An Automated Procedure for Forecasting Clear-Air Turbulence

A computer program was written to obtain several parameters which have been useful in identifying clear-air turbulence (hereafter referred to as CAT) from upper-air data. These are: vertical and horizontal wind shear, vertical and horizontal gradient of kinetic energy and Richardson number. In addition, the distribution of CAT with respect to the jet stream and ridges and troughs was investigated. The Richardson number was judged to be the most promising parameter. An automated system was developed which 1) computes the Richardson number for each wind-shear layer for all North American soundings, 2) selects the minimum Richardson number for each sounding, 3) analyzes isopleths of minimum Richardson number, 4) draws these fields with the National Meteorological Center's curve drawer. Areas outlined by minimum Richardson numbers less than one are designated as forecast areas and are assumed to persist for twelve hours. Independent data indicate that the forecast areas explain approximately 40 per cent of the reported occurrences of moderate or severe CAT.