• Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North Africa. J. Atmos. Sci.,29, 77–90.

  • Druyan, L., and T. Hall, 1994: Studies of African wave disturbances with the GISS GCM. J. Climate,7, 261–276.

  • ——, and ——, 1996: The sensitivity of African wave disturbances to remote forcing. J. Appl. Meteor.,35, 1100–1110.

  • ——, P. Lonergan, and M. Saloum, 1996: African wave disturbances and precipitation at Niamey during July–August 1987 and 1988. Climate Res.,7, 71–83.

  • Duvel, J., 1988: Analysis of diurnal, interdiurnal and interannual variations during Northern Hemisphere summers using METEOSAT infrared channels. J. Climate,1, 471–484.

  • ——, 1990: Convection over tropical Africa and the Atlantic Ocean during northern summer. Part II: Modulation by easterly waves. Mon. Wea. Rev.,118, 1855–1868.

  • Fortune, M., 1980: Properties of African squall lines inferred from time-lapse satellite imagery. Mon. Wea. Rev.,108, 153–168.

  • Lamb, P., and R. Peppler, 1991: West Africa. Teleconnections: Linkages Between ENSO, Worldwide Climate Anomalies, and Societal Impacts, M. H. Glantz, R. W. Katz, and N. Nicholls, Eds., Cambridge University Press, 121–189.

  • Landsea, C., and W. Gray, 1992: The strong association between Western Sahelian monsoon rainfall and intense Atlantic hurricanes. J. Climate,5, 435–453.

  • Reed, R., D. Norquist, and E. Recker, 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE. Mon. Wea. Rev.,105, 317–333.

  • ——, A. Hollingsworth, W. Heckley, and F. Delsol, 1988a: An evaluation of the performance of the ECMWF operational system in analyzing and forecasting easterly wave disturbances over Africa and the tropical Atlantic. Mon. Wea. Rev.,116, 824–865.

  • ——, E. Klinker, and A. Hollingsworth, 1988b: The structure and characteristics of African easterly wave disturbances as determined from the ECMWF operational analysis/forecast system. Meteor. Atmos. Phys.,38, 22–33.

  • Trenberth, K. E., 1992: Global analyses from ECMWF. National Center for Atmospheric Research TN-373+STR, 191 pp. [Available from NCAR, P.O. Box 3000, Boulder, CO 80307-3000.].

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Case Studies of African Wave Disturbances in Gridded Analyses

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  • 1 Center for Climate Systems Research, Columbia University, New York, New York
  • | 2 Science Systems and Applications, Inc., NASA/Goddard Institute for Space Studies, New York, New York
  • | 3 Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts
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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.

Corresponding author address: Dr. Leonard M. Druyan, Center for Climate Systems Research, Columbia University, Armstrong Hall, 2880 Broadway, New York, NY 10025.

Email: ldruyan@giss.nasa.gov

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.

Corresponding author address: Dr. Leonard M. Druyan, Center for Climate Systems Research, Columbia University, Armstrong Hall, 2880 Broadway, New York, NY 10025.

Email: ldruyan@giss.nasa.gov

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