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Boundary-Layer Meteorology and Atmospheric Dispersion
Dr. J. D. Carlson Oklahoma State University Stillwater, Oklahoma
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Mechanisms of Heat Transfer in the Atmosphere-Earth System
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)
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in the Earth-Atmosphere System
RADIATION in the Earth-Atmosphere System
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Shortwave Radiation Longwave Radiation SUN EARTH
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Longwave Shortwave
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THE GREENHOUSE EFFECT
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MERCURY Sunlit Side = 800 F Dark Side = -279 F NO Greenhouse Effect
(no atmosphere)
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VENUS Surface Temp = 900 F Large Greenhouse Effect
(atmosphere is 97% CO2)
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RADIATION AT THE EARTH’S SURFACE
SW SW LW LW Shortwave (solar) radiation reaches a portion of the earth’s surface (SW ) A portion of that solar is reflected back (SW ) Albedo (α) = the fraction of solar radiation reflected (SW = α SW ) Albedo values: Dark soil Dry sand Meadow Forest Water Fresh snow Old snow The surface receives longwave (infrared) radiation from the sky (LW ) The surface emits longwave radiation to the sky (LW ) The sum of the four radiation terms is often called “Net Radiation” (R)
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(How is the net radiation partitioned at the earth’s surface ?)
SURFACE ENERGY BUDGET (How is the net radiation partitioned at the earth’s surface ?)
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SURFACE ENERGY BUDGET DAY NIGHT SW = shortwave radiation received
SW = shortwave radiation reflected LW = longwave radiation received LW = longwave radiation emitted H = sensible heat transfer by turbulence, advection, convection LE = 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 DAY LE G Energy Units = (surface warming) +9 -7 NIGHT LE G Energy Units = (surface cooling) -7 +5
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ATMOSPHERIC BOUNDARY LAYER Daily Behavior under High Pressure Regimes
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= LAPSE RATE ∂T T2 – T1 ∂z z2 – z1 Typical Vertical Profiles of
Wind and Temperature during the Course of a 24-h Fall Day with Clear Skies (note formation and growth of temperature inversion during the night) “Inversion” = temperature increases with height =
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T2, z2 LAPSE RATE ∂T T2 – T1 ∂z z2 – z1 T1, 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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Mechanical (wind-related) 2. Thermal (temperature-related)
TURBULENCE Mechanical (wind-related) 2. Thermal (temperature-related)
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MECHANICAL TURBULENCE
Speed shear Directional shear 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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without any heat exchange)
ADIABATIC LAPSE RATE (rate of temperature change that an air parcel experiences as it changes elevation without 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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TYPES OF ATMOSPHERIC DISPERSION
Weather Factors Side View (vertical dispersion) Top View (horizontal dispersion) UNSTABLE ATMOSPHERE NEUTRAL STABLE 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 dominates
Γ (adiabatic) environmental Larger scale convective turbulence dominates Strong solar heating with generally light winds Super-adiabatic lapse rates
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Neutral Atmosphere – Moderate Dispersion
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CONING Γ Near neutral conditions (adiabatic lapse rates)
Overcast days or nights Moderate to strong winds Small-scale mechanical turbulence dominates
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Stable Atmosphere – Poor Dispersion
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FANNING Γ Strong inversion (large positive lapse rate) at plume height
Extremely stable conditions (buoyancy suppression) Typical of clear nights with light winds
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Special Cases
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LOFTING Γ Inversion layer below plume
Pollutants dispersed downwind with minimal surface concentration Sometimes a transition to a fanning plume
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FUMIGATION Γ Opposite of lofting
Inversion lies above plume with unstable air below Typical of early morning as inversion breaks up from below Short duration, high surface concentrations
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TRAPPING Γ Subsidence inversion aloft (well above plume) with unstable air below Typical of weather conditions featuring high pressure
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Six types of plume behavior, under various conditions of
stability and instability. At left: broken lines: dry adiabatic lapse 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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Example of Complex Shear Flows along a Coastline
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. East Atlantic Ocean Shoreline Inland West
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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 from Pollutant Sources
Emission Rate (amount/time) Height of release Plume rise (thermal effects) Plume descent (gravitational effects)
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σz σy Diagram showing Gaussian distribution of pollutant plume. σy and σz are standard deviations of the horizontal and vertical concentration distributions, respectively.
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Physical system Model system Bounded space; plume Unbounded space; no reflection
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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. 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.
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OSU Dispersion Modeler
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The Oklahoma Dispersion Model
Gases and small particulates (no gravitational effects) Focus on surface concentrations within the plume at downwind distances of 0.25 to 3 miles
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1) Downwind concentrations (dispersion conditions)
2) Where the pollutant is headed (wind direction)
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Six Dispersion Categories
Excellent = (“EX”; dark green) Good = (“G”; green) Moderately Good = (“MG”’; light green) Moderately Poor = (“MP”; beige) Poor = (“P”; orange) Very Poor = (“VP”; red)
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TYPES OF ATMOSPHERIC DISPERSION
Weather Factors Side View (vertical dispersion) Top View (horizontal dispersion) UNSTABLE ATMOSPHERE NEUTRAL STABLE TYPES OF ATMOSPHERIC DISPERSION
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Dispersion Products on OK-FIRE (http://okfire.mesonet.org)
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Dispersion Conditions
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Wind Direction
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Dispersion and Wind Charts
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Dispersion and Wind Tables
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Fire Prescription Planner
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Prescribed Burn Example
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OK-FIRE Web Site: SMOKE Section “Fire Prescription Planner”
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