulation model. We show that the forecast errors may be divided into four catagories, two of which appear above the jet, and two of which appear at the jet stream level. They are 1) error growth above the jet near the Himalayas; 2) error growth above the jet initiated elsewhere followed by extensive advection; 3) tropical moisture hums burst appearing in equatorial regions and migrating northeastward to merge with and distort the jet; and 4) undulatory growth of waves in the meridional component of the jet stream velocity. Two forecasts for Southern Hemisphere winters show similar problems.
For the error growth near the Himalayas, we have performed additional tests. Initializations based on analyses prepared by NMC, GLA, and ECM/WF and special forecasts made using the ECMWF model showed qualitatively results Both a simple orographically induced gravity-wave parameterization and a model with increased stratospheric resolution yielded a reduced forecast error north of 30°N but yielded continued problems south of the Himalayas. We suggest parameterization of shear-induced gravity waves as a possible solution.
Abstract
Excessive wind speeds are present above the subtropical jets in all current general circulation model forecasts. In this paper we study the longitude and time dependence of these errors in 14 five-day winter forecasts using the NASA Goddard Laboratory for Atmospheres (GLA) fourth-order general circulation model. We show that the forecast errors may be divided into four catagories, two of which appear above the jet, and two of which appear at the jet stream level. They are 1) error growth above the jet near the Himalayas; 2) error growth above the jet initiated elsewhere followed by extensive advection; 3) tropical moisture hums burst appearing in equatorial regions and migrating northeastward to merge with and distort the jet; and 4) undulatory growth of waves in the meridional component of the jet stream velocity. Two forecasts for Southern Hemisphere winters show similar problems.
For the error growth near the Himalayas, we have performed additional tests. Initializations based on analyses prepared by NMC, GLA, and ECM/WF and special forecasts made using the ECMWF model showed qualitatively results Both a simple orographically induced gravity-wave parameterization and a model with increased stratospheric resolution yielded a reduced forecast error north of 30°N but yielded continued problems south of the Himalayas. We suggest parameterization of shear-induced gravity waves as a possible solution.
Abstract
Wind measurements have been obtained from commercial aircraft crossing the 1992 winter subtropical jet streams over southwest and east Asia. Comparisons of these data with new, high-resolution analyses from four of the major operational centers show that the peak analyzed winds are still too weak by about 10%. In addition, about 17% of the cases show larger errors in which the analyses substantially miss the location or magnitude of individual jet streaks. Both the aircraft data and the highest-resolution analysis (European Centre for Medium-Range Weather Forecasts, equivalent equatorial quarter-wave grid spacing of 47 km) show evidence of orographically generated gravity waves but differ in their vertical damping.
Abstract
Wind measurements have been obtained from commercial aircraft crossing the 1992 winter subtropical jet streams over southwest and east Asia. Comparisons of these data with new, high-resolution analyses from four of the major operational centers show that the peak analyzed winds are still too weak by about 10%. In addition, about 17% of the cases show larger errors in which the analyses substantially miss the location or magnitude of individual jet streaks. Both the aircraft data and the highest-resolution analysis (European Centre for Medium-Range Weather Forecasts, equivalent equatorial quarter-wave grid spacing of 47 km) show evidence of orographically generated gravity waves but differ in their vertical damping.
Abstract
We have obtained cruise-level wind data from commercial aircraft, and compared this data with operational jet stream analyses over southwest Asia, an area of limited conventional data. We present results from an ensemble of 11 cases during January 1989 and individual cases during December 1988–March 1989.
Our key results are: (a) European Centre for Medium-Range Weather Forecasts (ECMWF), National Meteorological Center (NMC), and United Kingdom Meteorological Office (UKMO) analyses of the subtropical jet in southwest Asia are 11%, 17%, and 17% weaker, respectively, than aircraft observations; (b) analyzed poleward shears range up to 1 f (7 ×10−5 s−1) compared with up to 3f(21 × 10−5 s−1 in the aircraft observations where f is the local Coriolis parameter; (c) the ECMWF errors are larger at the base of the jet; (d) the mean ECMWF core location is latitudinally correct but has an rms latitude variance of 1.5°; (e) isolated erroneous radiosondes produce unmeterorological structure in the analyzed subtropical jet stream; and (f) the increased utilization of automated aircraft reports is likely to produce a spurious secular increase in the apparent strength of the jets.
The magnitude and spatial extent of the errors seen are near limits of current general circulation model resolution (100 km) but should be resolvable. Our results imply that studies of general circulation model systematic jet stream wind errors in weather and climate forecasts must be interpreted with caution in this region.
Abstract
We have obtained cruise-level wind data from commercial aircraft, and compared this data with operational jet stream analyses over southwest Asia, an area of limited conventional data. We present results from an ensemble of 11 cases during January 1989 and individual cases during December 1988–March 1989.
Our key results are: (a) European Centre for Medium-Range Weather Forecasts (ECMWF), National Meteorological Center (NMC), and United Kingdom Meteorological Office (UKMO) analyses of the subtropical jet in southwest Asia are 11%, 17%, and 17% weaker, respectively, than aircraft observations; (b) analyzed poleward shears range up to 1 f (7 ×10−5 s−1) compared with up to 3f(21 × 10−5 s−1 in the aircraft observations where f is the local Coriolis parameter; (c) the ECMWF errors are larger at the base of the jet; (d) the mean ECMWF core location is latitudinally correct but has an rms latitude variance of 1.5°; (e) isolated erroneous radiosondes produce unmeterorological structure in the analyzed subtropical jet stream; and (f) the increased utilization of automated aircraft reports is likely to produce a spurious secular increase in the apparent strength of the jets.
The magnitude and spatial extent of the errors seen are near limits of current general circulation model resolution (100 km) but should be resolvable. Our results imply that studies of general circulation model systematic jet stream wind errors in weather and climate forecasts must be interpreted with caution in this region.
Abstract
A model description and numerical results are presented for a global atmospheric circulation model developed at the Goddard Institute for Space Studies (GISS). The model version described is a 9-level primitive-equation model in sigma coordinates. It includes a realistic distribution of continents, oceans and topography. Detailed calculations of energy transfer by solar and terrestrial radiation make use of cloud and water vapor fields calculated by the model. The model hydrologic cycle includes two precipitation mechanisms: large-scale supersaturation and a parameterization of subgrid-scale cumulus convection.
Results are presented both from a comparison of the 13th to the 43rd days (January) of one integration with climatological statistics, and from five short-range forecasting experiments. In the extended integration, the near-equilibrium January-mean model atmosphere exhibits an energy cycle in good agreement with observational estimates, together with generally realistic zonal mean fields of winds, temperature, humidity, transports, diabatic heating, evaporation, precipitation, and cloud cover. In the five forecasting experiments, after 48 hr, the average rms error in temperature is 3.9K, and the average rms error in 500-mb height is 62 m. The model is successful in simulating the 2-day evolution of the major features of the observed sea level pressure and 500-mb height fields in a region surrounding North America.
Abstract
A model description and numerical results are presented for a global atmospheric circulation model developed at the Goddard Institute for Space Studies (GISS). The model version described is a 9-level primitive-equation model in sigma coordinates. It includes a realistic distribution of continents, oceans and topography. Detailed calculations of energy transfer by solar and terrestrial radiation make use of cloud and water vapor fields calculated by the model. The model hydrologic cycle includes two precipitation mechanisms: large-scale supersaturation and a parameterization of subgrid-scale cumulus convection.
Results are presented both from a comparison of the 13th to the 43rd days (January) of one integration with climatological statistics, and from five short-range forecasting experiments. In the extended integration, the near-equilibrium January-mean model atmosphere exhibits an energy cycle in good agreement with observational estimates, together with generally realistic zonal mean fields of winds, temperature, humidity, transports, diabatic heating, evaporation, precipitation, and cloud cover. In the five forecasting experiments, after 48 hr, the average rms error in temperature is 3.9K, and the average rms error in 500-mb height is 62 m. The model is successful in simulating the 2-day evolution of the major features of the observed sea level pressure and 500-mb height fields in a region surrounding North America.
