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
Boundary-Layer Meteorology and
Atmospheric Dispersion
Dr. J. D. CarlsonOklahoma State
UniversityStillwater, Oklahoma
• 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
RADIATION in the Earth-Atmosphere System
Shortwave Radiation
Longwave Radiation
4T(K)E
T(K)constant / max
SUN EARTH
LongwaveShortwave
THEGREENHOUSE EFFECT
MERCURY
Sunlit Side = 800 F
Dark Side = -279 F
NO Greenhouse Effect(no atmosphere)
VENUS
Surface Temp = 900 F
Large Greenhouse Effect(atmosphere is 97% CO2)
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
SURFACE ENERGY BUDGET
(How is the net radiation partitionedat the earth’s surface ?)
+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
ATMOSPHERIC BOUNDARY LAYER
Daily Behavior under High Pressure Regimes
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
=
T2, z2
T1, z1 LAPSE RATE
∂T T2 – T1 ∂z z2 – z1
=
Surface Radiation Inversion
Temperature Profile
Radiation Inversion
Subsidence Inversion
HIGH PRESSURE
Temperature Profile
Subsidence Inversion
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
TURBULENCE
1. Mechanical (wind-related)
2. Thermal (temperature-related)
MECHANICAL TURBULENCE
1. Speed shear2. Directional shear3. Surface frictional effects
THERMAL TURBULENCE
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
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
ENVIRONMENTAL LAPSE RATE(actual rate of temperature change with height
of the current atmosphere)
(∂T/∂z)env = environmental lapse rate
z
T
(∂T/∂z)env < Γ
(∂T/∂z)env = Γ
(∂T/∂z)env > Γ
THERMAL STABILITY
(∂T/∂z)env < Γ Unstable
(∂T/∂z)env = Γ Neutral
(∂T/∂z)env > Γ Stable
WeatherFactors
Side View(vertical dispersion)
Top View(horizontal dispersion)
UNSTABLEATMOSPHERE
NEUTRALATMOSPHERE
STABLEATMOSPHERE
TYPES OF ATMOSPHERIC DISPERSION
PLUME BEHAVIOR
Unstable Atmosphere –Good Dispersion
LOOPING
Larger scale convective turbulence dominatesStrong solar heating with generally light windsSuper-adiabatic lapse rates
Γ (adiabatic)environmental
Neutral Atmosphere –Moderate Dispersion
CONING
Near neutral conditions (adiabatic lapse rates)Overcast days or nightsModerate to strong windsSmall-scale mechanical turbulence dominates
Γ
Stable Atmosphere –Poor Dispersion
FANNING
Strong inversion (large positive lapse rate) at plume heightExtremely stable conditions (buoyancy suppression)Typical of clear nights with light winds
Γ
Special Cases
LOFTING
Inversion layer below plumePollutants dispersed downwind with minimal surface
concentrationSometimes a transition to a fanning plume
Γ
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
TRAPPING
Subsidence inversion aloft (well above plume) with unstable air belowTypical of weather conditions featuring high pressure
Γ
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.
PLUME RISE
Minimal plume rise due to strong winds
DIFFERENT PLUME HEIGHTS
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
Types of Air Pollutants
• Gases • Particulate Matter
– PM10 (< 10 microns dia.)– PM2.5 (< 2.5 micron dia.)
Types of Emission Sources
GAUSSIAN PLUME MODELING
Emissions fromPollutant Sources
• Emission Rate (amount/time)• Height of release• Plume rise (thermal effects)• Plume descent (gravitational
effects)
Diagram showing Gaussian distribution of pollutant plume. σy and σz arestandard deviations of the horizontal and vertical concentration distributions,respectively.
σy
σz
Bounded space; plume Unbounded space; no reflection
Physical system Model system
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.
OSU Dispersion Modeler
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
1) Downwind concentrations (dispersion conditions)
2) Where the pollutant is headed(wind direction)
Six Dispersion Categories
• Excellent = 6.0 (“EX”; dark green)• Good = 5.0 (“G”; green)• Moderately Good = 4.0 (“MG”’; light
green)• Moderately Poor = 3.0 (“MP”; beige)• Poor = 2.0 (“P”; orange)• Very Poor = 1.0 (“VP”; red)
WeatherFactors
Side View(vertical dispersion)
Top View(horizontal dispersion)
UNSTABLEATMOSPHERE
NEUTRALATMOSPHERE
STABLEATMOSPHERE
TYPES OF ATMOSPHERIC DISPERSION
Dispersion Productson OK-FIRE
(http://okfire.mesonet.org)
Dispersion Conditions
Wind Direction
Dispersion and Wind Charts
Dispersion and Wind Tables
Fire Prescription Planner
Prescribed BurnExample
OK-FIRE Web Site:SMOKE Section
“Fire Prescription Planner”