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A Basic Introduction to Boundary Layer Meteorology

A Basic Introduction to Boundary Layer Meteorology. Luke Simmons. What is the Boundary Layer?. The part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a timescale of about an hour or less

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A Basic Introduction to Boundary Layer Meteorology

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  1. A Basic Introduction to Boundary Layer Meteorology Luke Simmons

  2. What is the Boundary Layer? • The part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a timescale of about an hour or less • Including frictional drag, ET, heat transfer, pollutant emission, and terrain induced flow modifications • 200 – 3000 meters thick

  3. Significance • Pollution is trapped in the BL • Crops are subject to the BL • Weather is changed and maintained in the BL • Turbulent transport and advection move water and oxygen to and from plants • You spend the majority of your life in it

  4. So how do things get around in the boundary layer? • Wind – 3 parts • Mean wind • Waves • Turbulence

  5. Each can exist on their own or in the presence of the others • Mean wind dominates horizontal transport • Turbulence dominates vertical transport

  6. Turbulent Transport • Eddy - Gusts, swirls of wind in the vertical plane caused by turbulence • Carry heat, momentum, water vapor, carbon dioxide, etc. • As large as the boundary layer, as small as a few molecules Big whorls have little whorls Which feed on their velocity And little whorls have lesser whorls And so on to viscosity

  7. Seconds 3.6*10^5 3.6*10^4 3600 360363.6 Eddy Frequency and Time Period

  8. Stability in the Boundary Layer • 3 States • Unstable, usually daytime, caused by swirling eddies rising off the heated surface because they are more buoyant than surrounding air • Stable, usually at night, only mean wind and waves, little turbulence causes only horizontal transport • Neutral, upper BL at night, turbulence at equal intensity in all directions

  9. Stability in Plumes

  10. Scan in Figure 1.7

  11. Micrometeorology • Space scales smaller than 3km and time scales less than 1 hour are on the micro scale • Mechanical turbulence, plumes, thermals, wakes, cumulus clouds, boundary layers • Mesometeorology includes • Fronts, thunderstorms, geographic disturbances, hurricanes • Stuff that can be numerically forecasted

  12. Agricultural Meteorology • Applied micrometeorology • Airborne transport of chemicals necessary to plant life governed by turbulence

  13. Surface Energy Balance Equation • Rn = LE + H + G • Rn = Net Radiation Flux • LE = Latent Heat Flux • H = Sensible Heat Flux • G = Soil Heat Flux • All in W/m2 • Measure each one individually, closure? • Or measure three, solve for one (usually LE) • LE is converted to depth of water, or consumptive water use of plants

  14. On a Daily Basis

  15. Net Radiation (Rn) • Q7.1 Rebs Net Radiometer • Accuracy • 6% @ 500W/m^2 (Twine et al, 2000) • Canopy height

  16. Soil Heat Flux (G) • HFT3 Rebs soil heat flux plates • Buried at 1 cm depth

  17. Sensible Heat(H) and Latent Heat(LE) • Methods of measuring • Eddy covariance (Swinbank,1951) • Overbar denotes a 30 minute averaging period, and prime(‘) indicates deviation from the mean, T = temp(K), w = vertical wind speed(m/s), ρ = air density(kg/m3), cp= specific heat of air(29.3 J/kg K), Lv=latent heat of vaporization (J/kg), ρw=water density(kg/m3), rw=water vapor density(kg/m3) Covariance -

  18. 3D Sonic eddy covariance(SEC) • CSAT 3d sonic anemometer (10Hz) • Measures wind speed and direction on three axes and Ts • Measurement height several meters above canopy for H • $8,300

  19. Hygrometers Rn = LE + H + G • Used to measure LE • KH20 Krypton Hygrometer(Campbell Scientific Inc) • Closure 3.2% (Stoughton et al, 2002) • $4,900 • LI7500 IRGA Carbon and water vapor flux densities (Licor) • Underestimating 30-40% • $13,400 • Corrected by KH20 • Software issues with matching the time delay for the sensor, but more likely that the sensor response is not as fast as the anemometer which reduces sensitivity to changes, or variance

  20. Correcting $14,000 instruments

  21. One propeller eddy covariance (OPEC) • Always have closure (only measure H, not LE) LE = Rn – H – G • Samples at 4 Hz • Compares well to SEC • $3,500 (Bawazir et al, 2000)

  22. Surface Renewal Analysis UsingStructure Functions α = weighting factor, ρ = air density, cp= specific heat of air, a = ramp amplitude(K), l+s = inverse ramp frequency(s), zc = measurement height (m) • Sampled at 4Hz

  23. Detecting Ramps Detection based on algorithms with established thresholds that mark the beginning and end of a ramp event

  24. SR eddies in this range Smallest 5s Seconds 3.6*10^5 3.6*10^4 3600 360363.6 Eddy Frequency and Time Period

  25. Compared to SEC H • 95% of SEC H • $2,000 (Spano et al, 1997)

  26. Conclusions • Surface Renewal method only reliable for daytime data at certain heights in canopy • When looking for water use data, that is the most important, night time data is difficult to measure because of condensation and stable conditions • Misses smaller eddies because of ramp detection process, needs scaling factor • Krypton hygrometer reliable to measure water use based on other energy balance measurements • Licor hygrometer underestimates water use • Can be used with a scaling factor • OPEC and SEC methods match up well • Errors in Rn and G measurements can make big errors in LE calculations for OPEC

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