As a radar pulse travels through the atmosphere, some of its energy is
lost. This occurs as the energy interacts with precipitation particles
(such as rain, snow, and hail), cloud droplets, gases, and lithometeors
(such as dust and smoke). Physically, electromagnetic energy interacts
with molecules and disturbs the motion of bound electrons. The
electrons absorb some of the electromagnetic energy, and the atom is
said to be in an excited state. Energy has been lost from the radar
beam to the molecule it has encountered. The amount of this loss,
called attenuation, depends primarily upon the wavelength of the radar
pulse relative to the size and composition of what it encounters, as
well as how long the interaction occurs.
Atmospheric attenuation is the loss of radar energy due to absorption and/or scattering as it passed through the atmosphere.
Energy is attenuated form the
radar pulse through scattering and absorption, and these are functions
of the wavelength relative to the particle size and composition. If
scattering occurs, energy of the same wavelength is emitted from the
atom or molecule. On the other hand, if absorption occurs, the atom or
molecule remains at a more excited state after emission and energy is
emitted at a different wavelength. As an example of absorption,
microwave ovens transmit radar wavelength energy into food that
contains water molecules; these water molecules absorb the energy and
heat up (i.e., they enter a more excited state).
The radar beam is attenuated through losses due to scattering and absorption by
- Gases
- Cloud droplets
- Dust and smoke
- Precipitation – mainly due to Rayleigh scattering
It is evident that attenuation does need to be considered, especially
when using the C-band (5 cm) radars. This is because the shorter the
wavelength of radar energy being used, the greater the attenuation is.
Attenuation occurs both on the outward path, and also on
the return path. For example for a C band radar, if a thunderstorm had
a 60 dBZ core of diameter 10 km, then the attenuation encountered (due
to traversing the core alone) on the outward path would be a loss of
10dBZ, and a further 10dBZ would be lost on the return path. The
farthest edge of the reflectivity core would thus read 20dBZ low.
Reflectivity returns for any precipitation further away from the radar
and on the same azimuth would also read about 20 dBZ lower than actual.
For an S band radar encountering the same core, attenuation would only
be 2dBZ. However in the case of very high reflectivities (giant hail)
similar attenuation could theoretically be experienced at S band for a
10km wide 75dBZ core, but this has rarely (if ever?) been observed.