Clear Air Mode on Radar is More than Just for Weather

DISCUSSION: When observing the radar at night, large circles of reflectivity usually surround the radar scan area. It is likely that this reflected feedback from the radar is ground clutter, although in other cases you might see something else. When there isn’t any active weather within a radar coverage area, the local radar is set to a slower antenna rotation which causes the radar to have a finer resolution and heightened sensitivity. This setting is called “clear air mode”. During this mode of operation, the radar will pick up reflected ground clutter from mountains, buildings and tall objects as well as clear air echoes such as dust particles, light drizzle, air mass boundaries and fronts. Clear air mode is also used by scientists to pick up on certain echoes that many wouldn’t think of being a possibility. These specific clear air echoes are birds and insects. This has been beneficial to scientists who have used clear air mode to study bird and insect migrations for years.

When the radar is in clear air mode, it has an advantage of picking up on small objects such as insects and birds. This is because of its increased sensitivity and resolution. When birds or insects cluster together in larger groups, they become increasingly visible by the radar. The more birds and insects in one area, a higher reflectivity is shown in the radar image. Scientists can determine the difference between bird and insect reflectivity in a multitude of ways. For birds, it depends on the time of day. Most species of migrating birds continue their travel at sunset and fly through most of the night before landing at sunrise. Using radar velocity imagery, scientists can also decipher between birds or insects by comparing their movement to the prevailing winds. If the reflectivity moves against or faster than the wind, it could be birds. It is challenging to determine if the reflectivity is caused by insects because they fly with the wind rather than against it. For insects, it depends on the time of year and location more so than the time of day and speed. Many insects in the spring and summer tend to swarm near rivers, lakes, fields and ocean shores where they hatch, use up resources and gather to reproduce.

In the image above, a radar signature posted by NWS Norman, Oklahoma shows grasshoppers and beetles as they move over agricultural fields in Quanah, Texas on July of 2015. The radar shows the movement of these insects to be northeast in the direction of Oklahoma. It is typical for grasshoppers and beetles to migrate from one area to another using up resources and then moving on to find more food elsewhere. Grasshoppers are especially one of the most burdensome migratory insects to agriculture. They eat most of the grain, cereal, tomato and onion crops until there is nothing left.

Radar is used in detecting density, location, direction, and speed of birds and insects. A study done by Dr. Sid Gauthreaux and Carroll Belser was able to quantify bird migration using radar reflectivity by interpreting magnitude of bird migration in terms of radar values measured in dBZ (decibels of Z, where Z represents the energy reflected back to the radar). Their findings are used today as a guide to scientists who record and predict migrating bird patterns. The guide is established as follows:

• Minimal migration: less than 5 dBZ
• Light migration: 5-10 dBZ
• Moderate migration: 10-20 dBZ
• Heavy migration: 20-30 dBZ
• Extreme migration: greater than 30 dBZ

Radar has been very beneficial to the science of studying bird and insect migration just as much as it has been for meteorologists who study and forecast the weather. One would not expect, aside from the normal radar imagery of precipitation in large and small storms, that they would have the ability to see insects and birds as well. Clear air mode allows for this and is a very useful tool in furthering scientific discoveries.

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©2018 Meteorologist Alexandria Maynard