Absorption The  atmosphere  absorbs  some  amount  of  EM energy, mainly by oxygen and water vapor. It becomes trapped  within  these  parcels  long  enough  to  become unrecognizable to the radar. Absorption, like any form of attenuation, results in lost power and decreased radar  performance.  Like  scattering,  the  degree  of absorption  is  dependent  on  particle  size,  particle composition,   and   wavelength.   The   longer   the wavelength,   the   smaller   the   attenuation   due   to absorption. Solar Effects Due  to  the  high  sensitivity  of  the  WSR-88D, anomalous returns near sunrise or sunset may occur. These false returns are generated because the sun radiates   energy   in   the   same   region   of   the electromagnetic spectrum as the WSR-88D. These echoes are recognized by their continuous, narrow "baseball  bat"  appearance. Q25. Q26. Q27. Q28. Q29. Q30. Q31. Q32. REVIEW  QUESTIONS What is refraction? What atmospheric parameter has the greatest impact on refraction and refractivity? What   normally   happens   to   N-units   with increasing altitude? If a radar beam is consistently overshooting targets, which refractive condition might be present ? The phenomenon of ducting is most likely to occur under what atmospheric condition? What is the cause of ground clutter? What type of scattering is NOT attenuation? What are the main absorbers of EM energy in the atmosphere? PRINCIPLES OF DOPPLER RADAR LEARNING OBJECTIVES: Define Doppler and Doppler shift. Define phase shift and radial velocity.  Recognize  the  effects  of  target  motion on radial velocity. Define velocity aliasing. Recognize the effects of velocity aliasing on Doppler  radar.  Compute  Nyquist  velocity. Identify  Doppler  dilemma. So  far,  we  have  discussed  basic  principles  of electromagnetic  energy  common  to  all  radar  systems. The  following  text  expands  on  the  theory  and principles already discussed and introduces concepts unique to Doppler radar. Doppler  is a means to measure motion. Doppler radars not only detect and measure the power received from a target, they also measure the motion of the target  toward  or  away  from  the  radar.  Although Doppler radar enjoyed widespread use for many years, cost made it an impractical tool for weather detection. Only   recently   has   expense   been   offset   by   the technological   breakthroughs   of   the   Doppler meteorological radar, WSR-88D. This shore-based radar has capabilities that far exceed those of older Doppler   systems.   These   capabilities   include   a complementary  mix  of  velocity  detection, increased power and sensitivity, and the integration of high tech computers. This automation provides forecasters with a wealth of information. The WSR-88D not only can detect target motion and velocity, but can also examine internal   storm   circulations   as   well   as   detect atmospheric motions in clear air. The WSR-88D excels in detecting severe weather events,   and   more   important,   increases   advance warning time. In addition, the increased sensitivity of the   WSR-88D   allows   various   meteorological boundaries to be identified. These boundaries include synoptic fronts, gust fronts, drylines, land and sea breeze fronts, and thunderstorm outflows. DOPPLER SHIFT In 1842, the Austrian physicist Johann Christian Doppler first related motion to frequency changes in light and sound. Doppler discovered that the shift in frequency  caused  by  moving  sources  of  sound  was directly  proportional  to  the  speed  of  the  source.  He then developed mathematical formulas to describe this effect called the Doppler Shift. While not given much thought,  you  experience  Doppler  shifts  many  times each day. The change in pitch of a passing train whistle and  a  speeding  automobile  horn  demonstrate  its effects. When you hear a train or automobile, you can determine  its  approximate  location  and  movement. Doppler radar accomplishes much the same thing, but to a higher degree of accuracy. As a target moves toward a radar, frequency is increased; if the target is moving away from a radar, the frequency is reduced. The radar then compares the received signal with the frequency of the transmitted signal and measures the 2-16


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