boundary-layer meteorology and atmospheric dispersion
DESCRIPTION
Boundary-Layer Meteorology and Atmospheric Dispersion. Dr. J. D. Carlson Oklahoma State University Stillwater, Oklahoma. Mechanisms of Heat Transfer in the Atmosphere-Earth System. Radiation (no conducting medium) Sensible Heat Transfer (large-scale movement of heated material) - PowerPoint PPT PresentationTRANSCRIPT
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Boundary-Layer Meteorology and
Atmospheric Dispersion
Dr. J. D. CarlsonOklahoma State
UniversityStillwater, Oklahoma
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• Radiation (no conducting medium)• Sensible Heat Transfer (large-scale
movement of heated material)• Latent Heat Transfer (change of
phase associated with water)• Conduction (molecule to molecule)
Mechanisms of Heat Transferin the Atmosphere-Earth
System
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RADIATION in the Earth-Atmosphere System
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Shortwave Radiation
Longwave Radiation
4T(K)E
T(K)constant / max
SUN EARTH
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LongwaveShortwave
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THEGREENHOUSE EFFECT
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MERCURYSunlit Side = 800 F
Dark Side = -279 F
NO Greenhouse Effect
VENUSSurface Temp = 900 F
Large Greenhouse Effect
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1. Shortwave (solar) radiation reaches a portion of the earth’s surface (SW )
2. A portion of that solar is reflected back (SW )Albedo (α) = the fraction of solar radiation reflected (SW = α SW )
Albedo values: Dark soil 0.05-0.15Dry sand 0.25-0.40Meadow 0.10-0.20Forest 0.10-0.46Water 0.05-0.10Fresh snow 0.7-0.9Old snow 0.4-0.7
3. The surface receives longwave (infrared) radiation from the sky (LW )
4. The surface emits longwave radiation to the sky (LW )
5. The sum of the four radiation terms is often called “Net Radiation” (R)
RADIATION AT THE EARTH’S SURFACE
LW LW SW SW
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SURFACE ENERGY BUDGET
(How is the net radiation partitionedat the earth’s surface ?)
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+9 -7
-7 +5
SURFACE ENERGY BUDGETSW = shortwave radiation receivedSW = shortwave radiation reflectedLW = longwave radiation receivedLW = longwave radiation emittedH = sensible heat transfer by turbulence, advection, convectionLE = latent heat transfer (change of phase: evaporation, condensation,
freezing, thawing)G = heat transfer through the submedium (conduction)
SW + SW + LW + LW + H + LE + G = rate of warming or cooling of surface
Energy Units +20 -4 +4 -11 -1 -4 -2 = +2 (surface warming)
Energy Units +4 -11 +1 +3 +1 = -2 (surface cooling)
DAY
NIGHT
G
G
LE
LE
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ATMOSPHERIC BOUNDARY LAYER
Daily Behavior under High Pressure Regimes
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T2, z2
T1, z1
LAPSE RATE
∂T T2 – T1 ∂z z2 – z1
Typical Vertical Profiles ofWind and Temperature duringthe Course of a 24-h Fall Day with Clear Skies
(note formation and growth oftemperature inversion during the night)
“Inversion” = temperatureincreases with height
=
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T2, z2
T1, z1 LAPSE RATE
∂T T2 – T1 ∂z z2 – z1
=
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Surface Radiation Inversion
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Temperature Profile
Radiation Inversion
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Subsidence Inversion
HIGH PRESSURE
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Temperature Profile
Subsidence Inversion
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ATMOSPHERIC DISPERSION
1. General mean air motion that transports the pollutant
a. horizontally - “advection”b. vertically - “convection”
2. Turbulence - random velocity fluctuations that disperse the pollutant in all directions
3. Molecular diffusion - due to concentration gradients
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TURBULENCE
1. Mechanical (wind-related)
2. Thermal (temperature-related)
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MECHANICAL TURBULENCE
1. Speed shear2. Directional shear3. Surface frictional effects
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THERMAL TURBULENCE
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DENSITY DEPENDS ON TEMPERATURE
Ideal Gas Law:PV = nRT
(P = pressure, V = volume, n = # moles, R = Universal gas constant, T = Absolute Temp)
Can be rewritten: P = rRT, where r= Density
For two air parcels at the same pressure, the warmer parcel has the lower density:
r = P / RT
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ADIABATIC LAPSE RATE(rate of temperature change that an air parcel
experiences as it changes elevationwithout any heat exchange)
(dT/dz)adiab = Γ = - g/cp = -1C/100 m = -5.4F/1000 ft
z
T
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ENVIRONMENTAL LAPSE RATE(actual rate of temperature change with height
of the current atmosphere)
(∂T/∂z)env = environmental lapse rate
z
T
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(∂T/∂z)env < Γ
(∂T/∂z)env = Γ
(∂T/∂z)env > Γ
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THERMAL STABILITY
(∂T/∂z)env < Γ Unstable
(∂T/∂z)env = Γ Neutral
(∂T/∂z)env > Γ Stable
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WeatherFactors
Side View(vertical dispersion)
Top View(horizontal dispersion)
UNSTABLEATMOSPHERE
NEUTRALATMOSPHERE
STABLEATMOSPHERE
TYPES OF ATMOSPHERIC DISPERSION
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PLUME BEHAVIOR
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Unstable Atmosphere –Good Dispersion
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LOOPING
Larger scale convective turbulence dominatesStrong solar heating with generally light windsSuper-adiabatic lapse rates
Γ (adiabatic)environmental
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Neutral Atmosphere –Moderate Dispersion
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CONING
Near neutral conditions (adiabatic lapse rates)Overcast days or nightsModerate to strong windsSmall-scale mechanical turbulence dominates
Γ
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Stable Atmosphere –Poor Dispersion
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FANNING
Strong inversion (large positive lapse rate) at plume heightExtremely stable conditions (buoyancy suppression)Typical of clear nights with light winds
Γ
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Special Cases
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LOFTING
Inversion layer below plumePollutants dispersed downwind with minimal surface
concentrationSometimes a transition to a fanning plume
Γ
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FUMIGATION
Opposite of loftingInversion lies above plume with unstable air belowTypical of early morning as inversion breaks up from belowShort duration, high surface concentrations
Γ
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TRAPPING
Subsidence inversion aloft (well above plume) with unstable air belowTypical of weather conditions featuring high pressure
Γ
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Six types of plume behavior, under various conditions of stability and instability. At left: broken lines: dry adiabaticlapse rate; full lines: existing environmental lapse rates.
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PLUME RISE
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Minimal plume rise due to strong winds
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DIFFERENT PLUME HEIGHTS
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Salem, Mass.
Oil-fired power plant looking south on a winter morning.
Lower steam plume from two 250-ft stacks trapped by inversion.
Upper plume from a 500-ft stack.
ARAC Model
Photo
East Atlantic Ocean Shoreline Inland West
Example of Complex Shear Flows along a Coastline
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Types of Air Pollutants
• Gases • Particulate Matter
– PM10 (< 10 microns dia.)– PM2.5 (< 2.5 micron dia.)
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Types of Emission Sources
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GAUSSIAN PLUME MODELING
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Emissions fromPollutant Sources
• Emission Rate (amount/time)• Height of release• Plume rise (thermal effects)• Plume descent (gravitational
effects)
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Diagram showing Gaussian distribution of pollutant plume. σy and σz arestandard deviations of the horizontal and vertical concentration distributions,respectively.
σy
σz
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Bounded space; plume Unbounded space; no reflection
Physical system Model system
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Thermal turbulence dominates (buoyancy enhancement)
Mechanical turbulence only
Thermal effects dominate (buoyancy suppression)Classes E-F
Class A results in the most dispersion, while Class F has the least.
Category A represents very unstable conditions; B, moderately unstable; C, slightly unstable; D, neutral; E, slightly stable; and F, stable. Night refers to the period from one hour before sunset to one hour after sunrise. The neutral category, D, should be used regardless of wind speed for overcast conditions, day or night.
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