Happy #WeatherWednesday everyone! Last time we talked about 5/27/1997 as a 20th-year-anniversary special post. Today we get back to a more technical lesson!
Radio Detection And Ranging, or “radar” for short, is our primary method of detecting precipitation type, location, intensity, and even more. The type of radar we will discuss today is the most typical radar used by NWS and media stations: the Doppler radar.
Doppler radar is composed of many different parts including a radar dish/antenna, transmitter components to produce electromagnetic waves, and wave guide/ feed horn to direct the EM waves into the dish for reflection into the atmosphere. It’s normally all inside an elevated white dome on a tower. The dish amplifies the wave to make it stronger and last longer. The radar beam can be directed in every direction horizontally and can be tilted to conduct a volume scan. That gives a basic picture of what a radar does minus the bloody details of EM pulse generation.
What purpose does radar serve to the weather community? In the mid 20th century, the US discovered that its military technology used to detect enemy naval and aircraft could also be used to detect snow, showers and storms, hail and more! Falling hydrometeors actually act as somewhat of a wall to emitted EM radiation and the “backscatter” gets returned to the antenna as an “echo”. Some of the pulses from the radar still makes it through rain or storm cells and can detect even more precipitation farther away. When too much of the pulse has been absorbed and scattered by heavy rain or hail, “attenuation” describes the weakness and inaccuracy of radar observations.
The radar will emit a pulse at a certain PRF (pulse repetition frequency) and then “listen” for an echo. The amount of time it takes for the echo to reach the radar determines the location of the weather target. The value of the backscatter radiation also determines the intensity of the weather target. More radiation received by the radar means those oranges, reds, and purples on the television screen during weather coverage!
The country is fairly well covered in the range of radars used by NWS, especially in more populated areas in the eastern 2/3rds. They are known as WSR-88Ds and have a multitude of capabilities beyond detecting hydrometeors. They can also detect wind velocity, precipitation shape (is it rain or is it hail or is it tornado debris?), storm heights, estimate rainfall rate, etc. Presumably, these will last another few decades with routine evolutions to improve the way we detect storms. It can be expected that in the future, satellites or more powerful new radar called phased array will replace the current Doppler system.