The Surface Energy Balance and Turbulent Fluxes

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The Surface Energy Balance and Turbulent The Surface Energy Balance and Turbulent Fluxes Fluxes Bonan, Ch. 13, Ch. 14; Kiehl and Bonan, Ch. 13, Ch. 14; Kiehl and Trenberth (1997) Trenberth (1997) Why The SEB? Why The SEB? What and How? What and How? a. a. SEB components (Rn, SH, LE, G, B, Tskin, SEB components (Rn, SH, LE, G, B, Tskin, ε ε , , α α , , examples) examples) b. b. ABL (neutral, stable, unstable, Ri, z/L, ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy covariance, bulk entrainment, LCL, eddy covariance, bulk formulations, examples) formulations, examples) c. c. SEB measurements SEB measurements d. d. SEB remote sensing SEB remote sensing e. e. SEB modeling (LSMs) SEB modeling (LSMs) f. f. International Programs (GEWEX) International Programs (GEWEX)

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The Surface Energy Balance and Turbulent Fluxes. Why The SEB? What and How? SEB components ( Rn , SH, LE, G, B, Tskin , ε , α , examples) ABL (neutral, stable, unstable, Ri , z/L, entrainment, LCL, eddy covariance, bulk formulations, examples) SEB measurements SEB remote sensing - PowerPoint PPT Presentation

Transcript of The Surface Energy Balance and Turbulent Fluxes

Page 1: The Surface Energy Balance and Turbulent Fluxes

The Surface Energy Balance and Turbulent FluxesThe Surface Energy Balance and Turbulent Fluxes

Bonan, Ch. 13, Ch. 14; Kiehl and Trenberth (1997)Bonan, Ch. 13, Ch. 14; Kiehl and Trenberth (1997)

Why The SEB?Why The SEB?What and How? What and How? a.a. SEB components (Rn, SH, LE, G, B, Tskin, SEB components (Rn, SH, LE, G, B, Tskin, εε, , αα, examples) , examples) b.b. ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy ABL (neutral, stable, unstable, Ri, z/L, entrainment, LCL, eddy

covariance, bulk formulations, examples) covariance, bulk formulations, examples) c.c. SEB measurementsSEB measurementsd.d. SEB remote sensing SEB remote sensing e.e. SEB modeling (LSMs)SEB modeling (LSMs)f.f. International Programs (GEWEX)International Programs (GEWEX)

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The Atmospheric Boundary LayerThe Atmospheric Boundary Layer

ABL = The part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a time scale of about an hour or less. See http://lidar.ssec.wisc.edu/papers/akp_thes/node6.htm http://apollo.lsc.vsc.edu/classes/met455/notes/section9/1.html

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The Atmospheric Boundary LayerThe Atmospheric Boundary Layer1. Definition: ABL = The part of the troposphere that is directly influenced by the presence of

the earth’s surface, and responds to surface forcings with a time scale of about an hour or less.

2. Structure: free atmosphere, entrainment zone, mixed layer (where U, θ, q almost constant with height), surface layer (where vertical fluxes of momentum, heat, and moisture are almost constant with height)

3. Thickness: typically 1 km; varying from 20 m to several km; deeper with strong solar heating, strong winds, rough surface, or upward mean vertical motion in the free troposphere.

4. Both structure and thickness have a strong diurnal cycle.5. Turbulent motions (opposite to laminar flow)

i. chaotic swirls; rapid chaotic fluctuations in winds, temperature, moisture, other massii. generated mechanically (in the presence of strong near surface mean winds), oriii. generated thermally (strong solar heating high buoyancy vertical motion) (mostly daytime, land; also common over the oceans)

6. ABL clouds: fog, fair weather cumulus, stratocumulus

               ,

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Potential TemperaturePotential Temperature

The potential temperature (θ) of a parcel of air at pressure P is the temperature that the parcel would acquire if adiabatically brought to a standard reference pressure P0 (= 1000 millibars).

where T = the current absolute temperature (in K) of the parcel, R = the gas constant of air, and cp = the specific heat capacity at a constant pressure. See GPC Appendix C for derivations.

θ is a more dynamically important quantity than T. Under almost all circumstances, θ increases upwards in the atmosphere, unlike T which may increase or decrease. θ is conserved for all dry adiabatic processes, and as such is an important quantity in the ABL (which is often very close to being dry adiabatic). The dry adiabatic lapse rate: Γd = g/cp = 9.8 °C/km

θ is a useful measure of the static stability of the unsaturated atmosphere.

stable, vertical motion is suppressed;

unstable, convection is likely

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Stüve diagramStüve diagram (Thermodynamic Diagram)Isotherms are straight and vertical, isobars are straight and horizontal and dry adiabats are also straight and have a 45 degree inclination to the left while moist adiabats are curved (see also GPC Appendix

C, Fig. C.1).

T=20°C,P=1000 mb θ= 20°C

T=20°C, P=900 mb θ= 28.96°C

A parcel with P, T, q Td =? q*=?, RH=?, LCL=? Δq=?

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Thermodynamics

http://hyperphysics.phy-astr.gsu.edu/Hbase/heacon.html#heacon

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Air Flow and Turbulent VorticesAir Flow and Turbulent Vortices

Air flow can be imagined as a horizontal flow of numerous rotating eddies, a turbulent vortices of various sizes, with each eddy having 3D components, including vertical components as well. The situation looks chaotic, but vertical movement of the components can be measured from the tower.

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Reynolds Decomposition and Eddy CovarianceReynolds Decomposition and Eddy Covariance

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Reynolds Decomposition and Eddy CovarianceReynolds Decomposition and Eddy Covariance

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Bulk Aerodynamic Formulas (Parameterizations)Bulk Aerodynamic Formulas (Parameterizations)

τ = ρ CDM Ur2

SH = cp ρ CDH Ur [Ts – Ta(zr)]

LE = L ρ CDE Ur [qs – qa(zr)]

CDN = [κ / ln(zr/z0)]2

CDM = CDN,M fM(RiB)CDH = CDN,H fH(RiB)CDE = CDN,E fE(RiB) or Eqn (14.31) in Bonan (2008)

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Global Distribution of Sensible Heat FluxGlobal Distribution of Sensible Heat Flux

http://www.cdc.noaa.gov/

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Global Distribution of Latent Heat FluxGlobal Distribution of Latent Heat Flux

http://www.cdc.noaa.gov/

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Regional Patterns of The Surface Energy BalanceRegional Patterns of The Surface Energy Balance

Yuma, AZ energy balance (ly/day)At the other extreme is Yuma, Arizona, a warm and dry climate. The most noticeable characteristic of this place is the lack of latent heat transfer. Though  ample radiation is available here, there is no water to evaporate. Nearly all net radiation is used for sensible heat transfer which explains the hot dry conditions at Yuma.

West Palm Beach, Fl energy balance (ly/day) West Palm Beach, Florida  is located in a warm and moist climate. Latent energy transfer into the air is greatest during the summer time which is the wettest period of the year, and when net radiation is the highest. During the summer, sensible heat transfer decreases as net radiation is allocated to evaporation and latent heat transfer.

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Modeling of The Surface Energy BalanceModeling of The Surface Energy Balance

NCAR CLM: http://www.cgd.ucar.edu/tss/clm/ for global climate modeling and projections

NCEP Noah LSM: for numerical weather predictions

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NCAR CLM 3.5Biogeochemistry

Ecosystem DynamicsHydrology

Biogeophysics

Niu, Yang, et al., 2007Niu, Yang, et al., 2005

Yang et al., 1997, 1999Niu & Yang, 2003, 2006

Yang & Niu, 2003

Collaborators: UT (Z.-L. Yang, G.-Y. Niu, R.E. Dickinson); NCAR (G.B. Bonan, K. Oleson, D. Lawrence)

2008 CCSM Distinguished Achievement

Award

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Collaborators: UT (Z.-L. Yang, G.-Y. Niu, D. Maidment), NCAR (Fei Chen, Dave Gochis); NCEP (Mike Ek, Ken Mitchell)

1-D ‘Noah’ Community 1-D ‘Noah’ Community Land Surface ModelLand Surface Model

Dynamical Routing MethodologiesDynamical Routing Methodologies

Explicit diffusive wave overland flow

Explicit saturatedsubsurface flow

Groundwater discharge, reservoir routing &

Explicit channel routing

• fully distributed flow/head• reservoir levels• distributed soil moisture• distributed land/atmo fluxes• distributed snow depth/SWE

Noah LSM with hydrological enhancements

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Observing The Surface Energy BalanceObserving The Surface Energy Balance

FLUXNEThttp://www.fluxdata.org/default.aspx

See also other flux measurement networks (e.g., Ameriflux, CarboEurope, Fluxnet Canada, and iLEAPS).

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International ProgramsInternational Programs

GEWEX http://www.gewex.org/