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

MERCURYSunlit Side = 800 F

Dark Side = -279 F

NO Greenhouse Effect

VENUSSurface Temp = 900 F

Large Greenhouse Effect

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.