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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.

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Meteorology

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  1. Meteorology Background concepts

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

  3. Introduction • Once emitted pollutants: • Transported • Dispersed • concentrated • By meteorological conditions

  4. Layer nomenclature in the atmosphere

  5. Light scattering

  6. Orographic rainfall

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

  8. Transport • Pollutants moved from source • May undergo physical and chemical changes • Smog – interaction of NOx, HC, and solar energy • Ozone formation

  9. 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

  10. 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

  11. Dispersion • General mean air motion • Turbulent velocity fluctuations • Diffusion due to concentration gradients – from plumes • Aerodynamic characteristics of pollution particles • Size • Shape • Weight

  12. 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

  13. Atmosphere • Relative humidity (RH): ratio of water content to air • Increases with increasing temperatures

  14. 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

  15. 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

  16. 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

  17. 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/

  18. 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

  19. 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)

  20. 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

  21. Wind Circulation Air flows directly from high to low pressure areas (fp)

  22. 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

  23. Note differences in flow between cells

  24. Sinking boundaries Rising Boundaries

  25. 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

  26. Rotation • Without rotation • Air flows directly from high to low pressure areas (fp) • Rotation of earth affects movement

  27. Effect of rotation on baseball thrown at North Pole • Space observer sees straight path • Catcher moves – ball appears to curve to the left

  28. Coriolis forces

  29. 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.

  30. 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

  31. 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.

  32. Hurricane

  33. Atmospheric regions & cells

  34. Regions and cells

  35. 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

  36. Isobar • Areas of equal pressure

  37. 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)

  38. Frictional Force

  39. 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

  40. 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

  41. 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

  42. Influence of Ground & Sea • Water adsorbs and transfer heat differently than rock & soil • Rock and soil radiate heat differently summer to winter

  43. 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

  44. 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

  45. 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

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