Land surface - boundary layer interactions Dennis Baldocchi University of California, Berkeley NCEAS Workshop, Santa Barbara 2009

Case Study: Energetics of a Grassland and Oak Savanna Measurements and Model

From a CO2 perspective, the oak woodland is a Greater C sink than the Annual Grassland

Savanna Woodland adjacent to Grassland Case Study: Savanna Woodland adjacent to Grassland Savanna 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-2

Landscape Differences On Short Time Scales, Grass ET > Forest ET Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, 2008 5

On Annual Time Scale, Forest ET > Grass ET Role of Land Use on ET: On Annual Time Scale, Forest ET > Grass ET Ryu, Baldocchi, Ma and Hehn, JGR-Atmos, 2008

4a. U* of tall, rough Savanna > short, smooth Grassland 4b. Savanna injects more Sensible Heat into the atmosphere because it has more Available Energy and it is Aerodynamically Rougher

5. Mean Potential Temperature differences are relatively small (0 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 Warmer Compare to Greenhouse Sensitivity ~2-4 K/(4 W m-2)

Conceptual Diagram of PBL Interactions H and LE: Analytical/Quadratic version of Penman-Monteith Equation

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 cooling

Forests Transpire effectively, causing evaporative cooling, which in humid regions may form clouds and reduce planetary albedo

Theoretical Difference in Air Temperature: Grass vs Savanna: Grass Tair is much cooler if we only consider albedo Summer Conditions

And Smaller Temperature Difference, like field measurements, if we consider PBL, Rc, Ra and albedo….!! And temperatures are about equal when albedo of the grass is 0.25 Summer Conditions

Tsfc can vary by 10 C by changing Ra and Rs Mean Tsfc field as a function of Ra and Rs for a range of albedos

Tsfc can vary by 10 C by changing albedo and Rs

Tair can vary by 3 C by changing albedo and Rs Diurnal pattern in Rg, sinusoidal and pbl model in time with variation in parameters

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

Conclusions To 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 services Water yield, habitat, grazing, recreation ‘We must not blame the forests for climate change if we can improve energy efficiency and reduce carbon emissions’ Sebastiaan Luyssaert

Feedbacks with Surface Temperature ESPM 129 Biometeorology