2.3 Attenuation
Attenuation is the gradual loss of power resulting from absorption and scattering as
the radar signal travels through precipitation. The amount of attenuation depends on
the precipitation particles present along the path of the radar beam and the radar
frequency. The main absorbing substances that cause attenuation of microwaves in
the atmosphere are water vapour and precipitation. Attenuation caused by precipitation increases steadily with radar frequency. At frequencies below 3 GHz (wavelengths greater than 10 cm), attenuation is relatively small. However, attenuation of
radar signals by precipitation is a significant problem and one that becomes increasingly severe at wavelengths shorter than 10 cm. For instance, for a uniform rain rate
of 20 mm/h on a 10-km path, the path integrated attenuation (PIA) is around 50 times
greater at X-band frequencies than at S-band frequencies. However, the amount of
PIA depends upon the rainfall rate and the length of the path. The attenuation effects
remain relatively moderate at the C-band radar (5.4-cm wavelength) with a factor of
less than four compared to S-band [33]. For a given wavelength, the amount of
attenuation grows proportionally with rainfall intensity, but its effects are cumulative
with range. In practice, heavy rainfall may lead to a complete loss of radar signal at
X-band frequencies, severely limiting the maximum detectable range, whereas at
C-band frequencies, the radar signals can still penetrate through even the most
intense precipitation. Furthermore, a thin film of water forms on the radome surface
in rain causing additional attenuation, particularly at shorter wavelengths. As a
result, rain attenuation and radome attenuation are important error sources that affect
the radar rainfall estimates. However, modern radomes have water-repellant coatings
(e.g. hydrophobic coating) that might help to reduce radome attenuation. It is
therefore important that steps are taken to mitigate attenuation effects if reliable
radar rainfall estimates are required. Technically, the choice of the radar system with
a longer wavelength (e.g. S-band) is a practical solution to mitigate this specific
issue. However, this comes at a high cost due to the larger antenna of S-band radar
and the higher transmitted power to retain a reasonable resolution and sensitivity.
Shorter wavelength radars (e.g. X-band) have their own advantages, including
smaller-sized antenna and higher sensitivity of the differential phase shift, which is
immune to attenuation and can be used to estimate rain rates in heavy precipitation.
However, several X-band radars are often required to measure precipitation over a
particular region in order to mitigate potential problems of radar signal loss due to
rain attenuation at these frequencies.
Different techniques have been developed to mitigate attenuation effects on radar
systems at shorter wavelengths (e.g. X-band or C-band) [47–50]. Attenuation correction algorithms that use reflectivity measurements only are known to be unstable
[48]. Early attenuation correction approaches were iterative, correcting the range
gate from the first resolution volume (where attenuation is considered negligible)
and moving to continuous range gates along the beam as it penetrates the precipitation cells. However, such gate-to-gate algorithms are inherently unstable and
certain constraints must be imposed on the maximum amount of attenuation
Precipitation Measurement with Weather Radars
241
Attenuation is the gradual loss of power resulting from absorption and scattering as
the radar signal travels through precipitation. The amount of attenuation depends on
the precipitation particles present along the path of the radar beam and the radar
frequency. The main absorbing substances that cause attenuation of microwaves in
the atmosphere are water vapour and precipitation. Attenuation caused by precipitation increases steadily with radar frequency. At frequencies below 3 GHz (wavelengths greater than 10 cm), attenuation is relatively small. However, attenuation of
radar signals by precipitation is a significant problem and one that becomes increasingly severe at wavelengths shorter than 10 cm. For instance, for a uniform rain rate
of 20 mm/h on a 10-km path, the path integrated attenuation (PIA) is around 50 times
greater at X-band frequencies than at S-band frequencies. However, the amount of
PIA depends upon the rainfall rate and the length of the path. The attenuation effects
remain relatively moderate at the C-band radar (5.4-cm wavelength) with a factor of
less than four compared to S-band [33]. For a given wavelength, the amount of
attenuation grows proportionally with rainfall intensity, but its effects are cumulative
with range. In practice, heavy rainfall may lead to a complete loss of radar signal at
X-band frequencies, severely limiting the maximum detectable range, whereas at
C-band frequencies, the radar signals can still penetrate through even the most
intense precipitation. Furthermore, a thin film of water forms on the radome surface
in rain causing additional attenuation, particularly at shorter wavelengths. As a
result, rain attenuation and radome attenuation are important error sources that affect
the radar rainfall estimates. However, modern radomes have water-repellant coatings
(e.g. hydrophobic coating) that might help to reduce radome attenuation. It is
therefore important that steps are taken to mitigate attenuation effects if reliable
radar rainfall estimates are required. Technically, the choice of the radar system with
a longer wavelength (e.g. S-band) is a practical solution to mitigate this specific
issue. However, this comes at a high cost due to the larger antenna of S-band radar
and the higher transmitted power to retain a reasonable resolution and sensitivity.
Shorter wavelength radars (e.g. X-band) have their own advantages, including
smaller-sized antenna and higher sensitivity of the differential phase shift, which is
immune to attenuation and can be used to estimate rain rates in heavy precipitation.
However, several X-band radars are often required to measure precipitation over a
particular region in order to mitigate potential problems of radar signal loss due to
rain attenuation at these frequencies.
Different techniques have been developed to mitigate attenuation effects on radar
systems at shorter wavelengths (e.g. X-band or C-band) [47–50]. Attenuation correction algorithms that use reflectivity measurements only are known to be unstable
[48]. Early attenuation correction approaches were iterative, correcting the range
gate from the first resolution volume (where attenuation is considered negligible)
and moving to continuous range gates along the beam as it penetrates the precipitation cells. However, such gate-to-gate algorithms are inherently unstable and
certain constraints must be imposed on the maximum amount of attenuation
Precipitation Measurement with Weather Radars
241