1. Air Pollution Potential and Fire Weather Forecasting Anthony R. Lupo Atms Sci 4310 / 7310 Lab 9

2. Air Pollution Potential Forecasting  Q: Who is interested in this? Why should I be?  A: The general public! Thus, entities like The Weather Channel, Air Quality Monitors (Health, Regulators), Forest Managers, and Government / Private Sector forecasters.

3. Air Pollution Potential Forecasting  Forecast Philosophy:  today, numerical models can provide more detailed analysis of where and when these type of conditions will exist.  Then, forecasts represent conditions for pollution (build-up) potential (air stagnation) not for specific pollutants and/or their concentrations, whether these are manmade or natural pollutants.

4. Air Pollution Potential Forecasting  Climatology  Eastern US: (east of the Rockies)  Summer (all) and fall (primarily south) are most favorable times for air stagnation.  The fall conditions may represent “Indian Summer”.

5.  A map…..

6. Air Pollution Potential Forecasting  Western US (west of Rockies and Rockies):  Can be quite common in the spring (Mar - May).

7. Air Pollution Potential Forecasting  Favorable meteorological conditions  1) slow moving surface high pressure, and or ridge aloft, with weak horizontal pressure gradients (height gradients)  2) light winds in the mixed layer (PBL - planetary boundary layer  what’s this?)  3) stable air the boundary layer ( e <  m )

8. Air Pollution Potential Forecasting  4) No precipitation! (precipitation will scavenge particulates)  NCEP Air Pollution Stagnation Areas   an area for which ALL the following conditions are met are said to possess a high potential for air stagnation.

9. Air Pollution Potential Forecasting  A) 850 hPa (5000 ft) windspeeds of less than 10 m/s or 20 kts (Red Flag for fire weather is > 25 kts at the surface)  this suggests a small “ventillation” rate and minimal mechanical mixing.  B) 850 hPa (5000 ft) Temperature change in the past 12 h is less than –5C, or weak advections.  this eliminates the possiblity of strong CAA and a change of airmass to cold/cool and clear.

10. Air Pollution Potential Forecasting  C) 500 hPa absolute vorticity less than 10 x 10 -5 s -1, (recall f = 10 x 10 -5 at ~43 o N),  this suggests that the relative Vorticity is less than 0, which is indicative of a ridge aloft, preferrably a quasi-stationary long wave ridge.  D) 500 hPa 12 –hr vorticity change of less than 3 x 10 -5 s -2  this identifies ridges than are not moving very much.

11. Air Pollution Potential Forecasting  E) Probability of precipitation is less than 45%  F) Red Flag is prolonged periods with less than 15% RH.  this means no surface fronts or particulate scavenging!  If we meet all these criterion, then we issue air stagnation advisories.

12. Air Pollution Potential Forecasting  Another technique often used is the “mixing height” (Miller – Holzworth) technique:  Higher mixing height would indicate more mixing, and less change of air stagnation (trapping of particles).  We will “calculate” (using empirical methods) a morning and afternoon mixing height and compare this to an empirically derived standard.

13. Air Pollution Potential Forecasting  Morning mixing height http://www.wrh.noaa.gov/sew/fire/olm/mxhgts.htm:  1) The height above the ground where the surface temperature + “heat island correction”, following the dry adiabat intersects sounding.  2) Correction: +3C if the Obs. Site is considered “urban”, (STL), and +5C “rural” (MCI)  3) Calculate transport wind (tw): (tw) is mean wind speed in the mixing layer.

14. Air Pollution Potential Forecasting  The criterion:  If the morning mixing height is less than 500 m (1640 ft) and a mean wind of less than 4 m/s (8kts), this is significant.  This indicates poor mixing and dispersion of pollution.  You can convert pressure to height using the hypsometric equation on thermodynamic diagram.

15. Air Pollution Potential Forecasting  Afternoon mixing height (http://www.wrh.noaa.gov/sew/fire/olm/m xhgts.htm):  1) height above the ground at which the dry adiabat following from the maximum temperature (forecast or observed), intersects the sounding.  2) Transport wind speed is the average wind speed through the afternoon mixing layer

16. Air Pollution Potential Forecasting  The criterion:  afternoon mixing height of less than 4920 ft and a transport wind speed of less than 4 m/s (8kts) are considered significant. Calculate height via hypsometric equation.

17. Air Pollution Potential Forecasting  Ventillation  Ventillation is mixing height x wind speed (in metres) (m 2 /s) is answer. It is a measure of how fast particles dispersed or removed.  This quantity is often used by fire weather forecasters.  Values of less than 6000 m 2 /s for 1 to 3 days or 8000 m 2 /s for more than 2 days are considered significant.

18. Air Pollution Potential Forecasting  The End!

19. Air Pollution Potential Forecasting  Questions?  Comments?  Criticisms?  LupoA@missouri.edu