Saharan Dust Longwave Radiative Forcing using GERB and SEVIRI

Imperial College London Saharan Dust Longwave Radiative Forcing using GERB and SEVIRI Vincent Gimbert 1, H.E. Brindley 1, Nicolas Clerbaux 2, J.E. Harries 1 1. Blackett Laboratory, Imperial College, London 2. Royal Meteorological Institute of Belgium, Brussels IUGG Conference, 10 July 2007, Perugia, Italy

Aims and Motivation Use geostationary broadband and spectral data to measure TOA Direct LW radiative forcing of Dust over the Sahara Model clear-sky TOA LW radiances and fluxes 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 2

Outline of Presentation Data and Methods Case Study: March 2004 Dust Storm Broadband forcing from GERB Spectral forcing from SEVIRI Clear-sky Model Evaluation Summary and Discussion 3

Geostationary Earth Radiation Budget - GERB GERB: European consortium led by the UK (Harries et al., 2005) Imperial College: PI (J.E. Harries), Instrument calibration and Science 4 instruments onboard Meteosat Second Generation Calibrated LW and SW Broadband radiances Resolution: ~ 50km at Nadir, 15 Minutes Edition 1 data available from Feb 2004 (12-14 years of data) 4

Case Study: March 2004 Dust Storm Spinning Enhanced Visible and InfraRed Imager (SEVIRI) RGB Composite IR12.0-IR10.8, IR10.8-IR8.7, IR10.8 3 March 2004-1200 UTC Dust storm associated with strong decrease in OLR. How much did the dust contribute? Ground-based and model data show strong changes in temperature and atmospheric composition. Cooling by cold air advection. (Knippertz and Fink, 2006) Need to model Clear-sky OLR to estimate dust forcing. Credit: EUMETSAT 5

Estimating the dust Direct Radiative Effect 6

RT Modelling, Data and Scene Identification Spectral Radiances modeled using MODTRAN 4 Temperature, humidity, ozone from ECMWF operational model analyses Unity surface emissivity 1 1 Degree resolution - 60 vertical levels Study region: [0º-40ºN] and [25ºW-60ºE] Comparison with GERB and SEVIRI measurement GERB Level 2 Average Rectified Geolocated (L2 ARG) product SEVIRI L1.5 Radiances (7 IR channels, 062,073,087,097,108,120,134) 6-hourly 0000, 0600, 1200, 1800 UTC (No GERB data at 0000) March 2004 Scene Identification from SEVIRI (NWCSAF) Cloud and Dust detection (Visible and infrared channels) 7

Broadband DRE from GERB Mean DRE = 22.2 ± 4.6 W.m -2 8

Spectral DRE from SEVIRI Similar approach with the 7 SEVIRI infrared channels Spectral integration of high resolution radiance spectra Strong emissivity correction in 8.7µm channel Significant DRE in CO 2 and O 3 channels Dust signature in 7.3 µm water vapour channel 9

Monthly Mean Daytime Clear-sky Model-GERB Large regional biases: Positive ~ [20-30N] and Arabian Peninsula: Land surface emissivity Negative ~ [10-15N]: Daytime underestimation of Land Surface Temperature 10

Model Evaluation Similar regional patterns in March 2006 Narrowband comparisons useful to understand model errors Very good agreement in WV channels Most Model-GERB errors arise from the land surface Surface emissivity shows strong spectral dependance ECMWF model underestimates amplitude of diurnal cycle of Ts 11

Summary and Discussion Combination of model and observations to infer DRE without information on dust properties and vertical distribution Average DRE of 22.2 ± 4.6 W.m -2 on 3 March 2004 at 1200 UTC Spectral DRE form SEVIRI Clear-sky errors Dust detection issues: - Detection of light dust loadings - Cloud/dust misidentification Geostationary satellite for diurnal cycle of DRE Ground-based model validation + Information on dust 12

Contact and references Thanks to: Rainer Hollmann, DWD, Germany Alessandro Ipe, RMIB, Belgium GERB International Science Team GERB Project: http://www3.imperial.ac.uk/spat/research/missions/atmos_missions/gerb References: Harries, J. E., et al. (2005), The Geostationary Earth Radiation Budget project, Bull. Am. Meteorol. Soc., 86, 945 960. Knippertz, P., and A. H. Fink (2006), Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak, Q. J. R. Meteorol. Soc., 132, 1153 1177. Please contact me: vincent.gimbert03@imperial.ac.uk 13

6-hourly Clear-sky Model-GERB Errors Surface Time 06 UTC 12 UTC 18 UTC LAND Mean (1σ), W.m -2 8.7 (4.4) 3.4 (11.0) 6.8 (4.8) WATER Mean (1σ), W.m -2 5.4 (1.8) 4.8 (1.9) 4.6 (1.9) 15

Additional Slides WV 6.2 µm, High/mid atm WV 7.3µm, Mid/Low Channel Time 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) 16