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Surface control on albedo and radiation balance over the Gourma site Laurent Kergoat, Olivier Samain, Françoise Guichard, Pierre Hiernaux, Franck Timouk,

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Presentation on theme: "Surface control on albedo and radiation balance over the Gourma site Laurent Kergoat, Olivier Samain, Françoise Guichard, Pierre Hiernaux, Franck Timouk,"— Presentation transcript:

1 Surface control on albedo and radiation balance over the Gourma site Laurent Kergoat, Olivier Samain, Françoise Guichard, Pierre Hiernaux, Franck Timouk, Colin Lloyd, Eric Mougin, François Lavenu, Patricia de Rosnay, Valerie Le Dantec, Josiane Seghieri et al. CESBIO, CNRM, CEH

2 Albedo is a key parameter, isn’t it ? Yes it is ! as is Rn – G = H + LE ≈  e Eltahir, Zeng

3 Albedo is STILL a key parameter, isn’t 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)

4 Gourma mesoscale Site (~300 x 100 km²) Agoufou local Site (15.3°N, 1.5°W) Bamba local Site (17.1°N, 1.3°W) Niger River Rainfall 50 mm 350 mm 450 mm 300 mm 100 mm in situ met stations

5 In situ : short term variability courtesy Samain et al. soil moisture effect aerosol apparent effect

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

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.

8 Seasonal cycle courtesy Samain et al.

9 Vegetation growth controls the summer decrease of albedo at seasonal and inter-annual time scales.

10 Both NIR and Vis spectral band decrease with biomass Straw + litter dynamics controls dry season albedo (bright soil) No ‘vegetation’ cycle in replaced by a two-year cycle ! courtesy Samain et al.

11 Interannual variability : rain drives peak biomass and dry season straw/litter fire

12 surface type, grazing pressure effect mainly in the dry season

13 Gourma meso scale

14 Some intriguing trends over Northern Gourma.... (NOT a calibration problem)

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

16 Seasonal R net over 4 years, mix of surface and atmosphere controls late start of rain, vegetation courtesy Guichard, Timouk

17 Radiation balance for Agoufou in August LW  LW  SW  SW  Rnet (W.m -2 ) LW  LW  SW  SW  Rnet (W.m -2 ) Rnet Difference dry year versus wet year ≈ 20 w due to LW losses Tsurface is cooled by ETR. ≈ 15 w due to albedo effect = highest Rnet +40% compared to

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 ?

19 STEP simulations of biomass, straw, litter and roots for Agoufou being couples to SETHYS svat model ground litter yellow leaves green leaves total leaves One STEP beyond courtesy Baup et al.

20 Any questions ?

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

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 courtesy Mougin et al.

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


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