Ground-Based Radar Flashcards
Map of ground radar systems in Canada
- Outer white circle is 240-250km from the station, called the CAPPI Zone (Constant Altitude and Planned Position Indicator)
- Inner gray circle is 120km from the station, known as the Doppler Radar Zone
The intensity of the return information you get from a radar is proportional to:
The number, size, and reflectivity of the particles or objects the beam hits
A large cluster of wet snow will be more efficient at reflecting the microwaves than a single dry snowflake (obviously)
Little tiny rain drops will reflect less than big fat raindrops
What are the three methods for obtaining the information from ground-based radars?
- Plan Position Indicator (PPI) (has mostly been phased out)
- Constant Altitude PPI (CAPPI)
- Doppler Radar
Description of a PPI (Planned Position Indicator) type of ground-based radar
- Oldest, simplest, and fastest display system
- Radar is elevated at a fixed angle (usually 3°) and rotates 360°
- Can be difficult to interpret as the radar beam gets higher and higher the farther away from the emitter it gets, so it wont see rain below that exact beam angle it is set at. Because of the curvature of the earth, eventually the beam just shoots off into space
- Only shows a narrow band of info
At an elevation angle of 3°, a ground-based radar beam would be approximately __ meters above the ground at 1km. What about at a distance of 240km?
50 meters
12 km off the ground at 240km
What is the CAPPI system for displaying ground-based radar info?
- Constant Altitude PPI.
- Unlike the PPI, it scans the sky horizontally and vertically (from 0.3° up to 30° or more)
- Takes a full ten minutes to do all the scans it does, so info has a ten min lag once you get it
- Can have a problem with attenuation or false returns due to ground clutter (usually becomes a problem after the 120km (doppler) mark)
What is the difference between Doppler Radar and PPI/CAPPI?
- Doppler can sense reflectivity as well as radial velocity (the speed at which objects are moving towards or away from the radar)
- Because it is so sensitive, Doppler Radar can detect very small targets like dust or insects if there is enough of them
- Doppler radar is a very powerful tool in severe weather situations
What types of things can give incorrect returns on a radar?
- Ground clutter
- Blocking beams
- Overshooting beams
- Electromagnetic interference
- Virga
- Attenuation
- Seasonal variations
- Anomalous Propagation
How do you know you are looking at ground clutter on a radar?
The radar returns don’t move when you move through the series of loops/playback (note the intensities can change through the playback as the radar beam sweeps upward, but they wont move horizontally across the map)
In what radar area will you typically see ground clutter on the radar?
Past the 120km mark, in the CAPPI ring. The doppler area will not display static returns as it knows it is ground clutter and so doesn’t display it
What are blocking beams?
The radar beams get blocked by mountains (the dark triangles in the image below)
What is anomalous propagation?
- When you put a straw in a glass of water and the straw appears to bend
- The more dense the air, the slower the beam travels and the more the beam bends downward towards the earth
What would be an extreme example of anomalous propagation?
The air near the ground is so cold and dense that a radar beam which starts out moving upward is bent all the way down to the ground.
This produces a strong echo at large distances from the radar and they returns appear to move around erratically. AP will usually also be deleted from the doppler area and be seen in the CAPPI ring
What are overshooting beams?
The radar beam does not return precip that is below the beam. The beam ‘overshoots’ the rain.
Why can virga cause incorrect or false returns on the radar?
The beam hits the rain and reports that the rain goes down to the surface, when it does not.
But even if you visually cant see any rain, intense downdrafts are found below virga
