Development of cell tracking method using multiple thresholds for obtaining accurate storm motion

   One representative indicator of the probability of a supercell is the storm relative environmental helicity (SREH), which is calculated from the storm motion and the horizontal vorticity produced by the storm's environmental winds. Generally, storm motion and horizontal vorticity are calculated from soundings, objective analysis, and numerical forecasts; SREH is a very sensitive indicator to storm motion, so calculating SREH from more accurate storm motion will increase the possibility of determining whether a developing cumulonimbus cloud will become a supercell. In this study, more accurate SREH is calculated for each cumulonimbus cloud by using cell tracking technology to calculate accurate storm motions.

   The cell tracking method used in this study is based on an automatic cumulonimbus cloud tracking algorithm (AITCC; Shimizu and Uyeda, 2012). Generally, when tracking a cell, a precipitation intensity or reflectivity intensity area exceeding a single threshold value is tracked as a cumulonimbus cloud. However, when a single threshold value is used, it is difficult to continuously track cells that merge or split or are detected at the very edge of the threshold value. Therefore, in this study, multiple threshold values (specifically, from 43 to 53 dBZ at 1 dBZ intervals) are used for cell tracking to develop a method to robustly track cells even when cell mergers or splits occur. Using the developed method, SREH for each cumulonimbus cloud can be calculated more accurately. In the future, we plan to use these methods to develop nowcasting methods for more accurate prediction of the probability of supercell occurrence for the next hour.