Doppler Radar Detects Vehicles on Thursday Morning

  Cool high pressure moved over the area early Thursday morning with morning lows in the mid 40s to lower 50s across many outlying areas.   As the high pressure moved overhead on Wednesday night, a clear sky allowed for full radiational cooling.  The atmosphere would decouple, or "separate" into a near-surface calm and stable layer below a more mixed layer.  In these cases, a temperature inversion often develops in-between.  To the upper right is an early Thursday morning weather balloon data plot (or sounding) from the NWS office in Lincoln, IL.  The temperature trace reveals a 16° climb in about 400 ft.  The wind barbs on the right show a very light wind regime, ideal for a strong temperature inversion to form overnight.

    Such a pronounced atmospheric decoupling and associated strong inversion can often cause superrefraction of the radar energy, which was the case early Thursday morning.  The energy from the radar was refracted at a higher rate than normal, thus striking non-meteorological targets on the ground.  While the radar can suppress a lot of these false echoes, it is more challenging on mornings with very strong inversions.  This is especially true when moving targets are indicated by the radar, such as turbines in a wind farm or in rare cases moving vehicles.  Such targets can "fool" the radar due to their movement.  With the upgrade to dual-polarization the radar received last autumn, the radar is much more able to classify these echoes as non-meteorological, and thus not calculate it as part of the estimated rainfall total.



7 am NWS Lincoln Weather Balloon Data

7 am Lincoln Sounding

Radar Refractions

Radar Refractions

Image courtesy of the NWS WDTB.


Thursday Morning NWS Chicago Radar Reflectivity Image

Radar Reflectivity Image

Reflectivity is showing the power reflected by targets back to radar.  All of these echoes at 7:11 am are "false", that is they are non-meteorological and are showing up due to superrefraction of the radar's energy.  This includes wind farms and vehicles on area interstates.  Are you able to identify four interstate corridors on the above image?

Thursday Morning NWS Chicago Radar Reflectivity Image:  Labeled

Radar Reflectivity Image Labeled

This image is the same as above, with interstates depicted in red and wind farms outlined in yellow.  Vehicles on parts of Interstates 39, 55, 57, and 80 are all visible.

   The Doppler characteristic of Doppler radar allows for information about the movement of radar scatterers to be determined by the radar.  This can be helpful for meteorologists to see rotation and strong wind speeds within thunderstorms.  On Thursday morning, the superrefraction allowed for the detection of high speeds of the targets on the interstate corridors, verifying they were indeed vehicles.

Thursday Morning NWS Chicago Radar Velocity Image

Radar Velocity Image

This 7:11 am image is Doppler Radar velocity.  Note the high speeds along the interstate corridors.  Here is a zoomed-in image on Grundy County, where Interstates 55 and 80 are visible due to the reflected energy of vehicles.


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