Transcript Comparison of methods for estimating ET In irrigated pecans

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
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•
•
•
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
Eddy Frequency and Time Period
3.6
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
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–
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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)
H   c p (w ' t ')
LE Lv w (w ' q ')
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 -
1 n
w 'T '   ( xi  ux )( yi  u y )
n i 1
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
1 April - 8 June
y = 1.7586x
R2 = 0.9504
1400
1200
1000
800
kh20 LE
600
400
200
12 June - 22 July
0
-200
-200
0
200
400
600
y = 1.4844x
R2 = 0.9551
800
1600
-400
1400
-600
1200
licor LE
1000
800
kh20 LE
-400
600
400
200
0
-400
-200
-200
0
200
400
-400
licor LE
600
800
1000
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 Using
Structure Functions
a
H   c p
zc
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
SR eddies in this range
Smallest 5s
Seconds 3.6*10^5
3.6*10^4
3600
360
36
Eddy Frequency and Time Period
3.6
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