(Online Course) Contemporary Issues for IAS Mains 2012- PIB : C-Band Polarimetric Doppler Radar- Better

C-Band Polarimetric Doppler Radar- Better

Radars will not tell you if it is going to rain tomorrow.
However, when something arises on the horizon and that cloud floats to your
city, the Polarimetric Doppler radar will be at your service with precise
information with estimated rain, storm structure, snow rates etc. The First
C-Band Polarimetric Doppler Radar of the country started working in Delhi on
January 15th, 2012 i.e. from the 137th Foundation Day of India Meteorological
Department. Most weather radars transmit radio wave pulses that have a
horizontal orientation. Polarimetric radars transmit radio wave pulses that have
both horizontal and vertical orientation. The horizontal pulses essentially give a measure of the horizontal dimension of cloud
(cloud water and
cloud ice) and precipitation (snow, ice pellets, hail and rain) particles while
the vertical pulses essentially give a measure of the vertical dimension. Since
the power returned to the radar is a complicated function of each particle size,
shape and ice density, this additional information results in improved estimates
of rain, snow rate, better detection of large hail location in storms, and
improved identification of rain/snow transition regions in winter storms.

Doppler radar has added capability of being able to measure a frequency shift
that is introduced into the reflected signal by the motion of the cloud and
precipitation particles. This frequency shift is then used to determine wind
speed.The C-Band Polarimetric Doppler Radar at Delhi has the latest state of the
art technology used for weather surveillance. It uses advanced algorithms to
generate information which is vital for detecting severe weather phenomena such
as rain, hail storm etc., with greater accuracy. Due to this additional
information it is very useful to issue weather forecasts and warnings for events which are likely to
occur within short period of time also known as now casting.

Variables Measured

The C-band Polarimetric radar has capability to measure
mainly four variables such as Differential Reflectivity, Correlation
Coefficient, Linear Depolarization ratio and Specific Differential Phase.
Besides this, total and horizontal Reflectivity, Radial velocity, Spectral width
and Hydromet classification will be measured. The Differential Reflectivity 
indicates ratio of the reflected horizontal and vertical power returns. Among
other things, it is a good indicator of drop shape. In turn the shape is a good
estimate of average drop size. Cross Correlation Coefficient  is a
statistical correlation between reflected horizontal and vertical power returns.
It is an indicator of regions of precipitation types, such as rain and snow. It
is prominently used for hydrometeor classification. Differential Phase 
and Specific Differential Phase is a comparisons of the returned phase
difference between horizontal and vertical pulses, caused by the difference in
number of wave cycles (wavelengths) along the propagation path for horizontal
and vertically polarized waves. This normalized to a standard displacement is
termed as Specific Differential Phase which directly correlates with the
“propagation effect” and a good estimator of rain rate as it is independent of
calibration, and other radar related parameters. It will measure Total
Reflectivity which is measured in the horizontal plane in logarithmic scale
equated relative to 1mm drop in meter cube, without any correction and filtering
of unwanted echoes. Horizontal Reflectivity  is the corrected Total
Reflectivity for ground returns, non perceptible echoes, interferences, and
enhancing the signal quality through signal qualifiers of LOG (weather data
gathered in a scientific, consistent manner), SQI ( Special Qualification
Identifiers), CSR (Coherent Scatter Radars which operate in the frequency range
between ionosondes and ISRs who scatter off thermal fluctuations in the plasma)
etc. Radial Velocity  will be measured which is the Reflectivity averaged
radial velocity of the scatterer. Colour coded analogous to frequency red shift
in astronomy, i.e. red colour echoes moving away and cool colours moving towards
radar will be analyzed. Spectral Width will be measured which is the spread
of the return signal attributed directly to the turbulence of the atmosphere. Hydromet
Classification  is a special Higher signal processing
capability with which radar classify the echoes during data acquisition, based
on other moments and proven algorithms employing fuzzy logic.

Benefits

The improvements associated with polarimetric Doppler radars
come with their ability to provide previously unavailable information on cloud
and precipitation particle size, shape, and ice density. It is independent of
calibration errors and has fine tuning. Itwill greatly help meteorologists,
hydrologists, aviation users and society. Polarimetric radar will significantly
improve the accuracy of estimates of amount of precipitation (snow, ice pellets,
hail and rain).It will now tell difference between very heavy rain and hail,
which will improve flash floods watches and warnings and disastermanagement. The
sudden, ferocious thunder storms lash the eastern part of India, particularly
the north-east region and states of Jharkhand, Orissa,West Bengal and Bihar
during themonth of Baisakh (April- May) at regular interval. This kal-Baisakhi
take great toll on lives and property. The C-band Polarimetric radar will be a
boon to give indication of kal-baisakhi. Itwill also contribute to increased
lead time in flash floods andwinter weather hazardwarnings and severe hazards
like thunder storms, kal- Baisakhis and cyclonic circulations. It has better
ability to correct signal attenuation due to atmosphere including attenuation
due to rain in cyclonic wall clouds. Due to advanced algorithms and software
which uses polarimetric data, it will generate better estimates of rainfall.
Rainfall in catchment areas of dams can be monitored. It will provide critical
rainfall estimation information for stream flow hence is very useful for
hydrological studies. It will be useful in water management. It will detect
aviation hazards such as birds (ornithological echoes), insects etc. It is able
to identify no-meteorological echoes with better accuracy than conventional
radars. (PIB Feautre).

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