Estimating the radiative impacts of aerosol using GERB and SEVIRI
Dust Longwave Forcing from GERB and SEVIRI Vincent Gimbert, H.E. Brindley, J.E. Harries Imperial...
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Transcript of Dust Longwave Forcing from GERB and SEVIRI Vincent Gimbert, H.E. Brindley, J.E. Harries Imperial...
Dust Longwave Forcing from GERB and SEVIRI
Vincent Gimbert, H.E. Brindley, J.E. Harries
Imperial CollegeLondon
GIST 26, 03 May 2007, RAL, Abingdon
Thanks to Rainer Hollmann, Nicolas Clerbaux, Alessandro Ipe and Richard Allan, Andy Smith
Motivation
Use GERB and SEVIRI to measure TOA LW radiative forcing of Dust aerosols over the Sahara
Model Clear-sky TOA LW Radiances Using ECMWF Analyses to separate out effect of Dust from that of variable meteorology
Test the ECMWF model against observations and provide uncertainties in estimate of Dust forcing
Outline of Presentation
Method, measurements, model
March 2004: LW forcing of dust storm from GERB
Model comparison with Clearsky observations from GERB and SEVIRI IR Channels
Dust spectral forcing
Results from March 2006
Conclusion and future work
GERB LW radiance 200403 12 UTC
Strong OLR perturbation associated with dust event
Daytime SAFNWC Dust detection
RT modelling / GERB-SEVIRI data
Spectral Radiances modelled using MODTRAN 4 from 3.5 μm to ∞• Temperature, humidity, ozone from ECMWF analyses• Unity surface emissivity• 1 * 1 Degree resolution - 60/91 vertical levels• Spectral integration over appropriate instrument/channel response
Comparison with GERB and SEVIRI measurement• GERB L2 ARG• SEVIRI L1.5 Radiances (7 IR channels, 062,073,087,097,108,120,134)• 6-hourly 00:00, 06:00, 12:00, 18:00 (No GERB data at 00:00)• March 2004 and 2006
12:00 GERB - Model difference
GERB/model anomaly coincides with dust front on 3rd March 2004
Anomalies over clear sky as well
GERB and Model 12UTC through March
Cloud/dust free pixels
averages over dust
front region
(~10*10 deg square)
Over dust front (20040303) : LWRF ~ 20(+/- 3) W/m2
clearLLBias
BiasLLLWRF
LclearLdustLWRF
ECMWFGERB
ECMWFGERB
)(
• Model warm bias at 06 and 18
• Larger errors at 12 (larger Std Dev than 06 and 18)
• Use SEVIRI to test model...
Time dependence of clearsky errors
Time
Channel
00 UTC 06 UTC 12 UTC 18 UTC
IR_062
Mean (1σ), K
-1.67 (0.8)
-1.59 (0.6)
-1.51 (0.6)
-1.53 (0.6)
IR_073
Mean (1σ), K
0.95 (0.8) 0.95 (0.7) 2.05 (1.0) 1.34 (0.8)
SEVIRI/model Water Vapour channels WV 6.2 μm, High/mid atm WV 7.3μm, Mid/Low
Window channels
• Persistent model warm bias at night (σ~1-2K)• Daytime cold bias (~3K), σ~3.3K• Discrepancies over LAND• Same results from 10.8μm channel
The case of 8.7μm Channel
12.0μm
8.7μm
Night Day
• Strong SEVIRI/model differences in 8.7μm channel• Same pattern at all time steps• Surface emissivity?
• Assume emissivity (12.0μm)~1 and T(8.7 μm) ~T(12.0 μm)• Separate surface emissivity signal from 8.7 μm channel
Method useful for removing consistent biasNo a priori knowledge of surface spectral emissivity needed
Latitudinal dependence of Clearsky errors
• No dependance in WV channels
12.0μm: Surface Temperature 8.7μm: Emissivity + Tsurf
Dust LW Spectral forcing
No correction: unrealistic forcing, No uncertainties6.2um: non- physical7.3um: Negative at 12UTC…8.7um: Too high (emissivity bias)
March 2006 Dust event
EUMETSAT Dust RGB colour scheme
20060307 12UTC
EUMETSAT Dust RGB colour scheme
20060308 12UTC
March 2006 Dust event
Dust LW Spectral forcing
• Maximum forcing on 20060308• Similar spectral dependence as March 2004• Consistent with models
Conclusion and future work
Mean 200403 Dust storm OLR forcing: ~20W/m2 Diurnal cycle of Clear-sky GERB-model errors Good agreement in SEVIRI WV channels (small biases) Land Emissivity and Temperature Regional Biases in Window
Channels Very similar results for March 2004 and 2006: Dust Spectral Forcing
Future work: Validation at Niamey ARM site: Dust detection reliability Diurnal cycle of Tsurf Consistence of GERB-SEVIRI vs Model biases
Diurnal cycle of forcing
Daytime Conditional Bias
Model does not reproduce the range of GERB OLR at 12
Similar finding as Trigo and Viterbo (2003) with ECMWF model compared to MS7Window channel
We look at possible sources of errors at the surface