Doppler Radar and Non-Meteorological Targets

One of the cool side benefits of the WSR-88D radars is the ability to detect non-meteorological targets.  Smoke from wild fires, flocks of birds, swarms on insects and even debris from tornadoes can all be seen on radar given the right circumstances.  On Saturday, August 2, 2014, the kids had gotten up extra early, presumably from the excitement of leaving for vacation the next day.  When I sat down at the kitchen table, I noticed that my radar program was still on from the night before.  Gulping down my morning coffee in an attempt to wake up, I noticed something pretty cool.  Here is a time-lapse sequence of what I saw....



See those expanding rings in northeast Dallas and down by Waco?  These are known as Roost Rings.  These rings are actually thousands of birds leaving their nests in their morning hunt for insects.

One way that we can tell that these echoes are biological in nature and not some kind of strange precipitation is by using something called Correlation Coefficient (CC).  This radar product looks at how diverse the radar returns are.  If the values are homogeneous, the higher the CC value will be.  The more values are different, the lower the CC value will be. 

As an example, look at this:

This is a great example of normal precipitation.  The radar beam sees the rain as similar sized drops (more or less) and as a result, the CC values show up with warm colors (reds and oranges). 

Now, look at the roost rings:

The roost rings have very low CC values - which tells us that the objects the radar is seeing are not uniform in size and shape and, more than likely, biological in nature.

Although these roost rings are not uncommon, you usually only see them during the morning hours.  Why is that?  Well, atmospheric conditions play a large role.  During the mornings, there is usually something called a temperature inversion present.  This means that the temperature near the ground is cooler than the air just above it.  As a result, the radar beam gets bent downward (an effect known as superrefraction), making objects near the surface more visible..