Improvement of Tornado Warnings by Doppler Radar Measurement of Mesocyclone Rotational Kinetic Energy

Abstract

A significant improvement in the reliability and timeliness of tornado warnings can be achieved through quantitative measurement by Doppler radar of selected mesocyclone features. The mesocyclone, a midsize circulation found in some of the most severe thunderstorms, is a frequent precursor of tornadoes, especially those of strong or violent intensity. Mesocyclones can be readily detected by Doppler radar, and their distinguishing characteristics measured. A small sample of 17 Oklahoma storms containing mesocyclones was investigated in an effort to find optimum tornado predictors. The mesocyclone characteristic that provided the basis for the longest and most accurate warnings for violent (F4 and FS) as well as strong (F2 and F3) tornadoes is excess rotational kinetic energy (ERKE), defined as the rotational kinetic energy of the mesocyclone core calculated after subtraction from mesocyclone rotational velocity of the product of core radius and a selected value of threshold mesocyclonic shear. Rotational velocity also offered a basis for excellent warnings of the strong tornadoes. Little or no skill was achieved for identification of weak (F0 and F1) tornadoes. However, the distinctive warnings possible for strong and violent tornadoes in this small sample, using ERKE, offer reasonable promise of similar results when next generation weather radar (NEXRAD) becomes operational.