Quantification of Moderate or Great CAT over Europe in the period 1950-2025

Abstract

Turbulence represents one of the main hazards for aviation. Among the different types of turbulence, this study focuses on Clear Air Turbulence (CAT), which represents a special threat, as it mostly happens in cloudless areas, being visually undetectable and difficult to forecast. Consequently, flights that encounter CAT experience violent changes in altitude and damage can be caused to the aircraft, crew and passengers, compromising the safety. Previous works have studied the increase of CAT in severity and frequency because of climate change, but only over North America and the North Atlantic. To assess the increase of CAT, prior investigations have used 21 different turbulent indices. Among these 21 indices, 7 of them have been selected for this study based on the consistent results shown in previous studies and their definitions, so different perspectives can be obtained. The 7 indices are Horizontal Divergence, Negative Richardson Number, North Carolina State University Index (NCSUI), Brown Index, variants 1 and 2 of the Ellrod Index and Potential Vorticity. To compute them, the present study uses the ERA 5 reanalysis data at 200 hPa and 225 hPa over Europe from 1950 to 2025. To evaluate the possible increase of CAT, four different analyses have been performed. Using annual and seasonal averages, horizontal cross sections of the indices over the domain and plots have been obtained to determine the trends. A correlation test was performed to determine whether the different indices show similar trends, and Fast Fourier Transforms were used to relate the results of the indices with other atmospheric phenomena such as solar cycles. The horizontal cross sections showed a possible relationship between the North Atlantic Oscillation (NAO) and the turbulent indices. Therefore, a correlation analysis has been carried out between the NAO and the indices. The results showed that for all the indices except the Negative Richardson number, that a medium correlation exists with the NAO. Different results regarding the trends were obtained depending on the index. Horizontal Divergence and variant 2 of the Ellrod Index did not show an increase in CAT. Negative Richardson Number showed a decrease, and variant 1 of the Ellrod Index, Brown Index, Potential Vorticity and NCSUI showed an increase of CAT in the studied period. In consequence, it is important to keep improving the ability to forecast CAT as well as conduct new research on the increase of this phenomenon and its impact on aviation.

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