Radar class

[Kevin Scharfenberg]

[Broken External Image]:http://cimms.ou.edu/~kscharf/ktlx_040701z.png
[Broken External Image]:http://cimms.ou.edu/~kscharf/ktlx_040701v.png


Today we shall have a pop quiz! Woohoo!

This is the KTLX (Oklahoma City) radar image at 12 degree elevation angle from 1440Z (9:40 am CDT) July 1st, 2004.

The ring of high reflectivities is the bright band, and it extends vertically from 11,500 to 14,000 feet.

1) The 12Z Norman sounding showed the freezing level above the area was at about 14,000 feet. Why does the bright band extend downward another 2,500 feet?

2) Why is there a narrow north-south break in the reflectivities in a zig-zag pattern centered on the radar?

3) Why is there an "annular" pattern of high/low velocities centered on the radar?

Pencils down...answers?

 

[Glen Romine]

Woo hoo, this looks fun!

1) The 12Z Norman sounding showed the freezing level above the area was at about 14,000 feet. Why does the bright band extend downward another 2,500 feet?

Because the ice/snow starts melting at 14,000 ft, the snow then aggregates and melts (getting a water coating and causing the high reflectivities) and completely melts once falling to around 11,500 ft.

2) Why is there a narrow north-south break in the reflectivities in a zig-zag pattern centered on the radar?

This lies along the zero dop velocity, so I'm guessing the ground clutter algorithm removed it.

3) Why is there an "annular" pattern of high/low velocities centered on the radar?

Let's see, since the elevation angle is fairly high, a considerable portion of the measured velocity here is the fall velocity of the precipitation particles. So, since the melting snow aggregates are large (often silver dollar sized or bigger), and relatively dense as they melt, this will lead to a downward acceleration and higher mean fall velocity through the bright band region, with a large and fast falling drop near the bottom of the bright band region. But, large raindrops are unstable, and tend to break up into many smaller drops, which would lead to a decrease in the mean fall velocity of the drops. Subtracting this pattern from the mean weak westerly shear should give a pattern similar to the one shown.

Does this seem reasonable?
Glen

 

[Kevin Scharfenberg]

You fail!

I would add to #1 that there might have been some potential in the morning sounding for evaporative cooling, but your answer addresses the major cause. #2 looks right on! #3 - I was especially interested in hearing opinions on this question. Your answer certainly looks reasonable. Any other thoughts?

 

[Glen Romine]

Originally posted by Kevin Scharfenberg
You fail!

I would add to #1 that there might have been some potential in the morning sounding for evaporative cooling, but your answer addresses the major cause. #2 looks right on! #3 - I was especially interested in hearing opinions on this question. Your answer certainly looks reasonable. Any other thoughts?

Ha, guess I should have actually looked at the sounding - thought you were just pointing out the freezing level height. As I look a second time at the velocity pattern - I was clumsy it seems, as the downward acceleration is happening in a ring outward of the bright band, so that would suggest the main accleration is due to snow aggregation prior to melting. The second velocity minima might be due to the process I mentioned before (big drops in the inner minima on the right, maxima left).

Glen