Glossary - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 161

active and passive path-averaging methods available. The opposite technique is
profiling or sounding. See also “tomography”.

Phased-array antenna Phased-array SODARs have a larger number of smaller
sound transducers (often 32 or 64), which are regularly arranged on a quadratic
plate with an area of about 1 m 2 . By using interference principles (enhancement
and extinction), slanted and vertical beams are formed by operating the various
sound transducers with specified time delays among each other. While with classical
SODARs the antennas simultaneously serve as a shield to protect the environ-
ment from the sound pulses as well as the SODAR from disturbing ambient noise,
phased-array SODARs need a shield around them.

Polarization Polarization means the orientation of the plane within which a wave
oscillates. This plane is perpendicular to the propagation direction of the wave.
Certain scattering processes lead to well-defined changes in the orientation of the
scattered waves, which can be used to identify properties of the scatterer.

Pressure broadening Collision between emitting particles or molecules disturbs
the emission process. This leads to non-zero line widths. The number of collisions
increases with increasing pressure. The shape of pressure broadened emission lines
is Lorentzian.

Profiling see “sounding”.

Pulsed RADAR RADARs which emit discontinuous signals (pulses of given
length and frequency) are called pulsed RADARs in contrast to continuous-wave
RADARs. Pulsed RADARs can easily determine the range from the time delay
between the emitted and the received signal.

Radome A radome (from the words RADAR and dome) is a usually spherical
weatherproof enclosure which protects RADAR antennas.

Raman scattering With normal elastic scattering, a molecule absorbs a pho-
ton, enters into an excited vibrational or rotational state, and immediately emits
(backscatters) the photon again when returning to its ground state. With inelas-
tic Raman scattering, the molecule does not return to the ground state, but to a
neighbouring energy level so that a photon with a slightly changed (usually greater)
wavelength is emitted (Stokes scattering). If the emitted wavelength is lower, the
process is called Anti-Stokes scattering. Raman scattering is distinguished from flu-
orescence by that it can happen for radiation of any wavelength and the excited
molecule does not enter into a resonance state.
The observed wavelength shift due to the excitation of rotation or vibration of
air molecules is characteristic for a given trace gas species. This scattering phe-
nomenon is named after the Indian physicist Chandrasekhara Venkata Raman,
who first reported the experimental discovery of this wavelength shift in 1928.
The disadvantage of the method is that inelastically backscattered signal intensi-
ties are two to three orders of magnitude lower than those from elastic Rayleigh
scattering.