Validation of the Space–Time Variability of African Easterly Waves Simulated by the CNRM GCM

J. P. Céron Ecole Nationale de la Météorologie–Unité de Météorologie Tropicale, Météo-France, Toulouse, France

Search for other papers by J. P. Céron in
Current site
Google Scholar
PubMed
Close
and
J. F. Guérémy Ecole Nationale de la Météorologie–Unité de Météorologie Tropicale, Météo-France, Toulouse, France

Search for other papers by J. F. Guérémy in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

The ability of the Centre National de Recherches Météorologiques (CNRM) general circulation model (GCM) to properly simulate the space–time variability of the African easterly waves (AEWs) has been examined over the period 1982–88. The relative vorticity at 850 hPa in the analyses of the European Centre for Medium-Range Weather Forecasts analyses has been compared with the CNRM GCM simulations carried out in the framework of the Atmospheric Model Intercomparison Project. Detailed results of two statistical diagnoses used in this study have been presented for the selected year of 1985 in order to evaluate against the results of extensive studies available for this year.

First, with the help of a space–time spectral analysis, it has been shown that the GCM produces only one main variance maximum for the waves located between the two analyzed variance maxima (consistent with the northern and southern components associated with AEWs). The waves in the GCM propagate more slowly than those from the analyses, mainly because of greater periods in the GCM. There is a qualitative agreement between the simulation and the analysis in terms of the seasonal evolution of the location and intensity of the variance maximum. Barotropic and baroclinic energy transfers seem to be quite correctly represented.

Second, using a complex empirical orthogonal functions analysis, the existence of two main modes of variability of AEWs has been illustrated. A first hypothesis is proposed for the two modes’ interpretation. The second mode seems to be well linked to the interannual rainfall variability over West Africa. A low-frequency modulation of the wave activity exists in the analyses. This modulation is only qualitatively duplicated in the GCM. Interannual variability of the variance shown by the analyses is quite well reproduced by the GCM, particularly the variance seesaw of the two main modes between dry and wet years. Nevertheless, the GCM does not seem able to correctly simulate the pattern of atypical years, such as 1987. Finally, by projecting the GCM dataset on the eigenvectors of the analyses, it has been shown that GCM time discrepancies could influence the quality of the space representation of the AEWs.

Corresponding author address: Dr. Jean-Pierre Céron, Ecole Nationale de la Météorologie–Unité de Météorologie Tropicale, 42 avenue G. Coriolis, F31057-Toulouse Cedex 1, France.

Email: Jean-Pierre.Ceron@meteo.fr

Abstract

The ability of the Centre National de Recherches Météorologiques (CNRM) general circulation model (GCM) to properly simulate the space–time variability of the African easterly waves (AEWs) has been examined over the period 1982–88. The relative vorticity at 850 hPa in the analyses of the European Centre for Medium-Range Weather Forecasts analyses has been compared with the CNRM GCM simulations carried out in the framework of the Atmospheric Model Intercomparison Project. Detailed results of two statistical diagnoses used in this study have been presented for the selected year of 1985 in order to evaluate against the results of extensive studies available for this year.

First, with the help of a space–time spectral analysis, it has been shown that the GCM produces only one main variance maximum for the waves located between the two analyzed variance maxima (consistent with the northern and southern components associated with AEWs). The waves in the GCM propagate more slowly than those from the analyses, mainly because of greater periods in the GCM. There is a qualitative agreement between the simulation and the analysis in terms of the seasonal evolution of the location and intensity of the variance maximum. Barotropic and baroclinic energy transfers seem to be quite correctly represented.

Second, using a complex empirical orthogonal functions analysis, the existence of two main modes of variability of AEWs has been illustrated. A first hypothesis is proposed for the two modes’ interpretation. The second mode seems to be well linked to the interannual rainfall variability over West Africa. A low-frequency modulation of the wave activity exists in the analyses. This modulation is only qualitatively duplicated in the GCM. Interannual variability of the variance shown by the analyses is quite well reproduced by the GCM, particularly the variance seesaw of the two main modes between dry and wet years. Nevertheless, the GCM does not seem able to correctly simulate the pattern of atypical years, such as 1987. Finally, by projecting the GCM dataset on the eigenvectors of the analyses, it has been shown that GCM time discrepancies could influence the quality of the space representation of the AEWs.

Corresponding author address: Dr. Jean-Pierre Céron, Ecole Nationale de la Météorologie–Unité de Météorologie Tropicale, 42 avenue G. Coriolis, F31057-Toulouse Cedex 1, France.

Email: Jean-Pierre.Ceron@meteo.fr

Save
  • Albignat, J. P., and R. J. Reed, 1980: The origin of African wave disturbances during phase III of GATE. Mon. Wea. Rev.,108, 1827–1839.

  • Barnett, T. P., 1983: Interaction of the monsoon and Pacific trade wind system at interannual time scales. Part I: The equatorial zone. Mon. Wea. Rev.,111, 756–773.

  • Bloomfield, P., and J. M. Davis, 1994: Orthogonal rotation of complex principal components. Int. J. Climatol.,14, 759–775.

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

  • ——, 1974: Characteristics of North African easterly waves during the summers of 1968 and 1969. J. Atmos. Sci.,31, 1556–1570.

  • ——, and R. J. Reed, 1982: Synoptic scale motions. The GATE Monograph, Garp Publ., No. 25, WMO/ICSU, 61–120.

  • Carlson, T. N., 1969a: Synoptic histories of three African disturbances that developed into Atlantic hurricanes. Mon. Wea. Rev.,97, 256–276.

  • ——, 1969b: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev.,97, 716–726.

  • Céron, J. P., and J. F. Guérémy, 1994: Numerical sensitivity studies on the West African monsoon: Initial conditions and land processes. Proc. Int. Conf. on Monsoon Variability and Prediction, Vol. 2, WCRP-84, WMO-TD 619, Trieste, Italy, WMO, 47–479.

  • Déqué, M., 1986: Analyse en composantes principales complexes. Note de travail de l’Etablissement d’Etudes et de Recherches Météorologiques, 152, 59 pp. [Available from Météo-France—CNRM/GMGEC, F31057 Toulouse, France.].

  • Desbois, M., T. Kayiranga, B. Gnamien, S. Guessous, and L. Picon, 1988: Characterization of some elements of the Sahelian climate and their interannual variations for July 1983, 1984 and 1985 from the analysis of METEOSAT ISCCP data. J. Climate,1, 867–904.

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

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

  • ECMWF, 1988: Data assimilation and the use of satellite data. ECMWF Seminar Proceedings, Vol. 1, Reading, United Kingdom, ECMWF, 314 pp.

  • Estoque, M. A., J. Shukla, and J. G. Jiing, 1983: African wave disturbances in a general circulation model. Tellus,35A, 287–295.

  • Gates, W. L., 1992: AMIP: The Atmospheric Model Intercomparison Project. Bull. Amer. Meteor. Soc.,73, 1962–1970.

  • Hastenrath, S., 1991: Climate Dynamics of the Tropics. Kluwer Academic, 488 pp.

  • Hayashi, Y., 1977: On the coherence between progressive and retroprogressive waves and a partition of space time power spectra into standing and traveling parts. J. Appl. Meteor.,16, 368–373.

  • ——, 1979: A generalized method of resolving transient disturbances into standing and traveling waves by space-time spectral analysis. J. Atmos. Sci.,36, 1017–1029.

  • ——, 1982: Space time spectral analysis and its applications to atmospheric waves. J. Meteor. Soc. Japan,60, 156–171.

  • Horel, J. D., 1984: Complex principal component analysis: Theory and examples. J. Climate Appl. Meteor.,23, 1660–1673.

  • Jenkins, G. M., and D.G. Watts, 1968: Spectral Analysis and Its Applications. Holden-Day Series, 517 pp.

  • Lare, A. R., and S. E. Nicholson, 1994: Contrasting conditions of surface water balance in wet years and dry years as a possible land surface–atmosphere feedback mechanism in the West African Sahel. J. Climate,7, 653–668.

  • Mahfouf, J. F., 1993: L’expérience d’intercomparaison AMIP: Simulation du climat 1979 1988 avec le modèle Emeraude. Note de travail du groupe de météorologie de grande échelle et climat 18, 32 pp. [Available from Météo-France—CNRM/GMGEC, F31057 Toulouse, France.].

  • Mass, C., 1979: A linear primitive equation model of African wave disturbances. J. Atmos. Sci.,36, 2075–2092.

  • Murakami, M., 1979: Large-scale aspects of deep convective activity over the GATE area. Mon. Wea. Rev.,107, 994–1013.

  • Nitta, T., and Y. Takayabu, 1985: Global analysis of the lower tropospheric disturbances in the tropics during the northern summer of the FGGE year. Part II: Regional characteristics of the disturbances. Pure Appl. Geophys.,123, 272–292.

  • Norquist, D. C., E. E. Recker, and R. J. Reed, 1977: The energetics of African wave disturbances as observed during the phase III of GATE. Mon. Wea. Rev.,105, 334–342.

  • Paradis, D., J.-P. Lafore, J.-L. Redelsperger, and V. Balaji, 1995: African easterly-waves and convection. Part I: Linear simulations. J. Atmos. Sci.,52, 1657–1679.

  • Payne, S. W., and M. M. McGarry, 1977: The relationship of satellite convective activity to easterly waves over West Africa and the adjacent ocean during phase III of GATE. Mon. Wea. Rev.,105, 413–420.

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

  • ——, A. Hollingsworth, W. A. 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.

  • Rennick, M. A., 1976: The generation of African waves. J. Atmos. Sci.,33, 1955–1969.

  • Saloum, M., 1993: Analysis of rain producing systems over Sahel region during the wet and dry period. Proc. First Int. Conf. of African Meteorological Society, Nairobi, Kenya, African Meteorological Society, 122–138.

  • Shaw, D. B., P. Lönnberg, A. Hollingsworth, and P. Undén, 1987: Data assimilation: The 1984/85 revisions of the ECMWF mass and wind analysis. Quart. J. Roy. Meteor. Soc.,113, 533–566.

  • Shove, D. J., 1946: A further, contribution to the meteorology of Nigeria. Quart. J. Roy. Meteor. Soc.,72, 105–110.

  • Thorncroft, C. D., and B. J. Hoskins, 1994a: An idealized study of African easterly waves. I: A linear view. Quart. J. Roy. Meteor. Soc.,120, 953–982.

  • ——, and ——, 1994b: An idealized study of African easterly waves. II: A nonlinear view. Quart. J. Roy. Meteor. Soc.,120, 983–1015.

  • Tiedke, M., W. A. Heckley, and J. Slingo, 1988: Tropical forecasting at ECMWF: The influence of physical parameterization on the mean structure of forecasts and analyses. Quart. J. Roy. Meteor. Soc.,114, 639–664.

  • Toledo Machado, L. A., J. P. Duvel, and M. Desbois, 1993: Diurnal variations and modulation by easterly waves of the size distribution of convective cloud clusters over West Africa and the Atlantic Ocean. Mon. Wea. Rev.,121, 37–49.

  • Unden, P., 1989: Tropical data assimilation and analysis of divergence. Mon. Wea. Rev.,117, 2495–2517.

  • Walker, J., and P. R. Rowntree, 1977: The effect of soil moisture on circulation and rainfall in a tropical model. Quart. J. Roy. Meteor. Soc.,103, 29–46.

  • Wallace, J. M., and R. E. Dickinson, 1972: Empirical orthogonal representation of time series in the frequency domain. Part I: Theoretical considerations. J. Appl. Meteor.,11, 887–892.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 265 35 8
PDF Downloads 88 65 20