Region  products  than  in  the  Mean  Radial  Velocity product. Due  to  beam  broadening  and  an  average mesocyclone size of 2.7 nmi, the range of detection is generally  limited  to  about  124  nmi.  Mesocyclones formed in vertical wind shear may not be detected beyond 38 nmi due to the small size vorticity maximum found   at   low   levels,   and   initial   formation   in   a precipitation-free   environment. Multiple  mesocyclone  cores  can  form  within  a storm complex, but not in the same cell. WIND  SHEAR A  major  hazard  to  aviation  is  the  presence  of low-level wind shear. Wind shear may result from a variety  of  phenomena  such  as  synoptic  fronts, boundaries  associated  with  thunderstorms,  terrain,  and nocturnal  inversions. Recognition of Wind Shear Wind shear will appear as a discontinuity in the doppler velocity field and is frequently associated with wide spectrum widths. Monitoring changes in the velocity field over time is likely the best approach to recognition  of  wind  shear  phenomena.  Low-level  wind shear can be detected close to the radar in the Mean Radial Velocity product. An indication of extreme shear will show up in a narrow band in the velocity field. This narrow band will likely not be identified in the Velocity Azimuth  Display  Wind  Profile  product.  However,  this product can be useful in keeping track of significant wind speed and direction changes within about 16 nmi of the radar and provide an indication of shear in the vertical. Variations in the spectrum width field are related to the mean wind shear across the radar beam; therefore, a shear layer(s) will usually show up as an enhanced area in the Spectrum Width product. Values of 14 kts, or higher, are usually associated with significant wind shear or turbulence, or both. Considerations The Combined Shear product could be useful for the detection of areas affected by strong wind shear. The product,  however,  tends  to  be  “noisy”  and,  in  the absence of strong signatures, may be of little use. The product does not take vertical shear into account. For indications of shear in the vertical, the Mean Radial Velocity or Spectrum Width Cross Section products may be  useful. CLOUD LAYERS The sensitivity of the WSR-88D provides the user the   capability   of   detecting   cloud   layers.   This information has application to such things as aviation forecasting   and   forecasting   the   evolution   of precipitation. The WSR-88D can detect large ice crystals that are present   in   middle   and   high-level   clouds.   These reflectivities may range as high as plus 20  dBXe.  The Velocity  Azimuth  Display  algorithm  will  use  doppler velocity measurements from middle and high level clouds  to  generate  profiles  into  the  middle  atmosphere. These  wind  profiles  allow  the  operator  to  monitor  the movement of synoptic and smaller scale waves and troughs. Recognition of Cloud Layers With the high sensitivity of the WSR-88D, it is possible to obtain reflectivity estimates of elevated clouds,  which  generally  reflect  at  levels  between  minus 12 to plus 5 dBZe. The depth of cloud layers maybe inferred from two processes. A four-panel display of the Reflectivity product for successively higher elevation scans can provide information on the depth as well as the structure of a layer. The top of a cloud layer and its depth can also be  inferred  from  a  Reflectivity  Cross  Section  product, depending  on  the  distance  from  the  radar  and  the viewing  angle.  However,  resolution  is  better  with  the Reflectivity product than the Cross Section product since  the  cross  section  integrates  returns  from  the surface to 70,000 ft. The following is a technique that will allow the determination of the top, base, and depth of a cloud layer.  Using  the  Reflectivity  product  for  the  elevation that intersects the cloud layer, place the cursor at the point where the base appears to be and get a readout on the screen of azimuth, range, elevation, and height. Then, place the cursor at the apparent top of the cloud layer and get the same information. The depth can be computed  from  the  difference  in  heights.  If  the  cloud base or top is not uniform, this technique will have to be repeated  several  times  to  get  average  heights  and thicknesses. This will also help ensure that the true radar cloud top and base are being observed. 12-10