A Climatological Analysis of the Variance of the Ellrod Turbulence Index based on Seasonality, the Diurnal Cyle, and the North Atlantic Oscillation

Clear Air Turbulence (CAT) can pose a major threat to the aviation industry in a number of ways. Severe turbulence can cause injury to passengers and crew, as well as potentially causing damage to the aircraft. The other major varieties of aviation turbulence (Convectively Induced Turbulence and Mountain-Wave Turbulence) have visual and instrumental clues for pilots to avoid, thus making CAT particularly challenging for the aviation industry. Scientists at forecasting centers around the world forecast for CAT because of this, and they still use the Turbulence Index 1 (TI1) developed by Ellrod and Knapp in 1992. This index is calculated by combining the stretching and shearing deformations of the wind field and finding the product of the combined deformations and vertical wind shear. Using the NCEP/NCAR Reanalysis 1 data, a 25-year climatology (1990-2014) is built of the TI1. Then, data is analyzed by 3-month averages to find seasonal maxima. The winter hemisphere was found to be more turbulent than the summer hemisphere. Since data was available at a resolution of four times daily, the diurnal cycle was analyzed, though the magnitude of the change was small over the course of a day. Finally, the North Atlantic Oscillation (NAO) Index was taken into consideration. While the effect is smaller than the seasonal variance, it is greater in magnitude than the diurnal cycle. The positive phase of the NAO exhibits more turbulent conditions over Europe and north Africa, while the negative phase is more turbulent over Canada and southern Alaska.