Dual-Pol Radar at NWS Chicago Captures Transitioning Precipitation

    In October, the NWS Weather Forecast Office in Chicago had a dual-polarization upgrade to the Doppler radar.  One of the exciting advantages of the dual-polarization (dual-pol) radar data was better determination of precipitation type at the level where the radar is sampling.  It is important to note where the radar is sampling as the radar beam still increases with height at further distances from the radar due primarily to the curve of the earth.  Thus, the Doppler radar base imagery will not necessarily be reflective of what is occurring at the ground at that point, especially at greater distances from the radar.  However, it often can give a very good estimate.   

    On Monday Night into Tuesday, northwest Indiana was grazed by a strong late autumn storm that produced heavy snow in northern Indiana and lower Michigan.  To the right is an NWS Chicago radar loop of 0.5° reflectivity from 11:15 am - 12:15 pm.  Reflectivity is basically the energy scattered back to the radar by targets. The 0.5° radar beam is generally at a height of 6,500 to 10,000 ft across these four counties. Some higher reflectivity over 40 dBZ, indicated by the yellow colors, can briefly be seen.  To meteorologists that could mean either the presence of water coated ice/snow meaning a melting layer, or just heavier rain.  At this time, concentrated rainfall was being observed across Benton and Jasper Counties, but a transition to snow was taking place across Tippecanoe and White Counties during this period. 


NWS Chicago Reflectivity Loop:  11:15 am - 12:15 pm

Reflectivity Loop

   Below are dual-pol element loops from this event, which help to shed some light on what was occurring over northwest Indiana at that time.  The images are of differential reflectivity and correlation coefficient and are also from the 0.5° for the same area and time.  This imagery is made possible from dual-pol radar because of the horizontal and vertical orientations of the radio wave pulses transmitted (hence the name dual-polarization).

   Differential reflectivity is used by meteorologists to help identify target shapes and thus type.  Rain drops tend to have values greater than 1, with drops having a greater horizontal aspect as they fall due to gravity.  Snow tends to have values around 1, with wetter snow or a mix favored with values of 1 to 2.  Note in the loop of differential reflectivity how values of 1.5 to 4 transition quickly to 0.5 to 1.5.  The precipitation shapes were becoming more spherical to randomly oriented, which tends to indicate something other than rain.

   Correlation coefficient can even further help distinguish precipitation type.  This variable describes the differences in shapes and type within the radar sample.  Values of 0.98 to 1 indicate a homogenous area, while lower values indicate a sample composed of varying sizes and potentially types of scatterers.  Within the correlation coefficient loop below, values of 0.92 to 0.95 quickly change to 0.99.  While the value change seems subtle, to meteorologists it means the sample is becoming much more homogenous at that height.  Given the atmosphere and the other radar variables, this indicates a change to snow at that height.

NWS Chicago Dual-Polarization Imagery

0.5° Differential Reflectivity

0.5° Correlation Coefficient

Differential Reflectivity

Correlation Coefficient

Click Images for Loops  (~2 MB each)

     If interested in learning more on the dual-pol radar, please visit this public training page by the NWS Warning Decision Training Branch.  For more detailed specifics behind the variables of dual-pol radar, please visit the NWS NSSL dual-pol information page here.

    The loops show the importance of dual-pol data in winter weather analysis and short term forecasting.  In this case, this data helps indicate that the precipitation aloft is quickly changing to snow. Indeed, in Lafayette and areas northeast of there, snow happened almost simultaneously or shortly after the radar indications. 

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