Initial results from RADAGAST

Nazim BharmalTony Slingo, Gary Robinson, Jeff Settle, Helen White

ESSC, University of Reading

GIST 26 3rd May 2007

• Radiative Atmospheric Divergence using ARM Mobile Facility, GERB data and AMMA stations.

• Using the AMF and GERB to study the divergence of radiation across the atmosphere.– Long-term & comprehensive surface measurements

of atmospheric radiation and structure in Africa.– Resolve disagreements between radiation codes and

data.– Study the radiative properties of aerosols (desert

dust, biomass), water vapour and clouds.

• AMF is ARM mobile facility.

• Niger was first overseas deployment.

• Two sites established;– Main site at Niamey (airport).– Subsidiary site at Banizoumbou (rural).

• Active (Niamey only) and passive measurements of atmospheric composition & radiation.

AMF, Niger during 2006

Geographical precis

Geographical precis

0.6m radiances, 2006/11

10.8 m radiances, 2006/11

● SEVIRI (365 days)● GERB ARG (338 days)● GERB HR (265 days)

GERB data and spatial resolutions

Surface overview

PWV

AOT

Surface flux overview

LW

SW

TOA flux overview

LW

SW

Modelling undertaken

• Modelling of radiation using ARM data.– Using Edwards-Slingo and Fu-Liou (flux only).

• Comparisons at ground and TOA, broadband and spectrally resolved.– At AMF, Niamey and GERB.

• Currently, LW is clear-sky & SW is clear-sky plus aerosol.

Modelling OLR; GERB & ES

OLR difference persist between ES model and GERB ARG.

Model comparisons ; ES & ECMWF

OLR comparison; ECMWF has better correspondence to GERB.

Model comparisons ; ES & ECMWF

DLR comparison; atmospheric composition is comparable.

GLR comparison; ECMWF has lower skin temperature.

Model comparisons ; ES & ECMWF

AMF site comparisons

GLR scatter OLR scatter

Banizoumbou c.f. Niamey has lower skin temperature

but equivalent OLR.

● Potential issues:● Inappropriate skin temperature.

● AMF, Niamey not representative.● Inaccurate emissivity.

● Sand/soil is measured ~0.95.● ECMWF uses 0.99.● ES uses 1.

● Lack of aerosol in atmosphere.● Significant clear periods in November.

● Inconsistent atmospheric profile.● Surface comparisons show to be adequate.

How to represent GERB “column”?

Changing emissivity and skin temperature

• Emissivity, 0.99, lowered to 0.95

• Tskin

lowered by 1.6 K

• Understand GERB fluxes for Niamey during 2006.• LW HR and ARG agree well over Niamey.

• Modelling shows both radiances and fluxes behave similarly c.f. GERB.

• SW HR and ARG disagreement due to heterogeneity.

• Heterogeneity issues affect modelling of OLR.• Emissivity is ~0.95.• AMF, Niamey skin temperature is too high.

• Accurate clear-sky fluxes permit cloud and aerosol forcing.• Complementary method to interpolation.

Conclusion and future intentions