Air Pollution Potential and Fire Weather Forecasting

Anthony R. LupoAtms Sci 4310 / 7310Lab 9

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.

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.

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”.

A map…..

Air Pollution Potential Forecasting

Western US (west of Rockies and Rockies):

Can be quite common in the spring (Mar - May).

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)

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.

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.

Air Pollution Potential Forecasting C) 500 hPa absolute vorticity less than 10 x 10-5 s-1,

(recall f = 10 x 10-5 at ~43o 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.

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.

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.

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.

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.

Air Pollution Potential Forecasting Afternoon mixing height

(http://www.wrh.noaa.gov/sew/fire/olm/mxhgts.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

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.

Air Pollution Potential Forecasting Ventillation

Ventillation is mixing height x wind speed (in metres) (m2/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 m2/s for 1 to 3 days or 8000 m2/s for more than 2 days are considered significant.

Air Pollution Potential Forecasting

The End!

Air Pollution Potential Forecasting

Questions?

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LupoA@missouri.edu