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Enhancing Weather Forecasting: Doppler Radar Insights

Discover the capabilities and applications of Doppler Weather Radar, from rainfall estimation to wind speed measurements. Explore the principles behind Doppler Effect and coherent radar operation, along with various meteorological products. Learn about the Doppler Dilemma and how it impacts radar performance optimization. Unveil the significance of key radar products like Reflectivity, Radial Velocity, and Spectrum Width in weather monitoring and forecasting.

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Enhancing Weather Forecasting: Doppler Radar Insights

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  1. What is Doppler Weather Radar?It is a state of art weather radar which provides:-1. Rainfall rate & total rainfall associated to a cloud,2. Intensity of cyclone, speed of winds, direction of movement of a system.3. Direction and speed of movement of the thunderstorms, tornadoes & cyclone.4. Estimation of wind speed accompanied with a cyclone, expected storm surge height and potential of destruction.

  2. Doppler Weather Radar • It works on the principle of “Doppler Effect” and measures the change in frequency of the return signal to determine if targets are moving towards or away from the radar station. • COHERENT radars provide detection of the phase difference between the outgoing and return pulses. • NON-COHERENT radars are unable to measure the difference between the outgoing pulse and the return pulse. • Doppler radar is a coherent radar.

  3. Volume scan Bins, Rays and Slice

  4. Base Products from DWR • Reflectivity (Z) : A measure of size and number of droplets in sample volume. • Mean Radial Velocity (V) : A measure of mean wind towards or away from radar in sample volume. • Spectrum width (W) : A measure of variability of radial velocity I.e. turbulence in sample volume.

  5. Standard Meteorological Products • Plan Position Indicator (PPI)(Z,V,W) • Range Height Indicator (RHI) (Z,V,W) • Max - Maximum display (MAX) (Z,V,W) • Constant Altitude PPI (CAPPI) (Z,V,W) • Pseudo CAPPI (PCAPPI) (Z,V,W) • Vertical Cut (VCUT) (Z,V,W) • Echo Top (ETOP) (Z) • Echo Base (EBAS) (Z)

  6. If the target is not moving towards or away from the radar, V can be replaced by radial component of V as follows: f = 2Vcos(α)/λ • Maximum Unambiguous Velocity • The maximum velocity a Doppler radar can detect correctly or unambiguously in given by the velocity which produces a phase shift of π radians. This is also called the Nyquist frequency or Nyquist velocity. • V max = PRF λ / 4

  7. Maximum Unambiguous Range We know that the electromagnetic radiation travels at the speed of light .The time it takes for a signal to go out and back from a target is • r max = c / ( 2.PRF) (3.2) • Doppler Dilemma • The combination of maximum unambiguous velocity and maximum unambiguous range form two constraints which must be considered in choosing the PRF for use with a Doppler Radar .Unfortunately, PRF appears in the both the equations (3.1) and (3.2), but in the denominator of one and in the numerator of the other. By solving both equations for PRF and equating them, we find that • V max r max = c. λ / 8 • This form what has been called the” Doppler Dilemma”. • The Doppler Dilemma: There is no single PRF that maximizes both Rmax and Vmax

  8. Base ProductsPlan Position Indicator (PPI) • PPI (Z) : It is quite similar to the conventional radar scope display of Z with colour coded schemes for display. This display is possible for all elevations at which data is collected. This product is available for display immediately on completion of the scan.

  9. Max – Maximum Reflectivity • (1) A top view as the highest measured values in Z-direction. This image shows the highest measured value for each vertical column, seen from the top of the Cartesian volume. • (2) N-S view of highest measured values in Y-direction. This image is appended above top view and show highest measured value for each horizontal line scan from north to south.

  10. Plan Position Indicator (Radial Velocity) PPI (V) : It gives the radial velocity on a PPI scope . The radial velocity towards radar site is taken as –ve and away from radar site is taken as +ve.

  11. Plan Position Indicator (Spectrum Width) • PPI (W) : It is a measure of turbulence and it is of immense use in nowcasting the occurrence of microburst, wind shear etc for aviation. Low value of spectrum width shows uniform wind flow and high value of spectrum width shows turbulence.

  12. Range Height Indicator (Reflectivity) It is another widely used form of display of base products. A display is generated with the range on the X-axis and the height on the Y-axis. It gives height of the system at different ranges. RHI (Z)

  13. Volume Velocity Processing_2 This product provides the vertical profile of the horizontal winds over an area of radius 40 km from the DWR. VVP_2 provides a very good estimate of the winds in the surrounding areas.

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