|Fig 1 -- 0.5 degree base reflectivity from KDMX at 405Z, Dec. 12, 2010.|
|Fig 2 -- 0.5 degree base velocity from KDMX at 405Z, Dec. 12, 2010.|
Let's go to a higher tilt (3.5 degrees) and zoom into the velocity image a bit.
|Fig 3 -- 3.5 degree base velocity from the KDMX radar at 412Z, Dec. 12, 2010.|
Anyhow, one thing we can do based on the image is draw the wind directions based on the location of the zero isodop. The zero isodop is the line of "zero" velocities (the gray line) that separates the inbound and outbound air (it can actually be any place where zero velocities are showing up but the zero isodop is usually the one I just described). The zero isodop follows the line where the wind velocity is perpendicular to the radar beam. Since the winds are roughly north to south in the image above, when the radar beam scans to the east or west, there will be a point when the winds are blowing perpendicular to the radar beam. Since the radar can only measure air moving toward or away from the radar, it measures no velocity when the air is moving perpendicular to the radar beam. Thus we get a line of zero velocity in every radar image that represents all the places where the wind is blowing perpendicular to the radar beam. We can see that in the above image, the "true" zero isodop is the gray line of zero velocities that squiggles all the way across the image and goes through the radar.
Since along this line we know the direction the radar beam is pointing, we can draw arrows perpendicular to the radar beam along the zero isodop and extract wind directions. Since the radar beam increases its height off the ground the further it goes out, we can extract a vertical profile of winds from this zero-isodop analysis. I've drawn a few sample wind direction arrows in the image below (I make no claims about the wind velocity, just the direction).
|Fig 4 -- 3.5 degree base velocity image from KDMX at 412Z, Dec. 12, 2010. Annotated with wind direction arrows.|
|Fig 5 -- 3.5 degree base velocity image from KDMX at 412Z, Dec. 12, 2010. Annotated with wind direction arrows and locations of directional shear.|
What's causing this? To be honest, I'm really not sure. Some sort of latent heat release from melting precip? The surface temperatures in central Iowa are very close to freezing, or even slightly above it. Perhaps some of the snow is melting right before it hits the surface and this is causing some kind of warming. It's an intriguing possibility. We don't see much of a melting layer in the reflectivity image:
|Fig 6 -- 3.5 degree base reflectivity image from KDMX at 412Z, Dec. 12, 2010. .|
The radar can also somewhat verify itself when it comes to areas of wind shear. There's another product called the spectrum width product. Think of this as a standard deviation of velocity measurements at a given point. Since the radar processor takes an average of several different measurements to determine the velocity at a particular point, the spectrum width is more or less just the standard deviation of all those measurements. Therefore areas of high spectrum width tend to be areas of higher uncertainty with regards to what the velocity is. This could mean that the wind velocity is changing rapidly in that bin, like in areas of high shear or turbulence. Here's the spectrum width image at the same time as the above images.
|Fig 7 -- 3.5 degree spectrum width image from KDMX at 412Z, Dec. 12, 2010.|
|Fig 8 -- 3.5 degree velocity image from KDMX at 412Z, Dec. 12, 2010. Annotated with wind directions, areas of wind shear, and lines corresponding to maximum bands in spectrum width.|
What I can't exactly describe is why the spectrum width bands seem to be spiraling. If there was one uiniform layer of veering winds and then another uniform layer of backing winds, I would have expected concentric circles, one at each level where the velocities were changing, and not a spiral. This probably says that there's some sort of tilt to these layers in the horizontal, but I can't say much more beyond that. If anyone else has any ideas, by all means, please let me know...
This sort of isodop analysis to determine winds is actually used by an algorithm in the radar product generator to create a product called VAD winds. It uses model soundings (and some math on the velocity field) to calculate a vertical profile of winds (both speed and direction) from the radar (though it assumes that the winds are horizontally uniform across the radar domain--not always the best assumption). The VAD profile can be looked at in "meteogram" format like shown below.
|Fig 9 -- VAD winds from KDMX at the times listed on Dec. 12, 2010.|
So my analysis of the image seems to be consistent. But my interpretation of the images is rather loose. What really is causing these subtle variations in the wind direction (since it's not just showing flat out cold air advection)? Is there some sort of warming due to precipitation melting near the surface? Or is this wind shift simply due to friction with the land causing odd wind direction shifts near the surface? Or is that the warm conveyor belt showing up as subtle shifts in the velocity profile? I have my random thoughts, as explained above. I'd welcome any thoughts anyone else has. I just thought that there was a really pretty spiral image in the spectrum width and this is where it led me...
Also, just for comparison, the sounding from Omaha from 00Z (four hours before and over 100 miles to the west--well into the cold air) looked like this:
|Fig 10 -- 00Z sounding from Omaha, NE on Dec. 12, 2010. From the HOOT website.|