Editorial Type:
Article
Article Type:
Research Article

Mesoscale Convective Systems over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

Brian Jackson Department of Meteorology, The Florida State University, Tallahassee, Florida

Search for other papers by Brian Jackson in
Current site
Google Scholar
PubMed Close
,
Sharon E. Nicholson Department of Meteorology, The Florida State University, Tallahassee, Florida

Search for other papers by Sharon E. Nicholson in
Current site
Google Scholar
PubMed Close
, and
Douglas Klotter Department of Meteorology, The Florida State University, Tallahassee, Florida

Search for other papers by Douglas Klotter in
Current site
Google Scholar
PubMed Close
Print Publication:
01 Apr 2009
DOI:
https://doi.org/10.1175/2008MWR2525.1
Page(s):
1272–1294
Restricted access

Abstract

This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria). The fourth coincides with a midtropospheric convergence maximum in the right entrance quadrant of the African easterly jet of the Southern Hemisphere (AEJ-S). This maximum is substantially stronger in the September–November rainy season, when the jet is well developed, than in the March–May rainy season, when the jet is absent. Lightning frequency and flashes per MCS are also greatest during September–November; maxima occur in the right entrance quadrant of the AEJ-S. The lightning maximum is somewhat south of the MCS maximum and coincides with the low-lying areas of central Africa. Overall, the results of this study suggest that large-scale topography plays a critical role in the spatial and diurnal patterns of convection, lightning, and rainfall in this region. More speculative is the role of the AEJ-S, but this preliminary analysis suggests that it does play a role in the anomalous intensity of convection in western equatorial Africa.

Corresponding author address: Sharon E. Nicholson, Department of Meteorology, The Florida State University, Tallahassee, FL 32308. Email: sen@met.fsu.edu

Abstract

This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria). The fourth coincides with a midtropospheric convergence maximum in the right entrance quadrant of the African easterly jet of the Southern Hemisphere (AEJ-S). This maximum is substantially stronger in the September–November rainy season, when the jet is well developed, than in the March–May rainy season, when the jet is absent. Lightning frequency and flashes per MCS are also greatest during September–November; maxima occur in the right entrance quadrant of the AEJ-S. The lightning maximum is somewhat south of the MCS maximum and coincides with the low-lying areas of central Africa. Overall, the results of this study suggest that large-scale topography plays a critical role in the spatial and diurnal patterns of convection, lightning, and rainfall in this region. More speculative is the role of the AEJ-S, but this preliminary analysis suggests that it does play a role in the anomalous intensity of convection in western equatorial Africa.

Corresponding author address: Sharon E. Nicholson, Department of Meteorology, The Florida State University, Tallahassee, FL 32308. Email: sen@met.fsu.edu

Save
Cover Monthly Weather Review
Monthly Weather Review

  • Ba, M. B., and S. E. Nicholson, 1998: Analysis of convective activity and its relationship to the rainfall over the Rift Valley lakes of East Africa during 1983–90 using the Meteosat infrared channel. J. Appl. Meteor., 37 , 1250–1264.

    • Search Google Scholar
    • Export Citation

  • Balas, N., S. E. Nicholson, and D. Klotter, 2007: The relationship of rainfall variability in West Central Africa to sea-surface temperature fluctuations. Int. J. Climatol., 27 , 1335–1349.

    • Search Google Scholar
    • Export Citation

  • Biggerstaff, M. I., and S. A. Listemaa, 2000: An improved scheme for convective/stratiform echo classification using radar reflectivity. J. Appl. Meteor., 39 , 2129–2150.

    • Search Google Scholar
    • Export Citation

  • Cecil, D. A., S. J. Goodman, D. J. Boccippio, E. J. Zipser, and S. W. Nesbitt, 2005: Three years of TRMM precipitation features. Part I: Radar, radiometric, and lightning characteristics. Mon. Wea. Rev., 133 , 543–566.

    • Search Google Scholar
    • Export Citation

  • Christian, H. J., R. J. Blakeslee, and S. J. Goodman, 1992: Lightning Imaging Sensor (LIS) for the Earth Observing System. NASA Tech. Memo. 4350, MSFC, Huntsville, AL, 36 pp.

    • Search Google Scholar
    • Export Citation

  • Cunningham, P., and D. Keyser, 2000: Analytical and numerical modeling of jet streaks: Barotropic dynamics. Quart. J. Roy. Meteor. Soc., 126 , 3187–3217.

    • Search Google Scholar
    • Export Citation

  • Dai, A., 2001: Global precipitation and thunderstorm frequencies. Part II: Diurnal variations. J. Climate, 14 , 1112–1128.

  • Dai, A., F. Giorgi, and K. E. Trenberth, 1999: Observed and model-simulated diurnal cycles of precipitation over the contiguous United States. J. Geophys. Res., 104 , 6377–6402.

    • Search Google Scholar
    • Export Citation

  • Duvel, J-P., 1989: Convection over tropical Africa and the Atlantic Ocean during northern summer. Part I: Interannual and diurnal variations. Mon. Wea. Rev., 117 , 2782–2799.

    • Search Google Scholar
    • Export Citation

  • Ekman, A. M. L., C. Wang, J. Wilson, and J. Strom, 2004: Explicit simulations of aerosol physics in a cloud-resolving model: A sensitivity study based on an observed convective cloud. Atmos. Chem. Phys., 4 , 773–791.

    • Search Google Scholar
    • Export Citation

  • Flohn, H., and K. Fraedrich, 1966: Tagesperiodische Zirkulation und Niederschlagsverteilung am Victoria-See (Ostafrika) (The daily periodic circulation and distribution of rainfall over Lake Victoria). Meteor. Rundsch., 19 , 157–165.

    • Search Google Scholar
    • Export Citation

  • Fraedrich, K., 1972: A simple climatological model of the dynamics and energetics of the nocturnal circulation at Lake Victoria. Quart. J. Roy. Meteor. Soc., 98 , 322–335.

    • Search Google Scholar
    • Export Citation

  • Futyan, J. M., and A. D. Del Genio, 2007: Deep convective system evolution over Africa and the tropical Atlantic. J. Climate, 20 , 5041–5060.

    • Search Google Scholar
    • Export Citation

  • Geerts, B., and T. Dejene, 2005: Regional and diurnal variability of the vertical structure of precipitation systems in Africa based on spaceborne radar data. J. Climate, 18 , 893–916.

    • Search Google Scholar
    • Export Citation

  • Grist, J. P., and S. E. Nicholson, 2001: A study of the dynamic factors influencing the interannual variability of rainfall in the West African Sahel. J. Climate, 14 , 1337–1359.

    • Search Google Scholar
    • Export Citation

  • Hodges, K. I., and C. D. Thorncroft, 1997: Distribution and statistics of African mesoscale convective weather systems based on the ISCCP Meteosat imagery. Mon. Wea. Rev., 125 , 2821–2837.

    • Search Google Scholar
    • Export Citation

  • Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77 , 437–471.

  • Keyser, D., and M. A. Shapiro, 1986: A review of the structure and dynamics of upper-level frontal zones. Mon. Wea. Rev., 114 , 452–499.

    • Search Google Scholar
    • Export Citation

  • Kinter III, J. L., M. J. Fennessy, V. Krishnamurthy, and L. Marx, 2004: An evaluation of the apparent interdecadal shift in the tropical divergent circulation in the NCEP–NCAR reanalysis. J. Climate, 17 , 349–361.

    • Search Google Scholar
    • Export Citation

  • Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and J. Simpson, 1998: The Tropical Rainfall Measuring Mission (TRMM) sensor package. J. Atmos. Oceanic Technol., 15 , 809–817.

    • Search Google Scholar
    • Export Citation

  • Kummerow, C., and Coauthors, 2000: The status of the Tropical Rainfall Measuring Mission (TRMM) after two years in orbit. J. Appl. Meteor., 39 , 1965–1982.

    • Search Google Scholar
    • Export Citation

  • Laing, A. G., and J. M. Fritsch, 1993a: Mesoscale convective complexes in Africa. Mon. Wea. Rev., 121 , 2254–2265.

  • Laing, A. G., and J. M. Fritsch, 1993b: Mesoscale convective complexes over the Indian monsoon region. J. Climate, 6 , 911–919.

  • Laing, A. G., and J. M. Fritsch, 1997: The global population of mesoscale convective complexes. Quart. J. Roy. Meteor. Soc., 123 , 389–405.

    • Search Google Scholar
    • Export Citation

  • Lin, X., D. A. Randall, and L. D. Fowler, 2000: Diurnal variability of the hydrologic cycle and radiative fluxes: Comparisons between observations and a GCM. J. Climate, 13 , 4159–4179.

    • Search Google Scholar
    • Export Citation

  • Liu, C., and E. J. Zipser, 2005: Global distribution of convection penetrating the tropical tropopause. J. Geophys. Res., 110 , D23104. doi:10.1029/2005JD006063.

    • Search Google Scholar
    • Export Citation

  • Liu, C., E. J. Zipser, and S. W. Nesbitt, 2007: Global distribution of tropical deep convection: Different perspective from TRMM infrared and radar data. J. Climate, 20 , 489–503.

    • Search Google Scholar
    • Export Citation

  • McCollum, J. R., A. Gruber, and M. B. Ba, 2000: Discrepancy between gauges and satellite estimates of rainfall in equatorial Africa. J. Appl. Meteor., 39 , 666–679.

    • Search Google Scholar
    • Export Citation

  • McGarry, M. M., and R. J. Reed, 1978: Diurnal variations in convective activity and precipitation during phases II and III of GATE. Mon. Wea. Rev., 106 , 101–113.

    • Search Google Scholar
    • Export Citation

  • Mekonnen, A., C. D. Thorncroft, and A. Aiyyer, 2006: Analysis of convection and its association with African easterly waves. J. Climate, 19 , 5405–5421.

    • Search Google Scholar
    • Export Citation

  • Mohr, K. I., 2004: Interannual, monthly, and regional variability in the wet season diurnal cycle of precipitation in sub-Saharan Africa. J. Climate, 17 , 2441–2453.

    • Search Google Scholar
    • Export Citation

  • Mohr, K. I., and E. J. Zipser, 1996a: Mesoscale convective systems defined by their 85-GHz ice-scattering signature: Size and intensity comparison over tropical oceans and continents. Mon. Wea. Rev., 124 , 2417–2437.

    • Search Google Scholar
    • Export Citation

  • Mohr, K. I., and E. J. Zipser, 1996b: Defining mesoscale convective systems by their 85-GHz ice-scattering signatures. Bull. Amer. Meteor. Soc., 77 , 1179–1189.

    • Search Google Scholar
    • Export Citation

  • Mohr, K. I., and C. Thorncroft, 2006: Intense convective systems in West Africa and their relationship to the African easterly jet. Quart. J. Roy. Meteor. Soc., 132 , 163–176.

    • Search Google Scholar
    • Export Citation

  • Mohr, K. I., J. S. Famiglietti, and E. J. Zipser, 1999: The contribution to tropical rainfall with respect to convective system type, size, and intensity estimated from 85-GHz ice-scattering signature. J. Appl. Meteor., 38 , 596–606.

    • Search Google Scholar
    • Export Citation

  • Nesbitt, S. W., and E. J. Zipser, 2003: The diurnal cycle of rainfall and convective intensity according to three years of TRMM measurements. J. Climate, 16 , 1456–1475.

    • Search Google Scholar
    • Export Citation

  • Nesbitt, S. W., E. J. Zipser, and D. J. Cecil, 2000: A census of precipitation features in the Tropics using TRMM: Radar, ice scattering, and ice observations. J. Climate, 13 , 4087–4106.

    • Search Google Scholar
    • Export Citation

  • Nesbitt, S. W., R. Cipelli, and S. A. Rutledge, 2006: Storm morphology and rainfall characteristics of TRMM precipitation features. Mon. Wea. Rev., 134 , 2702–2721.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 1986: The spatial coherence of African rainfall anomalies: Interhemispheric teleconnections. J. Climate Appl. Meteor., 25 , 1365–1381.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 2008: The intensity, location and structure of the tropical rainbelt over West Africa as factors in interannual variability. Int. J. Climatol., 28 , 1775–1785.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 2009a: A low-level jet along the Benguela coast, an integral part of the Benguela Marine Ecosystem. Climatic Change, in press.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., 2009b: On the factors modulating the intensity of the tropical rainbelt over West Africa. Int. J. Climatol., 29 , 673–689.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., and D. Entekhabi, 1987: Rainfall variability in equatorial and southern Africa: Relationships with sea surface temperatures along the southwestern coast of Africa. J. Climate Appl. Meteor., 26 , 561–578.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., and J. P. Grist, 2003: On the seasonal evolution of atmospheric circulation over West Africa and equatorial Africa. J. Climate, 16 , 1013–1030.

    • Search Google Scholar
    • Export Citation

  • Nicholson, S. E., and P. J. Webster, 2007: A physical basis for the interannual variability of rainfall in the Sahel. Quart. J. Roy. Meteor. Soc., 133 , 2065–2084.

    • Search Google Scholar
    • Export Citation

  • Oki, T., and K. Musiake, 1994: Seasonal change of the diurnal cycle of precipitation over Japan and Malaysia. J. Appl. Meteor., 33 , 1445–1463.

    • Search Google Scholar
    • Export Citation

  • ORSTOM, 1978: Précipitations journalières de l’origine des stations à 1972. Etienne Julien, Paris, Vol. 1, 512 pp.

  • Petersen, W. A., and S. A. Rutledge, 2001: Regional variability in tropical convection: Observations from TRMM. J. Climate, 14 , 3566–3586.

    • Search Google Scholar
    • Export Citation

  • Poccard, I., S. Janicot, and P. Camberlin, 2000: Comparison of rainfall structures between NCEP/NCAR reanalysis and observed data over tropical Africa. Climate Dyn., 16 , 897–915.

    • Search Google Scholar
    • Export Citation

  • Reed, R. J., and J. D. Jaffe, 1981: Diurnal variation of summer convection over West Africa and the tropical eastern Atlantic during 1974 and 1978. Mon. Wea. Rev., 109 , 2527–2534.

    • Search Google Scholar
    • Export Citation

  • Sherwood, S. C., 2002a: Aerosols and ice particle size in tropical cumulonimbus. J. Climate, 15 , 1051–1063.

  • Sherwood, S. C., 2002b: A microphysical connection among biomass burning, cumulus clouds, and stratospheric moisture. Science, 295 , 1271–1275.

    • Search Google Scholar
    • Export Citation

  • Sherwood, S. C., and R. Wahrlich, 1999: Observed evolution of tropical deep convective events and their environment. Mon. Wea. Rev., 127 , 1777–1795.

    • Search Google Scholar
    • Export Citation

  • Sherwood, S. C., T. J. Phillips, and J. S. Wettlaufer, 2006: Small ice crystals and the climatology of lightning. Geophys. Res. Lett., 33 , L05804. doi:10.1029/2005GL025242.

    • Search Google Scholar
    • Export Citation

  • Thorncroft, C. D., and M. Blackburn, 1999: Maintenance of the African easterly jet. Quart. J. Roy. Meteor. Soc., 125 , 763–786.

  • Thorncroft, C., and K. Hodges, 2001: African Easterly Wave variability and its relationship to Atlantic tropical cyclone activity. J. Climate, 14 , 1166–1179.

    • Search Google Scholar
    • Export Citation

  • Toracinta, E. R., and E. J. Zipser, 2001: Lightning and SSM/I-ice-scattering mesoscale convective systems in the global tropics. J. Appl. Meteor., 40 , 983–1002.

    • Search Google Scholar
    • Export Citation

  • Trenberth, K. E., and C. J. Guillemot, 1995: Evaluation of the global atmospheric moisture budget as seen from analyses. J. Climate, 8 , 2255–2272.

    • Search Google Scholar
    • Export Citation

  • Tripoli, G. J., 1986: A numerical investigation of an orogenic mesoscale convective system. Ph.D. dissertation, Colorado State University, 290 pp.

  • Tripoli, G. J., and W. R. Cotton, 1989a: Numerical study of an observed orogenic mesoscale convective system. Part I: Simulated genesis and comparison with observations. Mon. Wea. Rev., 117 , 273–304.

    • Search Google Scholar
    • Export Citation

  • Tripoli, G. J., and W. R. Cotton, 1989b: Numerical study of an observed orogenic mesoscale convective system. Part II: Analysis of governing dynamics. Mon. Wea. Rev., 117 , 305–328.

    • Search Google Scholar
    • Export Citation

  • Uccellini, L. W., and D. R. Johnson, 1979: The coupling of upper and lower tropospheric jet streaks and implications for the development of severe convective storms. Mon. Wea. Rev., 107 , 682–703.

    • Search Google Scholar
    • Export Citation

  • Wallace, J. M., 1975: Diurnal variations in precipitation and thunderstorm frequency over the conterminous United States. Mon. Wea. Rev., 103 , 406–419.

    • Search Google Scholar
    • Export Citation

  • Yang, G. Y., and J. Slingo, 2001: The diurnal cycle in the Tropics. Mon. Wea. Rev., 129 , 784–801.

  • Yang, S., and E. A. Smith, 2006: Mechanisms for diurnal variability of global tropical rainfall observed from TRMM. J. Climate, 19 , 5190–5226.

    • Search Google Scholar
    • Export Citation

  • Yin, X., and S. E. Nicholson, 1998: The water balance of Lake Victoria. J. Hydrol. Sci., 43 , 789–811.

  • Zipser, E. J., D. J. Cecil, C. Liu, S. W. Nesbitt, and D. P. Yorty, 2006: Where are the most intense thunderstorms on earth? Bull. Amer. Meteor. Soc., 87 , 1057–1071.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 1604 509 21
PDF Downloads 941 209 9