Download - Lecture 2: Biophysical interactions between land and atmosphere Elena Shevliakova & Chip Levy
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Lecture 2: Biophysical interactions between land and atmosphere
Elena Shevliakova & Chip Levy
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Faq 1.1
from IPCC (2007)
Energy Flows in the Atmosphere
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Generalized scope of interactions
GB Bonan 2002, Ecological Climatology
time-scale
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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 CO2
• Temperature – Regulates rates of biochemical and enzymatic reactions – Determines if water is gas, liquid or solid
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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
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Land Surface-Atmosphere Coupling
*for natural fires and re-growth in boreal region.
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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.
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Major Radiation Components
• Absorbed• Reflected• Transmitted
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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 = 1for a surface, transmittance = 0
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General Surface Reflectance Curves
from Klein, Hall and Riggs, 1998: Hydrological Processes, 12, 1723 - 1744 with sources from Clark et al. (1993); Salisbury and D'Aria (1992, 1994); Salisbury et al. (1994)
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MODIS Broadband Albedo, 10/1986
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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
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Pitman 2003
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GLDAS
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LAI Biophysical Interactions
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Surface Roughness Length
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Roughness Length Interaction with Biophysics
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Image adapted from an illustration which originally appeared in Scientific American (September 1989, p. 82). http://www.globalchange.umich.edu/globalchange1/current/labs/water_cycle/water_cycle.html
thousands of km3 per year
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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 CO2, 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
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Desertification Positive Feedback (soil moisture)
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Foley et al. 2005
Natural/Potential Vegetation vs Land Use (Human Impact)
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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:
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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
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MODIS Broadband Albedo, 10/1986
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Bonan 2008.
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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;
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Land Surface-Atmosphere Coupling
*for natural fires and re-growth in boreal region.
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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
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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
CO2;– Coupling between clouds, meso-scale dynamics, and atmospheric
circulation (oceans play a role).
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Potential natural land cover distribution
Tropical deforestation experiment Historical land cover change experiment
Land cover disturbances
Experiments discussed in Findell et al. (2006, 2007, 2009)
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Strong local response,Weak remote response
Change in annual net radiation (W/m2), 1990-NatVeg
• Local responses to both perturbations are generally significant– Less Rnet, 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
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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.
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Source: AK Betts
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Pitman 2003
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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).