Boundary Layer Climatology

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Boundary Layer Climatology

• ATMOS/GEOG 622.01
• Basics of Electromagnetic Radiation
• Part 1

Colors across the visible spectrum
Max Karl Ernst Ludwig Planck Nobel Prize Physics
1918
The Sun, NASA
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Sun Angles

• Sun Angle
• Angle of sun above horizon
• Solar Zenith Angle
• Angle between sun and zenith
• Solar Azimuth Angle
• Angle in horizontal plane, zero toward North
• To calculate, need
• Time of day
• Latitude
• Longitude

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Angle Measurement Degrees or Radians

• Default angles in most computer languages and
  scientific calculators are specified in radians
• Deg or Rad on calculator
• X degrees Xp/180 radians
• p/2 rad. 90 deg.
• p rad. 180 deg.
• 3/2p rad. 270 deg.
• 2p rad. 360 deg.

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Common Trigonometric Functions

• Sin
• Sin(0p)0
• Sin(p)0
• Sin(p/2)1
• Cos
• Cos(0p)1
• Cos(p)-1
• Sin(p/2)0
• Irradiance incident on surface, given by I0
  cos(sza)
• sza solar zenith angle

p
p/2
2p
3/2p
p
3/2p
p/2
2p
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Electromagnetic (EM) Radiation

• Radiation directional energy transfer by rapid
  oscillations of energy fields
• treated as discrete particles
• and/or as rays in geometric optics
• EM Radiation characterized by Wavelength (l),
  frequency (n)
• lc/n
• Where c speed of light
• 3 x 108 m s-1

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More on waves

• Wavelength is the distance between any next equal
  position on the wave, e.g. wave crests or where
  i0
• Units mm, nm, m
• Frequency is the number of waves passing a point
  per unit time
• Units s-1, i.e. Hz

i
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The Electromagnetic Spectrum

• Near IR (0.75 mm - 4 mm)
• Thermal IR (4 mm - 150 mm)
• Parts of Ultraviolet (UV) spectrum, UVA, UVB,
  UVC
• visible radiation, also termed shortwave or
  optical
• thermal radiation, also termed longwave or
  terrestrial radiation or earthshine

Source Oke (1987)
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• Central wavelengths of green, blue, and red light
• Green 510 nm
• Blue 475 nm
• Red 650 nm

Source NASA Langley ASDC User Services
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Ultra Violet (UV)

• UVA 0.400 mm 0.320 mmUVB 0.320 mm 0.290 mm
  UVC 0.290 mm 0.100 mm
• UVC blocked by stratospheric ozone (O3) absorption

Kipp Zonen Broadband UV (UV a-c) Pyranometer
Radiometer
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Solar Spectrum and Atmospheric Absorption
Arya (2001)
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Radiation Quantities

• Radiant flux rate of energy transfer by
  electromagnetic radiation. J s-1, or W. For
  example, the sun's radiant flux is 3.9 x 1026 W.
• Irradiance radiant flux divided by area through
  which it passes e.g. for the sun, with visible
  radius about 7 x 108 m.
• Monochromatic irradiance irradiance covering an
  infinitesimal wavelength interval of the
  electromagnetic spectrum. Units W m-2 µm-1.
• Radiance irradiance per unit solid angle (d?)
  W m-2 ster-1
• often referred to as intensity I (or B when
  referring to the Planck function). L is the
  quantity measured by a passive remote sensors

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Radiance and Irradiance

• Radiance is the radiant intensity (flux) into or
  out of a cone illustrated in this figure
• solid angle with angular radius (W) or (w)
• Irradiance is when the angular diameter of this
  cone p (or 180 degrees)
• can be represented as the flux through a plane of
  unit area
• flux density

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Blackbody Radiation Laws

• A body with T gt 0 K emits radiation
• If the body emits the max possible radiation over
  its surface at all wavelengths, it is a perfect
  radiator or a blackbody

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Emission by wavelength (Rl)

• Plancks Law for monochromatic emittance
• hp Planck constant
• 6.626068 10-34 m2 kg s-1
• b Boltzmann constant
• 1.3806503 10-23 m2 kg s-2 K-1
• Stefan-Boltzmann Law for Emittance over all
  wavelengths for a perfect radiator
• Integral of Plancks Law
• (E) EPR sT4
• Stefan-Boltzmann constant (s 5.67x 10-08 W m-2
  K-4)
• T temperature K
• Kelvins deg. C 273.15
• Units W m-2 l-1
• L E, L stands for longwave radiation
• Outward longwave L?
• E is diffuse, isotropic

Sun
Earth
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Measuring L?

• Pyrgeometer
• Kipp and Zonen CG4
• Eppley PIR (gt 3 µm - 150 µm)
• L? depends on effective atmospheric temperature
• Effective Blackbody Temperature
• Teff (L?/s )1/4

Kipp and Zonen CG4
Eppley PIR
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Measuring Ts

• Extrapolate temperature profile measurements near
  the surface
• Large errors, unless very windy
• Infrared thermometer
• (remotely sensed)
• Pyrgeometer
• Then must know emissivity for surface
• Fine wire thermocouple (in situ)
• Campbell Scientific FW3 is 0.003" in diameter,
  FW1 is 0.001", FW05 is 0.0005"
• Small sensors do not absorb as much solar
  radiation

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• Wiens Law
• lmax w/T
• lmax units mm
• w 2897 mm K-1

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Radiative Properties of Natural Surfaces

• emissivity (e)
• Reflectivity (r)
• Absorptivity (a)
• Transmissivity (tl)
• Kirchoffs Law a r t 1
• For opaque object, what isnt absorbed is
  reflected.
• a r 1
• Absorbed irradiance E(1-r)
• For natural surfaces, a, r, and t vary with l
• Table 3.1 (Arya, 2001) Radiative Properties of
  Natural Surfaces

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Instruments for Solar Radiation
Sun

• A Pyranometer measures hemispheric irradiance
• Measures direct and diffuse components.

Atmosphere
Diffuse
Direct
Kipp and Zonen CM3
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Solar Radiation measured by the Pyranometer
Kipp and Zonen CM3 Pyranometer
LI-COR 200sZ Photoelectric Diode Pyranometer
Eppley Precision Spectral Pyranometer
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Solar Shortwave Radiation (S)

• Wavelength range 0.28 0.75 mm
• Solar irradiance composed of direct and diffuse
  components
• Direct, Collimated parallel rays, S0
• Diffuse Radiation , SDiff
• From scattering
• Global Irradiance S0 SDiff

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Measuring Diffuse Solar Irradiance

• Pyranometer with shadowband

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Direct Beam Solar Radiation
Sun

• Pyrheliometer
• Measures direct component, i.e. radiance
• Units W m-2 st-1

Direct
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Albedo

• Albedo reflectance in visible wavelengths
• ? S?/S?
• Some Albedo Values
• Forest 0.15
• Fresh snow 0.84
• See Table 3.1 (Arya, 2001), pg. 32

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Types of Reflection

• Lambertian Reflection
• scatters evenly in all directions, i.e. is
  isotropic and hemispheric
• Roughness l
• Specular Reflection
• Angle of incidence angle of exitance
• Mirror
• Water surface with high solar zenith angles

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Net Radiation (RN)

• Net vertical irradiance over all wavelengths in
  solar and terrestrial spectrum.
• Wavelength range 0.2 150 mm
• Components RN
• Net Shortwave (S)
• S S? - S?
• Net Longwave (L)
• L L? - L?
• Radiation Balance
• RN S L
• RN (S ? - S ?) (L ? - L ?)
• RN (S ? (1- a)) (L)

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Net Radiation (RN)
Sun
Atmosphere
S? direct
S? diffuse
L ?
L?esT4 (1-e)L ?
S?(1-a) or S?
Ground
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Net Radiation
Kipp and Zonen 4-Component CNR1
Kipp and Zonen NR Lite Pyradiometer
REBS Pyradiometer
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Diurnal Surface Radiation Budget
Arya (2001)
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Electromagnetic Radiation - The hotter sun
emits shortwave radiation (ultraviolet, visible
and shortwave infrared) - The cooler Earth emits
longwave radiation (infrared) - much is absorbed
by the Earths atmosphere before it leaves (e.g.
by carbon dioxide)
Strahler and Strahler, Introduction to Physical
Geography, Figure 2.4, p. 55
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Atmospheric Absorption
Arya (2001)
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Earths Thermal Emission and The Atmospheres
Absorption Spectrum

• Important atmospheric window near 9-12 mm,
  near Earths peak emittance

Earths emittance curve including transmission
windows and opaque spectral regions
atmospheric window
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Downward LW is responsive to humidity and air
temperature
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Energy Budgets of the Earth Atmosphere and Surface
49 of insolation direct radiation (radiation
that goes directly to Earths surface) 31 of
insolation reflected back to space (3 by scatter
19 by clouds, 9 by ground)
Strahler and Strahler Figure 2.15, p. 67
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Reflection of Radiation

• Scattering
• Re-directing of light
• Scattering is strongly wavelength dependant.
• Absorption is also wavelength dependant.
• Atmospheric Attenuation (Absorption)
• Extinction Absorption Scattering
• Reflectance
• Spectral, i.e. f(l)
• Anisotropic, i.e. not Lambertian

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Spectral Reflectance of Natural Surfaces
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Scattering

• Molecular (Rayleigh) Scattering
• Mainly by O2
• Occurs when particle radius (r) lt 0.1 l
• i.e., l gtgt r
• Scattering inversely proportional l4
• Particle (Mie) Scattering
• For particles 0.1l lt r lt 25l
• Scattering of visible and UV light caused by
  aerosols and cloud droplets, i.e. particles

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Beer-Lambert Law

• The intensity of light entering a homogeneous
  translucent medium (atmosphere) decreases with
  distance into the medium according to
• S(z)S0e- kz
• S0 is the intensity of the incident light
• S(z) is the intensity after passing through the
  material
• z is the distance that the light travels through
  the material (the path length)
• k a l cx
• cx is the concentration of absorbing species in
  the material and
• a is the absorption coefficient of the absorber.
• acx extinction coefficient (k)
• Extinction Scattering Absorption

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Results of Beer-Lambert Law
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Radiative Flux Divergence

• Rate of warming or cooling of a layer of air due
  to the change in net radiation with height
• Heating Rate (dT/dt)R(1/raircp)(dRN/dz)
• Heating or cooling by radiation at microscales is
  diabatic
• Can evaluate dRN/dz by measurements of RN at
  different heights or by modeling

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Radiative Transfer Model Streamer v3.0
Approximation method(s) DISORT Spectral
resolution 24 shortwave bands20 cm-1
bandwidth Clouds Flexible specification of
cloud physical properties multiple ice cloud
particle types user-specified optical
properties Aerosols Six optical models, some
user control Gas absorption Principle gases (H2
O, O3, CO2, and O2) Trace gases (CH4, N2O,
and CO. ) Atmospheric profiles Standard and
user-specified Surface characteristics Lambertian
and BRDF, built-in spectral albedo models and
user-specified BRDF Output Radiance/reflectance
/ or flux User interface Input file with
command language interactive mode web interface
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RT Model Input
Cloud Layer
Cloud Layer
Choose significant layers
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Vertical Radiation Fluxes Cloudy Case
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Vertical Radiation Fluxes Clear Sky Case