Regional Model Simulations of the Bodélé Low-Level Jet of Northern Chad during the Bodélé Dust Experiment (BoDEx 2005)

Abstract

The low-level jet (LLJ) over the Bodélé depression in northern Chad is a newly identified feature. Strong LLJ events are responsible for the emission of large quantities of mineral dust from the depression, the world's largest single dust source, and its subsequent transport to West Africa, the tropical Atlantic, and beyond. Accurate simulation of this key dust-generating atmospheric feature is, therefore, an important requirement for dust models. The objectives of the present study are (i) to evaluate the ability of regional climate models (RCMs) and global analyses/reanalyses to represent this feature, and (ii) to determine the driving mechanisms of the LLJ and its strong diurnal cycle. Observational data obtained during the Bodélé Dust Experiment (BoDEx 2005) are utilized for comparison. When suitably configured, the fifth-generation Pennsylvania State University-NCAR Mesoscale Model (MM5) RCM can represent very accurately many of the key features of the jet including the structure, diurnal cycle, and day-to-day variability. Surface winds are also well reproduced, including the peak winds, which activate dust emission. Model fidelity is, however, strongly dependent on the boundary layer parameterization scheme, surface roughness, and vertical resolution in the lowest layers. A model horizontal resolution of a few tens of kilometers is sufficient to resolve most of the key features of the LLJ, while in global analyses/reanalyses many features of the LLJ are not adequately represented. Idealized RCM simulations indicate that under strong synoptic forcing the surrounding orography of the Tibesti and Ennedi Mountains acts to focus the LLJ onto the Bodélé and to accelerate the jet by ϳ40%. From the RCM experiments it is diagnosed that the pronounced diurnal cycle of the Bodélé LLJ is largely a result of varying eddy viscosity, with elevated heating/cooling over the Tibesti Mountains to the north as a second-order contribution.

Multi‐platform observations of a springtime case of Bodélé and Sudan dust emission, transport and scavenging over West Africa

“…(iii) Strong northeasterly low-level winds were also seen at 0000 UTC to blow between the Tibesti and Ennedi mountains in Chad, and over the Bodélé depression, where dust emission events are associated with an enhanced low-level jet (LLJ) feature Washington et al, 2006;Todd et al, 2008). At 1200 UTC, the channelling of the low-level flow through the gap between the Tibesti and the Ennedi is still visible, even though much weaker than at 0000 UTC.…”

“…that dust emission from the Bodélé is related to surface wind speed maxima which occur in the hours after sunrise. This phase shift in the diurnal cycle of the LLJ and surface winds is associated the downward transfer of momentum from the nocturnal LLJ to the surface due to turbulent mixing after solar heating commences each day (Todd et al, 2008). The AOD over West Sudan increased from 9 to 12 June (from 0.72 to 1.13), and decreased afterwards to 0.73 ( Figure 5(a)).…”

A process-based evaluation of dust-emitting winds in the CMIP5 simulation of HadGEM2-ES

“…This result is surprising as winter is the time of year when the channeling of the prevailing north-easterly Harmattan winds cause favorable conditions for NLLJ formation Todd et al 2008;Fiedler et al 2013). Todd et al (2008) provide observational evidence that a stronger synoptic-scale pressure gradient across the Bodélé Depression causes high wind speeds throughout the day while the NLLJ mechanism enhances the winds during the following morning. It is likely that the vertical wind shear below the larger geostrophic winds in HadGEM2-ES leads to so much vertical mixing that the formation of NLLJs is prevented in some nights.…”

“…The evaluation shows a good agreement for the dust source activation frequency compared to satellite data from Schepanski et al (2009) in most areas, but does not allow a direct conclusion on the emitted dust amounts . Using the classification of the dust storm severity observed in the Bodélé Depression during the BoDEx campaign (Todd et al 2008) points to a good performance in dust emission magnitudes from T-EI-O . Figure 2 shows the fraction of dust emission per season for H2ES-I and all offline dust emission calculations with the model by Tegen et al (2002).…”

“…Parker et al 2005;Bain et al 2010;Pospichal et al 2010) and interaction of the atmospheric flow with mountains (e.g. Washington and Todd 2005;Todd et al 2008). Hourly output from HadGEM2-ES in combination with the automated detection tool for NLLJs by Fiedler et al (2013) offers the unique opportunity to statistically analyze this dust-emitting process on sub-daily scales over a climatological time period of 30 years for new insights into the model performance.…”