Thursday, December 16, 2010

How Radar VCPs Really Make a Difference

Since my post(s) talking about different volume coverage patterns or scanning strategies on the NEXRAD radars, I've been asked by several people just what the big deal is about these different VCPs. Why do we care what scanning strategy the radar is using?  And is there really that big of a difference between clear air and precipitation mode?

To answer this, I scoured through my collection of saved radar images to find examples of times when radars changed VCP to illustrate what the difference is between different scanning strategies, particularly between a precipitation and a clear air mode.  Just to review, what makes clear air mode different?
  1. The radar spins more slowly and uses and sometimes uses slightly longer pulse (longer in VCP 31 than in VCP 32--the two clear air mode VCPs) which give it greater sensitivity to reflectivity measurements.  The receiver assembly itself is also tuned to be more sensitive.
  2. The radar has fewer vertical tilts, so it's only focused on the lower parts of the atmosphere.  Even so, the average clear air mode VCP takes around 10 minutes to complete because of the slower rotation rate.
  3. Because of the longer pulse length, the Nyquist velocity (the maximum unambiguously detectable velocity) is lower in VCP 31 (where the Nyquist velocity is only 25 knots.  In VCP 32, this velocity is around 53 knots).  This creates difficulty in measuring wind speeds in high wind events.
  4. Something I haven't mentioned before--because we are trying for increased sensitivity in clear air mode, the receiver is tuned to receive a lot more return power than it normally would (since it takes a lot more power to see finer structures of very low reflectivities).  As such, the receiver tends to saturate with power if the return gets to be too much. Because of this, there's actually a maximum reflectivity value detectable in both clear air mode VCPs.  Any return higher than that won't be measured.  So if you're using a clear-air VCP to measure a hail core (something that's going to return a LOT of power...), the actual maximum reflectivity of the hail core would not be measured--the radar would saturate at its threshold value and that's the maximum value you would see. (I'm not sure about the exact value of this threshold...I'll have to look it up).
Anyhow, here are a couple images to illustrate how the different fields change switching between precipitation and clear-air mode.  First is a slide with the base reflectivity, base velocity, and spectrum width images from the Fort Drum, NY radar on May 5, 2009.  This is when the radar is in VCP 121--precipitation mode.
Fig 1 -- Base moments from the KTYX radar at 1931Z on May 5, 2009.
Refectivity is on the upper left, spectrum width is on the upper right and velocity is on the bottom.  Now, five minutes later on the next volume scan, the meteorologists controlling the Fort Drum radar had changed to clear air mode--VCP 32.
Fig 2 -- Base moments from the KTYX radar at 1936Z, May 5, 2009.
 The differences between the two images in terms of sensitivity are actually pretty significant.  To compare them, I like to click on the images and open them in new browser tabs and then switch between the two tabs.  You can really see what a difference the increased sensitivity makes.  Not only are the finer details of the precipitation resolved, but the aerial coverage of the precipitation area is increased.  Furthermore, because this is VCP 32 (which still has a rather high Nyquist velocity), the increased sensitivity gave a much more coherent picture of the velocity field.  Notice how the spectrum width values (which can be thought of as a measure of how uncertain the velocity measurements are) decreased significantly once the VCP switched to clear air mode--and, consequently, the velocity measurements became much smoother.  Using a clear air VCP in this case works because--
  1. This is a slowly-evolving event--these look like showers and stratiform rain and there is no fast-developing convection here.  Therefore the 10 minute update cycle of a clear air VCP is acceptable.  In a fast-developing convective environment, a lot of important things can happen in those 10 minutes...
  2. Also as a consequence of this not being a very convective environment, the depth of these precipitating clouds is not expected to be too deep.  Therefore we can get away with having fewer vertical tilts of the radar in a clear-air VCP.
  3. The maximum reflectivity values we saw even in precip mode were not that high--certainly no hail cores or very heavy downpours.  Therefore we're not as worried about reaching that saturation reflectivity value and having inaccurate estimates of reflectivity (and consequently rainfall).
Since this was VCP 32 (which has a higher Nyquist velocity than VCP 31), we're not as concerned with the velocity estimates.  But, let's look at another case where a radar went from precipitation mode to VCP 31.  We'll start with the reflectivity measurements from the Oklahoma City radar (KTLX) in VCP 21 (precipitation mode) on January 27, 2010.
Fig 3 -- 0.5 degree base reflectivity from KTLX at 1549Z on January 27, 2010.
We see some light showers in eastern Oklahoma.  The reflectivity values certainly aren't very high and we're not expecting these to be rapidly-evolving or convective showers.  So, the switch was made to VCP 31.
Fig 4 -- 0.5 degree base reflectivity from KTLX at 1555Z on January 27, 2010.
Because it uses a longer pulse, VCP 31 has much more sensitivity than even VCP 32.  You can see this increased sensitivity dramatically shown in the two radar images above.  Note how much the areal coverage of the precipitation increases--it's probably lightly raining (or snowing!) in many more areas than the radar implied in VCP 21.  Notice that we also picked up more clutter near the radar, though.  That's another hazard of clear-air mode VCPs--they are more sensitive to everything--not just precipitation.  So if there is clutter, you'll tend to see MORE clutter in a clear-air mode VCP as opposed to a precipitation mode VCP.

Of course, remember that to get this very high sensitivity by using a longer pulse, the radar in VCP 31 sacrifices its ability to resolve high velocities well.  As a result, compare the velocity image before:
Fig 5 -- 0.5 degree base velocity image from KTLX at 1549Z on January 27, 2010.
To the velocity image after the switch to VCP 31:
Fig 6 -- 0.5 degree base velocity from KTLX at 1555Z on January 27, 2010.
The increase in areal coverage is once again very noticeable.  But where we had a very nice, coherent velocity field in VCP 21, we now suffer from some chaotic-looking velocity measurements in VCP 31. Since the wind speed above the ground are probably well over the Nyquist velocity of about 25 knots in VCP 31, the radar starts aliasing the velocities (note how we start getting alternating bands of red and green instead of simply two coherent blobs of red and green on either side of the radar.  Trying to sort out actual velocities from this image can be difficult (but not impossible).  But, if these really are just light showers, then we're probably not as concerned with the wind field.  In that case, the enhanced reflectivity sensitivity of VCP 31 may be desired, particularly if we're trying to find where exactly it's raining--even only a little.

So there are a couple of graphical examples of the difference between clear-air and precipitation VCPs.  I hope this helps those people who were asking me about this to better visualize the difference.  As always, if you have any questions or comments, please feel free to email me at

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