The Basics of Doppler Radar

The Doppler on Wheels (DOW) is spending some time on Long Island to do what our local National Weather Service radar can't-- if the weather won't come to the radar, then the radar can go to the weather! Radar is most commonly known since the 1940's for indicating where there are areas of precipitation and how heavy that precipitation is. However, there are a lot of other complexities associated with understanding radar data as well much more information besides precipitation characteristics that can be gained. Let's explore some frequently asked questions encountered since the DOW has been on campus.

How does a radar work?
A radar sends out pulses of energy at a specific frequency and "listens" for its return, or echo. If the pulse of energy encounters an object (raindrop, cloud droplet, insect, building, etc.) then part of the energy will be scattered back, or reflected, to the radar. A general rule of thumb is that the larger the object is or the more densely-packed the objects are, then the stronger the echo, or reflectivity, will be. What is interesting about Doppler radar is that it can also keep track of the phase (shape, position, and form) of the energy so that it can tell how it has changed phase to indicate whether the object intercepted is moving towards or away from the radar. This provides a very useful and less widely known product known as "Doppler velocity" which can provide a detailed picture of how the winds are moving near the radar, specifically inbound and outbound.

So if you have the NWS radar, why do you need the DOW exactly?
Firstly, the DOW has a much higher spatial and temporal resolution than the NWS radar. Radars send out a pulse of energy at a specified angle relative to the ground. Through geometrical relationships, the farther the pulse travels from the radar than the higher the pulse will be above the ground. This can cause problems for forecasters when a storm is far away from the radar but the radar can only "see" the very top of it. For this reason, the DOW is a very important tool for trying to look at phenomena that may be just out of the NWS operational radar's range. Operational radars scan a complete circle at a constant elevation angle and after completing a complete rotation, move up in elevation. This takes about 5 minutes to complete rotations for all elevation angles. The DOW has the capability for the user to have complete control over doing circular scans or vertical slices, or even a combination of the two. Therefore, if you have one storm off to your south, you can focus the radar on just that sector.

What sort of data are you interested in from the DOW?
The DOW will be positioned at various points across Long Island to get a good "view" of whatever phenomenon is being studied at the time. When positioned at the Floyd Bennett Field site, parts of NYC can be measured. When positioned at a site on the North Shore, storms over the Sound can be measured. The most important data that will be collected using the DOW will be reflectivity and velocity. The field campaign will target any convection (storms) that fire up around Long Island to study their evolution in detail. This includes looking at the reflectivity data to understand the types of hydrometeors (rain, hail, etc.) that exist within the storms and velocity data to understand the complex winds and circulations within the storms. The students don't get to rest on clear days thanks to an interesting coastal phenomenon known as a sea breeze. The temperature difference between the land and ocean (or Sound) can cause the winds to converge over Long Island, rise, and sometimes form clouds. The DOW will be used to measure the reflectivity of the clouds formed to understand their vertical depth and motion and the velocity data will be used to show the wind shift at the convergence boundary. 

For more information on how Doppler radar works please visit the following sites:
- http://www.crh.noaa.gov/mkx/?n=using-radar
- http://www.srh.noaa.gov/srh/jetstream/doppler/doppler_intro.htm