Slide 1

Surface control on albedo and radiation balance over the Gourma site Laurent Kergoat, Olivier Samain, Franoise Guichard, Pierre Hiernaux, Franck Timouk, Colin Lloyd, Eric Mougin, Franois Lavenu, Patricia de Rosnay, Valerie Le Dantec, Josiane Seghieri et al. CESBIO, CNRM, CEH Slide 2 Albedo is a key parameter, isnt it ? Yes it is ! as is Rn G = H + LE e Eltahir, Zeng... 1 2 3 4 Slide 3 Albedo is STILL a key parameter, isnt it ? Few in situ measurements in the Sahel : SEBEX, HAPEX,... No multi-year multi-sites data. No multi-year validation of remotely sensed of Sahelian albedo No links with ecosystem monitoring data in the Sahel. As a result : Causality links between vegetation, albedo, and radiation budget poorly known and (maybe ?probably ? certainly ?) poorly represented in land surface models. A case study in Sahelian Mali (aka Gourma mesoscale site) Slide 4 Gourma mesoscale Site (~300 x 100 km) Agoufou local Site (15.3N, 1.5W) Bamba local Site (17.1N, 1.3W) Niger River Rainfall 50 mm 350 mm 450 mm 300 mm 100 mm in situ met stations Slide 5 In situ : short term variability courtesy Samain et al. soil moisture effect aerosol apparent effect Slide 6 In situ : short term variability Outside the rainy season, the short-term variations of the albedo are mainly the result of the incident radiation spectral composition (aerosols). Models need spectral albedo = aerosol effect Slide 7 Validation of MODIS albedo Very good agreement on - intra-seasonal - seasonal - interannual time scales. MODIS albedo can be used to analyse albedo variability over the 40 vegetation monitoring sites drought fire courtesy Samain et al. Slide 8 Seasonal cycle courtesy Samain et al. Slide 9 Vegetation growth controls the summer decrease of albedo at seasonal and inter-annual time scales. Slide 10 Both NIR and Vis spectral band decrease with biomass Straw + litter dynamics controls dry season albedo (bright soil) No vegetation cycle in 2004.... replaced by a two-year cycle ! courtesy Samain et al. Slide 11 Interannual variability : rain drives peak biomass and dry season straw/litter fire Slide 12 surface type, grazing pressure effect mainly in the dry season Slide 13 Gourma meso scale Slide 14 Some intriguing trends over Northern Gourma.... (NOT a calibration problem) Slide 15 Rnet = SW - SW + LW - LW atmosphere clouds, albedo, water vapour albedo surface temperature LE cooling more efficient than H more H means more LW vegetation growth drying period courtesy Timouk, Lloyd From albedo to Net radiation sensible Heat flux Slide 16 2003 2004 2005 2006 Seasonal R net over 4 years, mix of surface and atmosphere controls late start of rain, vegetation courtesy Guichard, Timouk Slide 17 Radiation balance for Agoufou in August 2002-2006 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 2002 2003 2004 2005 LW LW SW SW Rnet (W.m -2 ) -40 -30 -20 -10 0 10 20 LW LW SW SW Rnet (W.m -2 ) -400 +460 -250 +50 -140 Rnet Difference dry year versus wet year 20 w due to LW losses Tsurface is cooled by ETR. 15 w due to albedo effect. 2006 = highest Rnet +40% compared to 2004 2006 2002 2003 2004 2005 2006 Slide 18 rain albedo 24 h vegetation growthalbedo decrease 1 month decrease Long Wave losses 1 month increase straw, decrease albedo 6 months decrease Sahelian aerosol ? straw specific composition 1-5 years : effects unknown rain+ grazing surface controls of SW radiative budget in the Gourma timescales and processus fire albedo 5 days straw dynamics 6 month tree density ETR 5 days pre onset conditions What are models able to do ? Slide 19 STEP simulations of biomass, straw, litter and roots for Agoufou being couples to SETHYS svat model 20022003 2004 ground litter yellow leaves green leaves total leaves One STEP beyond courtesy Baup et al. Slide 20 Any questions ? Slide 21 Combined use of remote sensing information and models Validation of products A deliverable is 80% written, you can still contribute ( instead of going to the gala dinner ). Leaf Area Index Soil moisture Albedo Shortwave Land use mesoscale maps (important effort for SPOT HR imagery) + radiometry (AVHRR, SEVIRI, MODIS...) + ETR, Surface Temperature Slide 22 Very good phenology, absolute maximum a bit low, noise. inter-site work in progress Leaf Area Index : 1 km in situ measurements versus MODIS- LAI product Agoufou 2005-2006 courtesy Mougin et al. Slide 23 Combined use of remote sensing information and models Forcings for ALMIP Validation ? ALMIP veg ? ALMIP 2 Veg ? Interannual variability for EOP ? Diagnostics studies ? what do we see, what can model do ? (albedo, memory effects NDVI/rainfall, trends), +/- anomalies