Meteorology and dust in the central Sahara: Observations from Fennec supersite‐1 during the June 2011 Intensive Observation Period

Abstract

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.

Status and future of Numerical Atmospheric Aerosol Prediction with a focus on data requirements

“…This is also partly related to an underestimation of turbulent 580 dust emission during the day (Klose and Shao, 2012). Another substantial problem is the lack of dust generation related to cold pools (haboobs) associated with moist convection over the Sahel and Sahara (and many other desert areas), a process largely absent in models with parameterized convection (Marsham et al (2011);Heinold et al (2013); Pantillon et al (2015Pantillon et al ( , 2016). This leads to even reanalyses missing the summertime maximum in dust generating winds in the central Sahara 585 (Roberts et al, 2017).…”

“…Given the large diurnal cycle and the short lifetime of some dust-raising mechanisms, particularly moist convection, an hourly or better time resolution 595 would be desirable (Cowie et al, 2015;Bergametti et al, 2017). A first step in creating such a network was undertaken during the recent Fennec project (Hobby et al, 2013;Roberts et al, 2017).…”

“…Menut (2008) of the Saharan heat low, which is crucial for the large-scale circulation over northern Africa and thus a dominating factor for dust generation, can vary substantially between different analyses or model simulations with different resolution (Marsham et al, 2011). A much denser network of high-quality pressure observations is needed to better constrain models in this regard.…”

New developments in the representation of Saharan dust sources in the aerosol–climate model ECHAM6-HAM2

“…Aged cold pools and intermittent mixing of momentum from LLJs frequently produce dust-generating winds at night Fiedler et al, 2013). The cold outflow from moist convection, however, is not expected to occur in COSMO-MUSCAT, as moist convection is parameterized (Reinfried et al, 2009;Marsham et al, 2013). It is more likely that the dust emissions between noon and afternoon are modeled because of the reasons above and because of the downward mixing of momentum from a strong-wind layer in the free troposphere once the daytime boundary layer has grown sufficiently thick (Fiedler et al, 2015).…”

“…Further uncertainties in the MSG observations exist due to the sensitivity to atmospheric water vapor, the altitude of the dust layer, and the low contrast in the infrared signal between desert surface and dust at night (Ashpole and Washington, 2012;Brindley et al, 2012). At least for summer, ground-based observations in the central Sahara Marsham et al, 2013) and convection-permitting simulations for West Africa show a much larger contribution (30-50 %) by convective cold pools in the late afternoon and evening. Taking this into account, the model underestimation of morning dust emission events compared to the MSG observations appears less dramatic.…”