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Abstract
A nine-layer, primitive equation (PE) model of the global atmosphere developed at the Goddard Institute for Space Studies (GISS) has been used to generate six 48-hr forecasts during December 1972 and January 1973. Operational analyses north of 18N and experimental global analyses made available by the National Meteorological Center (NMC), NOAA, were used as the initial conditions; the operational analyses were used to verify the forecasts at 12-hr intervals over the northern hemisphere north of 22N. The combined analyses were used to verify the forecasts in the global domain.
Root-mean-square errors of the sea-level pressure, 1000-mb heights, and vector geostrophic winds, and 500-mb heights and vector geostrophic winds indicate that the GISS forecasts have skill comparable to those made by operational PE models.
A summary of the 36-hr evolution of extratropical cyclones shows that their speed of propagation is systematically too slow and their central pressures are systematically too high, as has already been documented for the NMC PE model forecasts.
Forecasts of the surface temperature, computed by vertical extrapolation from the model's two lowest levels, and verified quantitatively over North America and qualitatively over the United States, show considerable skill.
Abstract
A nine-layer, primitive equation (PE) model of the global atmosphere developed at the Goddard Institute for Space Studies (GISS) has been used to generate six 48-hr forecasts during December 1972 and January 1973. Operational analyses north of 18N and experimental global analyses made available by the National Meteorological Center (NMC), NOAA, were used as the initial conditions; the operational analyses were used to verify the forecasts at 12-hr intervals over the northern hemisphere north of 22N. The combined analyses were used to verify the forecasts in the global domain.
Root-mean-square errors of the sea-level pressure, 1000-mb heights, and vector geostrophic winds, and 500-mb heights and vector geostrophic winds indicate that the GISS forecasts have skill comparable to those made by operational PE models.
A summary of the 36-hr evolution of extratropical cyclones shows that their speed of propagation is systematically too slow and their central pressures are systematically too high, as has already been documented for the NMC PE model forecasts.
Forecasts of the surface temperature, computed by vertical extrapolation from the model's two lowest levels, and verified quantitatively over North America and qualitatively over the United States, show considerable skill.
Abstract
A computer procedure, described in an earlier study, uses the wind speed field near the ocean surface in combination with a small number of observations of pressure and wind velocity to specify the maritime sea-level pressure field. An improved version was used to analyze the pressure distribution over the North Pacific Ocean for eleven synoptic times in February 1967. Independent knowledge of the central pressures of lows is shown to reduce the analysis errors for very sparse data coverage. The application of planned remote sensing of sea-level wind speeds is shown to make a significant contribution to the quality of the analysis especially in the high gradient mid-latitudes and for sparse coverage of conventional observations (such as over Southern Hemisphere oceans). Uniform distribution of the available observations of sea-level pressure and wind velocity yields results far superior to those derived from a random distribution. A generalization of the results indicates that the average lower limit for analysis errors is between 2 and 2.5 mb based on the perfect specification of the magnitude of the sea-level pressure gradient from a known verification analysis, and 15 uniformly distributed, high-quality buoy, weather ship or island observations of the pressure and wind velocity. (A less than perfect specification will derive from wind-pressure relationships applied to satellite-observed wind speeds.) Analysis errors computed using poorly defined wind fields indicate the procedure's potential for sparse data analysis even without supplementary satellite data.
Abstract
A computer procedure, described in an earlier study, uses the wind speed field near the ocean surface in combination with a small number of observations of pressure and wind velocity to specify the maritime sea-level pressure field. An improved version was used to analyze the pressure distribution over the North Pacific Ocean for eleven synoptic times in February 1967. Independent knowledge of the central pressures of lows is shown to reduce the analysis errors for very sparse data coverage. The application of planned remote sensing of sea-level wind speeds is shown to make a significant contribution to the quality of the analysis especially in the high gradient mid-latitudes and for sparse coverage of conventional observations (such as over Southern Hemisphere oceans). Uniform distribution of the available observations of sea-level pressure and wind velocity yields results far superior to those derived from a random distribution. A generalization of the results indicates that the average lower limit for analysis errors is between 2 and 2.5 mb based on the perfect specification of the magnitude of the sea-level pressure gradient from a known verification analysis, and 15 uniformly distributed, high-quality buoy, weather ship or island observations of the pressure and wind velocity. (A less than perfect specification will derive from wind-pressure relationships applied to satellite-observed wind speeds.) Analysis errors computed using poorly defined wind fields indicate the procedure's potential for sparse data analysis even without supplementary satellite data.
Abstract
A computer-based procedure is developed that combines a field of simulated satellite-derived wind speeds with a limited amount of conventional surface data so as to recover the surface pressure field and the vector wind field over the North Pacific Ocean. Wind speeds are determined from an objective analysis of high spatial density ship observations in order to simulate the anticipated output of a proposed satellite-mounted radar-radiometer system. The conventional surface data consist of sparse observations from ocean-going vessels, observations from several tropical stations, and boundary pressures from analyses over coastal areas.
The simulated speeds are combined with the conventional network for various spatial distributions of ship data. The average rms departure of sea-level pressure fields analyzed by deleting from 75–94% of the available ship observations from the maximum data analysis is from 3.0–4.0 mb. Comparison of the wind components implied by the isobaric patterns to those of the withheld ship observations yields average rms differences of from 8.7–9.4 kt for a range of 75–94% data deletion.
Abstract
A computer-based procedure is developed that combines a field of simulated satellite-derived wind speeds with a limited amount of conventional surface data so as to recover the surface pressure field and the vector wind field over the North Pacific Ocean. Wind speeds are determined from an objective analysis of high spatial density ship observations in order to simulate the anticipated output of a proposed satellite-mounted radar-radiometer system. The conventional surface data consist of sparse observations from ocean-going vessels, observations from several tropical stations, and boundary pressures from analyses over coastal areas.
The simulated speeds are combined with the conventional network for various spatial distributions of ship data. The average rms departure of sea-level pressure fields analyzed by deleting from 75–94% of the available ship observations from the maximum data analysis is from 3.0–4.0 mb. Comparison of the wind components implied by the isobaric patterns to those of the withheld ship observations yields average rms differences of from 8.7–9.4 kt for a range of 75–94% data deletion.
Abstract
Throughout the month of June 1965 tetroons ballasted for 150–300 m altitude were released and radar-tracked in New York City and environs. The study evaluates the skill with which data from both a dense mesoscale network of surface wind observations and a less dense network of balloon-derived wind observations in the planetary boundary layer can be used to reconstruct the tetroon trajectories.
Root-mean-square errors in predicting 2- and 4-hr tetroon positions from surface-wind-derived trajectories are reduced by the addition of a vector correction to account for vertical wind shear; this correction also randomizes the direction of the errors. Corrected surface wind trajectories, when compared with the tetroon trajectories, are slightly better than those computed from the balloon-derived winds.
The best results obtained yield rms 2-hr prediction errors of 15 km; the median error of this distribution expressed as a percentage of the range of each tetroon flight was 36%.
Abstract
Throughout the month of June 1965 tetroons ballasted for 150–300 m altitude were released and radar-tracked in New York City and environs. The study evaluates the skill with which data from both a dense mesoscale network of surface wind observations and a less dense network of balloon-derived wind observations in the planetary boundary layer can be used to reconstruct the tetroon trajectories.
Root-mean-square errors in predicting 2- and 4-hr tetroon positions from surface-wind-derived trajectories are reduced by the addition of a vector correction to account for vertical wind shear; this correction also randomizes the direction of the errors. Corrected surface wind trajectories, when compared with the tetroon trajectories, are slightly better than those computed from the balloon-derived winds.
The best results obtained yield rms 2-hr prediction errors of 15 km; the median error of this distribution expressed as a percentage of the range of each tetroon flight was 36%.
Abstract
A coarse-mesh, global climate model developed at the Goddard Institute for Space Studies (GISS) has been used to assess the influence of ocean surface temperature (OST) gradient and continentality on the Walker circulation. The basic model climate was established by a five-year integration in which the prescribed seasonal cycle in OST distribution was identical for each year. In the model climate, the Walker circulation is characterized in the zonal plane by three pairs of clockwise and counterclockwise cells to the troposphere.
Three separate winter experiments were performed in which the normal west-to-east OST gradients in the tropical Pacific were replaced by a uniform distribution in the band from 8°N to 16°S. Each experiment was characterized by OSTs set at the warmest, coldest, or mean temperatures in the band. The model response features statistically significant changes in the intensity of the various cells and branches with small shifts in the east-west extent. The overall structure in the zonal plane for the experiments with the coldest or mean temperatures, however, remained unchanged. A major disruption of the six-cell structure did result for the experiment with the warmest temperature and resultant net heat source.
The fourth prescribed changed experiment involved the replacement of the South American continent by an ocean with the OSTs linearly interpolated from the eastern Pacific to the western Atlantic. In this case, a dramatic change in the structure of the Walker circulation also took place as the upward branch over South America was reduced sufficiently to eliminate the corresponding counterclockwise cell and thereby allow two clockwise cells to merge into one large cell. The Hadley cell was less intense and shifted northward with the South American continent removed.
In summary, these experiments with the GISS model seem to indicate that both continentality and OST gradient are important as forcing mechanism of the overall structure of the Walker circulation and the intensity of the individual cells. The details of the forcing, however, are likely to be different for the two mechanisms.
Abstract
A coarse-mesh, global climate model developed at the Goddard Institute for Space Studies (GISS) has been used to assess the influence of ocean surface temperature (OST) gradient and continentality on the Walker circulation. The basic model climate was established by a five-year integration in which the prescribed seasonal cycle in OST distribution was identical for each year. In the model climate, the Walker circulation is characterized in the zonal plane by three pairs of clockwise and counterclockwise cells to the troposphere.
Three separate winter experiments were performed in which the normal west-to-east OST gradients in the tropical Pacific were replaced by a uniform distribution in the band from 8°N to 16°S. Each experiment was characterized by OSTs set at the warmest, coldest, or mean temperatures in the band. The model response features statistically significant changes in the intensity of the various cells and branches with small shifts in the east-west extent. The overall structure in the zonal plane for the experiments with the coldest or mean temperatures, however, remained unchanged. A major disruption of the six-cell structure did result for the experiment with the warmest temperature and resultant net heat source.
The fourth prescribed changed experiment involved the replacement of the South American continent by an ocean with the OSTs linearly interpolated from the eastern Pacific to the western Atlantic. In this case, a dramatic change in the structure of the Walker circulation also took place as the upward branch over South America was reduced sufficiently to eliminate the corresponding counterclockwise cell and thereby allow two clockwise cells to merge into one large cell. The Hadley cell was less intense and shifted northward with the South American continent removed.
In summary, these experiments with the GISS model seem to indicate that both continentality and OST gradient are important as forcing mechanism of the overall structure of the Walker circulation and the intensity of the individual cells. The details of the forcing, however, are likely to be different for the two mechanisms.
Abstract
Ensembles of three simulations each, forced by June–September 1987 and 1988 sea surface temperatures, respectively, were made with a new version of the general circulation model of the National Aeronautics and Space Administration/Goddard Institute for Space Studies. Time series of 6-h meridional winds at about 780 mb over West Africa were spectrally analyzed to detect African wave disturbances, whose properties for the two ensembles are compared and contrasted. The realistically simulated, stronger 1988 tropical easterly jet and the associated stronger upper-tropospheric divergence are components of interannual differences in the SST-forced planetary circulation, which correspond to higher amplitudes of African wave activity and concomitant excesses in 1988 Sahel rainfall rates. Results do not show, however, that most of the heavier precipitation was spatially organized by African wave structures. The excess rainfall is associated with stronger mean southerly circulation in the lower troposphere, which carried more moisture into the Sahel. Nevertheless, because waves modulate winds, convergence, humidity, and precipitation, the study suggests that they serve as a teleconnection mechanism, whereby extreme Pacific Ocean SST anomalies are able to influence climate variability in Africa's Sahel.
Abstract
Ensembles of three simulations each, forced by June–September 1987 and 1988 sea surface temperatures, respectively, were made with a new version of the general circulation model of the National Aeronautics and Space Administration/Goddard Institute for Space Studies. Time series of 6-h meridional winds at about 780 mb over West Africa were spectrally analyzed to detect African wave disturbances, whose properties for the two ensembles are compared and contrasted. The realistically simulated, stronger 1988 tropical easterly jet and the associated stronger upper-tropospheric divergence are components of interannual differences in the SST-forced planetary circulation, which correspond to higher amplitudes of African wave activity and concomitant excesses in 1988 Sahel rainfall rates. Results do not show, however, that most of the heavier precipitation was spatially organized by African wave structures. The excess rainfall is associated with stronger mean southerly circulation in the lower troposphere, which carried more moisture into the Sahel. Nevertheless, because waves modulate winds, convergence, humidity, and precipitation, the study suggests that they serve as a teleconnection mechanism, whereby extreme Pacific Ocean SST anomalies are able to influence climate variability in Africa's Sahel.
Abstract
Simulations made with the general circulation model of the NASA/Goddard Institute for Space Studies (GISS GCM) run at 4° latitude by 5° longitude horizontal resolution are analyzed to determine the model's representation of African wave disturbances. Waves detected in the model's lower troposphere over northern Africa during the summer monsoon season exhibit realistic wavelengths of about 2200 km. However, power spectra of the meridional wind show that the waves propagate westward too slowly, with periods of 5–10 days, about twice the observed values. This sluggishness is most pronounced during August, consistent with simulated 600-mb zonal winds that are only about half the observed speeds of the midtropospheric jet. The modeled wave amplitudes are strongest over West Africa during the first half of the summer but decrease dramatically by September, contrary to observational evidence. Maximum amplitudes occur at realistic latitudes, 12°–20°N, but not as observed near the Atlantic coast. Spectral analyses suggest some wave modulation of precipitation in the 5–8-day band, and compositing shows that precipitation is slightly enhanced east of the wave trough, coincident with southerly winds. Extrema of low-level convergence west of the wave troughs, coinciding with northerly winds, were not preferred areas for simulated precipitation, probably because of the drying effect of this advection, as waves were generally north of the humid zone. The documentation of African wave disturbances in the GISS GCM is a first step toward considering wave influences in future GCM studies of Sahel drought.
Abstract
Simulations made with the general circulation model of the NASA/Goddard Institute for Space Studies (GISS GCM) run at 4° latitude by 5° longitude horizontal resolution are analyzed to determine the model's representation of African wave disturbances. Waves detected in the model's lower troposphere over northern Africa during the summer monsoon season exhibit realistic wavelengths of about 2200 km. However, power spectra of the meridional wind show that the waves propagate westward too slowly, with periods of 5–10 days, about twice the observed values. This sluggishness is most pronounced during August, consistent with simulated 600-mb zonal winds that are only about half the observed speeds of the midtropospheric jet. The modeled wave amplitudes are strongest over West Africa during the first half of the summer but decrease dramatically by September, contrary to observational evidence. Maximum amplitudes occur at realistic latitudes, 12°–20°N, but not as observed near the Atlantic coast. Spectral analyses suggest some wave modulation of precipitation in the 5–8-day band, and compositing shows that precipitation is slightly enhanced east of the wave trough, coincident with southerly winds. Extrema of low-level convergence west of the wave troughs, coinciding with northerly winds, were not preferred areas for simulated precipitation, probably because of the drying effect of this advection, as waves were generally north of the humid zone. The documentation of African wave disturbances in the GISS GCM is a first step toward considering wave influences in future GCM studies of Sahel drought.
Abstract
The sources of sub-Saharan precipitation are studied using diagnostic procedures integrated into the code of the GISS climate model. Water vapor evaporating from defined source region is “tagged,” allowing the determination of the relative contributions of each evaporative source to the simulated July rainfall in the Sahel. Two June–July simulations are studied to compare the moisture sources, moisture convergence patterns and the spatial variations of precipitation for rainy and drought conditions. Results for this eau study indicate that patterns of moisture convergence and divergence over northern Africa had a stronger influence on model rainfall over the sub-Sahara than did evaporation rates over the adjacent oceans or moisture advection from ocean to continent. While local continental evaporation contributed significant amounts of water to sahelian precipitation in the “rainy” simulation, moisture from the Indian Ocean did not precipitate over the Sahel in either case.
Abstract
The sources of sub-Saharan precipitation are studied using diagnostic procedures integrated into the code of the GISS climate model. Water vapor evaporating from defined source region is “tagged,” allowing the determination of the relative contributions of each evaporative source to the simulated July rainfall in the Sahel. Two June–July simulations are studied to compare the moisture sources, moisture convergence patterns and the spatial variations of precipitation for rainy and drought conditions. Results for this eau study indicate that patterns of moisture convergence and divergence over northern Africa had a stronger influence on model rainfall over the sub-Sahara than did evaporation rates over the adjacent oceans or moisture advection from ocean to continent. While local continental evaporation contributed significant amounts of water to sahelian precipitation in the “rainy” simulation, moisture from the Indian Ocean did not precipitate over the Sahel in either case.
Abstract
The response of the NASA/Goddard Institute for Space Studies GCM to large tropical sea surface temperature (SST) anomalies is investigated by evaluating model simulations of the particularly contrasting summer monsoon seasons 1987 and 1988. These years are representative of the warm and cold phases, respectively, of a recent ENSO event. An ensemble averaging the results of three simulations was considered for each season, using monthly mean observed SST anomalies for June–August 1987 and 1988 as lower boundary forcing. Consistent with the ECMWF-analyzed winds, the simulators based on 1988 as compared to 1987 SST exhibit stronger upper-tropospheric irrational circulation between the monsoon regions and the Southern Hemispheric subtropical anticyclones, a stronger Pacific Walker cell and a weaker subtropical westerly jet over the South Pacific. In the same vein, the modeled precipitation, indicating a more northerly position of the Pacific ITCZ in 1988 compared with 1987, is supported by satellite observations of outgoing longwave radiation and highly reflective clouds.
Abstract
The response of the NASA/Goddard Institute for Space Studies GCM to large tropical sea surface temperature (SST) anomalies is investigated by evaluating model simulations of the particularly contrasting summer monsoon seasons 1987 and 1988. These years are representative of the warm and cold phases, respectively, of a recent ENSO event. An ensemble averaging the results of three simulations was considered for each season, using monthly mean observed SST anomalies for June–August 1987 and 1988 as lower boundary forcing. Consistent with the ECMWF-analyzed winds, the simulators based on 1988 as compared to 1987 SST exhibit stronger upper-tropospheric irrational circulation between the monsoon regions and the Southern Hemispheric subtropical anticyclones, a stronger Pacific Walker cell and a weaker subtropical westerly jet over the South Pacific. In the same vein, the modeled precipitation, indicating a more northerly position of the Pacific ITCZ in 1988 compared with 1987, is supported by satellite observations of outgoing longwave radiation and highly reflective clouds.
Abstract
African wave disturbances (AWDs), an important trigger of Sahel summer rainfall, are studied using ECMWF gridded datasets for July and August 1987 and 1988. Power spectra of time series of 700-mb meridional winds near Niamey taken from analyses at both 2° × 2.5° and 4° × 5° horizontal resolution are compared to spectra based on Niamey station data. In addition, spatial distributions of meteorological fields at both resolutions are discussed for three case studies, including the synoptic features of several AWDs. Additional examples are presented from GCM simulations at comparable horizontal resolutions. While vertical motion and divergence centers were more extreme at 2° × 2.5°, many of the deduced characteristics of an AWD were similar at both resolutions. The higher-resolution analyses and simulation show a sharp transition across wave troughs between lower-tropospheric convergence (uplift) on the west and divergence (subsidence) on the east for several AWDs. For the two more southerly AWDs analyzed here, uplift associated with the convergence ahead of the trough appears to be displaced to the southwest at midtropospheric altitudes. Twice-daily July–September precipitation at Niamey is weakly, but significantly, correlated with corresponding time series of ECMWF analyzed vertical motion at nearby grid points.
Abstract
African wave disturbances (AWDs), an important trigger of Sahel summer rainfall, are studied using ECMWF gridded datasets for July and August 1987 and 1988. Power spectra of time series of 700-mb meridional winds near Niamey taken from analyses at both 2° × 2.5° and 4° × 5° horizontal resolution are compared to spectra based on Niamey station data. In addition, spatial distributions of meteorological fields at both resolutions are discussed for three case studies, including the synoptic features of several AWDs. Additional examples are presented from GCM simulations at comparable horizontal resolutions. While vertical motion and divergence centers were more extreme at 2° × 2.5°, many of the deduced characteristics of an AWD were similar at both resolutions. The higher-resolution analyses and simulation show a sharp transition across wave troughs between lower-tropospheric convergence (uplift) on the west and divergence (subsidence) on the east for several AWDs. For the two more southerly AWDs analyzed here, uplift associated with the convergence ahead of the trough appears to be displaced to the southwest at midtropospheric altitudes. Twice-daily July–September precipitation at Niamey is weakly, but significantly, correlated with corresponding time series of ECMWF analyzed vertical motion at nearby grid points.
