Dust emissions over the Sahel associated with the West African Monsoon inter-tropical discontinuity region: a representative case study

Question 1

……………………….. is the world’s largest source of mineral dust.

Answer 1

West Africa

Question 2

West Africa is the world’s largest source of mineral dust. Satellite sen- sors consistently indicate that

Answer 2

dust aerosol plumes are the most widespread, persistent and dense found on Earth

Question 3

The radiative effects of dust (both direct and indirect) can

Answer 3

modify the general circula- tion at climate time scales but also at shorter (synoptic) time scales

Question 4

Dust emissions from West Africa follow a

Answer 4

distinct annual cycle

Question 5

Dust emissions from West Africa follow a distinct annual cycle and are connected with

Answer 5

the occurrence of high wind speeds at the surface.

Question 6

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

Answer 6

strong harmattan winds

Question 7

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

Answer 7

discrete outbreaks of several days duration

Question 8

the summer season is prone to

Answer 8

high dust activity

Question 9

the summer season is prone to high dust activity and is the period when

Answer 9

most of the intense dust outbreaks over the North Atlantic occur

Question 10

Occasional strong dust outbreaks have been docu- mented to be associated with

Answer 10

• 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

Question 11

In the Bode ́le ́ Depression, dust outbreaks have been reported in association with

Answer 11

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.

Question 12

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

Answer 12

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

Question 13

Over the Sahel, prop- agating mesoscale convective systems (MCSs), and their associated ………………. offer ………………..

Answer 13

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

Question 14

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

Answer 14

not related to changes in mean surface wind strength but is linked to small-scale high-wind events.

Question 15

They evidenced that the dust loads over West Africa are highest around

Answer 15

the monsoon onset period in June,

Question 16

They evidenced that the dust loads over West Africa are highest around the monsoon onset period in June, in coincidence with

Answer 16

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

Question 17

In this study we present observational evidence of dust emission over western Niger associated with

Answer 17

the winds and turbulence existing along the leading edge of the monsoon flow in the ITD region.

Question 18

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

Answer 18

shortly after sunrise, when the nocturnal monsoon flow was still behaving as an intruding density current†, advancing into the harmattan flow.

Question 19

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 …………………

Answer 19

(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.

Question 20

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

Answer 20

summer season, being located in the vicinity of mountain foothills where fluvial sediment provides fine material for deflation.

Question 21

The objective of the mission was to document the

Answer 21

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.

Question 22

The mission was performed in the

Answer 22

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.

Question 23

The airborne Doppler lidar WIND operates at a laser wavelength of

Answer 23

10.6 um

Question 24

The airborne Doppler lidar WIND operates at a laser wavelength of 10.6 μm and is thus

Answer 24

sensitive to large aerosol particles in the μm-range

Question 25

The vertical profile of the horizontal wind vector is determined by a

Answer 25

conical scan using the Velocity Azimuth Display (VAD) technique.

Question 26

The accuracy of wind estimates depends on

Answer 26

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)

Question 27

reflectivity is sensitive to

Answer 27

aerosol optical properties and concentration, as well as relative humidity in the case of hygroscopic aerosols.

Question 28

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

Answer 28

desert dust particles are generally considered to be hygrophobic

Question 29

Therefore, reflectivity asso- ciated with desert dust is generally not expected to be

Answer 29

sensitive to relative humidity fluctuations

Question 30

In addition to providing information on the temperature and water vapour fields, dropsonde data were used to compute the

Answer 30

vertical profiles of the bulk Richardson number to assess where mechan- ical shear induced turbulence was important.

Question 31

The bulk Richardson number was computed as:

Answer 31

Question 32

Dropsonde-derived near surface measurements were also used to

Answer 32

estimate the theoretical depth and speed of the monsoon leading edge in an idealized density current framework.

Question 33

The depth and speed of a steady-state density current moving into an unstratified atmosphere with no vertical wind shear may be expressed as

Answer 33

Question 34

At 925 hPa the southern half of West Africa was under the influence of

Answer 34

the south-west monsoon,

Question 35

At 925 hPa the northern half of west africa

Answer 35

the wind field was under the influence of the Libyan High and was modulated by the topography.

Question 36

Strong northerly winds were seen over Libya, which were

Answer 36

deflected around the northern flank of the Hoggar towards the Atlas

Question 37

Strong northeasterly winds blew

Answer 37

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.

Question 38

Niger was under the influence of

Answer 38

strong opposing winds, separated by a northwest-southeast oriented line of weak winds corresponding to the ITD

Question 39

strong south-westerly winds were associated with

Answer 39

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.

Question 40

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,

Answer 40

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).

Question 41

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

Answer 41

(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.

Question 42

The AEJ intensity was

Answer 42

moderate in the analysis (15-20 m s−1), with a maximum over Burkina-Faso.

Question 43

Over western Niger, the AEJ exhibited

Answer 43

a marked north-easterly component, in connection with the broad anticyclonic circulation above the Saharan Heat Low

Question 44

along the aircraft flight track, the ITD was located at about

Answer 44

18.5◦N

Question 45

along the aircraft flight track, the ITD was located at about 18.5◦N, as delineated by

Answer 45

• 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.

Question 46

The upper-level flow is also quite perturbed by the

Answer 46

orography

Question 47

The upper-level flow is also quite perturbed by the orography. As a result ……………………………. exhibit a large horizontal vari- ability

Answer 47

the potential temperature and water vapour mixing ratio fields

Question 48

the potential temperature and water vapour mixing ratio fields

Answer 48

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.

Question 49

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,

Answer 49

the moist monsoon air is separated from the dry northerly air by an even drier airmass.

Question 50

South of the ITD

Answer 50

the isentropes are sloping upward

Question 51

South of the ITD, the isentropes are sloping upward (Fig. 2a). A similar behaviour is seen in

Answer 51

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.

Question 52

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

Answer 52

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

Question 53

The near-surface wind reversal associated with the ITD is observed to be around

Answer 53

18.7◦N along the outgoing transect

Question 54

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

Answer 54

the WIND-derived zero isotach of the wind component perpendicular to the flight track.

Question 55

The shape of the monsoon-harmattan interface (not just the ITD) stands out clearly in the WIND data as the region of

Answer 55

very light winds separating the two opposing flows

Question 56

The WIND data stresses that the slope of the monsoon-harmattan interface was

Answer 56

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).

Question 57

South of 18◦N, the monsoon-harmattan interface was almost

Answer 57

horizontal

Question 58

The depth of the south- westerly monsoon flow was on the order of

Answer 58

1.8 km, away from the leading edge.

Question 59

The AEJ is clearly seen in the WIND and dropsonde data, extending throughout

Answer 59

the entire aircraft leg

Question 60

The AEJ is clearly seen in the WIND and dropsonde data, extending throughout the entire aircraft leg, with the largest wind speeds

Answer 60

below 6 km msl

Question 61

North of the ITD, WIND data evidence the existence of

Answer 61

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).

Question 62

Isentropes within the monsoon flow (309 K and below) were observed to slope

Answer 62

downwards with latitude

Question 63

Isentropes within the monsoon flow (309 K and below) were observed to slope downwards with latitude (Fig. 4b). A similar behaviour was observed for

Answer 63

isentropes and iso-contours of water vapour mixing ratio associated with the harmattan flow, both north of the ITD and above the monsoon.

Question 64

The monsoon flow was characterized by water vapour mixing ratio values of

Answer 64

9 g kg−1 or more

Question 65

The stratification in the AEJ was

Answer 65

quite different from that of the monsoon and harmattan layers, potential temperature and water vapour mixing ratio being more mixed in the vertical.

Question 66

On the return flight to Niamey, the thermodynamical structure of the atmosphere

Answer 66

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