Meteorological and dust aerosol conditions over the western Saharan region observed at Fennec Supersite‐2 during the intensive observation period in June 2011

Abstract

International audienceWe describe observations from the Fennec supersite-2 (SS2) at Zouerate, Mauritania during the June 2011 Fennec Intensive Observation Period. These provide an improved basis for understanding and evaluating processes, models and remote sensing. Conditions during June 2011 show a marked distinction between; (i) a 'Maritime phase' during the early part of the month when the western sector of the Sahara experienced cool northwesterly maritime flow throughout the lower troposphere with shallow daytime boundary layers, very little dust uplift/transport or cloud cover. (ii) A subsequent 'heat low' phase which coincided with a marked and rapid westward shift in the Saharan Heat Low towards its mid-summer climatological position and advection of a deep hot, dusty air layer from the central Sahara (the 'Saharan Residual Layer'). This transition affected the entire western-central Sahara. Dust advected over SS2 was primarily from episodic low-level jet (LLJ)-generated emission in the northeasterly flow around surface troughs. Unlike Fennec SS1, SS2 does not often experience cold pools from moist convection, and associated dust emissions. The diurnal evolution at SS2 is strongly influenced by the Atlantic Inflow (AI), a northwesterly flow of shallow, cool and moist air propagating overnight from coastal West Africa to reach SS2 in the early hours. The AI cools and moistens the western Saharan and weakens the nocturnal LLJ, limiting its dust-raising potential. We quantify the ventilation and moistening of the western flank of the Sahara by (i) the large-scale flow and (ii) the regular nocturnal AI and LLJ meso-scale processes

A meteorological and chemical overview of the DACCIWA field campaign in West Africa in June–July 2016

“…The inflow of cool maritime air from the Atlantic Ocean leads to an abrupt ventilation of the SHL, causing a weakening of its intensity and rapid eastward shift in the centre (Fig. 4) with some resemblance to the situation Todd et al (2013) refer to as the "maritime phase". According to http://misva.sedoo.fr, the intra-seasonal SHL index reached a distinct maximum on 17 and 18 June 2016.…”

“…4, this section aims to discuss the impact of these variability patterns on low clouds and atmospheric composition, two particular scientific interests of the DACCIWA project (Knippertz et al, 2015a). Evaluating the behaviour of low clouds is difficult over SWA in summer due to a relatively sparse observational network at the surface and regular obscuring by mid-and high-level clouds (van der Linden et al, 2015). Figure 17 shows the fraction of low clouds (defined here as cloud-top pressure of 800 hPa and lower) daily at 00:00 UTC in the 5-10 • N, 8 • W-8 • E box (see Fig.…”

The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

“…A distinction can be made between the cooler, drier, less dusty Saharan "maritime" phase from around 8 to 12 June to a hotter, moister, dustier "heat low" phase from around 13 to 30 June (Fig. 2a), during which time both synoptic-scale monsoon surges and mesoscale convective cold-pool events transported both water vapour and dust into the heart of the SHL (see Ryder et al, 2015;Todd et al, 2013 for full details). For comparison, profiles of water vapour from ERA-I reanalysis are shown in Fig.…”

“…This information is necessary to resolve the processes that govern the Figure 1. Climatological state of the Saharan heat low region (mean of June, 1979June, -2013: SHL location, low-level circulation, and dust load. Shaded: the mean position of heat low region (occurrence frequency of 90 % of low-level atmospheric thickness); arrows: mean 925 hPa wind; blue line: the mean position of the intertropical discontinuity from ERA-Interim reanalysis data and aerosol optical depth (AOD) from Multi-angle Imaging Spectroradiometer (MISR) satellite data (contour intervals are 0.4, 0.6, and 0.8 for grey, white, and cyan lines).…”

How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

“…In North Africa the heat low moves from positions near the equator in the east between November and March towards West Africa between April and October (Lavaysse et al, 2009). The Saharan heat low during summer is typically quasi-stationary over several days to weeks (Lavaysse et al, 2009;Todd et al, 2013) and coincides with high concentrations of dust aerosol (e.g. Knippertz and Todd, 2010).…”