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- Author or Editor: Leonard M. Druyan x
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Abstract
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Abstract
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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 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
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.
Abstract
The performance of the NCAR Weather Research and Forecasting Model (WRF) as a West African regional-atmospheric model is evaluated. The study tests the sensitivity of WRF-simulated vorticity maxima associated with African easterly waves to 64 combinations of alternative parameterizations in a series of simulations in September. In all, 104 simulations of 12-day duration during 11 consecutive years are examined. The 64 combinations combine WRF parameterizations of cumulus convection, radiation transfer, surface hydrology, and PBL physics. Simulated daily and mean circulation results are validated against NASA’s Modern-Era Retrospective Analysis for Research and Applications (MERRA) and NCEP/Department of Energy Global Reanalysis 2. Precipitation is considered in a second part of this two-part paper. A wide range of 700-hPa vorticity validation scores demonstrates the influence of alternative parameterizations. The best WRF performers achieve correlations against reanalysis of 0.40–0.60 and realistic amplitudes of spatiotemporal variability for the 2006 focus year while a parallel-benchmark simulation by the NASA Regional Model-3 (RM3) achieves higher correlations, but less realistic spatiotemporal variability. The largest favorable impact on WRF-vorticity validation is achieved by selecting the Grell–Devenyi cumulus convection scheme, resulting in higher correlations against reanalysis than simulations using the Kain–Fritch convection. Other parameterizations have less-obvious impact, although WRF configurations incorporating one surface model and PBL scheme consistently performed poorly. A comparison of reanalysis circulation against two NASA radiosonde stations confirms that both reanalyses represent observations well enough to validate the WRF results. Validation statistics for optimized WRF configurations simulating the parallel period during 10 additional years are less favorable than for 2006.
Abstract
The performance of the NCAR Weather Research and Forecasting Model (WRF) as a West African regional-atmospheric model is evaluated. The study tests the sensitivity of WRF-simulated vorticity maxima associated with African easterly waves to 64 combinations of alternative parameterizations in a series of simulations in September. In all, 104 simulations of 12-day duration during 11 consecutive years are examined. The 64 combinations combine WRF parameterizations of cumulus convection, radiation transfer, surface hydrology, and PBL physics. Simulated daily and mean circulation results are validated against NASA’s Modern-Era Retrospective Analysis for Research and Applications (MERRA) and NCEP/Department of Energy Global Reanalysis 2. Precipitation is considered in a second part of this two-part paper. A wide range of 700-hPa vorticity validation scores demonstrates the influence of alternative parameterizations. The best WRF performers achieve correlations against reanalysis of 0.40–0.60 and realistic amplitudes of spatiotemporal variability for the 2006 focus year while a parallel-benchmark simulation by the NASA Regional Model-3 (RM3) achieves higher correlations, but less realistic spatiotemporal variability. The largest favorable impact on WRF-vorticity validation is achieved by selecting the Grell–Devenyi cumulus convection scheme, resulting in higher correlations against reanalysis than simulations using the Kain–Fritch convection. Other parameterizations have less-obvious impact, although WRF configurations incorporating one surface model and PBL scheme consistently performed poorly. A comparison of reanalysis circulation against two NASA radiosonde stations confirms that both reanalyses represent observations well enough to validate the WRF results. Validation statistics for optimized WRF configurations simulating the parallel period during 10 additional years are less favorable than for 2006.
Abstract
This paper evaluates the performance of the Weather Research and Forecasting (WRF) Model as a regional atmospheric model over West Africa. It tests WRF’s sensitivity to 64 configurations of alternative parameterizations in a series of 104 twelve-day September simulations during 11 consecutive years, 2000–10. The 64 configurations combine WRF parameterizations of cumulus convection, radiation, surface hydrology, and the PBL. Simulated daily and total precipitation results are validated against Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) data. Particular attention is given to westward-propagating precipitation maxima associated with African easterly waves (AEWs). A wide range of daily precipitation validation scores demonstrates the influence of alternative parameterizations. The best WRF performers achieve time–longitude correlations (against GPCP) of between 0.35 and 0.42 and spatiotemporal variability amplitudes only slightly higher than observed estimates. A parallel simulation by the benchmark Regional Model version 3 achieves a higher correlation (0.52) and realistic spatiotemporal variability amplitudes. The largest favorable impact on WRF precipitation validation is achieved by selecting the Grell–Devenyi convection scheme, resulting in higher correlations against observations than using the Kain–Fritch convection scheme. Other parameterizations have less obvious impacts. Validation statistics for optimized WRF configurations simulating the parallel period during 2000–10 are more favorable for 2005, 2006, and 2008 than for other years. The selection of some of the same WRF configurations as high scorers in both circulation and precipitation validations supports the notion that simulations of West African daily precipitation benefit from skillful simulations of associated AEW vorticity centers and that simulations of AEWs would benefit from skillful simulations of convective precipitation.
Abstract
This paper evaluates the performance of the Weather Research and Forecasting (WRF) Model as a regional atmospheric model over West Africa. It tests WRF’s sensitivity to 64 configurations of alternative parameterizations in a series of 104 twelve-day September simulations during 11 consecutive years, 2000–10. The 64 configurations combine WRF parameterizations of cumulus convection, radiation, surface hydrology, and the PBL. Simulated daily and total precipitation results are validated against Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) data. Particular attention is given to westward-propagating precipitation maxima associated with African easterly waves (AEWs). A wide range of daily precipitation validation scores demonstrates the influence of alternative parameterizations. The best WRF performers achieve time–longitude correlations (against GPCP) of between 0.35 and 0.42 and spatiotemporal variability amplitudes only slightly higher than observed estimates. A parallel simulation by the benchmark Regional Model version 3 achieves a higher correlation (0.52) and realistic spatiotemporal variability amplitudes. The largest favorable impact on WRF precipitation validation is achieved by selecting the Grell–Devenyi convection scheme, resulting in higher correlations against observations than using the Kain–Fritch convection scheme. Other parameterizations have less obvious impacts. Validation statistics for optimized WRF configurations simulating the parallel period during 2000–10 are more favorable for 2005, 2006, and 2008 than for other years. The selection of some of the same WRF configurations as high scorers in both circulation and precipitation validations supports the notion that simulations of West African daily precipitation benefit from skillful simulations of associated AEW vorticity centers and that simulations of AEWs would benefit from skillful simulations of convective precipitation.
Abstract
Six cases of two-week numerical weather prediction experiments, begun from and verified against actual data, are presented to illustrate the extended-range forecasting capability of a global circulation model. The forecasts, all for Northern Hemisphere winter, are analyzed for both transient and time-mean properties of the predicted fields of wind, temperature, pressure, and precipitation. Rms temperature and sea-level pressure errors rise above persistence level during the first week, but forecast tropospheric zonal winds and 500 mb heights are superior to persistence throughout the two-week period. Time-mean forecasts display the model's climatological bias, but show skill in the prediction of surface temperature and synoptic-scale circulation patterns representing an improvement over climatology. Skill in precipitation forecasting is demonstrable for about one week.
Abstract
Six cases of two-week numerical weather prediction experiments, begun from and verified against actual data, are presented to illustrate the extended-range forecasting capability of a global circulation model. The forecasts, all for Northern Hemisphere winter, are analyzed for both transient and time-mean properties of the predicted fields of wind, temperature, pressure, and precipitation. Rms temperature and sea-level pressure errors rise above persistence level during the first week, but forecast tropospheric zonal winds and 500 mb heights are superior to persistence throughout the two-week period. Time-mean forecasts display the model's climatological bias, but show skill in the prediction of surface temperature and synoptic-scale circulation patterns representing an improvement over climatology. Skill in precipitation forecasting is demonstrable for about one week.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Synoptic weather features over West Africa were studied in simulations by the regional simulation model (RM) at the NASA Goddard Institute for Space Studies. These pioneering simulations represent the beginning of an effort to adapt regional models for weather and climate prediction over West Africa. The RM uses a Cartesian grid with 50-km horizontal resolution and 15 vertical levels. An ensemble of four simulations was forced with lateral boundary conditions from ECMWF global analyses for the period 8–22 August 1988. The simulated midtropospheric circulation includes the skillful development and movement of several African wave disturbances. Wavelet analysis of midtropospheric winds detected a dominant periodicity of about 4 days and a secondary periodicity of 5–8 days. Spatial distributions of RM precipitation and precipitation time series were validated against daily rain gauge measurements and International Satellite Cloud Climatology Project satellite infrared cloud imagery. The time–space distribution of simulated precipitation was made more realistic by combining the ECMWF initial conditions with a 24-h spinup of the moisture field and also by damping high-frequency gravity waves by dynamic initialization. Model precipitation “forecasts” over the central Sahel were correlated with observations for about 3 days, but reinitializing with observed data on day 5 resulted in a dramatic improvement in the precipitation validation over the remaining 9 days. Results imply that information via the lateral boundary conditions is not always sufficient to minimize departures between simulated and actual precipitation patterns for more than several days. In addition, there was some evidence that the new initialization may increase the simulations' sensitivity to the quality of lateral boundary conditions.
Abstract
Synoptic weather features over West Africa were studied in simulations by the regional simulation model (RM) at the NASA Goddard Institute for Space Studies. These pioneering simulations represent the beginning of an effort to adapt regional models for weather and climate prediction over West Africa. The RM uses a Cartesian grid with 50-km horizontal resolution and 15 vertical levels. An ensemble of four simulations was forced with lateral boundary conditions from ECMWF global analyses for the period 8–22 August 1988. The simulated midtropospheric circulation includes the skillful development and movement of several African wave disturbances. Wavelet analysis of midtropospheric winds detected a dominant periodicity of about 4 days and a secondary periodicity of 5–8 days. Spatial distributions of RM precipitation and precipitation time series were validated against daily rain gauge measurements and International Satellite Cloud Climatology Project satellite infrared cloud imagery. The time–space distribution of simulated precipitation was made more realistic by combining the ECMWF initial conditions with a 24-h spinup of the moisture field and also by damping high-frequency gravity waves by dynamic initialization. Model precipitation “forecasts” over the central Sahel were correlated with observations for about 3 days, but reinitializing with observed data on day 5 resulted in a dramatic improvement in the precipitation validation over the remaining 9 days. Results imply that information via the lateral boundary conditions is not always sufficient to minimize departures between simulated and actual precipitation patterns for more than several days. In addition, there was some evidence that the new initialization may increase the simulations' sensitivity to the quality of lateral boundary conditions.
