Meteorology by huangyuarong

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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:
  n   Transported
  n   Dispersed
  n   concentrated
  By meteorological conditions
Layer nomenclature in the atmosphere
Light scattering


                    
                                 
Orographic rainfall
Air Pollutant Cycle

                     Transport




        Emission                  Diffusion or
                                  concentration


    Deposition onto
    vegetation, livestock,
    soil, water, or escape into
    space
Transport
  Pollutants moved from source
  May undergo physical and chemical
  changes
  n   Smog – interaction of NOx, HC, and solar
      energy
  n   Ozone formation
Concentration & Dispersion
 Disperse based on meteorological &
 topographic conditions
 Concentration --- usually stagnant conditions
 Dispersion
 n   Topological conditions
      w Affected by presence of large buildings
 n   Meteorological conditions
 n   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
  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
 General mean air motion
 Turbulent velocity fluctuations
 Diffusion due to concentration gradients –
 from plumes
 Aerodynamic characteristics of pollution
 particles
  n   Size
  n   Shape
  n   Weight
Atmosphere
 Gas composition (changes very little with time
 or place in most of atmosphere
 n   78% nitrogen
 n   21% oxygen
 n   1% argon & other trace gases
 Moisture content
 n   Water vapor
 n   Water droplets
 n   Ice crystals
Atmosphere
 Relative humidity (RH): ratio of water
 content to air
 Increases with increasing temperatures
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
 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
 n   Absorbed by higher atmosphere
 n   Reflected by clouds
 n   Back-scattered by atmosphere
 n   Reflected by earth’s surface
 n   Absorbed by water vapor & clouds
 47% adsorbed by land and water
Insolation
  Quantity of solar radiation reaching a
  unit area of the earth’s surface
  n   Angle of incidence
  n   Thickness of the atmosphere
  n   Characteristics of surface
  Albedo: fraction of incident radiation
  that is reflected by a surface
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
 Sun, earth, and atmosphere form dynamic
 system
 Differential heating of gases leads to
 horizontal pressure gradients ® horizontal
 movement
 Large scale movement
 n   Poles
 n   Equator
 n   Continents
 n   oceans
 Small scale movement
 n   Lakes
 n   Different surfaces
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
  n   Like air circulation from a heater in a room
  Without rotation
  Air flows directly from high to low pressure
  areas (fp)
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
  n   Rises from equator, sinks at poles
  n   Equator to pole at high altitudes
  n   Pole to equator at low altitudes
  n   Like air circulation from a heater in a room
  Wind Circulation

Air flows directly from
  high to low pressure
        areas (fp)
Wind Circulation
  Same principle as room heater but not
  as neat because atmosphere is so thin
  n   Height vs width
  n   Flow is mechanically unstable
  n   Breaks into cells
Note differences in flow between cells
      Sinking
      boundaries




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

   Areas of equal
  pressure
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)
Frictional Force
        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
             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
Figure 5-2, simplistic
representation
In reality, land & water
do not respond to solar
heating similarly
 Terrain is uneven
n   Highest mountains rise
    above most of atmosphere
n   Large mountain ranges are
    major barriers to
    horizontal winds
n   Even small mountain
    ranges influence wind
    patterns
Influence of Ground & Sea
  Water adsorbs and transfer heat
  differently than rock & soil
  Rock and soil radiate heat differently
  summer to winter
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
    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

  Important characteristic of atmosphere is
  ability to resist vertical motion: stability
  Affects ability to disperse pollutants
  When small volume of air is displaced upward
  n   Encounters lower pressure
  n   Expands to lower temperature
  n   Assume no heat transfers to surrounding
      atmosphere
  n   Called adiabatic expansion
Adiabatic expansion
 To determine the change in temp. w/ elevation due to adiabatic
 expansion
  n Atmosphere considered a stationary column of air in a
    gravitational field
  n Gas is a dry ideal gas

  n 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 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 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
    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 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
  Factors vary somewhat
  n   M
  n   Cp
  Meteorologists and aeronautical engineers
  have defined
  n   “standard atmosphere”
  n   Represents approximate average of all
      observations over most of the world
       w Summer & winter
       w Day & night
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
  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
  Lapse rate for “standard atmosphere”
  Troposphere:
   n   0 to 36,150 feet
   n   Temperature decreases linearly
   n   75% of atmospheric mass
  Not applicable above troposphere
  Stratosphere
   n   36,150 to 65,800 feet
   n   Temperature does not decrease further with
       increasing height
   n   Chemical reaction occur to absorb heat from the
       sun
   n   Adiabatic assumption is not followed
Atmospheric Stability
  Affects dispersion of pollutants
  Temperature/elevation relationship principal
  determinant of atmospheric stability
  Stable
  n   Little vertical mixing
  n   Pollutants emitted near surface tend to stay there
  n   Environmental lapse rate is same as the dry
      adiabatic lapse rate
  4 common scenarios
Neutral
n   Environmental lapse rate is same as the dry
    adiabatic lapse rate
n   A parcel of air carried up or down will have same
    temp as environment at the new height
n   No tendency for further movement
Superadiabatic --- Unstable
n   Environmental lapse rate > Γ
n   i.e. Actual temp. gradient is more negative
n   Small parcel of air displaced approximates adiabatic
    expansion
n   Heat transfer is slow compared to vertical movement
n   At a given point, Tparcel > Tsurrounding air
      w less dense than surrounding air
n   Parcel continues upward
Subadiabatic --- Stable
n   Environmental lapse rate < Γ
n   greater temp. gradient
n   No tendency for further vertical movement due to temp.
    differences
n   Any parcel of air will return to its original position
n   Parcel is colder than air above – moves back
Inversion --- Strongly Stable
n   Environmental lapse rate is negative
n   Temp. increases with height
n   No tendency for further vertical movement due to temp.
    differences
n   Any parcel of air will return to its original position
n   Parcel is colder than air above – moves back
n   Concentrates pollutants
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:
  n   Time of year
  n   Topography
  n   Presence of water or lakes
  n   Time of day
                                   Image source:
                                   http://www.unc.edu/courses/2005fall/geog/011/001/AirPoll
                                   ution/AirPollution.htm
                                          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”
                                                     Image and text source:
                                                     http://www.sparetheair.org/teach
                                                     ers/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”
                                            Image and text source:
                                            http://www.sparetheair.org/teach
                                            ers/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
Radiation Inversion
n   Surface layers receive heat by
    conduction, convection, and
    radiation from earth’s surface
Subsidence Inversion
n   Cloud layer absorbs incoming
    solar energy or high-pressure
    region with slow net
    downward flow or air and
    light winds
n   Sinking air mass increases in
    temp and becomes warmer
    than air below it
n   Usually occur 1,500 to 15,000
    feet above ground & inhibit                 Subsidence Inversion
    atmospheric mixing               Image Source:
n   Common in sunny, low-wind        http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_con
                                     ditions.html
    situations
Two more Types of Inversion
 Cold Air Flowing Under
 n   Nighttime flow of cold air down valleys
 n   Col air flows under warm air
 n   Winter
 n   Presence of fog blocks sun and inversion persists
 n   Sea or lake breezes also bring cold air under warm
     air
 Warm Air Flowing Over
 n   Same as above but warm air flows over cold
     trapping it
 n   Warm air frequently overrides colder more dense
     air
Stability Classes
  Developed for use in dispersion models
  Stability classified into 6 classes (A – F)
  n   A:   strongly unstable
  n   B:   moderately unstable
  n   C:   slightly unstable
  n   D:   neutral
  n   E:   slightly stable
  n   F:   moderately stable
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 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 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 Velocity Profile
Wind Direction
 Does the wind blow from my house towards a
 smelly feedlot or the other way?
 High and low-pressure zones
 n   Formed from large scale instabilities
 n   Often near boundaries of circulation zones
 n   Air is rising or sinking
 n   Major storms often associated with low-pressure
 Topography
 n   Air heats and cools differently on different
     surfaces, causes air from
 n   Lake to shore, etc.
 n   Mountains block low-level wind
Predicting Wind Direction
 Need to know distribution of wind direction
 for estimating pollution concentrations
 Need speed and direction
 Wind Rose
 n   Average of wind speed and direction over time
 n   Shows
      w Frequency
      w Speed
      w direction
 n   Wind direction is direction from which the wind is
     coming
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 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
  n   Where commercial air lines fly, air clear and non
      turbulent
  n   Very clear boundary
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
  Not always completely understood
  2 types
   n Atmospheric heating

       w Causes natural convection currents --- discussed
       w Thermal eddies
  n   Mechanical turbulence
       w Results from shear wind effects
       w Result from air movement over the earth’s surface,
         influenced by location of buildings and relative
         roughness of terrain
General Characteristics of
Stack Plumes
  Dispersion of pollutants
  n   Wind – carries pollution downstream from source
  n   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
  Looping:
  n   high degree of
      convective turbulence
  n   Superadiabatic lapse rate
      -- strong instabilities
  n   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 Behavior
  Coning:
  n   Occurs under neutral
      conditions
  n   Stable with small-scale
      turbulence
  n   Associated with overcast
      moderate to strong winds
  n   Roughly 10° cone
  n   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 Behavior
  Fanning:
  n   Occurs under large negative
      lapse rate
  n   Strong inversion at a
      considerable distance above
      the stack
  n   Extremely stable atmosphere
  n   Little turbulence
  n   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 Behavior
  Fumigation:
  n   Stable layer of air lies a
      short distance above
      release point with
      unstable air beneath
  n   Usually early morning
      after an evening with a
      stable inversion
  n   Significant ground level
      concentrations may be        Image Source:
                                   http://apollo.lsc.vsc.edu/classes/met130/
      reached                      notes/chapter17/fav_conditions.html
Six Classes of Plume Behavior
  Lofting
  n   Opposite conditions of fumigation
  n   Inversion layer below with unstable
      layer through and above
  n   Pollutants are dispersed downwind
      without significant ground-level
      concentration
  Trapping
  n   Inversion above and below stack
  n   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
  Problems:
  n   3.7
  n   3.9
  n   3.14
  n   Due Thu Feb. 3rd

								
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