Clear air turbulence analysis over the South Pacific region for a five-day period in December 1964

As part of the ICAO short-term high-level turbulence reporting programme, data has been compiled and analysed for flights at or above 20,000 ft. Areas of non-occurrence as well as areas of occurrence of high level clear air turbulence (CAT) were determined over the South Pacific Region during a selected five-day period in December 1964. Latitude-longitude 'squares' of 2.5° were used to tabulate the flight data. Flights were divided into (1) flights at or below 30,000 ft and (2) flights above 30,000 ft. Frequency distributions and percentages of CAT occurrence were computed over individual 'squares', for flight elevations and also for various parameters mainly of a meteorological nature. The percentage of CAT occurrences increases fairly uniformly with increasing "average" vertical wind shear and horizontal wind shear, and is more frequent near than away from the jet stream. No significant relationship was evident between CAT and Richardson's Number, maximum wind and the height and temperature of the tropopause. Most parameters were more successful for flights at and below 30,000 ft where the majority of CAT occurred. No significant relationship was evident between CAT and flight direction but the presence of clouds above the flight level appears to be significant. Mean airspeed fluctuations at the onset of turbulence increase steadily with increasing severity of turbulence. Six parameters - Endlich's turbulence index, vorticity, vorticity advection, "average" vertical wind shear, Richardson's number and the Colson-Panofsky turbulence index - were investigated in a study restricted to flights in the area bounded by latitudes 221/2 and 40 degrees south and longitudes 1121/2 and 160 degrees east. Average vertical wind shear and Richardson's Number gave the best reslts.