Meteorology

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”


“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 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


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 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


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


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



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



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

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Meteorology