The NOAA NEXRAD radar network has a set of protocols for how the radars scan the environment. Each WSR-88D radar (this is their model name--it stands for Weather Surveillance Radar-model 1988 Doppler) has a set of nine pre-defined scanning "patterns" it can use to scan. These patterns define a series of elevation angles, or "tilts", the speed at which the antenna rotates and the length of radar pulse that is used. These are called "Volume Coverage Patterns" or VCPs. The nine available VCPs can further be subdivided into "precipitation" and "clear air" VCPs. When no precipitation is expected, the radar doesn't need to scan as frequently or with as much vertical resolution, but it does need to be sensitive to pick up any small disturbances in the atmosphere that could signal the beginning of precipitation. So clear air VCPs tend to take longer to complete, have fewer elevation tilts, but have more sensitive pulse patterns.
The vast majority of the time, when there's no precipitation going on, the radar will be left in VCP 32--one of two clear-air VCPs. (The 3 in the number represents clear-air mode and the 2 represents the "number 2" pulse pattern). Take a look at a map of what VCP all the radars were in this afternoon.
|Fig 1 -- CONUS NEXRAD VCP status from 2048Z, Nov 28, 2010. From the WDSS-II website.|
But what about where things are going on? Currently the biggest weather story is areas of snow associated with a shortwave moving through Montana eastern Idaho and northern Utah. Some of the heaviest snows are occurring in central Montana, between Billings and Helena. But look at the status map above--both the Billings and the Great Falls radars are in VCP 31. But--31 (since it begins with a number 3) is a clear-air mode VCP. Why are we using a clear-air VCP when precipitation is falling?
Let's take a step back and look at what these clear-air VCPs are like. Below is a diagram of the scanning pattern of VCPs 31 and 32.
|Fig 2 -- Scanning pattern of NEXRAD radars in clear-air mode. From the NEXRAD Radar Operations Center website.|
However, if we're not expecting deep thunderstorms, all we're really concerned about is what's near or falling toward the ground. Therefore all we need are these low elevation angles, and that's why clear-air mode VCPs only have low-level elevation angles. There are other differences in clear-air mode. For instance, the radar antenna spins much more slowly than in precipitation mode. This allows us to spend a longer time looking at things and increases the accuracy of measurements. However, the trade off is that the radar takes a much longer time to finish a full cycle of elevation tilts--10 minutes in clear air mode as opposed to four and a half minutes in the fastest precipitation mode VCP.
What about the difference between VCP 31 and VCP 32? VCP 32 uses "short" pulses and VCP 31 uses "long" pulses. A doppler radar doesn't scan with a continuous beam of energy. It uses short pulses of emissions to measure both velocity and reflectivity. For a given volume of space, the radar takes the average return of several of these pulses to obtain the reflectivity measurement for that area. How long each pulse lasts has an effect on these measurements. A longer pulse length (leaving the beam "on" for longer during each pulse) allows for a lot more energy to interact with each volume of space and, by taking a longer time, increases our sampling of reflectivity and in turn provides a much more accurate and more sensitive reflectivity measurement. Basically, a longer pulse length (like in VCP 31) gives a much more accurate, much more coherent, much more sensitive reflectivity measurement, particularly in areas of low reflectivity to begin with. Here's an example of the VCP 31 image of snow from Billings, Montana.
|Fig 3 -- Base Reflectivity from Billings, Montana, KBLX radar, 2019Z, Nov 28, 2010.|
So why don't we use VCP 31 more often? If longer pulses provide a better reflectivity measurement, why would we ever use shorter pulses (like in VCP 32)? Unfortunately longer pulses have a trade-off. It turns out (for some rather technical reasons) that the longer the pulse time, the less accurate the velocity measurements become. For a given pulse length, there's a maximum unambiguous velocity that can be detected called the Nyquist velocity. If velocities become larger than the Nyquist velocity, the radar (as it's designed) can't tell what the correct velocity should be (though we can make a good guess based on continuity with the surrounding field, and there are algorithms that do this).
For VCP 31, the Nyquist velocity happens to be about 11.5 meters per second, or about 26 miles per hour. This is incredibly low when we start looking at wind speeds. We regularly see winds of well over 26 miles per hour at just a few hundred feet above the surface. Take a look at the velocity image that corresponds with the above reflectivity image.
|Fig 4 -- Base Velocity from Billings, Montana, KBLX radar, 2019Z, Nov 28, 2010.|
I mentioned that there are some algorithms that try and fix that by guessing at the correct velocity measurement. If we try applying one of these algorithms to the image above, we get this:
|Fig 5 -- Base Reflectivity from Billings, Montana, KBLX radar, 2019Z, Nov 28, 2010. With velocity dealiasing applied.|
So... in summary:
- NEXRAD radars have nine different scanning patterns or VCPs, separated into clear air and precip modes.
- Clear air VCPs are more sensitive and scan more slowly, but they have fewer elevation angles and take much longer to complete a full scan than precipitation VCPs.
- Longer pulses mean more sensitive reflectivity measurements, but much lower maximum unambiguous velocities.
- When velocities exceed the maximum unambiguous velocity (the Nyquist velocity), the radar can misinterpret the true wind velocity which results in velocity aliasing.
- Algorithms can try to correct velocity aliasing, but they don't always get it right
There's a whole lot more about choosing the correct VCP for the situation--after all, there are nine total. I'm sure in future blog posts I'll talk about other VCP differences.