Top

The top column in the storm attribute table corresponds to the highest vertical height of a 30 dBZ echo for a certain thunderstorm cell.

The National Weather Service radars originally display a height in composite reflectivity as a one-digit or two-digit number with a one-digit decimal. Multiply this number by one thousand to correctly format the output. For example, if a radar displays the height as 15.6, the actual height is 15,600.

Depending on the distance from the radar site, and the tilt of the beams, the echo top can be greatly overestimated or underestimated to the order of five thousand to ten thousand feet.

The radar may not always be able to resolve the 30 dBZ echo top. When this error occurs, the output reading will have a "<" or ">" in front of it, meaning the top is less than or greater than, respectively, the number that follows. For example, if the reading <26.8 is shown, then the top is less than 26,800 feet.

The storm attribute table's top column does not produce the same readings that are on the "Echo Tops" product because the product is not reliant on the 30 dBZ echo, but the column's output is. In addition, the product is generally less precise than the column. The reason for this lies on the fact that the algorithm associated with the "Echo Tops" product measures the echo top with the radar beam that is closest to it. At far distances, it can appear on the product that the height of the clouds sharply varies between two levels five thousand feet apart, whereas the variance between tops is more gradual.

A radar cannot detect the highest vertical 30 dBZ height well if a thunderstorm is close to the radar site. Beams are unable to extend into the updraft at such a degree from the radar's transmitter.


CLOSE WINDOW

Jordan Gerth, May 2007