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Basic Meteorology

Basic Meteorology. Prepared by: Clinton P. MacDonald and Charley A. Knoderer Sonoma Technology, Inc. Petaluma, CA The 2005 National Air Quality Conference Forecasting Short Course San Francisco, CA February 13, 2005. Goal. To explain how to interpret basic weather

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Basic Meteorology

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  1. Basic Meteorology Prepared by: Clinton P. MacDonald and Charley A. Knoderer Sonoma Technology, Inc. Petaluma, CA The 2005 National Air Quality Conference Forecasting Short Course San Francisco, CA February 13, 2005

  2. Goal To explain how to interpret basic weather information to forecast air quality using • Soundings • Weather charts

  3. Box Model Concept Key Processes • Source location, density, and strength • Dispersion (horizontal mixing) - wind speed • Sunlight • Stability • Vertical mixing - inversion Wind speed (WS) Concentration  S/WS S Vertical mixing (VM) S Concentration  S/VM Courtesy of New Jersey Department of Environmental Protection

  4. Soundings • Observed: Typically measured using instrument called a rawinsonde • Instruments carried by a balloon up through the atmosphere, equipped with sensors to measure meteorological variables (pressure, temperature, relative humidity, wind, etc.), and provided with a radio transmitter for sending this information to the observing station • Launched at0000 and 1200 UTC • Forecast: RUC, Eta, Meso-eta, GFS, UKMet • RUC, every 1 hour through 12 hours • Eta, every 3 hours through 84 hours • NGM, every 12 hours through 48 hours • Aviation, every 6 hours through 180 hours, every 12 hours 180 hours through 384 hours • For air quality forecasting, focus on 700 mb (~3000 m) and below

  5. 6 AM 12 AM 9 AM Height Height Height Warm Warm Cool Warm layer Cool Cool Surface Warming Warming Temperature Temperature Temperature Sunset 3 PM 12 NOON Height Height Height Maximum Warming Cooling Temperature Temperature Temperature Diurnal Temperature Soundings

  6. 10OC Cold 2 km Parcel Sinks to 1 km Parcels are same temperature when they reach 1 km 20OC 1 km Parcel Rises to 1 km 30OC Ground Warm Lapse Rates • Adiabatic Lapse Rate: The rate at which an unsaturated air parcel cools as it rises. It is minus 9.8ºC per km. • Moist Adiabatic Lapse Rate: The rate at which a saturated air parcel cools as it rises. • It varies with the original air temperature of the parcel. • A commonly used value is 6˚C/km.

  7. Rawinsonde Plots Winds Temperature Temperature grid Dry adiabat grid: rate at which an unsaturated air parcel cools as it rises. Dew point temperature Moist adiabat grid: The rate at which a saturated air parcel cools as it rises. Pressure scale (mb) Mixing ratio grid: the mass of the water vapor in a parcel to the mass of dry air Ground level Pressure level grid Temperature scale (˚C)

  8. Interpreting Rawinsondes • Temperature • Inversions • Stability • Mixing height • Winds • Clouds

  9. Interpreting Rawinsondes – Temperature • Warm aloft temperature leads to stable conditions and poor air quality • Determine the relationship for your area by season 850-mb temperature = 10.5˚C

  10. Interpreting Rawinsondes – Inversions A layer of very stable air over a short vertical distance produced by warmer air above cooler air

  11. Subsidence inversion Nocturnal inversion Ground level Interpreting Rawinsondes – Inversions • Types • Subsidence • Created by sinking air associated with ridges • Can limit daytime mixing depth and plays important role in daytime pollutant concentrations • Nocturnal • Created by cooling ground at night • Strongest with clear skies, light winds, and long nights • Can trap emissions, released during the overnight hours, close to the ground • Advection • Created when warm air aloft moves over cooler air below • Can occur ahead of an approaching cold front • Can cause poor air quality, despite the lack of an aloft ridge • Stable conditions in a temperature profile can exist without an inversion

  12. Dry adiabatic lapse rate Observed temperature profile When released Tparcel < Tair Height Parcel falls to original position because it is cooler than the surrounding air As parcel rises it cools at -9.8˚C/km Temperature Interpreting Rawinsondes – Stable Atmosphere

  13. Dry adiabatic lapse rate Parcel keeps rising because it is warmer than the surrounding air Observed temperature profile Height When released Tparcel > Tair Temperature Interpreting Rawinsondes – Unstable Atmosphere

  14. 2000 m 2000 m 1500 m 1500 m Estimated mixing height T T 1000 m 1000 m Dry adiabat Dry adiabat Estimated peak mixing height 500 m 500 m Forecasted max. temp. Forecasted max. temp. Increasing temperature Interpreting Rawinsondes – Estimating Mixing Heights from Morning Soundings • Holtzworth Method – Starting at the forecasted maximum temperature, follow the dry adiabat (dashed line) until it crosses the morning sounding. This is the estimated peak mixing height for the day. • The dry adiabatic rate is how an unsaturated air parcel cools as it rises. It is defined as -9.8ºC per km.

  15. Estimated mixing height = 1800 m or about 815 mb Estimated peak afternoon surface temperature Interpreting Rawinsondes – Estimating Mixing Heights

  16. Interpreting Rawinsondes – Clouds, Fog, and Wind Clouds occur when dew point temperature equals temperature Clouds Decoupled winds Fog Time Winds Weather Sky Cover 12:51 Z N 13 Light Rain Fog/Mist 11:51 Z N 12 Light Rain BKN006 OVC011 10:51 Z N 10 Light Rain Fog/Mist OVC004

  17. Rawinsonde Exercise • Determine the following • 850-mb temperature • Inversions • Afternoon mixing height based on morning temperature sounding, assuming a forecasted high temperature of 18°C • Afternoon mixing height on afternoon sounding • Based on findings • Did the mixing height estimate from the morning sounding match the mixing height determined from the afternoon sounding?

  18. Rawinsonde Exercise

  19. Actual afternoon mixing height =~600 mb or ~4205 m Inversions Rawinsonde Exercise 850-mb temperature = ~3OC Forecasted afternoon mixing height=~630 mb or ~3800 m

  20. Weather Charts (1 of 5) • Depict upper-air and surface meteorological patterns as a horizontal slice of the atmosphere • Show forecasted meteorological variables at a particular time on a particular pressure level

  21. Weather Charts (2 of 5) 850-mb heights and tempera- ture 500-mb heights and vorticity 700-mb heights and vertical velocity Surface pressure

  22. Weather Charts (3 of 5) • Surface fronts • Transition zone between air masses • Warm air ahead of fronts is often polluted • Cold air behind fronts is often clean • Surface pressure and winds • Good indicator of horizontal dispersion • Can be used to help determine transport • Locations where pressure contours are widely spaced have light winds and low dispersion • If pollutant concentrations upwind of a location are high and the winds are moderately strong, forecasters should evaluate the possibility of transport

  23. Weather Charts (4 of 5) • 850-mb temperature • Good indicator of stability • 700-mb vertical velocity • Downward vertical motion (negative on charts shown here) indicates stable conditions and is associated with poor air quality • Upward vertical motion (positive on charts shown here) indicates unstable conditions and is associated with good air quality • 500 mb heights – the height of the 500-mb pressure surface • Ridges associated with poor air quality • Troughs associated with good air quality

  24. Weather Charts (5 of 5) • Analyses and forecasts (various models) • Eta: NCEP short-range forecast model Now running at 12-km resolution with 60 vertical layers Runs 4 x day (00, 06, 12, 18 UTC) Forecasts for 00, 06, 12 and 18 UTC out 84 hours • Time in weather products • UTC/Zulu/GMT • Example: 00Z Feb 2 = 1800 CST Feb 1 (For Daylight Savings, 1900 CDT) • Forecast time periods • Forecast validation time • Based on model initialization time (00 or 12 UTC) + Forecast periods 12, 24, 36, 48, 72, 96 hrs., etc. • Example: 24-hr forecast from a 12 UTC model run is valid at 12 UTC (7 AM EST) the next day

  25. Interpreting Weather Charts – Surface (1 of 3) • Strong pressure gradient • Strong winds can lead to good dispersion and good air quality or can result in transport of poor air quality • Weak pressure gradient • Light winds can lead to poor dispersion and poor air quality

  26. Interpreting Weather Charts – Surface (2 of 3) • Fronts • Transition zones between two air masses of differing densities (e.g., temperature, pressure, and humidity) • Importance to air quality • Warm air ahead of front is often polluted • Cold air behind front is usually clean air

  27. At the surface, winds flow counterclockwise and inward toward a center of low pressure, and clockwise and outward around a center of high pressure. Interpreting Weather Charts – Surface(3 of 3)

  28. COLD Cold Air Advection Warm Courtesy of San Jose State University Meteorology Department Interpreting Weather Charts – 850-mb Temperature • Warm 850-mb temperatures can stabilize the atmosphere, which can lead to poor air quality by reducing vertical mixing • Cool 850-mb temperatures can destabilize the atmosphere, which can lead to good air quality by enhancing vertical mixing

  29. Interpreting Weather Charts – 700-mb Vertical Velocity • Downward vertical motion stabilizes the atmosphere which can lead to poor air quality • Upward vertical motion destabilizes the atmosphere which can lead to good air quality Courtesy of San Jose State University Meteorology Department

  30. Increasing Height Ridge Ridge 500 mb 500 mb Trough Trough Surface Very warm column Cool column Warm column Very cool column Interpreting Weather Charts – 500-mb Ridges and Troughs (1 of 5) • Mountains and valleys of warm and cool air • The height of the 500-mb pressure altitude depends on the relative temperature of the column

  31. Wave movement Fast wind Fast 500 mb Ridge Slow Convergence Divergence Convergence Trough Surface Interpreting Weather Charts – 500-mb Ridges and Troughs (2 of 5) • Waves (ridges and troughs) generally move west to east • Winds travel faster around ridges and slower around troughs • Areas of aloft convergence and divergence are created.

  32. Convergence 500 mb Ridge Convergence Divergence Trough Rising Sinking Sinking Surface High Pressure Low Pressure High Pressure Interpreting Weather Charts – 500-mb Ridges and Troughs (3 of 5) • Aloft divergence causes rising motion and surface low • Aloft convergence causes sinking motion and a surface high • Surface pressure patterns are offset from aloft patterns

  33. Interpreting Weather Charts – 500-mb Ridges and Troughs(4 of 5) • Effects on air quality • Ridges and associated sinking motion • Warm the air • Create a temperature inversion • Reduce vertical mixing • Create clear skies • Are associated with poor air quality • Troughs and associated rising motion • Cool the air • Break inversions • Increase mixing • Cause cloud cover • Are associated with good air quality

  34. Interpreting Weather Charts – 500-mb Ridges and Troughs (5 of 5) • Ridges are associated with poor air quality • Troughs are associated with good air quality Ridge Trough Courtesy of San Jose State University Meteorology Department

  35. Interpreting Weather Charts – 500-mb Vorticity (1 of 2) • Vorticity is a measure of rotation • It captures smaller-scale aloft features within larger patterns • Subtle changes in an upper-level pattern can have a large influence on air quality • Negative vorticity advection is associated with sinking motion (less than 10 1x10-5 s-1 on charts) • Positive vorticity advection is associated with rising motion (greater than 10 1x10-5 s-1 on charts)

  36. Interpreting Weather Charts – 500-mb Vorticity (2 of 2) Positive vorticity is associated with rising motion and good air quality Negative vorticity is associated with sinking motion and poor air quality Courtesy of San Jose State University Meteorology Department

  37. Weather Charts Exercise (1 of 2) • For Atlanta, Georgia, identify • Ridge or trough • 850-mb temperature • Surface wind direction and strength • Vertical velocity

  38. Weather Charts Exercise (2 of 2) Surface: temperature (color contours), pressure (solid lines, mb), and winds (barbs, knots). 850 mb: temperature (color contours), geopotential heights (solid lines, dm), and winds (barbs, knots) °C °C Atlanta, GA 750 mb: vertical velocity (color contours), geopotential heights (solid lines, dm), and winds (barbs, knots) 500 mb: temperature (color contours), geopotential heights (solid lines, dm), and winds (barbs, knots) -µB/s °C Atlanta, GA

  39. Conclusions • Soundings and weather charts are the cornerstone of conceptual air quality forecasting • Soundings • Aloft temperature and winds • Stability and Mixing • Weather Charts • Surface winds and fronts • 850 temperature • 700 vertical velocity • 500 heights and vorticity

  40. Links • Soundings • http://weather.uwyo.edu/upperair/sounding.html • http://www.rap.ucar.edu/weather/upper/ • http://vortex.plymouth.edu/uacalplt.html • Surface Analyses • http://www.hpc.ncep.noaa.gov/dailywxmap/ • http://www.rap.ucar.edu/weather/progs/ • http://weather.unisys.com/index.html • Models • http://www.met.sjsu.edu/weather/models.html • http://weather.unisys.com/index.html • http://ggweather.com/loops/ncep_loops.htm • http://www.arl.noaa.gov/ready/cmet.html • http://weather.uwyo.edu/models/ • http://www.rap.ucar.edu/weather/model/

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