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Dennis BaldocchiUniversity of California, BerkeleyNCEAS Workshop, Santa Barbara 2009Land surface - boundary layer interactions
Case Study:
Energetics of a Grassland and Oak Savanna
Measurements and Model2
From a CO2 perspective, the oak woodland is a Greater C sink than the Annual Grassland
Case Study:Savanna Woodland adjacent to GrasslandSavanna absorbs much more Radiation (3.18 GJ m-2 y-1) than the Grassland (2.28 GJ m-2 y-1) ; DRn: 28.4 W m-24
Landscape DifferencesOn Short Time Scales, Grass ET > Forest ETRyu, Baldocchi, Ma and Hehn, JGR-Atmos, 2008
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Role of Land Use on ET:On Annual Time Scale, Forest ET > Grass ET
Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, 20086
4a. U* of tall, rough Savanna > short, smooth Grassland4b. Savanna injects more Sensible Heat into the atmosphere because it has more Available Energy and it is Aerodynamically Rougher
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5. Mean Potential Temperature differences are relatively small (0.84 C; grass: 290.72 vs savanna: 291.56 K); despite large differences in Energy Fluxes--albeit the Darker vegetation is WarmerCompare to Greenhouse Sensitivity ~2-4 K/(4 W m-2)
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Conceptual Diagram of PBL Interactions
H and LE: Analytical/Quadratic version of Penman-Monteith Equation9
The Energetics of afforestation/deforestation is complicated
Forests have a low albedo, are darker and absorb more energy
But, Ironically the darker forest maybe cooler (Tsfc) than a bright grassland due to evaporative cooling10
Forests Transpire effectively, causing evaporative cooling, which in humid regions may form clouds and reduce planetary albedo11
Theoretical Difference in Air Temperature: Grass vs Savanna:Grass Tair is much cooler if we only consider albedoSummer Conditions12And Smaller Temperature Difference, like field measurements, if we consider PBL, Rc, Ra and albedo.!!Summer Conditions
And temperatures are about equal when albedo of the grass is 0.2513
Tsfc can vary by 10 C by changing Ra and RsMean Tsfc field as a function of Ra and Rs for a range of albedos14
Tsfc can vary by 10 C by changing albedo and Rs
Tair can vary by 3 C by changing albedo and RsDiurnal pattern in Rg, sinusoidal and pbl model in time with variation in parameters16
Tair can vary by 3 C by changing Ra and Rs
Rnet can by 100 W m-2 by changing albedo and Rs
Rnet can by 30 W m-2 by changing Ra and Rs
ConclusionsTo understand the role of vegetation on climate we must consider more that C sink and Albedo!Tair =f(PBL..f(H)..f(Rnet, LE, Tsfc, Gstor)..f(a, Rc, Ra))We must consider all the ecosystems servicesWater yield, habitat, grazing, recreationWe must not blame the forests for climate change if we can improve energy efficiency and reduce carbon emissionsSebastiaan LuyssaertESPM 129 BiometeorologyFeedbacks with Surface Temperature