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
Model-Based Soundings • 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
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
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.”
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.