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Meteorology. Background concepts. Meteorology chapter 3 of text. Meteorology is the study and forecasting of weather changes resulting from large scale atmospheric circulation. CCE 524 January 2011. Introduction. Once emitted pollutants: Transported Dispersed concentrated

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meteorology

Meteorology

Background concepts

meteorology chapter 3 of text

Meteorologychapter 3 of text

Meteorology is the study and forecasting of weather changes resulting from large scale atmospheric circulation

CCE 524

January 2011

introduction
Introduction
  • Once emitted pollutants:
    • Transported
    • Dispersed
    • concentrated
  • By meteorological conditions
air pollutant cycle
Air Pollutant Cycle

Transport

Diffusion or concentration

Emission

Deposition onto vegetation, livestock, soil, water, or escape into space

transport
Transport
  • Pollutants moved from source
  • May undergo physical and chemical changes
    • Smog – interaction of NOx, HC, and solar energy
    • Ozone formation
concentration dispersion
Concentration & Dispersion
  • Disperse based on meteorological & topographic conditions
  • Concentration --- usually stagnant conditions
  • Dispersion
    • Topological conditions
      • Affected by presence of large buildings
    • Meteorological conditions
    • prevailing wind speed & direction
  • Pollutants disperse over geographic area
  • Any location receives pollutants from different sources in different amounts
  • Need to understand how pollutants disperse to predict concentrations and predict violations at a particular location
prediction
Prediction
  • Mathematical models of local atmosphere determine transport and dispersion patterns
  • With emission data – predict concentrations throughout region
  • Should correlate with data from monitoring locations
  • Effect of sources can be estimated & regulations set
dispersion
Dispersion
  • General mean air motion
  • Turbulent velocity fluctuations
  • Diffusion due to concentration gradients – from plumes
  • Aerodynamic characteristics of pollution particles
    • Size
    • Shape
    • Weight
atmosphere
Atmosphere
  • Gas composition (changes very little with time or place in most of atmosphere
    • 78% nitrogen
    • 21% oxygen
    • 1% argon & other trace gases
  • Moisture content
    • Water vapor
    • Water droplets
    • Ice crystals
atmosphere16
Atmosphere
  • Relative humidity (RH): ratio of water content to air
  • Increases with increasing temperatures
atmosphere17
Atmosphere
  • Has well-defined lower boundary with water & land
  • Upper boundary becomes increasingly thinner
  • 50% of atmospheric mass is within 3.4 miles of earth
  • 99% is within 20 miles of earth
  • Large width & small depth
  • Most motion is horizontal
  • Vertical motion ~ 1 to 2x less than horizontal
solar radiation
Solar Radiation
  • At upper boundary of atmosphere, vertical solar radiation = 8.16 J/cm2min (solar constant)
  • Maximum intensity at λ = 0.4 to 0.8 μm = visible portion of electromagnetic spectrum
  • ~ 42% of energy
    • Absorbed by higher atmosphere
    • Reflected by clouds
    • Back-scattered by atmosphere
    • Reflected by earth’s surface
    • Absorbed by water vapor & clouds
  • 47% adsorbed by land and water
insolation
Insolation
  • Quantity of solar radiation reaching a unit area of the earth’s surface
    • Angle of incidence
    • Thickness of the atmosphere
    • Characteristics of surface
  • Albedo: fraction of incident radiation that is reflected by a surface
solar incidence angle
Solar Incidence Angle
  • angle between sun’s rays and an imaginary line perpendicular to the surface (0º)
  • maximum solar gain is achieved when incidence angle is 0º
  • Tangent in morning and approximately perpendicular
  • angle depends on surface

Information and image source: http://www.visualsunchart.com/VisualSunChart/SolarAccessConcepts/

wind circulation
Wind Circulation
  • Sun, earth, and atmosphere form dynamic system
  • Differential heating of gases leads to horizontal pressure gradients  horizontal movement
  • Large scale movement
    • Poles
    • Equator
    • Continents
    • oceans
  • Small scale movement
    • Lakes
    • Different surfaces
wind circulation22
Wind Circulation
  • Average over a year, solar heat flow to the earth’s surface at equator is 2.4x that at poles
  • Air moves in response to differences
  • Heat transports from equator to poles
    • Like air circulation from a heater in a room
  • Without rotation
  • Air flows directly from high to low pressure areas (fp)
wind circulation23
Wind Circulation
  • Average over a year, solar heat flow to the earth’s surface at equator is 2.4x that at poles
  • Air moves in response to differences
  • Heat transports from equator to poles
    • Rises from equator, sinks at poles
    • Equator to pole at high altitudes
    • Pole to equator at low altitudes
    • Like air circulation from a heater in a room
wind circulation24
Wind Circulation

Air flows directly from high to low pressure areas (fp)

wind circulation26
Wind Circulation
  • Same principle as room heater but not as neat because atmosphere is so thin
    • Height vs width
    • Flow is mechanically unstable
    • Breaks into cells
slide29

Sinking boundaries

Rising Boundaries

wind circulation30
Wind Circulation
  • Rising air cools & produces rain
  • Sinking air is heated and becomes dry
  • Rising boundaries are regions of of higher than average rainfall
    • Equator
    • Rain forests
    • Temperate forests
  • Sinking boundaries are regions of lower than average rainfall
    • Most of world’s deserts
    • Poles – small amounts of precipitation remains due to low evaporation
rotation
Rotation
  • Without rotation
  • Air flows directly from high to low pressure areas (fp)
  • Rotation of earth affects movement
slide32
Effect of rotation on baseball thrown at North Pole
  • Space observer sees straight path
  • Catcher moves – ball appears to curve to the left
inertial atmospheric rotation
Inertial atmospheric rotation

Schematic representation of inertial circles of air masses in the absence of other forces, calculated for a wind speed of approximately 50 to 70 m/s. Note that the rotation is exactly opposite of that normally experienced with air masses in weather systems around depressions.

low pressure area flows
Low-pressure area flows

Schematic represen-tation of flow around a low-pressure area in the Northern hemi-sphere. The Rossby number is low, so the centrifugal force is virtually negligible. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows

low pressure system
Low-pressure system

If a low-pressure area forms in the atmosphere, air will tend to flow in towards it, but will be deflected perpendicular to its velocity by the Coriolis force.

This low pressure system over Iceland spins counter-clockwise due to balance between the Coriolis force and the pressure gradient force.

rotation40
Rotation
  • Coriolis force – horizontal deflection force (fcor)
  • Acts at right angles to the motion of the body
  • Is proportional to the velocity of the moving body
  • Northern hemisphere turns body to the right
  • Southern hemisphere turns body to the left
isobar
Isobar
  • Areas of equal pressure
frictional force
Frictional Force
  • Movement of air near surface is retarded by effects of friction (ff) due to surface roughness or terrain
  • Opposite to wind direction
  • Wind direction is perpendicular to Coriolis
  • Directly reduces wind speed and consequently reduces Coriolis force (which is proportional to wind speed)
slide44

Frictional Force

  • Friction force is maximum at earth’s surface
  • Decreases as height increases
  • Effect on tall stack not consistent
  • Effect negligible with strong winds > 6 m/s
  • Effect at lower speeds < 6 m/s more significant
slide45

Frictional Force

  • Ф = 5 to 15° over ocean
  • Ф = 25 to 45° over land
  • As pollutants move downstream they diffuse outwardly in y direction
  • Disperse vertically in the z direction
influence of ground sea
Influence of Ground & Sea
  • Figure 5-2, simplistic representation
  • In reality, land & water do not respond to solar heating similarly
  • Terrain is uneven
    • Highest mountains rise above most of atmosphere
    • Large mountain ranges are major barriers to horizontal winds
    • Even small mountain ranges influence wind patterns
influence of ground sea47
Influence of Ground & Sea
  • Water adsorbs and transfer heat differently than rock & soil
  • Rock and soil radiate heat differently summer to winter
vertical motion
Vertical Motion
  • Any parcel of air less dense than surrounding air will rise by buoyancy
  • any parcel more dense will sink
  • Most vertical movement is due to changes in air density
  • The pressure at any point in the atmosphere = pressure required to support everything above that point
properties of gases
Properties of Gases
  • If volume of gas is held constant and heat is applied, temperature and pressure rise
  • if volume is not held constant and pressure is held constant, gas will expand and temperature will rise
  • Adiabatic expansion or contraction: an amount of gas is allowed to expand or contract due to a change in pressure (such as it would encounter in the atmosphere) assuming no heat transfer with atmosphere
lapse rate
Lapse Rate
  • Important characteristic of atmosphere is ability to resist vertical motion: stability
  • Affects ability to disperse pollutants
  • When small volume of air is displaced upward
    • Encounters lower pressure
    • Expands to lower temperature
    • Assume no heat transfers to surrounding atmosphere
    • Called adiabatic expansion
adiabatic expansion
Adiabatic expansion
  • To determine the change in temp. w/ elevation due to adiabatic expansion
    • Atmosphere considered a stationary column of air in a gravitational field
    • Gas is a dry ideal gas
    • Ignoring friction and inertial effects

(dT/dz)adiabatic perfect gas = - vpg

Cp

T = temperature

z = vertical distance

g = acceleration due to gravity

p = atmospheric density

v = volume per unit of mass

Cp = heat capacity of the gas at constant pressure

adiabatic expansion52
Adiabatic expansion
  • If the volume of a parcel of air is held constant and an incremental amount of heat is added to the parcel, temperature of the parcel will rise by an amount dT
  • Resultant rise in temperature produces a rise in pressure according to ideal gas law
  • If the parcel is allowed to expand in volume and have a change in temperature, while holding the pressure constant, the parcel will expand or contract and increase or decrease in temp.
  • Parcel rises or falls accordingly
adiabatic expansion53
Adiabatic expansion

SI:

(dT/dz)adiabatic perfect gas = -0.0098°C/m

American:

(dT/dz)adiabatic perfect gas = -5.4°F/ft

Change in temp. with change in height

slide54

Lapse rate

  • Lapse rate is the negative of temperature gradient
  • Dry adiabatic lapse rate =

Metric:

Γ = - 1°C/100m or

SI:

Γ = - 5.4°F/1000ft

lapse rate55
Lapse rate
  • Important characteristic of atmosphere is ability to resist vertical motion: stability
  • Comparison of Γ to actual environment lapse rate indicates stability of atmosphere
  • Degree of stability is a measure of the ability of the atmosphere to disperse pollutants
  • Determines if rising parcel of air will rise high enough for water to condense to form clouds
international lapse rate
International lapse rate
  • Factors vary somewhat
    • M
    • Cp
  • Meteorologists and aeronautical engineers have defined
    • “standard atmosphere”
    • Represents approximate average of all observations over most of the world
      • Summer & winter
      • Day & night
international lapse rate57
International Lapse Rate

SI:

Γ = - 6.49°C/km or 0.65 oC/100m

American:

Γ = - 3.45°F/1000ft

About 66% of adiabatic lapse rate

lapse rate example
Lapse Rate Example
  • Assuming the surface temperature is 15° at the surface of the earth, what is the temperature at 5510.5 m?

Γ = 6.49°C/km

Solution:

5510.5 m = 5.5105 km

For each km the temperature decreases 6.49°

So the temperature decreases:

5.5105 x 6.49 = 35.76°

Original temp was 15°, temp at 5.5105 km =

15° - 35.76° = -20.76°C

temperature change due to atmospheric height
Temperature change due to atmospheric height
  • Lapse rate for “standard atmosphere”
  • Troposphere:
    • 0 to 36,150 feet
    • Temperature decreases linearly
    • 75% of atmospheric mass
  • Not applicable above troposphere
  • Stratosphere
    • 36,150 to 65,800 feet
    • Temperature does not decrease further with increasing height
    • Chemical reaction occur to absorb heat from the sun
    • Adiabatic assumption is not followed
atmospheric stability
Atmospheric Stability
  • Affects dispersion of pollutants
  • Temperature/elevation relationship principal determinant of atmospheric stability
  • Stable
    • Little vertical mixing
    • Pollutants emitted near surface tend to stay there
    • Environmental lapse rate is same as the dry adiabatic lapse rate
  • 4 common scenarios
slide62
Neutral
    • Environmental lapse rate is same as the dry adiabatic lapse rate
    • A parcel of air carried up or down will have same temp as environment at the new height
    • No tendency for further movement
slide63
Superadiabatic --- Unstable
    • Environmental lapse rate > Γ
    • i.e. Actual temp. gradient is more negative
    • Small parcel of air displaced approximates adiabatic expansion
    • Heat transfer is slow compared to vertical movement
    • At a given point, Tparcel > Tsurrounding air
      • less dense than surrounding air
    • Parcel continues upward
slide64
Subadiabatic --- Stable
    • Environmental lapse rate < Γ
    • greater temp. gradient
    • No tendency for further vertical movement due to temp. differences
    • Any parcel of air will return to its original position
    • Parcel is colder than air above – moves back
slide65
Inversion --- Strongly Stable
    • Environmental lapse rate is negative
    • Temp. increases with height
    • No tendency for further vertical movement due to temp. differences
    • Any parcel of air will return to its original position
    • Parcel is colder than air above – moves back
    • Concentrates pollutants
inversions
Inversions
  • Stability lessens exchange of wind energy between air layers near ground and high altitude winds
  • Horizontal & vertical dispersions of pollutants are hampered
  • Influenced by:
    • Time of year
    • Topography
    • Presence of water or lakes
    • Time of day

Image source: http://www.unc.edu/courses/2005fall/geog/011/001/AirPollution/AirPollution.htm

slide67

Image and text source:

http://www.sparetheair.org/teachers/bigpicture/IIIA1a.html

From San Francisco Bay area: “Pollutants are carried from the ocean through mountain passes on an almost daily basis during the summer months”

slide68

Image and text source:

http://www.sparetheair.org/teachers/bigpicture/IIIA1a.html

“Streams of air carrying Bay Area emissions mix with locally generated pollution from automobile traffic, small engine exhaust, industry, and agriculture in the Valley and are diverted both north and south”

slide69

Image and text source:

http://www.sparetheair.org/teachers/bigpicture/IIIA1a.html

“A warm inversion layer acts like a blanket on the smog layer, preventing it from dissipating higher in the atmosphere. Because of high pressure, the Central Valley regularly experiences these thermal inversions. The Valley, which is nearly at sea level, often fills at night with cool heavy air underneath a layer of warmer air. The cool air layer grows through the night reaching up to 3000 feet thick. “

two types of inversion
Two Types of Inversion
  • Radiation Inversion
    • Surface layers receive heat by conduction, convection, and radiation from earth’s surface
  • Subsidence Inversion
    • Cloud layer absorbs incoming solar energy or high-pressure region with slow net downward flow or air and light winds
    • Sinking air mass increases in temp and becomes warmer than air below it
    • Usually occur 1,500 to 15,000 feet above ground & inhibit atmospheric mixing
    • Common in sunny, low-wind situations

Subsidence Inversion

Image Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html

two more types of inversion
Two more Types of Inversion
  • Cold Air Flowing Under
    • Nighttime flow of cold air down valleys
    • Col air flows under warm air
    • Winter
    • Presence of fog blocks sun and inversion persists
    • Sea or lake breezes also bring cold air under warm air
  • Warm Air Flowing Over
    • Same as above but warm air flows over cold trapping it
    • Warm air frequently overrides colder more dense air
stability classes
Stability Classes
  • Developed for use in dispersion models
  • Stability classified into 6 classes (A – F)
    • A: strongly unstable
    • B: moderately unstable
    • C: slightly unstable
    • D: neutral
    • E: slightly stable
    • F: moderately stable
wind velocity profile
Wind Velocity Profile
  • Friction retards wind movement
  • Friction is proportional to surface roughness
  • Location and size of surface objects produce different wind velocity gradients in the vertical direction
  • Area of atmosphere influenced by friction – planetary boundary layer – few hundred m to several km above earth’s surface
  • Depth of boundary layer > unstable than stable conditions
wind velocity profile75
Wind Velocity Profile
  • Wind speed varies by height
  • International standard height for wind-speed measurements is 10 m
  • Dispersion of pollutant is a function of wind speed at the height where pollution is emitted
  • But difficult to develop relationship between height and wind speed
wind velocity profile76
Wind Velocity Profile
  • Power law of Deacon

u/u1 = (z/z1)p

U: wind speed at elevation z

z: elevation

p: exponent based on terrain and surface cover and stability characteristics

wind direction
Wind Direction
  • Does the wind blow from my house towards a smelly feedlot or the other way?
  • High and low-pressure zones
    • Formed from large scale instabilities
    • Often near boundaries of circulation zones
    • Air is rising or sinking
    • Major storms often associated with low-pressure
  • Topography
    • Air heats and cools differently on different surfaces, causes air from
    • Lake to shore, etc.
    • Mountains block low-level wind
predicting wind direction
Predicting Wind Direction
  • Need to know distribution of wind direction for estimating pollution concentrations
  • Need speed and direction
  • Wind Rose
    • Average of wind speed and direction over time
    • Shows
      • Frequency
      • Speed
      • direction
    • Wind direction is direction from which the wind is coming
mixing height
Mixing Height
  • Vigorous mixing to a certain height (z) and little effect above that
  • Rising air columns mix air vertically & horizontally
  • Rising air mixes and disperses pollutants
  • Only mixes to “mixing” height no above it
  • Different in summer vs winter, morning vs evening
  • For inversions, mixing height can be close to 0
  • Thermal buoyancy determines depth of convective mixing depth
mixing height85
Mixing Height
  • Usually corresponds to tops of clouds
  • Different shapes but reach about same height
  • Up to mixing height unstable air brings moisture up from below to form clouds – above mixing height there is no corresponding upward flow
  • Strong delineation at stratosphere/troposphere boundary
  • Stratosphere very stable against mixing
    • Where commercial air lines fly, air clear and non turbulent
    • Very clear boundary
mechanics of mixing height
Mechanics of Mixing Height
  • Parcel heated by solar radiation at earth’s surface
  • Rises until temperature T’ = T
  • T’ = particle’s temp
  • T = atmospheric temp
  • Achieves neutral equilibrium, no tendency for further upward motion
turbulence
Turbulence
  • Not always completely understood
  • 2 types
    • Atmospheric heating
      • Causes natural convection currents --- discussed
      • Thermal eddies
    • Mechanical turbulence
      • Results from shear wind effects
      • Result from air movement over the earth’s surface, influenced by location of buildings and relative roughness of terrain
general characteristics of stack plumes
General Characteristics of Stack Plumes
  • Dispersion of pollutants
    • Wind – carries pollution downstream from source
    • Atmospheric turbulence -- causes pollutants to fluctuate from mainstream in vertical and cross-wind directions
  • Mechanical & atmospheric heating both present at same time but in varying ratios
  • Affect plume dispersion differently
six classes of plume behavior
Six Classes of Plume Behavior
  • Looping:
    • high degree of convective turbulence
    • Superadiabatic lapse rate -- strong instabilities
    • Associated with clear daytime conditions accompanied by strong solar heating & light winds

Image Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html

six classes of plume behavior93
Six Classes of Plume Behavior
  • Coning:
    • Occurs under neutral conditions
    • Stable with small-scale turbulence
    • Associated with overcast moderate to strong winds
    • Roughly 10° cone
    • Pollutants travel fairly long distances before reaching ground level in significant amounts

Image Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html

six classes of plume behavior94
Six Classes of Plume Behavior
  • Fanning:
    • Occurs under large negative lapse rate
    • Strong inversion at a considerable distance above the stack
    • Extremely stable atmosphere
    • Little turbulence
    • If plume density is similar to air, travels downwind at approximately same elevation

Image Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html

six classes of plume behavior95
Six Classes of Plume Behavior
  • Fumigation:
    • Stable layer of air lies a short distance above release point with unstable air beneath
    • Usually early morning after an evening with a stable inversion
    • Significant ground level concentrations may be reached

Image Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html

six classes of plume behavior97
Six Classes of Plume Behavior
  • Lofting
    • Opposite conditions of fumigation
    • Inversion layer below with unstable layer through and above
    • Pollutants are dispersed downwind without significant ground-level concentration
  • Trapping
    • Inversion above and below stack
    • Diffusion of pollutants is limited to layer between inversions

Image Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html

assignment 3
Assignment 3
  • Problems:
    • 3.7
    • 3.9
    • 3.14
    • Due Thu Feb. 3rd