Dust emissions over the Sahel associated with the West African Monsoon inter-tropical discontinuity region: a representative case study Flashcards
……………………….. is the world’s largest source of mineral dust.
West Africa
West Africa is the world’s largest source of mineral dust. Satellite sen- sors consistently indicate that
dust aerosol plumes are the most widespread, persistent and dense found on Earth
The radiative effects of dust (both direct and indirect) can
modify the general circula- tion at climate time scales but also at shorter (synoptic) time scales
Dust emissions from West Africa follow a
distinct annual cycle
Dust emissions from West Africa follow a distinct annual cycle and are connected with
the occurrence of high wind speeds at the surface.
Satellite-based climato- logical studies (e.g. Evan et al., 2006) have demonstrated that dust exports from the Sahara during winter are asso- ciated with
strong harmattan winds
Dust emissions from West Africa follow a distinct annual cycle and are connected with the occurrence of high wind speeds at the surface. Satellite-based climato- logical studies (e.g. Evan et al., 2006) have demonstrated that dust exports from the Sahara during winter are asso- ciated with strong harmattan winds and take place in
discrete outbreaks of several days duration
the summer season is prone to
high dust activity
the summer season is prone to high dust activity and is the period when
most of the intense dust outbreaks over the North Atlantic occur
Occasional strong dust outbreaks have been docu- mented to be associated with
- the penetration of an upper- level trough to low latitudes,
- to low-level cold fronts or
- to density currents caused by evaporational cool- ing along precipitating cloud-bands over the northern Sahara and along the Sahara side of the Atlas Mountain chain in southern Morocco
In the Bode ́le ́ Depression, dust outbreaks have been reported in association with
strong surface winds generally occurring after sunrise, as tur- bulence in the growing planetary boundary layer (PBL) mixes the momentum of the nocturnal low-level jet down- ward.
In the Bode ́le ́ Depression, dust outbreaks have been reported in association with strong surface winds generally occurring after sunrise, as tur- bulence in the growing planetary boundary layer (PBL) mixes the momentum of the nocturnal low-level jet down- ward. This jet forms above
the radiatively cooled surface layer in the lee of the constriction between the Ennedi and the Tibesti and decays in the course of the day
Over the Sahel, prop- agating mesoscale convective systems (MCSs), and their associated ………………. offer ………………..
density currents
the most efficient mech- anism for dust lifting and injection to altitudes favourable for long-range transport, particularly at the beginning of the monsoon season, before the growing vegetation rapidly inhibits local dust emission
Recently, using a satellite-derived dust index and reanalysed atmospheric fields, Engelstaedter and Wash- ington (2007) have shown that the annual dust cycle in the West African dust hot spots is
not related to changes in mean surface wind strength but is linked to small-scale high-wind events.
They evidenced that the dust loads over West Africa are highest around
the monsoon onset period in June,
They evidenced that the dust loads over West Africa are highest around the monsoon onset period in June, in coincidence with
the northward displacement of the inter-tropical discontinuity (ITD, the near-surface position of the interface between the monsoon and the har- mattan, referred to as the “convergence zone on the north- ward bound” of the ITCZ by Engelstaedter and Washing- ton, 2007) to dust hot spots in the Sahel
In this study we present observational evidence of dust emission over western Niger associated with
the winds and turbulence existing along the leading edge of the monsoon flow in the ITD region.
In this study we present observational evidence of dust emission over western Niger associated with the winds and turbulence existing along the leading edge of the monsoon flow in the ITD region. The dust emissions were observed
shortly after sunrise, when the nocturnal monsoon flow was still behaving as an intruding density current†, advancing into the harmattan flow.
In this study we present observational evidence of dust emission over western Niger associated with the winds and turbulence existing along the leading edge of the monsoon flow in the ITD region. The dust emissions were observed shortly after sunrise, when the nocturnal monsoon flow was still behaving as an intruding density current†, advancing into the harmattan flow. The region over which the dust emissions were evidenced (……….) is characterised by …………………
(i.e. to the south of the Hoggar and to the west of the A ̈ır Mountains)
he existence of a huge system of ephemeral rivers and streams that drain the Hoggar and A ̈ır massifs, thereby defining a complex array of dust sources consisting of fluvial deposits.
A recent study by Schepanski et al. (2007), based on the Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infra- Red Imager (SEVIRI) dust index, has shown that this area is a prominent dust source in the
summer season, being located in the vicinity of mountain foothills where fluvial sediment provides fine material for deflation.
The objective of the mission was to document the
vertical structure of the Saharan PBL and the aerosol distribution, as well as the structure of the ITD, using the high horizontal and vertical resolution lidar-derived atmospheric reflectivity and horizontal wind vector fields, together with temperature, water vapour and wind profiles derived from dropsondes.
The mission was performed in the
early morning between 0600 UTC and 0900 UTC (between 0700 and 1000 LT, i.e. shortly after sunrise which was around 0545 LT in the region of operations), when the monsoon flow is typically strong and the ITD is well marked.
The airborne Doppler lidar WIND operates at a laser wavelength of
10.6 um
The airborne Doppler lidar WIND operates at a laser wavelength of 10.6 μm and is thus
sensitive to large aerosol particles in the μm-range
The vertical profile of the horizontal wind vector is determined by a
conical scan using the Velocity Azimuth Display (VAD) technique.
The accuracy of wind estimates depends on
the aerosol loading of the atmospheric layers sounded by the lidar, and varies from about 0.5 m s−1 (high aerosol loading) to about 2 m s−1 at worst (weak aerosol loading)
reflectivity is sensitive to
aerosol optical properties and concentration, as well as relative humidity in the case of hygroscopic aerosols.
Furthermore, reflectivity is sensitive to aerosol optical properties and concentration, as well as relative humidity in the case of hygroscopic aerosols. However, over the African continent, close to the sources
desert dust particles are generally considered to be hygrophobic
Therefore, reflectivity asso- ciated with desert dust is generally not expected to be
sensitive to relative humidity fluctuations
In addition to providing information on the temperature and water vapour fields, dropsonde data were used to compute the
vertical profiles of the bulk Richardson number to assess where mechan- ical shear induced turbulence was important.
The bulk Richardson number was computed as:
Dropsonde-derived near surface measurements were also used to
estimate the theoretical depth and speed of the monsoon leading edge in an idealized density current framework.
The depth and speed of a steady-state density current moving into an unstratified atmosphere with no vertical wind shear may be expressed as
At 925 hPa the southern half of West Africa was under the influence of
the south-west monsoon,
At 925 hPa the northern half of west africa
the wind field was under the influence of the Libyan High and was modulated by the topography.
Strong northerly winds were seen over Libya, which were
deflected around the northern flank of the Hoggar towards the Atlas
Strong northeasterly winds blew
between the Tibesti and Ennedi Mountains in Chad, and over the Bode ́le ́ Depression, an area known to be one of the world’s most productive dust sources.
Niger was under the influence of
strong opposing winds, separated by a northwest-southeast oriented line of weak winds corresponding to the ITD
strong south-westerly winds were associated with
the monsoon flow being seen north of Niamey just east of the Mali-Niger border, while strong north-easterly winds were observed to the north of the ITD, in the lee of the constriction between the TibestiandHoggarmassifs,aswellasbetweentheA ̈ır and Hoggar massifs.
strong south-westerly winds were associated with the monsoon flow being seen north of Niamey just east of the Mali-Niger border, while strong north-easterly winds were observed to the north of the ITD, in the lee of the constriction between the TibestiandHoggarmassifs,aswellasbetweentheA ̈ır and Hoggar massifs. As a result,
a sharp gradient of water vapour mixing ratio was observed at 925 hPa across the ITD over Niger (Fig. 1b), with high moisture values to the south-west (in excess of 14 g kg−1) and drier conditions to the north-east (less than 6 gkg−1).
As a result, a sharp gradient of water vapour mixing ratio was observed at 925 hPa across the ITD over Niger (Fig. 1b), with high moisture values to the south-west (in excess of 14 g kg−1) and drier conditions to the north-east (less than 6 gkg−1). At higher levels
(600 hPa, Fig.1c), western Niger was under the influence of the African Easterly Jet (AEJ), which covered most of the Sahel and Soudanian zones.
The AEJ intensity was
moderate in the analysis (15-20 m s−1), with a maximum over Burkina-Faso.
Over western Niger, the AEJ exhibited
a marked north-easterly component, in connection with the broad anticyclonic circulation above the Saharan Heat Low
along the aircraft flight track, the ITD was located at about
18.5◦N
along the aircraft flight track, the ITD was located at about 18.5◦N, as delineated by
- the wind reversal at low level, characteristic of the transition from the monsoon to the harmattan flow
- trong near-surface convergence and a strong updraft reaching approximately 6 km msl are seen at the ITD.
- North of the ITD the north-easterly flow interacts with the topography resulting in strong downslope winds below 2 km msl in the northern and southern parts of the Hoggar.
The upper-level flow is also quite perturbed by the
orography
The upper-level flow is also quite perturbed by the orography. As a result ……………………………. exhibit a large horizontal vari- ability
the potential temperature and water vapour mixing ratio fields
the potential temperature and water vapour mixing ratio fields
a tongue of dry, warm air is seen to extend towards the surface, reaching down to 1 km msl as the result of the interaction between the northeasterly flow and the topography.
Just to the north of the ITD, a tongue of dry, warm air is seen to extend towards the surface, reaching down to 1 km msl as the result of the interaction between the northeasterly flow and the topography. Hence,
the moist monsoon air is separated from the dry northerly air by an even drier airmass.
South of the ITD
the isentropes are sloping upward
South of the ITD, the isentropes are sloping upward (Fig. 2a). A similar behaviour is seen in
the water vapour mixing ratio field
Though the sloping is similar, the potential temperature is increasing with height whereas the water vapour mixing ratio is decreasing.
A similar behaviour is seen in the water vapour mixing ratio field (Fig.2b). Though the sloping is similar, the potential temperature is increas- ing with height whereas the water vapour mixing ratio is decreasing. The ECMWF analysis suggests
the existence of a closed circulation south of the leading edge of the monsoon flow, roughly centred at 17◦N, reminiscent of the circulation observed in density current heads
The near-surface wind reversal associated with the ITD is observed to be around
18.7◦N along the outgoing transect
The near-surface wind reversal associated with the ITD is observed to be around 18.7◦N along the outgoing transect (Fig. 3a), as determined from
the WIND-derived zero isotach of the wind component perpendicular to the flight track.
The shape of the monsoon-harmattan interface (not just the ITD) stands out clearly in the WIND data as the region of
very light winds separating the two opposing flows
The WIND data stresses that the slope of the monsoon-harmattan interface was
gentle near the leading edge of the monsoon flow, on the order of 17 m km−1 (the height of the monsoon layer top increased from 500 to 1700 m msl between 18.7◦ and 18◦N).
South of 18◦N, the monsoon-harmattan interface was almost
horizontal
The depth of the south- westerly monsoon flow was on the order of
1.8 km, away from the leading edge.
The AEJ is clearly seen in the WIND and dropsonde data, extending throughout
the entire aircraft leg
The AEJ is clearly seen in the WIND and dropsonde data, extending throughout the entire aircraft leg, with the largest wind speeds
below 6 km msl
North of the ITD, WIND data evidence the existence of
a thin near-surface layer of strong south- easterly winds (12-14 m s−1 ), separated from the north- easterly AEJ (16 ms−1 or more) by a layer of weaker winds (8 m s−1).
Isentropes within the monsoon flow (309 K and below) were observed to slope
downwards with latitude
Isentropes within the monsoon flow (309 K and below) were observed to slope downwards with latitude (Fig. 4b). A similar behaviour was observed for
isentropes and iso-contours of water vapour mixing ratio associated with the harmattan flow, both north of the ITD and above the monsoon.
The monsoon flow was characterized by water vapour mixing ratio values of
9 g kg−1 or more
The stratification in the AEJ was
quite different from that of the monsoon and harmattan layers, potential temperature and water vapour mixing ratio being more mixed in the vertical.
On the return flight to Niamey, the thermodynamical structure of the atmosphere
did not change significantly, with the notable exception of the location of the leading edge of the monsoon which was observed from WIND measurements to have slightly progressed north, i.e. to roughly 18.9◦N
