Meteorology Background concepts Meteorology chapter 3 of text

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 n n Transported Dispersed concentrated By meteorological conditions

Layer nomenclature in the atmosphere

Light scattering

Orographic rainfall

Air Pollutant Cycle Transport Emission Deposition onto vegetation, livestock, soil, water, or escape into space Diffusion or concentration

Transport Pollutants moved from source May undergo physical and chemical changes n n Smog – interaction of NOx, HC, and solar energy 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 n 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 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 n n Size Shape Weight

Atmosphere Gas composition (changes very little with time or place in most of atmosphere n n n 78% nitrogen 21% oxygen 1% argon & other trace gases Moisture content n n n Water vapor Water droplets 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 2 x less than horizontal

Solar Radiation At upper boundary of atmosphere, vertical solar radiation = 8. 16 J/cm 2 min (solar constant) Maximum intensity at λ = 0. 4 to 0. 8 μm = visible portion of electromagnetic spectrum ~ 42% of energy n n n 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 water

Insolation Quantity of solar radiation reaching a unit area of the earth’s surface n n n 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 0º Tangent in morning and approximately perpendicular angle depends on surface Information and image source: http: //www. visualsunchart. com/Visual. Sun. Chart/Solar. Access. Concepts/

Wind Circulation Sun, earth, and atmosphere form dynamic system Differential heating of gases leads to horizontal pressure gradients horizontal movement Large scale movement n n Poles Equator Continents oceans Small scale movement n n Lakes Different surfaces

Wind Circulation Average over a year, solar heat flow to the earth’s surface at equator is 2. 4 x 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. 4 x that at poles Air moves in response to differences Heat transports from equator to poles n n 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 n Height vs width Flow is mechanically unstable 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 n n Equator Rain forests Temperate forests Sinking boundaries are regions of lower than average rainfall n n Most of world’s deserts 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 representation of flow around a low-pressure area in the Northern hemisphere. 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 This lowforce. pressure Coriolis system over Iceland spins counterclockwise due to balance between the Coriolis force and the pressure gradient

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 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 & 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 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 n Encounters lower pressure Expands to lower temperature Assume no heat transfers to surrounding atmosphere 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 (d. T/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 d. T 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: (d. T/dz)adiabatic perfect gas = -0. 0098°C/m American: (d. T/dz)adiabatic perfect gas = -5. 4°F/ft Change in temp. with change in height

Lapse rate is the negative of temperature gradient Dry adiabatic lapse rate = Metric: Γ = - 1°C/100 m SI: Γ = - 5. 4°F/1000 ft or

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 n M Cp Meteorologists and aeronautical engineers have defined n n “standard atmosphere” 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 o. C/100 m American: Γ = - 3. 45°F/1000 ft 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 n n 0 to 36, 150 feet Temperature decreases linearly 75% of atmospheric mass Not applicable above troposphere Stratosphere n n 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 pollutants Temperature/elevation relationship principal determinant of atmospheric stability Stable n n n 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 n n n 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 n n n 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 w less dense than surrounding air Parcel continues upward

Subadiabatic --- Stable n n n 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 n n n 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 winds Horizontal & vertical dispersions of pollutants are hampered Influenced by: n n Time of year Topography Presence of water or lakes Time of day Image source: http: //www. unc. edu/courses/2005 fall/geog/011/001/Air. Poll ution/Air. Pollution. htm

Image and text source: http: //www. sparetheair. org/teachers/bigpicture/ IIIA 1 a. 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/teac hers/bigpicture/IIIA 1 a. 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/teac hers/bigpicture/IIIA 1 a. 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 n 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/met 130/notes/chapter 17/fav_con ditions. html

Two more Types of Inversion Cold Air Flowing Under n n n 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 n n 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 models Stability classified into 6 classes (A – F) n n n A: B: C: D: E: F: strongly unstable moderately unstable slightly unstable neutral slightly stable 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 windspeed 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/u 1 = (z/z 1)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 n Formed from large scale instabilities Often near boundaries of circulation zones Air is rising or sinking Major storms often associated with low-pressure Topography n n n Air heats and cools differently on different surfaces, causes air from Lake to shore, etc. 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 n Average of wind speed and direction over time 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 n 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 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 n Wind – carries pollution downstream from source Atmospheric turbulence -- causes pollutants to fluctuate from mainstream in vertical and crosswind directions Mechanical & atmospheric heating both present at same time but in varying ratios Affect plume dispersion differently

Six Classes of Plume Behavior Looping: n n n 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/met 130/ notes/chapter 17/fav_conditions. html

Six Classes of Plume Behavior Coning: n n n 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/met 130/ notes/chapter 17/fav_conditions. html

Six Classes of Plume Behavior Fanning: n n n 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/met 130/ notes/chapter 17/fav_conditions. html

Six Classes of Plume Behavior Fumigation: n n n 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/met 130/ notes/chapter 17/fav_conditions. html

Six Classes of Plume Behavior Lofting n n n Opposite conditions of fumigation Inversion layer below with unstable layer through and above Pollutants are dispersed downwind without significant ground-level concentration Trapping n n Inversion above and below stack Diffusion of pollutants is limited to layer between inversions Image Source: http: //apollo. lsc. vsc. edu/classes/met 130/ notes/chapter 17/fav_conditions. html

Assignment 3 Problems: n n 3. 7 3. 9 3. 14 Due Thu Feb. 3 rd