Glossary - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 156

Glossary

Absorption Uptake of energy without backscatter or fluorescence. The gained
energy is radiated away as thermal radiation later. Wavelength-dependent absorp-
tion of radiation by trace gases may be utilized to measure the concentration of
trace gases in the atmosphere.

Active remote sensing Remote-sensing methods based on the emission of a
discontinuous (pulsed) or continuous signal and the subsequent receiving of the
backscattered signal are called active remote-sensing methods. The opposite method
is passive remote sensing.

Acoustic temperature The speed of s ound c depends on air p ressure p , air density
ρ

20.063 √ T with the specific gas
constant R , the specific heat for constant pressure, c p and the specific heat for con-
stant volume, c v .As

ρ = √ γ

p

and the ratio γ = c p / c v , i.e. c = γ RT ≈

and R slightly depend on atmospheric humidity, the speed of
sound is humidity-dependent as well. The temperature, T a , which, when entered into
the definition for the speed of sound for dry air, gives the true sound speed in humid
air, is called acoustic temperature. This temperature can be computed from the dry-
bulb temperature T and the specific humidity q by T a =

γ

0.51 q (Schotanus et al.
1983 ). It can be determined directly from measurements of the sound speed. Its defi-
nition is similar to the one for the virtual temperature. In cold and dry environments,
the differences between acoustic and virtual temperature are negligible, in warm and
moist environments the differences become important.

T +

Anti-Stokes scattering see “Raman scattering”.

Aperture Aperture usually defines the diameter of the opening of a optical system.

Attenuation Attenuation is the gradual depletion of energy along the propagation
path of a signal, usually due to absorption or scattering. It is sometimes also called
extinction.

Backscatter Scattering back into the direction from which the signal has arrived
is called backscatter.

Baffles Lateral shields which protect the environment from acoustic signals
coming from a SODAR are called baffles.