Boundary Layer Climatology

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Boundary Layer Climatology

• ATMOS/GEOG 622.01
• Surface Energy Budgets

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Surface Energy Budget/Balance

• Balance of vertical fluxes of energy
• Conservation of energy
• Properties of an ideal surface allows us to make
  a 1D model
• Extensive
• Horizontal
• Homogeneous
• Opaque to radiation
• Flat
• Stationary
• Temperature, humidity, turbulence

Stationarity - A form of homogeneity in a
single characteristic. Local stationarity occurs
when two or more adjacent, locally homogeneous
samples yield similar values of the property of
interest.
Surface Energy Budgets
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Surface Energy Budget Components

• Net Radiation (RN)
• Net vertical radiation flux in wavelengths
  significant for surface heating and cooling
• Wavelength range 0.2 150 mm
• from UV to thermal IR
• Turbulent Fluxes
• sensible heat flux (H)
• latent heat flux (HL)
• Energy transfer from evaporation/condensation or
  sublimation/freezing
• Conductive energy (HG)
• i.e. in subsurface, ground heat flux
• Change in stored heat (DHS)
• others
• Heat from precipitation (HP)
• Heat from fuel combustion (HF)
• e.g. cars, power plants, homes, etc.

Surface Energy Budgets
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turbulent sensible heat flux (H)

• Arises from vertical temperature and wind speed
  differences in the PBL
• H can be approximated by H rair cp K dT/dz
• cp specific heat of the air, constant (1001 J
  kg-1 K)
• K exchange coefficient
• K is a function of wind speed and air viscosity
• Typical Case
• During daytime, a net radiation surplus exists
  (radiative heating)
• surface usually loses turbulent sensible to the
  atmosphere
• During night, a net radiation deficit exists
  (radiative cooling)
• surface gains turbulent sensible heats from the
  atmosphere
• Atypical Case
• Surface can receive turbulent sensible heat
  energy at day
• warm air advection over cold surface
• Surface can lose turbulent sensible heat energy
  at night
• cold air advection over warm surface

Surface Energy Budgets
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Measuring the Sensible Heat Flux (H)
H -rair cp K dT/dz
T2, z2
dT/dz
T1 , z1
You always need to know the wind speed profile.
Without wind, turbulent fluxes can only be by
free convection and are thus (usually) very small!
Surface Energy Budgets
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turbulent latent heat flux (HL)

• Arises from humidity (q) and wind speed (and
  temperature) vertical differences in atmosphere
• Is associated with phase-changes of H2O.
• HL rair Lv,scp K dq/dz
• q specific humidity
• cp specific heat of the air, constant (1001 J
  kg-1 K)
• L latent heat of vaporization (Lv) or
  sublimation (Ls)
• LV 2.501 x 106 J kg-1 at 0 degrees C
• LV is a weak function of temperature
• LV 2.501 x 106 2400 T, where T is in C
  units
• LS 2.835 x 106 J kg-1 at lt 0 degrees C
• Latent heat of fusion (LF) 0.335 x 106 J kg-1
  at 0 degrees C
• LS LV LF
• K exchange coefficient, including dynamic
  viscosity and eddy diffusivity, here also a
  function of temperature

Surface Energy Budgets
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Water Vapor Exchange and Latent Heat Flux
Atmosphere gains latent heat energy from surface
Surface
Atmosphere
More Kinetic Energy of Molecules
Less Kinetic Energy of Molecules
Atmosphere loses latent heat energy to surface
Mass exchanges and phase transitions are
accompanied by heat energy exchanges
Surface Energy Budgets
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Measuring Evapotranspiration with a Lysimeter
Rietholzbach Research catchment Swiss Federal
Institute of Technology (ETH)
Under the Lysimeter
Surface Energy Budgets
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Ground Heat Flux (HG)

• Heat conduction into the sub-surface
• HG -k dT/dz
• k thermal conductivity thermal diffusivity
  density specific heat
• Note that density specific heat heat
  capacity
• k is a function of
• Density
• Porosity
• Liquid water content, etc.
• See Arya (2001), pg. 50 for Table of C,cp,k,etc
• Diurnal cycles and annual cycles define an active
  layer

Surface Energy Budgets
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Measuring HG
Surface Energy Budgets
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The Active Layer
Measurements from Greenland Ice Diamonds
constant depth Crosses actual temperature
measurement depths
The active layer is the depth over which there is
a significant daily or seasonal temperature
cycle, also considered depth where freeze-thaw
takes place
Surface Energy Budgets
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Sign Convention

• Height coordinates increase away from the surface
• Sign of sensible heat flux (H)
• with reference to the surface
• DT T2 T1
• If T2 gt T1, H gt 0
• atmosphere heats the surface
• i.e. inversion, nighttime or winter clear sky
  scenario
• If T2 lt T1, H lt 0
• ground heats atmosphere
• lapse, midday scenario
• with reference to the atmosphere
• DT T2 T1
• If T2 gt T1, H lt 0
• In this case, equation for H has negative sign
  inserted
• H -rair cp K dT/dz
• Sign convention from Arya (2001).
• All radiative fluxes directed toward the surface
  are positive.
• non-radiative fluxes directed toward the surface
  are negative.
• consistent with with reference to the atmosphere

Surface Energy Budgets
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Example Day and Night Surface Energy Flow
no clouds in both cases. Note that clouds can
give a positive RN at night
Surface Energy Budgets
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Energy Balance of a Dry Surface
Arya (2001)
Nighttime Temperature inversion promotes sensible
heating of surface
Surface heats atmosphere at day, negative sign to
balance with net radiative flux
Sun heats ground at day, negative sign to balance
with net radiative flux
Nighttime radiative loss by heat stored in ground
Surface Energy Budgets
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Energy Budget of a Vegetated Surface
Here, no sign convention is adopted for this graph
Evaporative cooling (HL) is the major heat sink
that balances radiation input at day
Arya (2001)
Surface Energy Budgets
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Energy Budget of a Melting Snow Surface
Strong winds
Strong radiative fluxes, but high albedo
Strong latent heat flux, surface cooling,
atmospheric heating
Continual loss of sensible heat from atmosphere
to surface, temperature inversion maintained by
surface cooling
Here, sign convention with reference to the
atmosphere is adopted for this graph
Day of Year, 2000
Surface Energy Budgets
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Surface Energy Balance and Conservation of Energy

• Neglecting energy from rain QP and fuel combusion
  QF
• H HL HG HS - RN
• Over timescales of 1 day or 1 year HS may be
  negligible
• H HL HG HS RN 0
• SEB closure
• SEB closure is difficult to attain experimentally
  owing to measurement errors of the different
  components of the SEB, i.e. radiation fluxes,
  turbulent fluxes, conductive fluxes.

Surface Energy Budgets
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OSU Airport Measurement Site
Surface Energy Budgets
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S?
u2 u1
QH QE
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S?
u2 u1
QH QE
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T2 T1
QH QE
Surface Energy Budgets
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Bowen Ratio

• Ratio of sensible to latent heat flux
• QH/QE
• Which is a typical value for a desert, 0.1 or 10?
• If the turbulent fluxes are not known, but there
  is some idea of how dry a surface is, the
  surface energy balance may be closed
• not to be confused with Bowen ratio method,
  discussed later

Surface Energy Budgets
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Equating Energy for Snowmelt (HM) with SEB
conservation

• Remember that H HL HG HS RN 0
• Normally, heat surpluses, e.g. from H are
  balanced by RN or by changes in surface
  temperature.
• However, over a snow surface, if energy surplus
  brings snow/ice to melting point, excess energy
  is sunk (used) by melting.
• H HL HG HS RN HM
• If melting point not reached, need to compute how
  much energy needed to first bring the ice to the
  melting point.

Surface Energy Budgets