Abstract
AbstractDuring the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) field campaign ∼900 radiosondes were launched from 12 stations in southern West Africa from 15 June to 31 July 2016. Subsequently, data‐denial experiments were conducted using the Integrated Forecasting System of the European Centre for Medium‐range Weather Forecasts (ECMWF) to assess the radiosondes' impact on the quality of analyses and forecasts. As observational reference, satellite‐based estimates of rainfall and outgoing long‐wave radiation (OLR) as well as the radiosonde measurements themselves are used. With regard to the analyses, the additional observations show positive impacts on winds throughout the troposphere and lower stratosphere, while large lower‐tropospheric cold and dry biases are hardly reduced. Nonetheless, downstream, that is farther inland from the radiosonde stations, we find a significant increase (decrease) in low‐level night‐time temperatures (monsoon winds) when incorporating the DACCIWA observations, suggesting a possible linkage via weaker cold air advection from the Gulf of Guinea. The associated lower relative humidity leads to reduced cloud cover in the DACCIWA analysis. Closer to the coast and over Benin and Togo, DACCIWA observations increase low‐level specific humidity and precipitable water, possibly due to changes in advection and vertical mixing. During daytime, differences between the two analyses are generally smaller at low levels. With regard to the forecasts, the impact of the additional observations is lost after a day or less. Moderate improvements occur in low‐level wind and temperature but also in rainfall over the downstream Sahel, while impacts on OLR are ambiguous. The changes in precipitation appear to also affect high‐level cloud cover and the tropical easterly jet. The overall rather small observation impact suggests that model and data assimilation deficits are the main limiting factors for better forecasts in West Africa. The new observations and physical understanding from DACCIWA can hopefully contribute to reducing these issues.
Highlights
The climate of southern West Africa is characterized by the West African monsoon (e.g. Fink et al, 2017)
Vogel et al (2018) evaluate ensemble forecasts from nine global models participating in The International Grand Global Ensemble (TIGGE) project (Bougeault et al, 2010) for northern tropical Africa and demonstrate that even after sophisticated statistical post-processing, forecast skill barely exceeds that of climatology-based probabilistic approaches
While the main aim of Dynamics–Aerosol–Chemistry– Cloud Interactions in West Africa (DACCIWA) was to improve the understanding of the interaction between anthropogenic and natural emissions, clouds, radiation, rainfall and the circulation over southern West Africa, the extensive radiosonde measurements during the campaign offer a unique opportunity for data-denial experiments similar to those by Tompkins et al (2005) and Agustí-Panareda et al (2010)
Summary
The climate of southern West Africa is characterized by the West African monsoon (e.g. Fink et al, 2017). While the main aim of DACCIWA was to improve the understanding of the interaction between anthropogenic and natural emissions, clouds, radiation, rainfall and the circulation over southern West Africa, the extensive radiosonde measurements during the campaign offer a unique opportunity for data-denial experiments similar to those by Tompkins et al (2005) and Agustí-Panareda et al (2010). This way an evaluation can be provided of whether and in what sense the ECMWF IFS has improved over the last 10 years, here with a specific focus on southern West Africa.
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