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plications, including meteorological observations and weather and climate forecasting, air
quality pollution forecasting, power energy plants design, geodesy and long-baseline interferometry,
communications and satellite data validation, air-sea interaction, and fundamental
atmospheric molecular physics study.
Radiometers can be operated continuously – on time scales of seconds to minutes – and
unattended mode under almost all weather conditions to continuously measure the temperature
profiles and temperature gradients (Troitsky et al., 1993; Westwater et al., 1998). The
remote sensing measurements (e.g. radiometer, RASS, sodar, lidar, etc.) have characteristics
that could be largely different from those taken by in situ instruments as radiosondes,
tethered balloon or traditional sensors. Remote sensing measurements are representative of a
volume (e.g. related to a radio antenna’s beam width or pulse length), whereas in situ measurements
are usually only local point measurements. These differences must be considered
in the comparison, interpretation and/or validation of data, and their use in models.
14.3 Upward-lookingradiometrictemperatureprofilemeasurements
The classical form of the radiative transfer theory describes the intensity of radiation propagating
in a general class of medium which absorbs, emits, and scatters the radiation. The
fundamental quantity measured by a radiometer is the radiant power, which is related to the
specific intensity Iν defined as the instantaneous radiant power that flows in a given point
in the medium, per unit area, per unit-frequency interval at a specified frequency ν, and in a
given direction per unit solid angle. As illustrated in Figure 172, its variation dIν at a point s
along a elementary segment ds in the direction of propagation is obtained by considering the
sources and sinks of the radiation in a elementary volume along that direction (in literature
optical path) per unit solid angle.
Figure 172:The specific intensityIν is the radiant energyflowingat each point in the medium
per unit area normal to the flux, per unit solid angle, in the frequency range ν e ν+dν. The
variation of intensity with position is governed by an equation of transfer that takes into
account the sinks and sources of radiation
This leads to a followingbalancingpowerdifferential form of the radiative transfer equation
(RTE):
dIν = −αν(s)Iν(s)ds+Sν(s)ds, (326)
where alphaν(s) is the local extinction coefficient and Sν(s) is a local contributive source
term at point s, which respectively describe the loss and gain of energy along the direction
into the considered given elementary (Janssen, 1993).
The thermal radiation emitted from an ideal blackbody at a definite frequency ν, depends
only on its thermodynamic temperature T: higher the temperature of the body more is its
DTU Wind Energy-E-Report-0029(EN) 261