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 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 = 1for 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

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

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)

Foley et al. 2005

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

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).

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)

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

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).