VIL values will change with the seasons and from location to location. For instance, warm, moist air masses  exhibit  higher  VIL  values  during  severe weather occurrences than cooler, dry air masses do. Therefore, the product must be tailored to the area of interest. In addition, strongly tilted or fast-moving storms  may  produce  unrepresentative  VIL  values. VAD Wind Profile (VWP) Product One  of  the  most  useful  and  unique  products generated by the WSR-88D is the VAD Wind Profile (VWP). VAD stands for velocity/azimuth display, and this program produces current upper wind information on a continuous basis. The VAD Wind Profile is like having real-time rawinsonde data at your disposal every few minutes. It shows wind velocities at various altitudes above the earth in l,000-foot increments, up to 70,000 feet (fig. 2-39). VWP  provides  easy-to-read  climb  winds  for  pilot briefings. The wind plots of VWP are similar to the wind plots on the Skew-T diagram and are updated each volume scan. VWP depicts environmental flow around  the  RDA  and  provides  valuable  insight  into general circulation patterns. Be aware that changes in the vertical wind profile are clues to the location of turbulence, inversions, boundary layers, and shear. They’re also helpful in determining frontal slope, thermal advection, and the evolution of jet streams. Different colors are used to indicate the reliability of  the  wind  data.  For  instance,  green  wind  barbs indicate that plenty of scatterers were available at that particular azimuth and elevation, thus highly accurate wind values are displayed. VWP is unique in that the mere absence of data (plotted as “ND”) can itself be an indicator. It suggests fewer scatterers, which implies dryer air (common behind cold fronts). This same technique also allows operators to view changes in overall  cloud  coverage  above  the  radar’s  viewing  area. REVIEW  QUESTIONS Q56. Products from the WSR-88D are grouped into what three areas of application? Q57. What is the purpose of the Routine Product Set (RPS) list? Q58. What is the purpose of an alert-paired product? Q59. What does the Base Reflectivity product (REF) display? Q60. Q61. Q62. Q63. Q64. Q65. A hook echo signature is associated with what phenomena? What does the color blue represent on the Base Velocity  product  (VEL)? Velocities  from  the  Base  Velocity  product  (VEL) are relative to what major component of the WSR-88D? What  is  the  most  serious  drawback  to  the Composite Reflectivity (CR) product? What is the primary purpose of the Vertical Integrated Liquid (VIL) product? What might a lack of data indicate when using the VAD Wind Profile (VWP) product? Echo Tops (ET) Product Echo Tops (ET) is yet another specialty product derived  from  base  reflectivity.  Its  simplistic  display makes  for  easy  interpretation,  but  also  limits  the product’s usefulness. The ET product indicates heights of echoes, in hundreds of feet above mean sea level (MSL). The ET product  shows  only  the  highest  echoes  over  a  given location and uses different colors to represent heights (fig.  2-40).  Targets  returning  less  than  18.0  dBZ (clouds) are completely omitted from the display. ET provides forecasters with a valuable “first look” tool. It allows   them   to   monitor   the   highest   echo   tops throughout  the  radar  viewing  area.  Echo  heights  paint  a very  good  picture  of  current  weather  and  make excellent indicators of things to come. For this reason, ET is of particular interest to forecasters and aviators. For example, rapidly increasing echo tops might warn of storm intensification, while decreasing tops indicate weakening. A rapid collapse of echo tops may indicate the onset of downburst conditions at the surface. Other products should be used for additional guidance in understanding the cause of such changes. The  ET  algorithm  does  not  tally  a  column’s reflectivity like VIL, nor does it depict the strongest returns like CR. ET merely depicts the highest echoes observed. The ET product can also be deceiving. Plotted heights are not actual cloud tops, but rather the tops of  precipitation.  The clouds themselves are probably less than 18.0 dBZ, and thus extend well beyond the ET heights. 2-43