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– Including frictional drag, ET, heat transfer, pollutant emission, and

terrain induced flow modifications

• 200 – 3000 meters thick

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

So how do things get around in the boundary layer?

• Wind – 3 parts– Mean wind– Waves – Turbulence

• Each can exist on their own or in the presence of the others

• Mean wind dominates horizontal transport• Turbulence dominates vertical transport

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

Seconds 3.6*10^5 3.6*10^4 3600 360 36 3.6

Eddy Frequency and Time Period

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

Stability in Plumes

• Scan in Figure 1.7

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

Agricultural Meteorology

• Applied micrometeorology– Airborne transport of chemicals necessary to plant life

governed by turbulence

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

On a Daily Basis

Net Radiation (Rn)

• Q7.1 Rebs Net Radiometer

• Accuracy– 6% @

500W/m^2 (Twine et al, 2000)

• Canopy height

Soil Heat Flux (G)

• HFT3 Rebs soil heat flux plates

• Buried at 1 cm depth

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)

( ' ')pH c w t

( ' ')v wLE L w q

1

1' ' ( )( )

n

i x i yi

w T x u y un

Covariance -

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

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

Correcting $14,000 instruments

12 June - 22 July y = 1.4844x

R2 = 0.9551

-400

-200

0

200

400

600

800

1000

1200

1400

1600

-400 -200 0 200 400 600 800 1000

licor LE

kh20

LE

1 April - 8 June y = 1.7586x

R2 = 0.9504

-600

-400

-200

0

200

400

600

800

1000

1200

1400

-400 -200 0 200 400 600 800

licor LE

kh

20

LE

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)

Surface Renewal Analysis UsingStructure Functions

p c

aH c z

l s

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

Detecting Ramps

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

Seconds 3.6*10^5 3.6*10^4 3600 360 36 3.6

Eddy Frequency and Time Period

SR eddies in this range

Smallest 5s

Compared to SEC H• 95% of SEC H

– $2,000

(Spano et al, 1997)

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