Gravity wave parameters over the west African area derived from the AMMA campaign (African Monsoon Multidisciplinary Analyses)

Using local (rainfall data) and global satellite observations (atmospheric fields reanalysis, OLR data, satellite-observed brightness temperature data and cloud cluster data), the activity of convective equatorial waves revealed to be strongly coupled with the West African Monsoon (WAM) variability. The present study analyzed convective gravity wave characteristics in the lower stratosphere (19-23 km) above the tropical and equatorial West African zone (4°N–20°N and 20°W–15°E). Wave activity and spectral characteristics (period, vertical and horizontal wavelengths) are determined for the WAM period from May to September in 2006 using high vertical resolution radiosonde data obtained during the African Monsoon Multidisciplinary Analyses (AMMA) campaign. The annual cycle of total energy density above the tropical sites reveals weak values during the dry period and a peak of activity at the highest activity of convection above the tropical sites (Kafando et al., 2008). As opposed two peaks of enhanced activity matching with the period of intense convection are observed above the equatorial area. Monthly values of spectral parameters, derived from the Stokes parameter technique (Chane-Ming et al., 2009), suggest the presence of inertia-gravity wave over the tropical area with periods of 1.1–2.1 days and horizontal wavelengths lying from 1300 km to 3900 km. Mixed tropical waves are detected over the equatorial zone with periods of 2.1–5.1 days and horizontal wavelengths of 1400–5200 km. Spectral analyses of ECMWF wind reanalyses confirm wave period values derived from radiosonde observations.