Sunday, November 28, 2010

Snow in VCP 31

Back from my Thanksgiving break trip to Portland, Oregon.  I've had a few requests to talk a bit more about weather radar interpretation.  Radar interpretation has long been an area of interest of mine, and I worked in that area of study for the past two years.  So today I'm going to look at some snow observations with radar.

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.
Most of the radars in the eastern half of the country are in VCP 32.  This is very normal, and is usually the case when not much is going on with the weather.

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.
In the chart above, the radar would be located in the lower left corner.  As we go out in range from the radar, we move to the right.  Each color represents a different "tilt" of the radar--the radar dish will make on complete circle at each tilt before moving up to the next highest tilt.  After it finishes the highest tilt, the radar goes back down to the lowest tilt and starts all over again.  The red numbers at the right of the chart represent the elevation angle in degrees of the center of the beam.  The radar beam spreads out as it moves away from the radar--that's why the colored wedges spread out.  On the left we can see the height above ground.  Anywhere in white is not scanned by the radar.  You can immediately see some of the difficulties with this scanning pattern.  The area directly over the radar is not scanned at all.  This region is referred to as the "cone of silence".  In a clear-air pattern like this one, the cone of silence is particularly large.  During severe thunderstorm events on the plains, thunderstorms can often top out at over 50,000 feet tall.  According to this chart, we wouldn't even begin to scan the tops of these thunderstorms (in clear air mode) until we got over 80 miles from the radar.  That's a huge area of no good coverage.

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.
Note how much structure and detail can be seen in that reflectivity image.  This comes from the lowest tilt (0.5 degrees).  The details of bands of snow and regions of enhanced snowfall really show up in this image.  Since snow in general has a much lower reflectivity than rain, the more sensitive scanning strategy of VCP 31 picks it up much better and with more detail than we would see if we used a "precipitation" mode VCP.

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.
Remember in radar velocity images that cool colors (green) indicate places where wind is blowing toward the radar and warm colors (red) indicate places where air is blowing away from the radar.  Notice how we generally have cool colors to the north and warm colors to the south, indicating air blowing from the north to the south.  But what about these blobs of the opposite color in the middles of these regions?  Is there really this big jet of air moving the opposite direction in the middle of the general flow?  Not at all!  These are areas where the wind velocity exceeds the Nyquist velocity and the radar has incorrectly assigned velocity values for that region.  Any time you see these random large blobs moving in the opposite direction as the surrounding flow, you must suspect that the velocity is exceeding the Nyquist velocity.  When the radar mis-assigns these values, it's called "velocity aliasing".

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.
Note how the algorithm has fixed much of the velocity field so that the colors are more coherent.  There's still an area to the south of the radar (the green blob in the middle of the red) where the algorithm didn't quite work.  It's an improvement, but it's not perfect.

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
So this is why we use clear-air mode VCP 31 for snow--its enhanced reflectivity measurements due to the longer pulses give us a better idea about the structure of snowfall, particularly since snow has such a low reflectivity to begin with.  Since we aren't worried about having fast updates (there are no fast-developing convective storms) and we're also not worried about very accurate velocity measurements, we can sacrifice time and velocity measurements to get the good reflectivity provided by VCP 31.

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.

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