The Lemon technique is well known to any radar operator. Even if you can't quote it chapter and verse, most of us operate under the assumption that with a standard atmosphere, a 50 to 55 dbz core originating at, or rising to, roughly 28 kft is a good precursor of severe weather once the core falls to the earth. Severe criteria are defined at hail .75" or more in diameter and/or 50 kt winds.
In the heart of summer, this height is often pushed to 35 kft, and in the early Spring and Fall can be as low as 20 kft. The idea of VIL density is often used when trying to support a warning decision based on volume products when the operator does not quite have enough confidence in the base products to issue a warning. However, if the operator is looking to achive maximum lead time, the use of VIL and VIL density should not be used as a substitute for proper interrogation of the base products.
This leads me to the point of this page, namely, those instances when using base data can be misleading. The diagram below is a crude attempt to illustrate a phenomena I see several times a year here in the southeast, particularly in the Summer months. When working radar, I will sometimes see a 45 dbz core suddenly show up at 35-40 kft. Often this is a rather fat core, and looks for all the world like it should do something. However, these higher reflectivities never appear to "descend" through the storm. Sometimes, as shown below, there is an area of higher reflectivites lower in the storm. This can also lead to the appearance that one very large core, extending to a great height, exists. However, I believe that frequently this is really a split between the heavier elements of the downdraft, and the considerable reflectivity elevated by the updraft. In an effort to achieve larger lead times I have issued warnings on these type of storms and nothing happened.
Of course, there are several other factors that can play a part. Large theta-e differences between the surface and mid levels of the storm can still lead to damaging winds even with these split reflectivity signatures. In this case, I think the verification occurs for the wrong reason from the radar operator's standpoint, as he assumes much of the contribution of the damage was from the high core, when in fact the lower core was at fault. Then, in cases with smaller theta-e deltas, nothing happens, and the radar operator is left scratching his head.
This is a very "quick and dirty" review. Basically all I want to know if any other radar operators have seem something similar to this, or if it is just me :)
-Bryan McAvoy
31 January 2002