Stable Atmosphere

Unstable Atmosphere

Mixing Height Definition

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

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

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

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

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)

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.

Convective VS Mechanical

Convective Mixing usually dominates over Mechanical Mixing

During very windy conditions smoke plumes will stay close to the ground

WHERE TO GET INFORMATION ON MIXING HEIGHTS

Holtzworth Techinque

NOAA models (such as ETA or SRI) can generate soundings for current time or future time for any location. www.arl.noaa.gov/ready/cmet.html

Model-Based Soundings

www.weather.nps.navy.mil/profiler/coastprof.html

DRI Web Page with Mixing Hts

www.cefa.dri.edu/Operational_Products/NCEP_Exp/exp_index.htm

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

Complex Terrain

Upper air data tend to be collected over simple terrain

Extrapolating mixing depths to elevated terrain is not straight forward

Mixing Depth Observationsat Flat-Land Sites in Bay Area

Issue #1

How to estimate mixing depth on site?

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

Issue #2

Burning before meeting the mixing height prescription.

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.”

•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.

Meeting Prescription

END

Holtzworth Technique