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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