RADAGAST status report andobservations of a major Saharan dust storm

Tony Slingo

Environmental Systems Science CentreUniversity of Reading, UK

• Brief background• Selected results to date• Major dust storm in March 2006• Summary and future work

GERB International Science Team, Met Office, Exeter, 23-25 October 2006

• Radiative Atmospheric Divergence using ARM Mobile Facility, GERB data and AMMA stations– collaboration between ARM program, ESSC, PNNL and other partners– led by Tony Slingo, ESSC

• Links the ARM Mobile Facility (Niamey) with GERB and AMMA (African Monsoon Multidisciplinary Analysis)

• Derive the divergence of radiation across the atmosphere:– first comprehensive surface measurements of atmospheric radiation

and vertical structure in Africa– study the radiative properties of aerosols (desert dust, biomass), water

vapour and clouds– opportunity to resolve disagreements between radiation codes and

observations

The RADAGAST project

ARM Mobile Facility

• AMF in Niger from late 2005 to end 2006

• Two sites:– main site at Niamey airport– subsidiary site at Banizoumbou, 60km distant

• Wide range of instruments available:– radiometers, Lidar, Radar, aerosol sampling,

infrared spectrometer and radiosondes

AMMA 2006

Overview of geography

2 E

13 N

Surface fluxes measured with AMF sites

Top of atmosphere fluxes measured with GERB and SEVIRI instruments on Meteosat

GERB resolution of 50km, but the ARCH product has a resolution of ~10km, whereas the AMF sites measure over ~0.01km

Overview of methodology

Long-term average annual cycles of climate parameters at Niamey, Niger

Why is RADAGAST based in Niamey?

Figure 4. Long-term dekad average position of West African Intertropical Front (ITF, 1974-2003) for the northward excursion phase (top panel), and the southward surge (bottom panel). ITF appears north of 10oN for the first time in early April and retreats completely below 10oN in mid-November. Note here the abrupt southward surge of the ITF (bottom panel) compared to the more gradual northward advance (top panel).

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NIAMEY

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Column water vapour over Niamey (cm)

Green: microwave radiometer Red: radiosondes

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Sonde humidity profiles in 2006

Observations of the impact of a major Saharan dust storm on the

Earth’s radiation balanceA. Slingo1, T.P. Ackerman2, R.P. Allan1, E.I. Kassianov2, S.A. Mcfarlane2, G.J. Robinson1, J.C. Barnard2, M.A. Miller3, J.E. Harries4, J.E.

Russell4 & S. Dewitte5

submitted to GRL

Animation of Meteosat dust productThe animation will be shown by Gary Robinson

1200GMT, 6 March 2006

This sequence of images shows the daily progression of the dust as it sweeps southwards across the Sahara.

In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.

On 6 March, unusually strong northerly winds bring cold air at low levels over the desert, creating a broad front of dust as the air moves southwards.

The location of Niamey is marked by a cross on each image.

1200GMT, 7 March 2006

The shallow layer of cold air cannot rise over the high mountain ranges of the central Sahara (light blue in colour), so it is forced to follow the valleys. Streaks appear where it accelerates through gaps in the topography.

The dust reached Niamey at 0930 on 7 March.

1200GMT, 8 March 2006

By 8 March, dust covers the whole of West Africa and is moving out over the Atlantic. Note again that the dust acts as a tracer of the flow, even showing evidence of instabilities on the jet-like features across the Hoggar massif in the middle of the image.

Dust product (upper) and GERB OLR (lower) for 1200UT on 8 March 2006

Cloud

Lake Chad

Dust

Onset of storm at 09h30, 2006/3/7

…and the next day, 2006/3/8

…webcam for 2006/3/8

…and dust settles, 2006/3/10

…webcam for 2006/3/12

Radiosonde ascent, 6 March (prior to storm)

Radio-sonde ascent, 2006/3/8 (during storm)

Dust storm: summary

• Increase in flux scattered back to space• Larger reduction in the downward flux at surface

– direct flux reduced virtually to zero (τaerosol ~ 3)– all the remaining downward flux is diffuse (as under thick cloud)

• Implies significant atmospheric absorption– absorption underestimated by the models, despite using dust

that is more absorbing than in other studies– so we may have a problem in modelling the absorption

Solar fluxes

Thermal fluxes• Significant effect of “cold”, dry air• But more thermal radiation emitted to the surface (dust)• Less OLR emitted to space• Increased atmospheric cooling

Solar effect is larger: net effect of aerosol is heating

Summary and future work• We continue to receive excellent data from the Mobile

Facility and from GERB and SEVIRI– the mobile facility will remain in Niamey for the RADAGAST project

until the end of 2006• Major dust storm in March 2006• Papers

– BAMS overview provisionally accepted: currently in revision– GRL paper on dust storm provisionally accepted: also in revision– ideas for other papers are being discussed

• Ongoing work at ESSC includes:– continue to monitor events and to identify test cases– exploit the data from the second site at Banizoumbou– develop the methodology to derive area-average surface fluxes– collaboration with project partners in PNNL, Met Office and

elsewhere