1. Stable Atmosphere

2. Unstable Atmosphere

3. Mixing Height Definition
The mixed layer can be defined as the altitude above ground level to which pollutants vertically disperse. Fearon

4. Mixing Height Definition
The height of the layer adjacent to the ground over which an emitted inert non-buoyant tracer will be mixed (by turbulence) with a time scale of about one hour or less. Beyrich

5. Mixing Height Definition
The thickness Z, of the mixed layer, defined as the location of a capping temperature inversion or statically stable layer of air.
Glossary of Meteorology

6. Reasons for Capping Inversion
Because of summertime Pacific High, average inversion ht along California coast is about 400 meters
Marine layer is usually 1500 to 2000 feet deep in Bay Area
Rising air cools, and descending air warms, making the mixed layer adiabatic

7. Types of Mixing MECHANICAL MIXING Predominates at nighttime
Turbulence due to wind shear
Turbulence due to obstacles
Turbulence is proportional to wind speed (ie. H= 125 U)

8. Types of Mixing Height CONVECTIVE MIXING HEIGHT
Is a daytime phenomenon - due to heating by the sun
On sunny or partly cloudy days, is dominate from one hour after sunrise to one hour before sunset (13 deg above horizon). On overcast days, figure two hours.

9. Convective VS Mechanical
Convective Mixing usually dominates over Mechanical Mixing
During very windy conditions smoke plumes will stay close to the ground

10. WHERE TO GET
INFORMATION
ON MIXING
HEIGHTS

11. Holtzworth Techinque

12. Model-Based Soundings
NOAA models (such as ETA or SRI) can generate soundings for current time or future time for any location.

14. DRI Web Page with Mixing Hts

15. Model Calculations
Model being developed by EPA (AIRMET) will calculated hourly mixing ht values using Monin Obukhov length and friction velocity
Need measurements of net radiation and heat flux, in addition to winds, temperature and RH

16. Complex Terrain
Upper air data tend to be collected over simple terrain
Extrapolating mixing depths to elevated terrain is not straight forward

17. Mixing Depth Observations at Flat-Land Sites in Bay Area

18. Issue #1
How to estimate mixing
depth on site?

19. Estimating Mixing Heights
Most plans say will use Oakland Sounding and test burns
Burn elevation may be higher than Oakland mixing depth
How is height of smoke determined during test burns

20. Issue #2
Burning before meeting the mixing height prescription.

21. TITLE 17, SECTION 80160, PART J
“Require the land manager or his/her designee conducting a prescribed burn to ensure that all conditions and requirements stated in the smoke management plan are met on the day of the burn event and prior to ignition.”

22. Meeting Prescription
Many plans prescribe a mixing depth of 1500 feet agl
There is a reference to this value in a table in the “Smoke Management Training Workshop Manual”
The problem is that this height is often the maximum mixing depth, and is not reached until 2 – 3 pm.

23. END

24. Holtzworth Technique