1. 2. 3. Northern Wisconsin Weather: Distance, heights, tilt of WSR-88D. 4. 12. 15. 16. 17. 18. 19. 20. 23. 24.

25. 26. Distance, heights, tilt of WSR-88D. 27. 28.

First off, I'm no pro concerning weather radar.

I've read some tutorials, I own and play with GRlevel3, and have tried to understand the extent and limitations of weather radar.

I've grabbed this chart (to the right) off the internet. Admittedly, I've no idea where I got it from... I simply found it in my files.

Each white diagonal line represents one tilt level of the radar beam as it sweeps around in a circle. Up to 14 tilts at various degrees make up one full set of radar images.

There are several different modes of operation for each radar.

This graphic (to the left) are cropped images from NWS's Jetstream Max, which show three of the main modes: Clear Air, Precipitation, and Severe.

Clear Air Mode uses 5 different angles and takes ten minutes to complete. To capture very small anomalies when precipitation is not present (e.g. wind shifts, smoke, bugs, lake-breezes), it is the slowest radar mode of operation.

Precipitation Mode scans 9 tilts in six minutes. When precipitation is detected by the radar, this mode is instantly turned on. It scans higher into the atmosphere than Clear Air Mode, so it better samples higher parts of thunderstorms and rain clouds.

Severe Weather Mode scans 14 slices in five minutes. It's very similar to Precipitation Mode, but the gaps are more completely filled in, so more of the atmosphere is scanned each time.

There are actually a couple other Modes being used, and I'm sure many more are coming in the next few years. The biggest challenge to overcome is how long it takes from one complete scan to another (a minimum of 5 minutes). When there are possible tornadoes five minutes can be too long. I have read that a Mode which can scan the same amount of sky in less time is rapidly being developed.

Onto what might interest someone in Oshkosh.

The lowest radar beam, in any of the various radar modes, is 0.5 degrees above horizontal. This means that as you increase your distance from a radar site, the radar beam is higher and higher above your head. On my awesome graphic (to the right) I've drawn in some black dots that show the base height over my guinea pigs... err, friends
: Derek in Polk County, OSNW3 in Oshkosh, and my location on the Bayfield Peninsula. The five radar sites I reference are KARX - La Crosse, KDLH - Duluth, KGRB - Green Bay, KMKX - Milwaukee, and KMPX - Minneapolis.

Examples of distance versus height.

The right-most column is the height of the radar beam in feet above ground level (agl).

For example,
above my head, the radar beam is approximately 4399 feet agl from KDLH. I used each radar site that scans the area above my three example locations.
Some implications...

Concerning lake-effect snow, which occurs under 4 or 5000 feet above the ground, the lowest radar beam will already be above that height more than 40 nautical miles from the radar site. The snowbelts of Wisconsin and UP of Michigan are beyond this threshold of KDLH and KMQT, so most lake-effect snow is miss simply by the location of each radar site.

Like lake-effect, any low level snowshowers will be falling at a level beneath any radar scan tilts.

I found a few reports online concerning the average heights above ground level where the maximum reflectivity (dBz) of hail are located (~9,500 ft agl), as well as the average height where mesoscale thunderstorm rotation occurs (~5,000 ft agl). Depending on how far from the radar site a severe weather event occurs, the radar beam may not be able to scan that feature.
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