Transcript ABL, Thermodynamics, Reynolds decomposition, Eddy covariance

Chapter 4 The Energy Balance of The Surface
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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
SEB modeling (LSMs)
International Programs (GEWEX)
Kiehl and Trenberth (1997)
The 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
The Atmospheric Boundary Layer
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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.
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)
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.
Both structure and thickness have a strong diurnal cycle.
Turbulent motions (opposite to laminar flow)
, temperature, moisture, other mass
i.
chaotic swirls; rapid chaotic fluctuations in winds,
ii.
generated mechanically (in the presence of strong near surface mean winds), or
iii. generated thermally (strong solar heating  high buoyancy  vertical motion)
(mostly daytime, land; also common over the oceans)
ABL clouds: fog, fair weather cumulus, stratocumulus
Potential 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
Stüve 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=?
Thermodynamics
http://hyperphysics.phy-astr.gsu.edu/Hbase/heacon.html#heacon
Air 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.
Determine Vertical Fluxes
Reynolds Decomposition and Eddy Covariance
Reynolds Decomposition and Eddy Covariance
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)
Global Distribution of Sensible Heat Flux
http://www.cdc.noaa.gov/
Global Distribution of Latent Heat Flux
http://www.cdc.noaa.gov/
Regional Patterns of The Surface Energy Balance
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.
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.
Modeling 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
2008 CCSM
Distinguished
Achievement
Award
Niu & Yang, 2003, 2006
Yang et al., 1997, 1999
NCAR CLM 3.5
Niu, Yang, et al., 2005
Niu, Yang, et al., 2007
Yang & Niu, 2003
Collaborators: UT (Z.-L. Yang, G.-Y. Niu, R.E. Dickinson); NCAR (G.B. Bonan, K. Oleson, D. Lawrence)
Noah LSM with hydrological enhancements
Collaborators: UT (Z.-L. Yang, G.-Y. Niu, D. Maidment), NCAR (Fei Chen, Dave Gochis);
NCEP (Ken Mitchell)
Explicit diffusive wave overland flow
Groundwater discharge,
reservoir routing &
Explicit channel routing
Dynamical Routing Methodologies
1-D ‘Noah’ Community
Land Surface Model
Explicit saturated
subsurface flow
• fully distributed flow/head
• reservoir levels
• distributed soil moisture
• distributed land/atmo fluxes
• distributed snow depth/SWE
Observing The Surface Energy Balance
FLUXNET
http://daac.ornl.gov/FLUXNET/
See also other flux measurement
networks (e.g., Ameriflux,
CarboEurope, Fluxnet Canada,
and iLEAPS).
International Programs
GEWEX http://www.gewex.org/
Many others http://www.gewex.org/links-org.htm