The Amma Radiosonde Program and its Implications for the Future of Atmospheric Monitoring Over Africa

Parker, Douglas J.; Fink, Andreas H.; Janicot, Serge; Ngamini, Jean-Blaise; Douglas, Michael W.; Afiesimama, Ernest; Agustı́-Panareda, Anna; Beljaars, Anton; Didé, Francis; Diedhiou, Arona; Lebel, Thierry; Polcher, Jan; Redelsperger, Jean‐Luc; Thorncroft, Chris D.; Wilson, George Ato

doi:10.1175/2008bams2436.1

Cited by 89 publications

(92 citation statements)

References 9 publications

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“…Using the latest ECMWF reanalyses product for the West African non-dipole wet year 2007, the conceptual model of rainfall enhancement factors during these years, as it was recently proposed by Nicholson (2009), is largely supported. Note that 2007 likely has the largest abundance of upperair data that went into the analyses ever since, due to the AMMA upper-air campaign (Parker et al 2008). One notable consistency with Nicholson's model is that the TEJ was much stronger and displaced southward.…”

Section: Discussionmentioning

confidence: 74%

Meteorological characteristics and potential causes of the 2007 flood in sub‐Saharan Africa

Paeth

Fink

Pohle

et al. 2011

Intl Journal of Climatology

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ABSTRACT:In August and September 2007 media and aid organizations reported on a severe flood in large parts of sub-Saharan Africa caused by a sequence of extreme rain events. However, the reporting was partly controversial and much was speculated about how unusual the flood event was. Here, we analyse various observational data sets in order to assess and to compare the spatio-temporal characteristics and intensity of the flood and the associated rain events. The return times are estimated from daily precipitation time series at fine spatial resolution, and the potential causes are disclosed in a global and regional context.Satellite data reveal that the flood was not large scale but confined to the main river basins in sub-Saharan West Africa. Nonetheless, abundant rainfall prevailed over large parts of Western Africa extending north into the Western Sahara, particularly during the second half of August and the beginning of September 2007. In detail, various precipitation data sets differ considerably in terms of the monthly anomalies, demonstrating the difficulty to delineate meteorological extreme events even at a sub-continental scale and during the most recent past. Return times typically range between 1 and 50 years with high spatial heterogeneity but amount to 1200 years in the regional mean over the Upper Volta Basin. Among the potential causes we identify a La Niña event in the tropical Pacific, anomalous heating in the tropical Atlantic coming along with a greater depth of the monsoonal westerlies, and enhanced activity of African easterly waves, both evident in 2-6 day zonal wind and outgoing longwave radiation variance anomalies.

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Section: Discussionmentioning

confidence: 74%

Meteorological characteristics and potential causes of the 2007 flood in sub‐Saharan Africa

Paeth

Fink

Pohle

et al. 2011

Intl Journal of Climatology

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“…This event occurred during one of the Special Observing Periods, SOP2, of the African Monsoon Multidisciplinary Analysis (AMMA) field campaign in West Africa (Redelsperger et al, 2006). As part of the SOPs, the upper-air network was greatly enhanced over the region, with unprecedented spatial coverage across West Africa, and much better transmission of data to the international weather prediction centres (Parker et al, 2008). Therefore, there has been no better-observed period for which to document AEW behaviour using upper-air measurements and the model analyses which make use of these measurements.…”

Section: Introductionmentioning

confidence: 99%

Anatomy of an observed African easterly wave in July 2006

Bain

Parker

Dixon

et al. 2011

Quart J Royal Meteoro Soc

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The detailed structure of an African easterly wave (AEW) observed during the AMMA field campaign is analysed. The wave was present from 25 to 29 July 2006. A complex circulation pattern was observed: the overall structure of convection and the positive vorticity of the trough region had an elongated inverted-V appearance, wrapped around an area of low winds and clear skies. Satellite imagery showed that the AEW was a significant influence on the modulation of convection on the large scale.The wave was identified initially through its strong signature on soil moisture and convection. The AEW structure observed was not anticipated and has not been discussed in previous literature. In addition, wave tracking using a Hovmöller diagram of meridional winds did not detect the wave, and a Hovmöller of vorticity showed the wave moved at a slower speed than other AEWs in July.New schematics explaining the structure are presented, describing the case as observed by satellites and analysed by a limited-area version of the Met Office Unified Model. It is proposed that the positive vorticity branches of the inverted-V can be regarded as analogous to atmospheric fronts, with characteristic gradients in winds and thermodynamic properties, acting as locations for enhanced convection. The implications of the new case are discussed in relation to previous theory and it is suggested that the accepted model of an idealised AEW is incomplete and should be extended to include more complex structures.

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“…The monsoon then retreats during and after September (Sultan and Janicot 2000). The position of the ITD also has a strong diurnal dependence (Parker et al 2005;Sultan et al 2007;Pospichal et al 2010). Dry convective turbulence produced by surface heating raises the turbulent viscosity of the monsoon flow, suppressing circulation during the day.…”

Section: Introductionmentioning

confidence: 99%

“…Dry convective turbulence produced by surface heating raises the turbulent viscosity of the monsoon flow, suppressing circulation during the day. At night the increased stability allows for the monsoon front to advance north under the influence of the Saharan heat low (SHL) (Parker et al 2005;Burton et al 2013). …”

Section: Introductionmentioning

confidence: 99%

“…Processes that are known to influence the position of the ITD include interactions with the extratropics Vizy and Cook 2009;Knippertz and Todd 2010;Roehrig et al 2011; Denotes Open Access content. Roberts and Knippertz 2014), changes to the strength of the SHL (Parker et al 2005), African easterly waves (AEWs) on the African easterly jet (AEJ) (Berry and Thorncroft 2005;Couvreux et al 2010;Bain et al 2011), convectively generated cold pools (Flamant et al 2007;Cuesta et al 2010;Marsham et al 2008Marsham et al , 2013bGarcia-Carreras et al 2013;Roberts and Knippertz 2014), and eastward-propagating convectively coupled Kelvin waves (Mounier et al 2007;Mekonnen et al 2008;Mera et al 2014). Increased boundary layer moisture south of the ITD, in regions where the monsoon flow is deep, allows for the production of mesoscale convective systems (MCSs).…”

Section: Introductionmentioning

confidence: 99%

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Disagreements in Low-Level Moisture between (Re)Analyses over Summertime West Africa

Roberts

Marsham

Knippertz

2015

Monthly Weather Review

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Reanalysis and operational analysis products are routinely used as the best estimates of the atmospheric state for operational and research purposes. However, different models, assimilation techniques, and assimilated datasets lead to differences between products. Here, such differences in the distribution of low-level water vapor over summertime West Africa are analyzed, as reflected in the zonal mean position of the leading edge of the West African monsoon [the intertropical discontinuity (ITD)] using five reanalyses [NCEP-NCAR, NCEP-Department of Energy (DOE), the Modern-Era Retrospective Analysis for Research and Applications (MERRA), the Climate Forecast System Reanalysis (CFSR), and the Interim ECMWF ReAnalysis (ERA-Interim)] and two operational analyses [Global Forecast System (GFS) and ECMWF] during the 11 monsoon seasons (April-September) from 2000 to 2010. Specific humidity differences regularly reach 50% of the mean value over areas spanning hundreds of kilometers and often coincide with northward excursions of the ITD that last several days and bring unusual rainfall to the Sahel and Sahara. The largest disagreements occur during the southward retreat of the ITD and are connected with anomalously high values of aerosol optical depth, consistent with the production of haboob dust storms. The results suggest that known errors in the representation of moist convection and cold pools may contribute to the identified disagreements. A large reduction in disagreement occurs in 2006, when upper-air observations were enhanced during the African Monsoon Multidisciplinary Analysis (AMMA) campaign, pointing to an insufficient observational constraint of the (re)analyses in other years. It is hoped that this work will raise awareness of the limited reliability of (re)analysis products over West Africa during the summer, particularly during northward surges of the ITD, and will instigate further work to improve their quality.

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The Amma Radiosonde Program and its Implications for the Future of Atmospheric Monitoring Over Africa

Cited by 89 publications

References 9 publications

Meteorological characteristics and potential causes of the 2007 flood in sub‐Saharan Africa

Meteorological characteristics and potential causes of the 2007 flood in sub‐Saharan Africa

Anatomy of an observed African easterly wave in July 2006

Disagreements in Low-Level Moisture between (Re)Analyses over Summertime West Africa

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