Lecture 2: Biophysical interactions between land and atmosphere Elena Shevliakova & Chip Levy

Faq 1.1 from IPCC (2007) Energy Flows in the Atmosphere

Generalized scope of interactions GB Bonan 2002, Ecological Climatology time-scale

Constraints of Climate on Plants Sunlight – Available sunlight drives photosynthesis. – ~1.4 g dry matter is produced for 1MJ of intercepted sunlight (2.5% efficiency). – Heats surface and evaporates Water Water – Hydrates cells – Causes tugor for growth and cell expansion – Transfers nutrients – Water vapor is lost as stomates open to acquire CO 2 Temperature – Regulates rates of biochemical and enzymatic reactions – Determines if water is gas, liquid or solid

Land cover effect on climate Radiation –Surface albedo –Surface temperature and emissivity Turbulent fluxes –Roughness –Stomatal conductance, Leaf area index (LAI) –Available moisture in soil and interception storage

Land Surface-Atmosphere Coupling *for natural fires and re-growth in boreal region.

Surface Energy Balance The land surface on average is heated by net radiation balanced by exchanges with the atmosphere of sensible and latent heat Rad_net = ShortWave_net + LongWave_net Sensible heat [SH] is the energy carried by the atmosphere in its temperature Latent heat [LH]is the energy lost from the surface by evaporation of surface water The latent heat of the water vapor is converted to sensible heat in the atmosphere through vapor condensation The condensed water is returned to the surface through precipitation.

Major Radiation Components Absorbed Reflected Transmitted

Radiative Properties of the Atmosphere, Leaves and Surface Conservation of energy: radiation at a given wavelength is either: –reflected — property of surface or medium is called reflectance or albedo (0-1) –absorbed — property is absorptance or emissivity (0-1) –transmitted — property is transmittance (0-1) reflectance + absorptance + transmittance = 1 for a surface, transmittance = 0

General Surface Reflectance Curves from Klein, Hall and Riggs, 1998: Hydrological Processes, 12, with sources from Clark et al. (1993); Salisbury and D'Aria (1992, 1994); Salisbury et al. (1994)

MODIS Broadband Albedo, 10/1986

Snow Albedo Feedback NH snow cover retreats rapidly as radiation and T increase Surface albedo is decreased and absorbed radiation is increased => enhanced warming Hall and Qu, 2005

Pitman 2003

GLDAS

LAI Biophysical Interactions

Surface Roughness Length

Roughness Length Interaction with Biophysics

Image adapted from an illustration which originally appeared in Scientific American (September 1989, p. 82). thousands of km 3 per year

Hydrological cycle and Climate Climate dynamics and physics depend on exchange of moisture between atmosphere, land and ocean –Water vapor acts as a greenhouse gas and nearly doubles effects of greenhouse warming CO 2, methane, and all other gases –~50% of net surface cooling* results from evaporation –~30% of thermal energy driving atmospheric circulations provided by latent heating in clouds –Clouds alter radiation budget * This is a little tricky

Desertification Positive Feedback (soil moisture)

Foley et al Natural/Potential Vegetation vs Land Use (Human Impact)

Land Cover Change and Climate Land use impacts the amount and partitioning of available energy at the earth’s surface. Model response is dependent on weighting of various parameter changes. In our model (LM2), a change from forest to grassland leads to:

Forests and Future Climate Change Biophysical forest-atmosphere interactions can dampen or amplify anthropogenic climate change –Tropical forests could mitigate warming through evaporative cooling –Boreal forests could increase warming through the low albedo –The evaporative and albedo effects of temperate forests are unclear Potential increase in forest growth and expansion will attenuate global warming through carbon sequestration

MODIS Broadband Albedo, 10/1986

Bonan 2008.

Land-atmosphere interactions: Amazonia (Betts & Silva Dias, 2009) Large seasonal variations in precipitation, cloud cover and radiation, not temperature Large changes in land use affecting, surface albedo and roughness, atmospheric composition from biomass burning, Large scale biosphere-atmosphere experiment (LBA) since the mid 1990s –long-term monitoring; –Intensive field campaigns; –data sets;

Land Surface-Atmosphere Coupling *for natural fires and re-growth in boreal region.

Betts, A.K., and M.A.F. Silva Dias, 2009: Progress in understanding land-surface-atmosphere coupling over the Amazon: a review. Submitted to J. Adv. Model. Earth Syst. Land-atmosphere interactions: tropics

Betts and Silvia Dias (2009) added new pathways to the Betts (1996) diagram: –Surface influence on the seasonal behavior of clouds, aerosols and precipitation; –Impact of diffuse radiation on net ecosystem exchange; –role of convection in the transport of atmospheric tracers, including CO 2 ; –Coupling between clouds, meso-scale dynamics, and atmospheric circulation (oceans play a role).

Potential natural land cover distribution Tropical deforestation experimentHistorical land cover change experiment Land cover disturbances Experiments discussed in Findell et al. (2006, 2007, 2009)

Strong local response, Weak remote response Change in annual net radiation (W/m 2 ), 1990-NatVeg Local responses to both perturbations are generally significant –Less R net, less evaporation, higher temperatures –Rainfall response not homogeneous Remote responses do not pass field significance tests Some globally and annually averaged fields do pass significance tests because of the strong local responses

The next two slides are a problem for the class. Please check the paper referenced in the next slide and explain to me why a surface albedo increase for pasture correlates with an increase in observed cloudiness.

Source: AK Betts

Pitman 2003

Summary Land and atmosphere are linked through exchanges of energy, moisture and chemical tracers (chemical link to be discussed). Snow/Ice-albedo feedback is a powerful regional climate feedback in most, if not all, climate models (Suki Manabe and many others) Surface albedo is a powerful climate knob (any climate model builder will tell you). Tropics have potential to mitigate climate change through evaporative cooling but the magnitude will depend on the future land use activities. The biophysical couplings are numerous, intertwined and not easy to unravel (this makes simplifications tricky in the scientific sense).