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On the synoptic climatology of summer rainfall over central Sudan

Synoptische Klimatologie des Sommerregens im mittleren Sudan

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Summary

The rainy season lasts from May through October in the southern parts of central Sudan, being concentrated to July and August in the latitude of Khartoum, while the extreme north of the country has no proper rainy season. In relation to this temporal and spatial variation of rainfall, development and decay of the summer circulation over the Sudan are studied on the basis of mean monthly pressure, relative humidity, surface streamline charts, 300 mb contour maps, and meridional-vertical cross-sections. The Intertropical Front (ITF) is identified as a discontinuity in the surface moisture and wind field.

From this analysis, the ITF is found to shift gradually northward, reaching its extreme positions of 18°N in July and 19°N in August. At this time of the year, an upper-tropospheric Easterly Jet develops somewhat south of the surface position of the ITF. The retreat of the rainy season from September onwards is characterized by a southward shift of the ITF in the lower layers and the disappearance of the Tropical Easterly Jet in the upper-troposphere. In contrast to the ten-year mean conditions, the ITF advanced as far as 19.5°N during the anomalously wet July 1958, while it stayed south of 17°N during the abnormally dry July 1966. Synoptic case studies for selected periods hint at the role of the upper-tropospheric circulation in hampering the rainfall activity, but hypotheses await testing on the basis of ampler observational data.

Zusammenfassung

Die Regenzeit dauert im südlichen Teil des mittleren Sudans von Mai bis Oktober und ist im Juli und August auf die Breite von Khartum konzentriert, während der nördliche Teil des Landes keine eigentliche Regenzeit hat. Mit Bezug auf diese zeitlichen und räumlichen Änderungen des Niederschlags werden die Entwicklung und der Verfall der sommerlichen Zirkulation über dem Sudan auf Grund von Monatsmittelwerten des Luftdrucks, der relativen Feuchtigkeit, Karten der Strömungslinien am Boden, Karten der 300mb-Topographie und vertikaler Meridianschnitte untersucht. Die Intertropikfront (ITF) wird als Diskontinuität der Feuchtigkeit am Boden und des Windfeldes festgelegt.

Die Analyse ergibt, daß die ITF sich allmählich im Juli bis 18°N und im August bis 19°N verlagert. In dieser Jahreszeit entwickelt sich ein oberer troposphärischer Ost-Jet Stream etwas südlich der ITF. Der Rückzug der Regenzeit ab September ist durch eine Südverlagerung der ITF in den unteren Schichten und durch ein Verschwinden des tropischen Ost-Jet Stream in der oberen Troposphäre charakterisiert. In Abweichung von den Verhältnissen im zehnjährigen Mittel hat sich im abnormal feuchten Juli 1958 die ITF bis 19,5°N verlagert, während sie im abnormal trockenen Juli 1966 südlich von 17°N blieb. Synoptische Untersuchungen ausgewählter Perioden deuten auf die Bedeutung der Zirkulation in der oberen Troposphäre für eine Hemmung der Niederschlagsaktivität hin; diese Hypothese bedarf aber noch der Prüfung durch weitere Beobachtungsdaten.

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Author notes

  1. O. E. Osman

    Present address: Sudan Meteorological Department, P. O. Box 574, Khartoum, Sudan

  2. S. L. Hastenrath

    Present address: Department of Meteorology, The University of Wisconsin, 1225 West Dayton Street, 53706, Madison, WI, USA

Authors and Affiliations

  1. Department of Meteorology, The University of Wisconsin, 1225 West Dayton Street, 53706, Madison, WI, USA

    O. E. Osman

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  1. O. E. Osman
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  2. S. L. Hastenrath
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Osman, O.E., Hastenrath, S.L. On the synoptic climatology of summer rainfall over central Sudan. Arch. Met. Geoph. Biokl. B. 17, 297–324 (1969). https://doi.org/10.1007/BF02243371

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