📄 earth_science

Climatology of Large-Scale Atmospheric Conditions Favorable to Clear-Air Turbulence over Europe (1950–2025): The Role of Quasi-Biennial Solar Modulation

This study analyzes 75 years of ERA5 reanalysis data to reveal that while large-scale atmospheric conditions favorable to Clear-Air Turbulence over Europe have generally increased with a shift toward reduced stability, these trends are index-dependent and modulated by a quasi-biennial solar cycle with a 2.45-year periodicity.

Original authors: Gabriel Pascual, Julia Soliva, Jordi Mazon, David Pino

Published 2026-06-26

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Gabriel Pascual, Julia Soliva, Jordi Mazon, David Pino

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Invisible Bump in the Sky

Imagine flying in a plane on a perfectly clear, sunny day. There are no clouds, no storms, and no lightning. Yet, suddenly, the plane hits a rough patch of air, jolting passengers and spilling coffee. This is Clear-Air Turbulence (CAT). It's like hitting an invisible pothole on a highway that you can't see until you're already in the air. Because it's invisible to the naked eye and radar, it's a major headache for pilots and airlines.

The Big Picture: A 75-Year Weather Diary

This study is like a historian looking back at a 75-year diary (from 1950 to 2025) of the sky over Europe and North Africa. Instead of trying to predict exactly when a single "pothole" will appear (which is too fast and chaotic to track with this data), the researchers looked at the big picture.

They asked: Is the "road" (the atmosphere) getting bumpier over time?

To answer this, they used a super-advanced digital weather map called ERA5. Think of this map as a high-resolution video game simulation of the atmosphere that has been running for decades. They zoomed in on the altitude where long-haul planes usually fly (about 35,000 feet up) and checked seven different "health monitors" to see if the air was becoming more unstable.

The Seven Health Monitors

The researchers didn't just look at wind speed; they used seven different "thermometers" to measure the air's mood. Here is what they found:

The "Stability" Gauge (Richardson Number): Imagine a stack of Jenga blocks. If the stack is stable, it doesn't fall. If it's unstable, it topples. This gauge measures how likely the air is to "topple." The study found this number is going down, which means the air is becoming less stable and more prone to tumbling.
The "Shear" Gauge (NCSU1 & Brown Index): Imagine two layers of a cake sliding past each other at different speeds. If the top layer moves much faster than the bottom, the cake gets messy. This measures that "sliding" effect. These gauges showed a slow increase, suggesting the air layers are sliding past each other more aggressively.
The "Spreading" Gauge (Horizontal Divergence): This measures if air is spreading out or bunching up. Surprisingly, this one showed no clear trend. It's like a heartbeat that hasn't changed its rhythm over 75 years.

The Main Takeaway: The Sky is Getting "Bumpier"

The study concludes that while the "spreading" of air hasn't changed much, the conditions that cause turbulence are getting worse.

The air is becoming less stable (like a wobbly Jenga tower).
The wind shear (the sliding layers) is getting stronger.

It's as if the atmosphere over Europe is slowly turning from a smooth, calm lake into a choppy sea, even though the overall weather patterns (like wind direction) haven't changed drastically.

The Sun's Secret Rhythm

One of the most fascinating parts of the paper is the discovery of a hidden rhythm.

The researchers used a mathematical tool (like a music equalizer) to listen to the "beat" of the atmosphere over 75 years. They found a dominant beat that repeats every 2.45 years.

The Analogy: Imagine the atmosphere is a drum. Most people think the drum is only beaten by the big 11-year solar cycle (like a slow, heavy drumbeat). But this study found a faster, quicker beat: a "quasi-biennial" rhythm (about every 2.5 years).
The Connection: This 2.45-year beat matches historical patterns of solar activity. The paper suggests that the Sun isn't just a steady light bulb; it has short-term "flickers" that might be nudging the Earth's upper atmosphere, making it more prone to turbulence every couple of years. It's like the Sun is tapping a rhythm on the atmosphere that we didn't fully realize was there.

What This Means (According to the Paper)

It's not about individual bumps: The study doesn't tell us when a specific flight will hit turbulence. It tells us that the environment is becoming more hostile to smooth flying.
Winter is the worst: Just like winter roads are icy, the winter atmosphere over Europe is the most turbulent time of year.
Solar influence: The Sun might be playing a bigger role in these "bumpy" years than we thought, specifically through these short 2.5-year cycles.

In short, the sky over Europe is slowly becoming a rougher place for airplanes, and the Sun might be tapping a secret, fast-paced rhythm that makes it even bumpier every few years.

Technical Summary: Climatology of Large-Scale Atmospheric Conditions Favorable to Clear-Air Turbulence over Europe (1950–2025)

Problem Statement
Clear-Air Turbulence (CAT) represents a critical hazard for aviation due to its undetectable nature in cloud-free regions of the upper troposphere and lower stratosphere. While previous research has established a link between anthropogenic climate change and the intensification of CAT, particularly over the North Atlantic and North America, there remains a need for a comprehensive, long-term climatological analysis of the macro-environmental conditions favoring CAT over the Euro-Mediterranean region. Existing studies often focus on transient microscale events or specific climate model projections; this study addresses the gap by analyzing the 75-year temporal evolution (1950–2025) of synoptic-scale atmospheric precursors to CAT, distinguishing between the "atmospheric climate" (the background flow) and individual turbulence events.

Methodology
The study utilizes monthly averaged ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) at 200 hPa and 225 hPa pressure levels, corresponding to standard commercial cruise altitudes. The geographical domain covers the Euro-Mediterranean region (75°N–20°N, 35°W–50°E).

Seven diagnostic indices were calculated to characterize turbulence potential:

Horizontal Divergence (HDG)
Negative Richardson Number (Ri)
North Carolina State University Index (NCSU1)
Brown Index (BRI)
Ellrod Indices (EI1 and EI2)
Potential Vorticity (PV)

Statistical analysis employed Spearman rank correlation to assess inter-index relationships, linear regression to determine long-term trends (annual and seasonal), and Fast Fourier Transform (FFT) to identify cyclical periodicities, specifically investigating potential solar modulation.

Key Contributions and Results

Index Inter-relationships: Correlation analysis revealed that deformation-based indices (EI1, EI2, NCSU1, BRI) are strongly inter-correlated ( $\rho > 0.70$ ), whereas HDG, Ri, and PV exhibit weak or negligible correlations with the deformation suite. This suggests that HDG, Ri, and PV capture distinct atmospheric processes not represented by shear-deformation metrics.
Long-Term Trends (1950–2025):
- NCSU1: Exhibited a statistically significant positive trend ( $p=0.05$ at 200 hPa; $p=0.041$ at 225 hPa), indicating a long-term expansion of atmospheric configurations prone to generating CAT.
- Richardson Number (Ri): Showed a statistically significant decreasing trend ( $p < 0.001$ ), reflecting a shift toward reduced static stability and enhanced vertical wind shear.
- Horizontal Divergence (HDG): Displayed no statistically significant long-term trend at either pressure level ( $p > 0.35$ ), suggesting that horizontal divergence patterns have remained relatively stable over the study period.
Seasonal Variability: Winter was identified as the most turbulent season across all indices, characterized by the highest values and greatest interannual variability. Conversely, summer conditions were the most quiescent. Notably, while annual trends for NCSU1 and Ri indicated increasing instability, seasonal analysis revealed divergent behaviors (e.g., spring showed a positive trend for Ri, while other seasons showed negative trends).
Solar Modulation: Spectral analysis via FFT identified a dominant periodicity of approximately 2.45 years in the HDG and PV indices. This cycle aligns with historical findings of quasi-biennial solar activity (distinct from the 11-year solar cycle), suggesting that large-scale background conditions driving regional CAT potential may be modulated by solar-driven stratospheric dynamics.

Significance and Claims
The paper asserts that the risk of CAT over Europe is not uniform and is highly dependent on the specific diagnostic index used. The findings underscore the necessity of distinguishing between macroscopic climate precursors and microscale turbulence events in aviation safety risk assessments.

The study claims that while domain-averaged indicators like NCSU1, BRI, PV, and EI1 suggest an increasing frequency of CAT-prone environments, others like HDG show no such trend. Furthermore, the identification of a 2.45-year periodicity in HDG and PV provides a novel, tentative link between regional CAT potential and quasi-biennial solar modulation mediated by stratospheric dynamics (such as the Quasi-Biennial Oscillation and Sudden Stratospheric Warmings). The authors conclude that while microscale turbulence cannot be directly resolved at monthly resolutions, long-term reanalysis averages successfully track the shifting climatological envelope of aviation safety risks, offering direct insights into operational safety and fuel efficiency in a changing climate.