Impact of assimilating AMMA soundings on ECMWF analyses and forecasts

TitleImpact of assimilating AMMA soundings on ECMWF analyses and forecasts
Publication TypeMiscellaneous
Year of Publication2009
AuthorsAgusti-Panareda, A, Beljaars, ACM, Cardinali, C, Genkova, I, Thorncroft, C
Secondary TitleTechnical Memorandum

The field experiment of the African Monsoon Multidisciplinary Analysis(AMMA) project during the 2006 wet monsoon season provided an unprecedented number of radiosonde data over the West African region. This paper explores the usage and impact of this invaluable dataset in the ECWMWF analyses and forecasts. Assimilation experiment comparison between pre-AMMA and AMMA radiosonde network shows that the extra AMMA soundings have a significant impact on the low-level temperature over the Sahel and on the structure of the African easterly jet. However, the impact of the extra AMMA data on the forecast disappears after 24 hours. The soundings reveal large model biases in boundary layer temperature over northern and eastern Sahel, which are consistent with the well-known model biases in cloud, rainfall and radiation. Large analysis increments in temperature lead to increments in divergence and subsidence which act to suppress convection. Thus, the analysis increments appear to have an undesirable feedback on the cloud and temperature model bias. The impact of the AMMA soundings on the African easterly jet is to enhance and extend the jet streak to 15$^o$E, i.e. towards the eastern part of the Sahel. No observations are assimilated east of 15$^o$E at the level of the African easterly jet to support the jet enhancement further east. Comparison with independent atmospheric cloud motion vectors indicate that the African easterly jet in the analysis is too weak over this data-scarse region. This could have implications for the development of African easterly waves in the model forecast.Further experimentation by assimilating atmospheric motion vectors – currently not used – could address this problem.