Clear Air Turbulence Research Articles

High-altitude balloon-launched uncrewed aircraft system measurements of atmospheric turbulence and qualitative comparison with infrasound microphone response

Abstract. This study investigates the use of a balloon-launched uncrewed aircraft system (UAS) for the measurement of turbulence in the troposphere and lower stratosphere. The UAS was a glider which could conduct an automated descent following a designated flight trajectory and was equipped with in situ sensors for measuring thermodynamic and kinematic atmospheric properties. In addition, this aircraft was equipped with an infrasonic microphone to assess its suitability for the remote detection of clear-air turbulence. The capabilities of the UAS and sensing systems were tested during three flights conducted in New Mexico, USA, in 2021. It was found that the profiles of temperature, humidity, and horizontal winds measured during descent were in broad agreement with those made by radiosonde data published by the US National Weather Service, separated by up to 380 km spatially and by 3 to 5 h temporally. Winds measured during controlled flight descent were consistent with the winds measured by global-positioning-system-derived velocity during balloon ascent. During controlled descent with this particular payload, a nominal vertical resolution on the order of 1 m was achieved for temperature, relative humidity, and pressure with a nominal vertical resolution of the wind velocity vector on the order of 0.1 m; the aircraft had a glide slope angle from 1 to 4° during this time. Analysis approaches were developed that provided turbulent kinetic energy and dissipation rate, but it was found that the corresponding Richardson number was sensitive to the methodology used to determine the vertical gradients from a single flight. The low-frequency content of the infrasonic microphone signal was observed to qualitatively align with long-wavelength wind velocity fluctuations detected at high altitude. Moreover, the microphone measured more broadband frequency content when the aircraft approached turbulence produced by the boundary layer.

A Numerical Study of Clear-Air Turbulence over North China on 6 June 2017

On 6 June 2017, four severe clear-air turbulence (CAT) events were observed over northern China within 3 h. These events mainly occurred at altitudes between 8.1 and 9.5 km. The characteristics and possible mechanisms of the CAT events in the different regions are investigated here using the weather research and forecasting (WRF) model. The simulated wind and temperature fields in a 27 km coarse domain were found to be in good agreement with those of the ERA5 (European Centre for Medium-Range Weather Forecasts Reanalysis v5) and the observed soundings of operational radiosondes over northern China. In terms of synoptic features, the region where the turbulence occurred is characterized by a southwest–northeast upper-level jet stream. The upper-level jet stream observed at an altitude of 10.4 km consistently moved eastwards, with a maximum wind speed of 61.7 m/s. Simultaneously, the upper-level front–jet system on the cyclonic shear side of the upper-level jet stream also exhibited an eastward motion. The developed upper-level front–jet system induced significant vertical wind shear (VWS) and tropopause folding in the vicinity of these CAT events. Despite the high stability resulting from tropopause folding, the presence of strong VWS (1.90 × 10−2 s−1–2.55 × 10−2 s−1) led to a low Richardson number (Ri) (0.24–0.88) and caused Kelvin–Helmholtz instability (KHI), which ultimately induced CAT. Although a standard numerical weather forecast resolution of tens of kilometers is adequate to capture turbulence for most CAT events, it is still necessary to use high-resolution numerical simulations (such as 3 km) to calculate more accurate CAT indices (such as Ri) for CAT prediction in some specific cases.

Impacts of a Cold‐Wave Event on Civil Aviation in China: A November 2022 Case Study

A cold wave is a hazardous event experienced in the middle latitudes during the winter that significantly influences civil aviation. This study utilized the fifth‐generation ECMWF reanalysis (ERA5) data and aerodrome routine meteorological reports (METARs) from 36 airports in China to analyze the impacts of a cold‐wave event from November 26 to 30, 2022. We assessed the effects on wind, visibility, thunderstorms, precipitation, ground‐icing conditions, in‐flight aircraft icing conditions, and clear air turbulence. The results indicated that regions affected by the cold wave shifted from northwest to southeast, with notable impacts on wind, precipitation, and icing conditions. Strong winds, particularly crosswinds, affected airports in coastal regions and narrow valleys, enhancing wind speeds and creating hazardous conditions. Ground‐icing conditions were prevalent, with 23 out of 36 airports experiencing significant icing, notably in Northeast, Central, southern, and eastern China. In‐flight icing conditions were identified in the lower and middle troposphere, primarily affecting Northwest, Northeast, and Central China and the middle and lower reaches of the Yangtze River. At cruising levels, the cold wave introduced light clear air turbulence. This study provides valuable insights for improving civil aviation operations during cold‐wave events, emphasizing the need for preventive measures and early warning systems to mitigate potential risks.

Fast upper-level jet stream winds get faster under climate change

Earth’s upper-level jet streams influence the speed and direction of travel of weather systems and commercial aircraft, and are linked to severe weather occurrence. Climate change is projected to accelerate the average upper-level jet stream winds. However, little is known about how fast (>99th percentile) upper-level jet stream winds will change. Here we show that fast upper-level jet stream winds get faster under climate change using daily data from climate model projections across a hierarchy of physical complexity. Fast winds also increase ~2.5 times more than the average wind response. We show that the multiplicative increase underlying the fast-get-faster response follows from the nonlinear Clausius–Clapeyron relation (moist-get-moister response). The signal is projected to emerge in both hemispheres by 2050 when considering scenario uncertainty. The results can be used to explain projected changes in commercial flight times, record-breaking winds, clear-air turbulence and a potential increase in severe weather occurrence under climate change.

Influence of airborne LiDAR wavelength on the detection distance of clear air turbulence

Influence of airborne LiDAR wavelength on the detection distance of clear air turbulence

An Airborne Visible Light Lidar’s Methodology for Clear Air Turbulence Detection Based on Weak Optical Signal

A clear air turbulence (CAT) detection method using a 532 nm visible light airborne laser radar (LiDAR) system is proposed to address the urgent challenge in the aviation safety field. This method is based on the indirect detection technique of atmospheric molecular density for CAT and utilizes the strong aerosol scattering absorption characteristics of the iodine molecular 1109 absorption line to eliminate the interference of aerosol scattering and extinction on the weak molecular backscattering signal caused by CAT. This enables CAT detection under conditions where traditional ultraviolet LiDAR systems fail to function properly due to aerosol presence. The influence of axial wind speed and atmospheric temperature variations on the molecular backscattering spectrum in the aircraft flight path is studied, and a formula for vertical wind speed inversion in the CAT field is derived. The 532 nm airborne LiDAR CAT detection theoretical model and system architecture are presented. Through simulation analysis, the CAT detection range of the visible light LiDAR system is evaluated under different aircraft cruising altitudes and turbulence intensities. The results indicate that, with the proposed LiDAR system, the aerosol scattering influence can be effectively suppressed, and CAT can be detected up to 7 km for light-to-moderate turbulence and 10 km for moderate turbulence ahead of the aircraft when traditional ultraviolet LiDAR systems fail as the backscattering coefficient ratio between aerosol and molecule reaches the 10−1 condition. Based on this finding, a suggestion is made to construct a dual-wavelength (ultraviolet-visible) LiDAR system for CAT detection, aiming to solve the full coverage problem of CAT detection under various aerosol conditions. This study has a reference value for promoting the early resolution of CAT detection in the aviation field.

Occurrence frequency of subcritical Richardson numbers assessed by global high-resolution radiosonde and ERA5 reanalysis

Abstract. Kelvin–Helmholtz instability (KHI) is most likely to be the primary source for clear-air turbulence, which is of importance in pollution transfer and diffusion and aircraft safety. It is indicated by the critical value of the dimensionless Richardson (Ri) number, which is predicted to be 1/4 from linear stability analysis. However, Ri is fairly sensitive to the vertical resolution of the dataset; a higher resolution systematically leads to a finer structure. The study aims to evaluate the performance of ERA5 reanalysis in determining the spatial–temporal variabilities in subcritical Ri by comparing it against a near-global high-resolution radiosonde dataset during the years 2017 to 2022 and further highlights the global climatology and dynamical environment of subcritical Ri. Overall, the occurrence frequency of Ri<1/4 is inevitably underestimated by the ERA5 reanalysis over all climate zones at all heights from the near-ground atmosphere up to 30 km, compared to radiosonde, due directly to the severe underestimation in wind shears. Otherwise, the occurrence frequency of Ri<1 in ERA5 is climatologically consistent with that from Ri<1/4 in radiosondes in the free troposphere, especially over the midlatitude and subtropics in the Northern Hemisphere and Southern Hemisphere. Therefore, we argue that the threshold value of Ri could be approximated as 1 rather than 1/4 when using ERA5-based Ri as a proxy for KHI. The occurrence frequency of subcritical Ri revealed by both datasets exhibits significant seasonal cycles over all climate zones. In addition, it is positively correlated with the standard derivation of orography at low levels and is exceptionally strong over the Niño-3 region at heights of 6–13 km. Furthermore, a high occurrence of subcritical Ri would likely be accompanied by strong wind speeds and intensive orographic or non-orographic gravity waves.

Clear-Air Turbulence (CAT) Encounters on 13 November 2019 over Central and Eastern China: Numerical Simulation and Generation Mechanism

Abstract On 13 November 2019, seven commercial aircraft of China Eastern Airlines encountered nine severe-or-greater clear-air turbulence (CAT) events over central and eastern China within 12 h (0000–1200 UTC). These events mainly occurred at altitudes between 6.0 and 6.7 km. A high-resolution nested numerical simulation is carried out using the Weather Research and Forecasting (WRF) Model to investigate the generation mechanism of these CAT events, with a horizontal resolution of 1 km over the inner domain. In addition, seven CAT diagnostics with outstanding performances are employed for the mechanism analysis. The WRF Model can reasonably reproduce both synoptic-scale systems (Siberian high and upper-level jet stream) and local vertical structures (temperature, dewpoint temperature, and wind field). The simulation indicates that an upper-level front–jet system with a remarkable meridional temperature gradient intensifies over central and eastern China, with the maximum wind speed increasing from 59.0 to 67.3 m s−1. The intensification of the front–jet system induces the tropopause folding, and nine localized CAT events occur in the region with large vertical wind shear (VWS) (1.55 × 10−2–2.53 × 10−2 s−1) and small Richardson numbers (Ri) (0.42–0.85) below the cyclonic side of the jet stream. Diagnostic analysis reveals that Kelvin–Helmholtz instability plays an important role in CAT generation, while convective and inertial instability is not directly associated with CAT generation in this study. A typical flight case with continuous CAT events also suggests that large VWS (greater than 1.3 × 10−2 s−1) accompanied with small Ri (less than 1) favors CAT generation in a front–jet system environment. Significance Statement A high-resolution nested numerical simulation is carried out using the Weather Research and Forecasting (WRF) Model to investigate the generation mechanism of nine severe-or-greater clear-air turbulence (CAT) events over central and eastern China. Intensification of a front–jet system induces tropopause folding, and nine CAT events occur in the region with large vertical wind shear (greater than 1.55 × 10−2 s−1) and small Richardson numbers (less than 0.85) below the cyclonic side of the jet stream. Kelvin–Helmholtz instability plays an important role in the CAT generation, rather than convective and inertial instability.

Non‐Orographic Gravity Waves and Turbulence Caused by Merging Jet Streams

AbstractJet streams are important sources of non‐orographic internal gravity waves and clear air turbulence (CAT). We analyze non‐orographic gravity waves and CAT during a merger of the polar front jet stream (PFJ) with the subtropical jet stream (STJ) above the southern Atlantic. Thereby, we use a novel combination of airborne observations covering the meso‐scale and turbulent scale in combination with high‐resolution deterministic short‐term forecasts. Coherent phase lines of temperature perturbations by gravity waves stretching along a highly sheared tropopause fold are simulated by the ECMWF IFS (integrated forecast system) forecasts. During the merging event, the PFJ reverses its direction from approximately antiparallel to parallel with respect to the STJ, going along with strong wind shear and horizontal deformation. Temperature perturbations in limb‐imaging and lidar observations onboard the research aircraft HALO during the SouthTRAC campaign show remarkable agreement with the IFS data. Ten hours earlier, the IFS data show an “X‐shaped” pattern in the temperature perturbations emanating from the sheared tropopause fold. Tendencies of the IFS wind components show that these gravity waves are excited by spontaneous emission adjusting the strongly divergent flow when the PFJ impinges the STJ. In situ observations of temperature and wind components at 100 Hz confirm upward propagation of the probed portion of the gravity waves. They furthermore reveal embedded episodes of light‐to‐moderate CAT, Kelvin Helmholtz waves, and indications for partial wave reflection. Patches of low Richardson numbers in the IFS data coincide with the CAT observations, suggesting that this event was accessible to turbulence forecasting.

Global response of upper-level aviation turbulence from various sources to climate change

Atmospheric turbulence at commercial aircraft cruising altitudes is a main threat to aviation safety worldwide. As the air transport industry expands and is continuously growing, investigating global response of aviation turbulence under climate change scenarios is required for preparing optimal and safe flying plans for the future. This study examines future frequencies of moderate-or-greater-intensity turbulence generated from various sources, viz., clear-air turbulence and mountain-wave turbulence that are concentrated in midlatitudes, and near-cloud turbulence that is concentrated in tropics and subtropics, using long-term climate model data of high-emissions scenario and historical condition. Here, we show that turbulence generated from all three sources is intensified with higher occurrences globally in changed climate compared to the historical period. Although previous studies have reported intensification of clear-air turbulence in changing climate, implying bumpier flights in the future, we show that intensification of mountain-wave turbulence and near-cloud turbulence can also be expected with changing climate.

Clear Air Turbulence Resolved by Numerical Weather Prediction Model Validated by Onboard and Virtual Flight Data

AbstractA clear air turbulence (CAT) occurred on 30 December 2020 over Tokyo, Japan. The CAT was largely generated by breaking Kelvin‐Helmholtz (KH) instability waves in the free atmosphere. A regional numerical weather prediction model simulated the event with fine resolution (35 m). Onboard‐recorded flight data and a flight simulation were utilized to validate the meteorological simulation. The locations of the reproduced strong turbulence agree well with the regions where flights encountered turbulence on that day. In a simulation with the finest resolution, the KH waves and their breaking were resolved. When the resolution was finer, the turbulent eddies were stronger, causing meteorological effects on the airplanes. The response of a virtual airplane to the simulated turbulence was estimated using a flight simulation. By comparing onboard‐recorded data with virtual flight data, we confirm that turbulent eddies are reasonably reproduced.

An Investigation of Sudden Plunging Motion Mechanisms for Transport Aircraft during Severe Clear-Air Turbulence Encounter

An Investigation of Sudden Plunging Motion Mechanisms for Transport Aircraft during Severe Clear-Air Turbulence Encounter

Possibility of Clear Air Turbulence Localization with Lidar

The ranges of the parameters of wind and temperature (refractive) turbulence are estimated corresponding to the four-point scale of turbulence intensity at flight altitudes in the free atmosphere. It is shown that the estimates of both the variance of the radial velocity measured with a coherent lidar in the flight direction and of the structural constant of the refractive index from the lidar measurements of the intensity of refractive turbulence in the flight direction make it possible to judge the potential danger of bumpy flight of aircraft in clear-air turbulence zones ahead of the aircraft.

Structure health inspection for aging transport aircraft

Purpose This study aims to present a diagnosis method to inspect the structure health for aging transport aircraft based on the postflight data in severe clear-air turbulence at transonic flight. The purpose of this method development is to assist certificate holder of aircraft maintenance factory as a complementary tool for the structural maintenance program to ensure that the transport aircraft fits airworthiness standards. Design/methodology/approach In this study, the numerical approach to analyze the characteristics of flight dynamic and static aeroelasticity for two four-jet transport aircraft will be presented. One of these two four-jet transport aircraft is an aging one. Another one is used to demonstrate the order of magnitude of the static aeroelastic behaviors. The nonlinear unsteady aerodynamic models are established through flight data mining and the fuzzy-logic modeling technique based on postflight data. The first and second derivatives of flight dynamic and static aeroelastic behaviors, respectively, are then estimated by using these aerodynamic models. Findings Although the highest dynamic pressure of aging aircraft is lower, the highest absolute value of static aeroelastic effects response to the wing of aging aircraft is about 3.05 times larger than normal one; the magnitude variations of angles of attack are similar for both aircrafts; the highest absolute value of the static aeroelastic effects response to the empennage of aging aircraft is about 29.67 times larger than normal one in severe clear-air turbulence. The stabilizer of aging aircraft has irregular deviations with obvious jackscrew assembly problems, as found in this study. Research limitations/implications A lack of the measurement data of vertical wind speed sensor on board to verify the estimated values of damping term is one of the research limitations of this study. This research involved potential problem monitoring of structure health for transport aircraft in different weights, different sizes and different service years. In the future research, one can consider more structural integrity issues for other types of aircraft. Practical implications It can be realized from this study that the structure of aging transport aircraft may have potential safety threat. Therefore, when the airline managed aging transport aircraft, it ought to be conducted comprehensive and in-depth inspections to reduce such safety risks and establish a complete set of safety early warning measures to deal with the potential problem of aircraft aging. Social implications It can be realized that the structure of aging transport aircraft has potential safety threat. The airline managed aging transport aircraft; it should conduct comprehensive and in-depth inspections to reduce safety risks and establish a complete set of safety early warning measures. Originality/value This method can be used to assist airlines to monitor aging transport aircraft as a complementary tool of structural maintenance program to improve aviation safety, operation and operational efficiency.

Clear-air turbulence trends over the North Atlantic in high-resolution climate models

Clear-air turbulence (CAT) has a large impact on the aviation sector. Our current understanding of how CAT may increase with climate change in future is largely based on simulations from CMIP3 and CMIP5 global climate models (GCMs). However, these models have now been superseded by high-resolution CMIP6 GCMs, which for the first time have grid lengths at which individual turbulence patches may start to be resolved. Here we use a multi-model approach to quantify projected moderate CAT changes over the North Atlantic using CMIP6 models. The influence of the model resolution on CAT projections is analysed. Twenty-one CAT diagnostics are used, in order to represent uncertainties in CAT production mechanisms. Each diagnostic responds differently in time, but the majority display an increase in moderate CAT between 1950 and 2050. Although winter is historically the most turbulent season, there is strong multi-model agreement that autumn and summer will have the greatest overall relative increase in CAT frequency. By 2050, summers are projected to become as turbulent as 1950 winters and autumns. The global-mean seasonal near-surface temperature is used as a comparative metric. For every 1 °C of global near-surface warming, autumn, winter, spring, and summer are projected to have an average of 14%, 9%, 9%, and 14% more moderate CAT, respectively. Our results confirm that the aviation sector should prepare for a more turbulent future.

Spatiotemporal characteristics of clear-air turbulence (CAT) potential in China during 1979–2020

Spatiotemporal characteristics of clear-air turbulence (CAT) potential in China during 1979–2020

Not All Clear Air Turbulence Is Kolmogorov—The Fine‐Scale Nature of Atmospheric Turbulence

AbstractWe study a strong clear air turbulence (CAT) event experienced by the German High‐Altitude Long‐Range research aircraft (HALO) during the Southern Hemisphere Transport, Dynamics, and Chemistry campaign. HALO encountered CAT leeward of the southern Andes Mountains, where tropospheric airflow favored vertically propagating mountain waves that were refracted southeastward into the core of tropopause jet. Turbulence is quantified using spectral quantities and structure functions computed from in situ 100 Hz flight level data. The detected CAT region exhibits strong patchiness, characterized by separated bursts in turbulent kinetic energy and energy dissipation rate. The high resolution in situ observations reveal different turbulent scaling within each patch, in both spectra and structure functions, and following Monin and Yaglom's conversion law. One patch follows power laws with exponents −1.71 ± 0.06, −1.771 ± 0.006, and −1.56 ± 0.05 for the velocity components w, v, and u, respectively, while another patch has exponents −2.17 ± 0.12, −2.50 ± 0.08, and −1.92 ± 0.09. These patches are mediated by a third patch with less clear scaling. While the patches can deviate from Kolmogorov scaling due to the anisotropy of the airflow, they still display evidence of CAT with enhanced energy dissipation rates.

Climatology of Clear‐Air Turbulence in Upper Troposphere and Lower Stratosphere in the Northern Hemisphere Using ERA5 Reanalysis Data

AbstractSpatial and temporal distributions of Clear‐Air Turbulence (CAT) in the Northern Hemisphere were investigated using 41 years (1979–2019) of the European Centre for Medium‐range Weather Forecast Reanalysis version 5 (ERA5) data. We used two groups of CAT diagnostics to determine occurrence frequencies: (a) commonly used empirical turbulence indices (TI1, TI2, and TI3) and their components [vertical wind shear (VWS), deformation, ‐divergence, and divergence tendency], and (b) theoretical instability indicators [Richardson number (Ri), potential vorticity (PV), and Brunt‐Vӓisӓlӓ frequency]. The empirical indices showed high frequencies of moderate‐or‐greater (MOG)‐level CAT potential over the East Asian, Eastern Pacific, and Northwestern Atlantic regions in winter. Over East Asia, the entrance region of strong upper‐level jets showed the highest frequencies in TI1, TI2, and TI3 due mainly to strong VWS. The Eastern Pacific and Northwestern Atlantic areas near the exit region of jets had relatively high frequencies of these indices and also Ri. PV frequency was high on the southern side of jet primarily due to negative relative vorticity. Long‐term increasing trends of MOG‐level CAT potential also appeared in those three regions mainly due to warming in lower latitudes. The most significant increasing trend was found over East Asia, due to the strengthening of the East Asian jet and increased VWS due to the strong meridional temperature gradients in the mid‐troposphere induced by warming in the tropics and cooling in eastern Eurasia. These trends over East Asia, if continued, are expected to be of importance to efficient aviation operations across the northwestern Pacific Ocean.

Movement mechanisms for transport aircraft during severe clear-air turbulence encounter

AbstractThe objective of this paper is to present the movement mechanisms of transport aircraft response to severe clear-air turbulence to obtain the loss of control prevention for pilot training in IATA – Loss of Control In-flight (LOC-I) program. The transport aircraft in transonic flight is subjected to severe clear-air turbulence, resulting in a sudden plunging motion with the abrupt change in flight attitude and gravitational acceleration. The comparative analyses of the flight environment and aircraft response to severe clear-air turbulence for two four-jet aircraft are studied. The one with a larger dropped-off altitude during the plunging motion will be chosen to construct the movement mechanism. The nonlinear unsteady aerodynamic model of the chosen transport is established through flight data mining and the fuzzy-logic modeling of artificial intelligence technique based on post-flight data. The crosswind before the turbulence encounter will easily induce a rolling motion and then the sudden plunging motion during the turbulence encounter. The influences of the varying vertical wind and crosswind on loss of control are presented. To formulate preventive actions, the situation awareness of varying crosswind encountering for the operational pilot will be studied further in the future. The present study is initiated to examine the possible mitigation concepts of accident prevention for the pilot training course of IATA – Loss of Control In-flight (LOC-I) program.

Исследование аэродинамических характеристик самолета при попадании в зону турбулентности ясного неба и вопросы безопасности полета

Shear flows in the atmosphere lead to formation of the clear-air turbulence (CAT) zones. These zones are having the characteristic horizontal size of 20 km to 2000 km and may be less than 1000 m thick making it difficult to detect them and create effective systems to find the bypass routes. Intense CAT zones could appear at the altitudes of up to 30 km and higher. The main CAT feature lies in the fact that it occurs not in the clouds, but in the clear sky with good visibility, where meteorological radar is unable to detect it, and the flight crew is unable to get prepared. Every year, about 750 cases of the aircraft entering the CAT zones are registered in the global civil aviation. This work constructs a grid solution that simulates the large-scale atmospheric turbulence based on the atmospheric profiles taken from the experiment within the framework of the boundary value problem for the Reynolds-averaged Navier – Stokes equations. On the basis of the obtained solution and within the frames of the engineering method, coherent vortex structures (CVS) are formed, interaction with which, assuming the hypothesis of the perturbed speed frozen field, makes it possible to evaluate possible increments in the aerodynamic forces and moments and to simulate a situation of the aircraft entering the CAT zones.