Meteorology
Background concepts
Meteorology
Meteorology is the study and forecasting of weather changes resulting from large scale atmospheric circulation
Introduction
Once emitted pollutants: Transported Dispersed concentrated
By meteorological conditions
Layer nomenclature in the atmosphere
Light scattering
Orographic rainfall
Emission
Transport
Diffusion or concentration
Deposition onto vegetation, livestock, soil, water, or escape into space
Air Pollutant Cycle
TransportPollutants moved from source May undergo physical and chemical changes Smog – interaction of NOx, HC, and
solar energy Ozone formation
Concentration & Dispersion
Disperse based on meteorological & topographic conditionsConcentration --- usually stagnant conditionsDispersion Topological conditions
Affected by presence of large buildings Meteorological conditions prevailing wind speed & direction
Pollutants disperse over geographic areaAny location receives pollutants from different sources in different amountsNeed 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 patternsWith emission data – predict concentrations throughout regionShould correlate with data from monitoring locationsEffect of sources can be estimated & regulations set
Dispersion
General mean air motionTurbulent velocity fluctuationsDiffusion due to concentration gradients – from plumesAerodynamic characteristics of pollution particles Size Shape Weight
AtmosphereGas 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
AtmosphereRelative humidity (RH): ratio of water content to airIncreases with increasing temperatures
AtmosphereHas well-defined lower boundary with water & landUpper boundary becomes increasingly thinner50% of atmospheric mass is within 3.4 miles of earth99% is within 20 miles of earthLarge width & small depthMost motion is horizontalVertical motion ~ 1 to 2x less than horizontal
Solar RadiationAt 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
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
angle between sun’s rays and an imaginary line perpendicular to the surface (0º) maximum solar gain is achieved when incidence angle is 0ºTangent in morning and approximately perpendicular angle depends on surface
Information and image source: http://www.visualsunchart.com/VisualSunChart/SolarAccessConcepts/
Wind CirculationSun, earth, and atmosphere form dynamic systemDifferential heating of gases leads to horizontal pressure gradients horizontal movementLarge scale movement Poles Equator Continents oceans
Small scale movement Lakes Different surfaces
Wind Circulation
Average over a year, solar heat flow to the earth’s surface at equator is 2.4x that at polesAir moves in response to differencesHeat transports from equator to poles Like air circulation from a heater in a room
Without rotationAir 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 polesAir moves in response to differencesHeat 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 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 Height vs width Flow is mechanically unstable Breaks into cells
Note differences in flow between cells
Sinking boundaries
Rising Boundaries
Wind CirculationRising air cools & produces rainSinking air is heated and becomes dryRising 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
Without rotationAir flows directly from high to low pressure areas (fp)
Rotation of earth affects movement
Effect of rotation on baseball thrown at North PoleSpace observer sees straight pathCatcher moves – ball appears to curve to the left
Coriolis forces
Inertial atmospheric rotationSchematic 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 flowsSchematic 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
RotationCoriolis force – horizontal deflection force (fcor)Acts at right angles to the motion of the bodyIs proportional to the velocity of the moving bodyNorthern hemisphere turns body to the rightSouthern 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 terrainOpposite to wind directionWind direction is perpendicular to CoriolisDirectly reduces wind speed and consequently reduces Coriolis force (which is proportional to wind speed)
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
Frictional ForceФ = 5 to 15° over oceanФ = 25 to 45° over landAs pollutants move downstream they diffuse outwardly in y directionDisperse vertically in the z direction
Influence of Ground & SeaFigure 5-2, simplistic representationIn 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 & Sea
Water adsorbs and transfer heat differently than rock & soilRock 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 sinkMost vertical movement is due to changes in air densityThe 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: stabilityAffects ability to disperse pollutantsWhen 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 expansionTo 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 = temperaturez = vertical distanceg = acceleration due to gravityp = atmospheric densityv = volume per unit of massCp = heat capacity of the gas at constant pressure
Adiabatic expansionIf 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 dTResultant rise in temperature produces a rise in pressure according to ideal gas lawIf 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 is the negative of temperature gradientDry adiabatic lapse rate =
Metric:Γ = - 1°C/100m or
SI:Γ = - 5.4°F/1000ft
Lapse rate
Lapse rate
Important characteristic of atmosphere is ability to resist vertical motion: stabilityComparison of Γ to actual environment lapse rate indicates stability of atmosphereDegree of stability is a measure of the ability of the atmosphere to disperse pollutantsDetermines if rising parcel of air will rise high enough for water to condense to form clouds
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 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 ExampleAssuming 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: 0 to 36,150 feet Temperature decreases linearly 75% of atmospheric mass
Not applicable above troposphereStratosphere 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
Affects dispersion of pollutantsTemperature/elevation relationship principal determinant of atmospheric stabilityStable 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
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
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
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
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
Stability lessens exchange of wind energy between air layers near ground and high altitude windsHorizontal & vertical dispersions of pollutants are hamperedInfluenced 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
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/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”
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. “
Image and text source:http://www.sparetheair.org/teachers/bigpicture/IIIA1a.html
Two Types of InversionRadiation 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 InversionImage Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html
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
Developed for use in dispersion modelsStability 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 ProfileFriction retards wind movementFriction is proportional to surface roughnessLocation and size of surface objects produce different wind velocity gradients in the vertical directionArea of atmosphere influenced by friction – planetary boundary layer – few hundred m to several km above earth’s surfaceDepth of boundary layer > unstable than stable conditions
Wind Velocity Profile
Wind speed varies by heightInternational standard height for wind-speed measurements is 10 mDispersion of pollutant is a function of wind speed at the height where pollution is emittedBut 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 zz: elevationp: exponent based on terrain and surface
cover and stability characteristics
Wind Velocity Profile
Wind DirectionDoes 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 DirectionNeed to know distribution of wind direction for estimating pollution concentrationsNeed speed and directionWind Rose Average of wind speed and direction over time Shows
Frequency Speed direction
Wind direction is direction from which the wind is coming
Mixing HeightVigorous mixing to a certain height (z) and little effect above thatRising air columns mix air vertically & horizontallyRising air mixes and disperses pollutantsOnly mixes to “mixing” height no above itDifferent in summer vs winter, morning vs eveningFor inversions, mixing height can be close to 0Thermal buoyancy determines depth of convective mixing depth
Mixing Height
Usually corresponds to tops of cloudsDifferent shapes but reach about same heightUp to mixing height unstable air brings moisture up from below to form clouds – above mixing height there is no corresponding upward flowStrong delineation at stratosphere/troposphere boundaryStratosphere very stable against mixing Where commercial air lines fly, air clear and non
turbulent Very clear boundary
Mixing Height
Mechanics of Mixing Height
Parcel heated by solar radiation at earth’s surfaceRises until temperature T’ = TT’ = particle’s tempT = atmospheric tempAchieves neutral equilibrium, no tendency for further upward motion
Turbulence
Not always completely understood2 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
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 ratiosAffect plume dispersion differently
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 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 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 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 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 inversionsImage Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter17/fav_conditions.html
Assignment 3
Problems: 3.7 3.9 3.14 Due Thu Feb. 3rd