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ARTICLE : C-Band Polarimetric Doppler Radar- Better Weather Predictions
Tuesday, 31 January 2012 02:37

C-Band Polarimetric Doppler Radar- Better  Weather Predictions

 

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 nowcasting.

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 (ZDR) 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 (Rho-HV) 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 (Phi-DP) 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 (KDP) 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 (TH) which is measured in the horizontal plane in logarithmic scale equated relative to 1 mm drop in meter cube, without any correction and filtering of unwanted echoes. Horizontal Reflectivity (ZH) is the corrected Total Reflectivity (ZH) 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 (V) will be measured which is the Reflectivity averaged radial velocity of thescatterer. Colour coded analogous to frequency red shift in astronomy, i.e. red colour echoes moving away and cool coloursmoving towards radar will be analyzed. Spectral Width (W) will be measured which is the spread of the return signal attributed directly to the turbulence of the atmosphere. Hydromet Classification (HCLASS) 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. It will 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 disaster management.  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 the month 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. It will also contribute to increased lead time in flash floods and winter weather hazard warnings 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.