Climatology of large-scale atmospheric conditions favorable to clear-air turbulence over Europe (1950–2025): the role of quasi-biennial solar modulation

Turbulence represents a primary hazard for aviation, with Clear-Air Turbulence (CAT) posing a unique threat due to its visually undetectable nature. This study evaluates the 75-year temporal evolution of the macro-environmental atmospheric conditions favorable to CAT over Europe and North Africa from 1950 to 2025. Using monthly ERA5 reanalysis data at 200 and 225 hPa, seven synoptic-scale diagnostic indices were analyzed: Horizontal Divergence (HDG), Richardson Number (Ri), NCSU1, Brown Index (BRI), Ellrod Index (EI1, EI2), and Potential Vorticity (PV). Results demonstrate that while domain-averaged indicators such as NCSU1, BRI, PV, and EI1 show a long-term increase in the frequency of CAT-prone environments, others, including HDG and EI2, exhibit no clear trend. In addition, the Richardson Number displays a statistically significant decreasing trend, reflecting a shift toward reduced atmospheric stability and enhanced shear at these upper-tropospheric levels. Furthermore, spectral analysis via Fast Fourier Transform identifies a dominant periodicity of 2.45 years in the HDG and PV indices. This cycle aligns with historical findings of quasi-biennial solar activity, suggesting that the large-scale background conditions driving regional CAT potential over Europe may be modulated by solar-driven stratospheric dynamics. These findings underscore the index-dependent nature of regional synoptic trends and the importance of distinguishing macroscopic climate precursors from microscale turbulence events in aviation safety risk assessments.