Permanent Echoes
Every radar is subject to permanent echoes i.e. backscatter and reflections from permanent obstacles in the field of illumination of the beam, such as trees, hills and buildings.
Radars located in metropolitan areas have extensive P.E.
clutter, which can make tracking significant weather very difficult.
Elimination of the P.E. pattern is therefore highly desirable and a
number of techniques are used for this. Clutter removal techniques will be discussed in the module on radar algorithms.
Example of the radar signature of a wind farm north of the Milwaukee, WI radar site
Wind farms can impact Doppler radars in three ways if the turbine blades are moving and they are within the radar's line of sight. If close enough (within a few kilometers) they can partially block a significant percentage of the beam and attenuate data down range of the wind farm. They can also reflect energy back to the radar and appear as clutter (AP) on the radar image and contaminate the base reflectivity data. The reflectivity data is used by radar algorithms to estimate rainfall and to detect certain storm characteristics. Finally, they can impact the velocity data, which are also used by radar operators and by a variety of algorithms in the radar's data processors to detect certain storm characteristics, such as mesocyclones, relative storm motion, turbulence, etc.