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Micrometeorology Instrumentation
Micrometeorology is the study of the atmosphere near the surface and its
associated turbulence and transport of scalars. It deals with the small horizontal
and short temporal scales of motion in meteorology. Typically, less than 1 km and
1 hr.
The atmospheric boundary layer is formed as a consequence of the interactions
between the atmosphere and the surface of earth. Micrometeorologists study
these interactions.
A variety of instrumentation is used to measure the atmospheric boundary layer
and related turbulence.
Boundary Layer Defined
• A boundary layer is the layer next to a physical boundary
such as the inside wall of a pipe. In our case the earth’s
surface.
• Transport processes modify the lowest 100 to 3000 m
AGL (above ground level) of the troposphere creating the
boundary layer.
• The atmosphere above the boundary layer is called the
free atmosphere, typically.
• The boundary layer air separates the atmosphere from
the earth’s surface. Actually, the atmosphere really
doesn’t know that the earth is below it.
Planetary Boundary Layer
The layer of air influenced by surface friction is called the
planetary boundary layer (PBL).
Define the boundary layer as that 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
~ 1 hr or less.
Background: definitions
What is turbulence?
•Simply defined as perturbation from the mean.
u  (u  u )
How do we measure u′?
u  (u  u )
A sonic anemometer measures at very high sampling rates.
This is typically at 10 or 20 Hz. Data from a high temporal
resolution time series is used to calculate the mean of the
time series and subsequently a perturbation from the mean.
Once the perturbations are calculated, turbulent statistics
can be calculated.
u  u  u
Turbulent Flux?
• Transport of a quantity by eddies or swirls.
• The covariance of a velocity component and any quantity.
1
cov( w ) 
N
N 1
1
(W  W )  (   ) 

N
i 0
N
 w   w 
i 1
i i
z
Eddy
Mixes some air down
And some air up
Turbulent Sensible Heat Flux
´= +
w´= neg.
´= neg.
0
w´= +

Net upward
heat flux
Variance
One statistical measure of the dispersion of data about the mean is the
biased variance
1
2
 
N
N 1
2
(
A

A
)
 i
i 0
It is a good measure of the dispersion of a sample of BL observations. However,
recall that a  A  A Substituting this into the biased definition of variance:
1
2
 
N
N 1
2
2


(
a
)

a
 i
i 0
Turbulence Kinetic Energy (TKE) During Fire
• is a measure of the intensity of turbulence
• simply the summed velocity variances

1 2
TKE  e  u   v  2  w 2
2

Friction Velocity
Friction velocity is the surface stress imposed on the
flow when turbulence is generated by wind shear near
the ground. The friction velocity is defined as

2
u*  u w  vw
u w 
v w 
2

1
4
are the u and v components of momentum fluxes.
Surface Energy Budget
The energy budget at the earth’s surface can be thought of as energy
transferred through a layer at the surface. This includes turbulent energy
transfer with the air above it, radiative transfer through the top of it, and
molecular energy transfer into the soil below it.
-Q* = QH + QE – QG + ΔQs
-Q* = net upward radiation at surface
QH = upward sensible heat flux
QE = upward latent heat flux
QG = upward molecular heat flux into the bottom
ΔQS = storage/residual
QH
sun
QE
-Q*
-QG
Radiation Definitions
Radiation flux: is the amount of radiation coming from a
source per unit time in W.
Radiant intensity: is the radiant flux leaving a point on the
source, per unit solid angle of space surrounding the
point. [W/steradian]
Radiance: is the radiant flux emitted by a unit area of a
source or scattered by a unit area of a surface.[W m-2 sr-1]
Irradiance: is the radiant flux incident on a receiving surface
from all directions, per unit area W m-2.
Absorptance, reflectance, transmittance: fractions of the
incident flux that are absorbed, reflected, or transmitted
by a medium.
Global Solar radiation: is the solar irradiance received on a
horizontal surface [W m-2]. This is the sum of direct solar
beam plus the diffuse component of skylight, and is the
physical quantity measured by a pyranometer.
Radiation Definitions
Direct solar radiation: is the radiation emitted from the
solid angle of the sun’s disc, received on a surface
perpendicular to the axis of this cone, comprising mainly
unscattered and unreflected solar radiation.
At the top of the atmosphere this is usually:
1367 W m-2 . The direct solar radiation at the earth’s surface
is the physical quantity measured by a pyrheliometer.
Diffuse Solar Radiation: (Sky radiation) is the downward
scattered and reflected radiation coming from the whole
hemisphere. Diffuse radiation can be measured by a
pyranometer mounted in a shadow band.
Radiation Definitions
Visible Radiation: is the spectral range of the
standard observer. Most of the visible radiation
lies between 400 nm and 730 nm.
Ultraviolet Radiation: is the radiation with
wavelengths in the range 100 to 400 nm. It is
subdivided into three ranges: UVA is 315-400 nm,
UVB is 280-315 nm, and UVC is 100-280 nm.
Infrared Radiation: is the radiation with
wavelengths longer than 730 nm.
Radiation Definitions
Photosynthetically Active Radiation (PAR): is the
band of solar radiation between 400-700 nm that
plants use in the photosynthesis process. PAR is
usually expressed in moles of photons, a mole
being Avogadro’s number of photons, 6.022 x
1023 photons.
Albedo: is the fraction of incoming solar radiation to
reflected solar radiation.
α = SWreflect / SWincoming
Methods of Measurement
Two primary methods in the measurement of radiation:
Thermal detection: response to heat gain or loss due to
absorption of incoming or emission of outgoing
radiation.
Photovoltaic detectors: convert absorbed radiation to a
voltage.
Shortwave or solar radiation is defined to be 0.3 μm to
4 μm BUT since high-quality glass windows are
transparent from 0.3 to 3 μm, an upper limit of 3 μm is
generally accepted.
Longwave radiation sensors have windows transparent
to radiation from 3 or 4 μm to at least 50 μm .
Types of instruments
Radiation measuring instruments can be classified according
to their use. The generic term for all radiation measuring
instruments is the radiometer.
A pyranometer is used to measure global solar radiation, so it
must respond to both the direct solar beam and to diffuse sky
radiation from the whole hemisphere. The sensing element
must be a horizontal flat surface.
A pyrgeometer is used to measure global, long-wave earth
radiation.
An instrument that measures the difference between incoming
and outgoing radiation is called a pyrradiometer or net
radiometer.
Pyranometers
Thermal detectors measure the temperature change induced
by the heat gain (loss) due to absorption (emission) of
radiation by a black surface.
The temperature change is measured relative to a white
surface or to the shell of the instrument.
A thermopile is a nested array of thermocouples, usually 10 to
100. The advantage of a thermopile over a single
thermocouple is that the number of thermocouple pairs in the
pile multiplies the output voltage.
The measurand is the incident radiation, irradiance. The
sensor is the thermal plate and the raw output is the
temperature difference between the absorbing plate
instrument shell.
Pyranometers
Typical sensitivity of a thermopile pyranometer is 4 μV W-1 m2
and the time constant 5 s.
Pyrgeometers
Measure only earth or longwave radiation and therefore glass
domes cannot be used. Typically, a silicon window (flat, not
domed) is used to measure from 3 to 50 μm and the field of
view is limited to 150 degrees.
Some sensors incorporate an electrical heater to prevent dew
or frost formation. Silicon windows are more stable and
reliable than polyethylene.
These instruments require temperature correction. The
simplest form involves a calibration with two terms:
E=a1V1 +a2Vt,
where a1 and a2 are constants. V1 is the raw sensor voltage
output, and Vt, is the voltage proportional to sensor
temperatures.
Turbulent Heat Fluxes from Tower at UH Coastal Center
(10 min Ave of Covariances)
1000
LE_irga
Hs
Qg_2_Avg
Rn_CNR1_Avg
800
600
400
200
0
-200
07/08/06
07/10/06
07/12/06
07/14/06
07/16/06
07/18/06
Surface Energy Balance: Q*=QH+QL+QG
07/20/06
07/22/06