Wednesday, November 2, 2011

DOW Adventures in the Transition Zone

The transition zone is the layer in which snow is warming, melting, and turning to rain in a storm (see our previous post, Skiing in the Transition Zone).

One of the powers of the Doppler on Wheels (DOW) radar system we are presently "storm chasing" with in Utah is that it is polarimetric, which allows us to observe some of the precipitation processes occurring in the transition zone.  A polarimetric radar sends and receives radio wave pulses that are oriented in both the horizontal and vertical planes.

Source: NOAA/NWS
The energy that is scattered back and received by the radar is converted to something called reflectivity, which is commonly plotted on many radar images available on the Internet and television.  In general, that reflectivity is based on radio wave pulses that are oriented in the horizontal plane (most National Weather Service radars presently transmit and receive only horizontally oriented radio-wave pulses).

We can mine far more information from the horizontally and vertically oriented radio-wave pulses used by polarimetric radars.  In particular, let's have a look at the transition zone we observed early in the morning during yesterday's storm.

We will be looking at vertical scans taken roughly to the north from our observing site along SR-111 (see red arrow below, the yellow arrow indicates the vertical scans discussed in the previous post).

The horizontal radar reflectivity in this scan shows the vertical structure of the storm.  The horizontal radar reflectivities generally increase as one moves downward in the storm (consistent with the snow crystal growth), but abruptly transition to much higher values where I've annotated a dotted line.

This transition to higher radar reflectivities denotes the top of the transition zone where snowflakes are beginning to melt.  The area of higher radar reflectivity, which is caused because wet snow and ice particles scatter more energy back to the radar, is commonly referred to as a bright band (or melting band).  Snow doesn't melt instantly, so the bright band has some depth to it.

Because the DOW is polarimetric, we can further examine the transition zone using something called the differential reflectivity, or ZDR for short.  The differential reflectivity is a measure of the ratio of the horizontal and vertical reflectivity and is sensitive to the shape and size of particles in the atmosphere.  It is zero if most of the particles are spherical, positive if they are wider than tall, and negative if they are taller than wide.  For example, due to the forces of the air as they fall, raindrops tend to be spherical when the are small, but more oblate when they are large.

Source:  Colorado State University Chill Radar Site
As a result, small raindrops have a small, near zero differential reflectivity, but large rain drops have a positive differential reflectivity.

Check out the differential reflectivity in yesterday's storm.  It is generally small except in or just below the base of the bright band/transition zone.

This suggests that we had big snowflakes melting into big raindrops as precipitation fell through the transition zone.  Note that the ZDR then decreases as one moves further downward, which is consistent with the breakup of those large drops into smaller raindrops that are more spherical.

Finally, we have the correlation coefficient, which is a measure of the statistical correlation between the horizontal and vertical energy received by the radar.  Typically this value is near 1 if the precipitation particles are all ice or water, but is smaller if the precipitation is mixed phase (both ice and water).  Indeed we saw the lowest values yesterday within the heart of the bright band where the precipitation was likely mixed rain and snow.

Thus is the life of precipitation falling through the transition zone.  The snow or ice becomes wet, producing a high radar reflectivity.  Some of the flakes melt faster than others, so one gets a mixture of rain and snow that produces a small correlation coefficient.  And finally, near the base of the transition zone, there are some big, oblate shaped raindrops that produce a large differential radar reflectivity.  These raindrops break up into smaller raindrops as they continue to fall.

Such observations of the transition zone are quite important when it is located near the surface or has large horizontal variability.  No meteorologist likes it when their forecast for 32F and 10" of snow turns into 36F and heavy rain.

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