Dust uplift and transport observed during the GERBILS campaign

John Marsham1, Doug Parker1, Christian Grams2,

Qian Huang1,3, Sarah Jones2, Jim Haywood4 and

Ben Johnson4

1 University of Leeds, UK2 IMK, Karlsruhe, Germany

3 Langzhou University, China4 Met Office, UK

GERBILS(GERB Intercomparison of Longwave and Shortwave radiation)

Model emits 50Wm-2 too much outgoing infared(Jim Haywood, Met Office)

Model – GERB outgoing longwave

0 10 20 30 40 50

Motivation

“Probably the best dry-adiabat in the world”

~550 hPa

Unique, deep (~5 km), near-neutral boundary layer. Poorly represented by models? Certainly poorly observed. Dust uplift a non-linear function of windspeed. Saharan Heat Low.

Shear stress at surface(~near surface windspeed)

Du

st u

pli

ft r

ate Windspeed3

The Saharan boundary-layer

• Observations suggest occasional complete vertical mixing .• Saharan Air Layer (SAL) allows dust to avoid rain-out.• Models are not designed for such deep boundary layers

(BL scheme of HadGAM was switched off above 2880 m)

Active well mixedboundary layer

Near neutral residual layers

Free troposphere

~5 km

Mixing from the boundary layer into the residual layer

Free troposphere

Residual layer

Active boundary layer

Orography Isentropic upgllideDeep dry convection

Albedo anomalies

(Parker et al)

Land-atmosphere coupling

Is this statistically significant?(Marsham et al submitted to ACPD)

Low albedoHigh land surface temperatureHigh boundary-layer air temperatureLow water vapour

Aircraft observations from GERBILS

Cospectral analysis

Along track windBL buoyancyBL WVMRBL windspeed

Relationship of variables with land surface temperature (LST) as a function of scale

Convergence towards warm BL over dark surfaces for scales > 10 km.

Implications for the SAL

(Marsham et al submitted to ACPD)

SAL weakly stratified, so small perturbations in BL buoyancy are expected to affect vertical mixing.

Large eddy modelling of this is ongoing work.

~ 4 K

Boundary-layer convection and dust uplift

• Boundary-layer convection contributes to uplift (especially if speed of mean wind ~ the uplift threshold).

Uplift rate with and without resolved large eddy model winds

Mean wind = 9.9 m/s

Mean wind = 4.9 m/s

Observations of dust in cold

pools (“haboobs”)

1200 UTC

Cold pool outflows

How typical is this?

What are the implications?

• Engelstaedter and Washington (2007) showed that:– Dustiness associated with convergence at head of monsoon.– Dustiness depends on gustiness more than synoptic-scale winds

(Engelelstaedter and Washington, 2007)– Dustier in monsoon onset than retreat. Why?

• Sterk 2003, Williams (2006), Bou Karam et al 2007, Flament et al 2007 – All show dust uplift by cold pools in this region at this time (also

Sutton 1925, 1931 for the Sudan)

Annual cycle of dust and cold poolsTOMS

(Marsham et al submitted to JGR)

Energy available to downdraughts (DCAPE) shows same asymmetry as TOMS Aerosol Index.

CAPE and deep convection do not show asymmetry.

(Marsham et al submitted to JGR)

Can this be explained by other annual

cycles?

Soil moisture and vegetation are only significant in the south.

(Marsham et al submitted to JGR)

Can this be explained by other cycles?

Conclusions• Saharan boundary layer is unique

– Deepest dry convection on Earth– Poorly observed. Poorly modelled?

• Mesoscale processes affect (control?) mixing between boundary layer and the SAL.

• Dust uplift is a function of local windspeeds. – Boundary-layer convection affects dust uplift.

• Downdraughts from storms are one of the major processes causing dust uplift in the western Sahara.– Must represent these, and so the monsoon flow, in

dust models.