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Todd M.C.,University of Sussex | Todd M.C.,Aeroqual Ltd | Allen C.J.T.,University of Oxford | Bart M.,University of Leeds | And 25 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

The climate of the Sahara is relatively poorly observed and understood, leading to errors in forecast model simulations. We 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 mesoscale processes. Key Points First detailed observations from western Sahara sector Intraseasonal shift in Saharan heat low drives meteorological/aerosol conditions Atlantic Inflow interaction with low level jet ©2013. American Geophysical Union. All Rights Reserved.


Marsham J.H.,University of Leeds | Hobby M.,University of Leeds | Allen C.J.T.,University of Oxford | Banks J.R.,Imperial College London | And 17 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

We describe observations from the Fennec supersite at Bordj Badji Mokhtar (BBM) made during the June 2011 Fennec Intensive Observation Period. These are the first detailed in situ observations of meteorology and dust from the central Sahara, close to the center of the Saharan heat low and the summertime dust maximum. Historically, a shortage of such Saharan observations has created problems for evaluating processes, models, and remote sensing. There was a monsoon influence at BBM before 8 June and after 12 June, with dry Harmattan winds in between. A split boundary layer, generated by ventilation from the Atlantic, persisted during the drier phase. Extensive cold pools (haboobs) and microburst-type events were regularly observed. Moisture reached BBM at night from the monsoon and the embedded haboobs. As well as the regularly occurring nocturnal low-level jet (LLJ), a Saharan upper boundary layer (650 hPa) jet was observed, where winds feel drag from dry convection in the afternoon. This jet is linked to the diurnal cycles of moisture and cloud. Most dust was observed in the cloudier monsoon-affected periods, and covarying dust and cloud amounts explain most of the variations in shortwave radiation that control the surface sensible flux. Dustiness is related to a standard parameterization of uplift using 10 m winds ("uplift potential"), and this is used to estimate uplift. Around 50% of uplift is nocturnal. Around 30% is from the LLJ, and 50% is from haboobs, which are mainly nocturnal. This demonstrates, for the first time from observations, the key role of haboobs, which are problematic for models. Key PointsFirst detailed observations from the central Sahara (upper BL jet observed)Most dust in moist air. Together, dust & cloud control surface energy balance~50% of dust uplift is nocturnal. ~30% from the LLJ and ~50% from haboobs ©2013. American Geophysical Union. All Rights Reserved.


Garcia-Carreras L.,University of Leeds | Marsham J.H.,University of Leeds | Parker D.J.,University of Leeds | Bain C.L.,UK Met Office | And 5 more authors.
Geophysical Research Letters | Year: 2013

Radiosonde data from Fennec supersite-1 in the remote central Sahara have been used to evaluate the impact of convectively generated cold pool outflows on model errors. Model predictions are too warm and dry, with cold pools contributing significantly to the mean bias. Although dust concentrations are high within cold pools, the sign of the errors is inconsistent with radiative impacts of dust. Cold pools cause 29% of the meridional humidity flux, but this contribution is absent in the forecast and analysis. Assimilating radiosondes reduces the errors, but significant temperature and meridional humidity flux biases remain at night. The model biases are consistent with the larger-scale heat low biases in the operational Met Office Unified Model and can be linked to known issues with convective parameterizations used in all global weather and climate models. This study suggests that the misrepresentation of moist convective processes can affect continental-scale biases, altering the West African monsoon circulation. Key Points First in-situ profiles from the central Sahara are used to evaluate model biases Lack of cold pools outflows from convection is main cause of warm dry model bias This error from convection has a regional impact, important in all global models. ©2013. American Geophysical Union. All Rights Reserved.

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