Transcript Biophysical interactions between land and atsmosphere

Lecture 2: Biophysical interactions between land and
atmosphere
Elena Shevliakova & Chip Levy
Energy Flows in the Atmosphere
Faq 1.1
from IPCC (2007)
Generalized scope of interactions
time-scale
GB Bonan 2002, Ecological Climatology
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
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, 1723 - 1744 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
thousands of km3 per year
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
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
Desertification Positive Feedback (soil moisture)
Natural/Potential Vegetation vs Land Use (Human Impact)
Foley et al. 2005
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.
Land-atmosphere interactions: tropics
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
CO2;
– Coupling between clouds, meso-scale dynamics, and atmospheric
circulation (oceans play a role).
Land cover disturbances
Potential natural land cover distribution
Tropical deforestation experiment
Historical land cover change experiment
Experiments discussed in Findell et al. (2006, 2007, 2009)
Strong local response,
Weak remote response
• 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
Change in annual net radiation (W/m2), 1990-NatVeg
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).