[cobirds] Birds on Weather Radar (LONG, scientific, and somewhat off topic)

COBirders,

This is a LONG email in response to a few queries I received from both Facebook and privately after posting about the radar echoes that I deemed migratory/dispersing birds.  There is a lot to explain and I will try my best to break this down unscientifically (there are no equations in this email thankfully). 

Radar IS able to see birds. However, the concentrations must be phenomenal to even get a slight whimper of an echo on a radar screen. I use this example to prove my point. 

http://www.atmos.uiuc.edu/~snodgrss/geese.gif

You are looking at a radar loop from Lincoln, IL from March 9. There are six parts to this animation that I want to make clear.  The contiguous areas of dark blue through light blue then green and in some areas yellow are all precipitation.  I see four different contiguous areas of precipitation on this image.  That doesn't mean they are reaching the ground though, especially the precipitation in the northwest part of the imagery (north being toward the top of the screen).  There is a lot of speckling around the center of the image that is called "ground clutter" by radar meteorologists.  The ONLY part of this image that is birds is the "explosion" coming from Lake Springfield southwest of the Lincoln radar (middle of the image). Notice that the birds are very small specks on the radar image. Physically, there were thousands of geese taking off from this location at this time moving north. You may notice that as the birds theoretically gain altitude, the specks become smaller and fade in intensity (color: from green to blue). Geese are large objects and in high concentrations. 

Radar from the other day though didn't show a pattern like this.  Here is a link to the reflectivity imagery from Tuesday night: http://homes.comet.ucar.edu/~guarente/birdweather/18Sep13/media/graphics/looper_images/reflectivity/index.htm

The pattern showed a large green area coming off of the Northern Front Range and then another "blob" coming off of Pikes Peak, plus a green blob out east.  Around all of this there is a blue aura.  None of this echo is precipitation.  It doesn't look like what precipitation normally looks like.  Precipitation on this loop is only seen in Wyoming and west of the divide in Colorado.  Precipitation on this loop is the yellow and orange areas wrapped in green.  That's a typical view of convective precipitation (i.e., cumulus clouds, puffy cottony clouds that produce rain/thunderstorms).  In winter that pattern will change, but that isn't important right now.  Later in the loop though, there are other patterns of yellow and orange, but not really wrapped in green or blue.  They are fine lines compared to the large scale blobs and precipitation.  Those fine lines are actually wind farms and mountains.  They can show up some days and not others (depending on the vertical profile of temperature on that day/night).  

If the geese showed up on the Illinois radar loop as small specks, this must mean that all of these specks are birds, and that must mean there were millions of birds migrating over us.  Look out the window... they must be going over right now... I am being sarcastic.

Unfortunately the explanation of why the radar image is lighting up like this is difficult and scientific. There are multiple reasons why a radar might show echoes where there isn't even a cloud.  

One of them is warmer temperatures as you go up in the atmosphere can cause this to occur as well but that will happen in a donut shape (It is called anomalous propagation). 

Another is Insects amassing at dusk and dawn can also cause large enough echoes due to their exceptionally large concentrations. These can often be seen at night, but not experienced at the surface, because they are concentrated at higher altitudes. Fly in a hot air balloon sometime at night and you will often find remarkable concentrations of moths, mosquitoes, and other insects at higher altitude, and it isn't just because you have a fire above your head. 

Other reasons are dust, smoke, military activities, bats, and, the important to us, birds.  And to make matters worse, all of these things can be happening at once, confusing the issue.

You might think though that since insects can cause high enough concentrations to make a radar echo, then birds must be able to cause high enough concentrations or be large enough to cause an echo as well. Unfortunately, there is more science here that we need to discuss to answer this question. 

A radar operates by sending out a pulse of electromagnetic energy and then listens for the energy to return after bouncing off of whatever objects are out there. The amount of energy that comes back is often only about 6% of the amount of energy sent out. But the energy that is returned has to bounce off an object that is less than half the size of the wavelength of the electromagnetic pulse sent out by the radar (10 cm for all NWS radars) to produce proper backwards reflection and thus return of energy to the radar. Thus anything smaller than 5 cm (half the wavelength) gives a good echo, while objects larger than 5 cm and especially 10 cm begin to not scatter energy backwards due to the way that the energy reflects/scatters off of the object. 

Insects are close to the same size as raindrops, which are what a radar is made to see, and they are significantly smaller than 5cm. Insects and raindrops are in the Rayleigh scattering regime, which is the scattering that radar hopes to take advantage. Rayleigh scattering is great because a bunch of the energy reflected/scattered by the object comes directly back along the direction the initial energy came from, thus back at the radar.  Birds though are much larger than raindrops, and can often stretch into another regime of scattering called Mie or Geometric Scattering, where energy continues in the same direction as the original pulse of energy, often returning 0% of the energy in the backward direction or toward the radar. 

So aside from experience, how did I know these echoes were birds/insects/bats instead of precipitation?  Good question.  Most of the answer is experience, but you can also see that the birds will often fly in a different direction than the winds.  Insects will almost always fly in the direction of the winds, and bats tend to be very erratic, but don't venture very far from their home site.  Precipitation tends to be more concentrated rather than over a large area at this time of year (summer), and it will be higher values of reflectivity, plus it has to move with the winds since it has no ability to do otherwise.  If you see something that is out of sync with the rest of the pattern on the radar it could be a biological target.  

Having I been not giving you the whole story so far?  Yes, because I can make this easier.  Now that we have upgraded to Dual-polarization radars, there is a product that can tell us what the radar thinks it is seeing.  That product is called Hydrometeor Classification.  You can find it here: http://weather.cod.edu/satrad/nexrad/index.php?type=FTG-HHC-0-24.  In that image, the color scale is different types of precipitation and echoes that radars can see.  Here is what the legend means:

HA = Hail
GR = Graupel
BD = Big Drops
HR = Heavy Rain
RA = Rain
WS = Wet Snow
DS = Dry Snow
IC = Ice Crystals
GC = Ground Clutter/Anomalous Propagation
BI = Biological Targets (Birds/Insects/Bats)

ND = No Data

That imagery isn't perfect, but can give you a clue of what on the imagery is biological.  Be careful though not to call all the biological targets birds though.  You will have to test your skills a little more to understand which is which.  

If you want to test your skills tonight, I suggest hitting this page for the reflectivity loop: http://weather.cod.edu/satrad/nexrad/index.php?type=FTG-N0Q-1-24 then after you have decided where the precipitation is, and where the biological targets are, test your hypothesis by clicking to the Hydrometeor Classification: http://weather.cod.edu/satrad/nexrad/index.php?type=FTG-HHC-0-24

How did you do?

If you want to learn even more about weather radars, there is some great training out there for both meteorological and ornithological purposes.  I can suggest both: 

https://www.meted.ucar.edu/training_module.php?id=960 (meteorological, but includes biological targets as well; free registration required; I have no direct financial gain by directing you to the website of my employer)

and

http://virtual.clemson.edu/groups/birdrad/comment.htm (ornithological but can't avoid some of the necessary meteorology; free)

Please feel free to contact me via email privately if you would like more information as we are leaving the realm of birds and entering into the realm of meteorology, which isn't what this list is made for. 

Thanks for reading all of this (if you didn't fall asleep already).

 

Bryan Guarente
Instructional Designer
The COMET Program
University Corporation for Atmospheric Research
Boulder, CO