Abstract
Abstract. Gravity wave activity is analysed in the lower stratosphere using 6 year radiosonde data (2001–2006) above two meteorological stations in the West African tropical region such as Niamey (13.47° N; 2.16° E) and Ouagadougou (12.35° N; 1.51° W). Monthly total energy density of gravity waves is computed with temperature and horizontal wind perturbations to highlight the West African Monsoon period from June to September. Comparison with monthly total energy density calculated with temperature only supports that observed small-scale temperature and wind perturbations are mostly associated with gravity waves in the lower stratosphere especially for large values during the wet season. Above the two sites, monthly evolution of gravity wave total energy density reveals a maximum intensity of gravity wave activity in July during the West African Monsoon period. Indicators of convective activity such as mean Outgoing Longwave Radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) rain rates reveal to be adequate monsoon proxies to be compared to gravity wave energy intensity during the West African Monsoon.
Highlights
Tropical Gravity Waves (GWs) are known to contribute to the coupling of the atmospheric circulation from the lower atmosphere to the upper atmosphere by the vertical transport of momentum, energy and to impact on chemical constituents distribution
No evidence of GW activity is observed during the West African Monsoon (WAM) as opposed to observations of variance of GWs with vertical wavelengths>10 km derived from radiance fluctuations observed by the Microwave Limb Sounder (MLS) experiment aboard the Upper Atmospheric Research Satellite (UARS) in the upper stratosphere (Jiang et al, 2004b)
Correlation coefficients indicate that observed temperature and horizontal wind perturbations above Niamey and Ougadougou in the Lower Stratosphere (LS) are mostly produced by GWs especially for large total energy density in www.ann-geophys.net/26/4081/2008/
Summary
Tropical Gravity Waves (GWs) are known to contribute to the coupling of the atmospheric circulation from the lower atmosphere to the upper atmosphere by the vertical transport of momentum, energy and to impact on chemical constituents distribution. Global maps of variation of GW potential energy in the Lower Stratosphere (LS), derived from the data of Global Positioning System/Meteorology (GPS/MET) satellites, suggest that atmospheric waves are actively generated by tropical convection (Tsuda et al, 2000). No evidence of GW activity is observed during the West African Monsoon (WAM) as opposed to observations of variance of GWs with vertical wavelengths>10 km derived from radiance fluctuations observed by the Microwave Limb Sounder (MLS) experiment aboard the Upper Atmospheric Research Satellite (UARS) in the upper stratosphere (Jiang et al, 2004b). In this last study, strong GW activities are well-correlated with deep convection in monsoon areas and at latitudes between 5◦ and 25◦ N above the Africa.
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