FORECASTING CLEAR-AIR TURBULENCE BY COMPUTER TECHNIQUES

Pilot reports of clear-air turbulence from four special five-day reporting periods are used to investigate relationships between turbulence probability and meteorological factors. The meteorological analyses are made by computer on a 2.5 degrees latitude/longitude grid, on the basis of standard upper-air observations. Analyses are made for the United States for altitude layers 50 mb (approximately 4000 ft) thick. Temperature analyses interpreted as thermal wind shear are used to compensate for wind observations missing in high-speed portions of the flow. The best meteorological indicators of turbulence are those related to the wind field. These indicators are vertical vector wind shear, deformation, and divergence. The analysis for turbulence is made from meteorological factors and pilot reports in terms of the probability of encountering significant turbulence (greater than light intensity) during flight segments 100 nautical miles long. Numerical forecasts of upper winds are prepared for the turbulence-reporting periods on the basis of advection and geostrophic departures. The departures are evaluated directly from the wind analyses by use of the balance equation. The advection uses upstream space differences and is tested in both explicit and implicit forms. (Author)