Fennec dust forecast intercomparison over the Sahara in June 2011

Abstract

Abstract. In the framework of the Fennec international programme, a field campaign was conducted in June 2011 over the western Sahara. It led to the first observational data set ever obtained that documents the dynamics, thermodynamics and composition of the Saharan atmospheric boundary layer (SABL) under the influence of the heat low. In support to the aircraft operation, four dust forecasts were run daily at low and high resolutions with convection-parameterizing and convection-permitting models, respectively. The unique airborne and ground-based data sets allowed the first ever intercomparison of dust forecasts over the western Sahara. At monthly scale, large aerosol optical depths (AODs) were forecast over the Sahara, a feature observed by satellite retrievals but with different magnitudes. The AOD intensity was correctly predicted by the high-resolution models, while it was underestimated by the low-resolution models. This was partly because of the generation of strong near-surface wind associated with thunderstorm-related density currents that could only be reproduced by models representing convection explicitly. Such models yield emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and the Aïr Mountains in the high-resolution forecasts. Over the western Sahara, where the harmattan contributes up to 80 % of dust emission, all the models were successful in forecasting the deep wellmixed SABL. Some of them, however, missed the large nearsurface dust concentration generated by density currents and low-level winds. This feature, observed repeatedly by the airborne lidar, was partly forecast by one high-resolution model only.

Importance of the Saharan heat low in controlling the North Atlantic free tropospheric humidity budget deduced from IASI <i>δ</i>D observations

“…In the summer, once the SHL settles in, the top of the Saharan air boundary layer (SABL) can reach an altitude of 6 to 7 km (∼ 550 hPa) due to the enhanced near-surface convergence in the SHL region. The SABL is fully developed in the late afternoon (Chaboureau et al, 2016). Dur- ing most of the day, while the mixed layer is developing, the characteristics of air masses in the upper part of the SABL (the residual layer) are controlled by advection from the east (Flamant et al, 2007Chaboureau et al, 2016).…”

“…The SABL is fully developed in the late afternoon (Chaboureau et al, 2016). Dur- ing most of the day, while the mixed layer is developing, the characteristics of air masses in the upper part of the SABL (the residual layer) are controlled by advection from the east (Flamant et al, 2007Chaboureau et al, 2016). As the warm and dry air moves off the African coast, the SABL rises and becomes the Saharan Air Layer (SAL) undercut by the cool and moist marine boundary layer.…”

Sensitivity of the WRF-Chem (V3.6.1) model to different dust emission parametrisation: assessment in the broader Mediterranean region

“…in summer), which tends to be fully mixed no earlier than around 18:00 LT. In the daytime, the SABL is composed of a convective mixing layer developing within a residual layer so that lidar and dropsonde data acquired around midday generally exhibit a two-layer structure (Ryder et al, 2015;Chaboureau et al, 2016). Dust concentrations within the lower half part of the SABL are representative of local emissions while the upper part is dominated by dust transport (Chaboureau et al, 2016).…”

A detailed characterization of the Saharan dust collected during the Fennec campaign in 2011: in situ ground-based and laboratory measurements

“…This project joined efforts to address open questions on atmospheric processes in central Sahara. Combining aircraft (Ryder et al, 2013a(Ryder et al, , 2015, ground-based Todd et al, 2013;Allen et al, 2013), modeling, and/or satellite observations (Banks et al, 2013;Chaboureau et al, 2016), the Fennec project successfully obtained a broad data set of meteorological conditions, atmospheric dynamics and structure, as well as dust emission and transport mechanisms for the central Saharan region (Washington et al, 2012).…”