Clear Air Turbulence Research Articles

Regional downscaled future change of clear‐air turbulence over East Asia under RCP8.5 scenario within the CORDEX‐EA‐II project

AbstractDriven by four global coupled atmosphere–ocean models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble (CNRM‐CM5, GFDL‐ESM2M, EC‐EARTH, and MPI‐ESM‐LR), future changes (2040–2059) of clear‐air turbulence (CAT) in winter over East Asia under Representative Concentration Pathway 8.5 (RCP8.5) scenario are projected with the regional climate model (RCM) RegCM4 and 18 CAT diagnostics based on the second phase of Coordinated Regional Downscaling Experiment East Asia (CORDEX‐EA‐II) framework. Evaluation of RegCM4 with ERA‐Interim shows that RegCM4 and diagnostics can well reproduce the spatial distribution of CAT potential. The probability of CAT potential is expected to increase over most East Asia between 35°N and 50°N, Southeast China, western Pacific, and North India in the future. The CAT potential that significantly affects aviation safety shows a remarkable increase over North and Northeast China, part of East China, and Tibet‐Plateau. In the whole domain of East Asia, the multi‐diagnostics ensemble‐average frequency increase is 6.9% for light CAT (with an intra‐ensemble range of 4.1–18%), 9.1% (5.0–24%) for light‐to‐moderate CAT, 12% (5.9–26%) for moderate CAT, 13% (7.2–29%) for moderate‐to‐severe CAT, and 15% (8.5–33%) for severe CAT. Stronger CAT is more likely to happen, which would be a constant threat, although greater scatter exists among these diagnostics, which implies the uncertainties are also significant in the projection.

The Importance of Convective Stage on Out-of-Cloud Convectively Induced Turbulence from High-Resolution Simulations

AbstractTurbulence (clear-air, mountain wave, convectively induced) is an aviation hazard that is a challenge to forecast due to the coarse resolution ultilized in operational weather models. Turbulence indices are commonly used to aid pilots in avoiding turbulence, but these indices have been designed and calibrated for midlatitude clear-air turbulence prediction (e.g., the Ellrod index). A significant limitation with current convectively induced turbulence (CIT) prediction is the lack of storm stage dependency. In this study, six high-resolution simulations of tropical oceanic and midlatitude continental convection are performed to characterize the turbulent environment near various convective types during the developing and mature stages. Second-order structure functions, a diagnostic commonly used to identify turbulence in turbulence prediction systems, are used to characterize the probability of turbulence for various convective types. Turbulence likelihood was found to be independent of region (i.e., tropical vs midlatitude) but dependent on convective stage. The probability of turbulence increased near developing convection for the majority of cases. Additional analysis of static stability and vertical wind shear, indicators of turbulence potential, showed that the convective environment near developing convection was more favorable for turbulence production than mature convection. Near developing convection, static stability decreased and vertical wind shear increased. Vertical wind shear near mature and developing convection was found to be weakly correlated to turbulence intensity in both the tropics and the midlatitudes. This study emphasizes the need for turbulence avoidance guidelines for the aviation community that are dependent on convective stage.

Study of Clear Air Turbulence Related to Tropopause Folding over the Romanian Airspace

Clear air turbulence (CAT) poses a significant threat to aviation. CAT usually occurs in the lower stratosphere and the upper troposphere. It is generally associated with large scale waves, mountain waves, jet streams, upper-level fronts and tropopause folds. Aircraft can experience CAT when flying in proximity of a tropopause fold. To better understand and diagnose tropopause fold- associated CAT we selected a series of cases from among those reported by pilots between June 2017 and December 2018 in the Romanian airspace. Data on turbulence were used in conjunction with meteorological data, satellite imagery, and vertical profiles. Additionally, a set of indices as Ellrod, horizontal temperature gradient, Dutton, and Brown were computed to diagnose CAT associated with tropopause folding. These indices were also analyzed to test the physics mechanisms that may explain the occurrence of severe turbulence. Results show that out of the 420 cases announced by pilots, severe turbulence was reported in 80 cases of which 13 were associated with tropopause folding.

Realistic Simulation of Tropical Atmospheric Gravity Waves Using Radar-Observed Precipitation Rate and Echo Top Height.

Gravity waves (GWs) generated by tropical convection are important for the simulation of large‐scale atmospheric circulations, for example, the quasi‐biennial oscillation (QBO), and small‐scale phenomena like clear‐air turbulence. However, the simulation of these waves still poses a challenge due to the inaccurate representation of convection, and the high computational costs of global, cloud‐resolving models. Methods combining models with observations are needed to gain the necessary knowledge on GW generation, propagation, and dissipation so that we may encode this knowledge into fast parameterized physics for global weather and climate simulation or turbulence forecasting. We present a new method suitable for rapid simulation of realistic convective GWs. Here, we associate the profile of latent heating with two parameters: precipitation rate and cloud top height. Full‐physics cloud‐resolving WRF simulations are used to develop a lookup table for converting instantaneous radar precipitation rates and echo top measurements into a high‐resolution, time‐dependent latent heating field. The heating field from these simulations is then used to force an idealized dry version of the WRF model. We validate the method by comparing simulated precipitation rates and cloud tops with scanning radar observations and by comparing the GW field in the idealized simulations to satellite measurements. Our results suggest that including variable cloud top height in the derivation of the latent heating profiles leads to better representation of the GWs compared to using only the precipitation rate. The improvement is especially noticeable with respect to wave amplitudes. This improved representation also affects the forcing of GWs on large‐scale circulation.

Detection and Avoidance of Atmospheric Aviation Hazards Using Infrared Spectroscopic Imaging

Atmospheric aviation hazards due to turbulence, poor visibility, high-altitude ice crystals and volcanic ash and gases are known problems for aviation and can cause both economic damage to engines and airframes as well as having the potential to cause the engines to stall in flight with possible loss of the aircraft. Current space- and ground-based assets allow observations of some of these hazards and their detection and movement can be forecast using modern computer weather forecasting and dispersion models. These largely strategic resources have proved very valuable but somewhat limited in the tactical sense, where commercial aviation must make rapid decisions in order to avoid an undetected or un-forecast hazardous cloud or atmospheric condition. Here we investigate the use of multi-spectral (two channels or more) infrared imaging from an aircraft perspective, and show that it is possible to use this information to provide tactical awareness tools for use by aviators and other stakeholders. This study has a strong focus on volcanic ash as an aviation hazard but also includes applications to some forms of clear air turbulence (CAT), to high-altitude ice crystals (HAIC) and windblown desert dust. For volcanic ash detection, the research shows that current two-channel satellite-based infrared techniques provide acceptable discrimination and quantification, but two-channel infrared imaging airborne solutions have significant drawbacks. Because of the limitation of two-channel methods, infrared spectroscopic techniques are investigated and it is shown they can significantly reduce the confusion caused by meteorological hydrometeors and potentially provide information on other atmospheric hazards to aviation, such as HAIC and some forms of turbulence. Not only are these findings important for on-going efforts to incorporate IR imaging onto commercial aircraft, but they also have relevance to the increasing use of drones for hazard detection, research and monitoring. Uncooled infrared bolometric imaging cameras with spectroscopic capabilities are available and we describe one such system for use on airborne platforms.

A Novel Approach for Satellite-Based Turbulence Nowcasting for Aviation

The predictability of aviation turbulence is influenced by energy-intensive flow patterns that are significantly smaller than the horizontal grid scale of current numerical weather prediction (NWP) models. The parameterization of these subgrid scale (SGS) processes is possible by means of an additional prognostic equation for the temporal change of turbulence kinetic energy (TKE), whereby scale transfer terms are used. This turbulence scheme has been applied operationally for 5 years in the NWP model ICON (Icosahedral Nonhydrostatic). The most important of the source terms parameterizes the Kelvin–Helmholtz instability, better known as clear air turbulence. This shear term was subjected to a nowcasting technique, is calculated with satellite data, and shifted forward in time using motion based on optical flow estimates and atmospheric motion vector (AMV). The nowcasts include turbulence altitude as determined by an adapted height assignment scheme presented here. The case studies illustrate that the novel approach for satellite-based turbulence nowcasting is a supplement to the NWP models.

Mesoscale modelling of optical turbulence in the atmosphere: the need for ultrahigh vertical grid resolution

ABSTRACT The high-fidelity modelling of optical turbulence is critical to the design and operation of a new class of emerging highly sophisticated astronomical telescopes and adaptive optics instrumentation. In this study, we perform retrospective simulations of optical turbulence over the Hawaiian islands using a mesoscale model. The simulated results are validated against thermosonde data. We focus on turbulence in the free atmosphere, above the atmospheric boundary layer. The free atmosphere is particularly important for adaptive optics performance and for sky coverage calculations and hence has significant impact on performance optimization and scheduling of observations. We demonstrate that a vertical grid spacing of 100 m or finer is needed to faithfully capture the intrinsic variabilities of observed clear air turbulence. This is a particularly timely study because the next generation of extremely large telescopes are currently under construction and their associated suite of instruments are in the design phase. Knowledge of the expected accuracy of optical turbulence simulations and real-time forecasts will enable the design teams to (i) test and develop instrument designs and (ii) formulate operational procedure.

A hybrid algorithm for wave-front corrections applied to satellite-to-ground laser communication

Laser communications hold accurate data rate for ground satellite links. The laser beam is transmitted through the atmosphere. The clear-air turbulence induces a number of phase distortions that damage wave-front. Adaptive optics (AO) treats wave front correction. The nature of AO systems is iterative; it can be integrated in metaheuristic algorithms such as genetic algorithm (GA). This paper presents improved version of algorithm for wave-front corrections. The improved algorithm is based on genetic algorithm (GA) and adaptive optics approach (OA). It is implemented in a computer simulation model called object-oriented matlab adaptive optics (OOMAO). The optimisation process involves best possible GA parameters as a function of population size, iteration count, and the actuators’ voltage intervals. Results show that the application of GA improves the performance of AO in wave-front corrections and the communication between satellite-to-ground laser links as well.

Increased shear in the North Atlantic upper-level jet stream over the past four decades.

Earth's equator-to-pole temperature gradient drives westerly mid-latitude jet streams through thermal wind balance1. In the upper atmosphere, anthropogenic climate change is strengthening this meridional temperature gradient by cooling the polar lower stratosphere2,3 and warming the tropical upper troposphere4-6, acting to strengthen the upper-level jet stream7. In contrast, in the lower atmosphere, Arctic amplification of global warming is weakening the meridional temperature gradient8-10, acting to weaken the upper-level jet stream. Therefore, trends in the speed of the upper-level jet stream11-13 represent a closely balanced tug-of-war between two competing effects at different altitudes14. It is possible to isolate one of the competing effects by analysing the vertical shear-the change in wind speed with height-instead of the wind speed, but this approach has not previously been taken. Here we show that, although the zonal wind speed in the North Atlantic polar jet stream at 250hectopascals has not changed since the start of the observational satellite era in 1979, the vertical shear has increased by 15 per cent (with a range of 11-17 per cent) according to three different reanalysis datasets15-17. We further show that this trend is attributable to the thermal wind response to the enhanced upper-level meridional temperature gradient. Our resultsindicate that climate change may be having a larger impact on the North Atlantic jet stream than previously thought.The increased vertical shear is consistent with the intensification of shear-driven clear-air turbulence expected from climate change18-20, which will affect aviation in the busy transatlantic flight corridorby creating a more turbulent flying environment for aircraft. We conclude that the effects of climate change and variability on the upper-level jet stream are being partly obscured by the traditional focus on wind speed rather than wind shear.

Development of Near-Cloud Turbulence Diagnostics Based on a Convective Gravity Wave Drag Parameterization

AbstractWe propose near-cloud turbulence (NCT) diagnostics for use in aviation turbulence forecasting, using a convective gravity wave drag (CGWD) parameterization scheme. The NCT diagnostics are obtained based on (i) CGWD and (ii) minimum Richardson number including the effects of convective gravity waves (CGWs). The feasibility of the NCT diagnostics is examined using numerical simulation results of real turbulence cases related to the breaking of CGWs, which occurred over eastern Missouri and southwestern Illinois in the United States on 9–10 March 2006, and near Fukuoka, Japan, on 2 September 2007. On 9–10 March 2006, several instances of moderate-or-greater (MOG)-intensity turbulence were reported above shallow but active convection over the central United States, while on 2 September 2007, severe turbulence was encountered above dissipating convection near Fukuoka, Japan. The high-resolution simulation results for both turbulence events show that CGWs and their breaking provide favorable environments for turbulence generation. For two simulated real cases, nonzero NCT diagnostics are reasonably well matched with observed turbulence encounters. The global distribution of CGWD calculated using global reanalysis data revealed a high potential of MOG turbulence in the tropics and the midlatitudes, which can be clearly distinguished from the traditional clear-air turbulence index where high potentials of MOG turbulence are diagnosed in the midlatitudes associated with the strong vertical wind shears near jet streams. These results imply that the proposed NCT diagnostics are useful for forecasting turbulence related to the breaking of CGWs, especially, in tropical regions.

Optimization of Lidar Systems Measurements for Detection of Clear Air Turbulence

Optimization of Lidar Systems Measurements for Detection of Clear Air Turbulence

Modification of Potential Vorticity near the Tropopause by Nonconservative Processes in the ECMWF Model

AbstractThe upper-level potential vorticity (PV) structure plays a key role in the evolution of extratropical weather systems. PV is modified by nonconservative processes, such as cloud latent heating, radiative transfer, and turbulence. Using a Lagrangian method, material PV modification near the tropopause is attributed to specific parameterized processes in the global model of the European Centre for Medium-Range Weather Forecasts (ECMWF). In a case study, several flow features identified in a vertical section across an extratropical cyclone experienced strong PV modification. In particular clear-air turbulence at the jet stream is found to be a relevant process (i) for the PV structure of an upper-level front–jet system, corroborating previous observation-based findings of turbulent PV generation; (ii) for the purely turbulent decay of a tropopause fold, identified as an effective process of stratosphere–troposphere exchange; and (iii) in the ridge, where the Lagrangian accumulated turbulent PV modification exhibits a distinct vertical pattern, potentially impacting the strength of the tropopause inversion layer. In contrast, cloud processes affect the near-tropopause PV structure above a warm conveyor belt outflow in the ridge and above cold-sector convection. In agreement with previous studies, radiative PV production dominates in regions with an anomalously low tropopause, where both radiation and convection act to increase the vertical PV gradient across the tropopause. The particular strengths of the Lagrangian diagnostic are that it connects prominent tropopause structures with nonconservative PV modification along the flow and that it quantifies the relative importance of turbulence, radiation, and cloud processes for these modifications.

Richardson number model for turbulence motion analysis around airport runway

Natural disasters now often occur in Indonesia, especially regarding atmospheric anomalies. The models of forecasting based convective cloud growth events are necessary to understand the anomaly symptoms of the atmosphere. This research solves a problem, we propose a method that can analyze the convective cloud with Weather Research and Forecasting (WRF) model. The potential of turbulence then needed turbulence index like Richardson Number (Ri). The Richardson Number Index (Ri) takes into account the value of Vertical Wind Shear and the stability of the air. This causes Ri to be considered ideal for describing Clear Air Turbulence (CAT). In this study, the negative Ri results tend to experience mild turbulence due to atmospheric conditions with Ri < 0.25 has a great chance of turbulence.

Evaluation of Multimodel-Based Ensemble Forecasts for Clear-Air Turbulence

AbstractTo test more consistent and reliable upper-level turbulence forecasts, seven global numerical weather prediction (NWP) model outputs are used to construct the multimodel-based ensemble forecasts for clear-air turbulence (CAT). We used the updated version of the well-known Ellrod index, the Ellrod–Knox index (EKI), which is currently an operational CAT diagnostic for the significant weather chart at one of the World Area Forecast Centers. In this study, we tested two types of ensemble forecasts. First is an ensemble mean of all EKI forecasts from the NWP models. Second is a probabilistic forecast that is computed by counting how many individual EKI values from the seven NWP models exceed a certain EKI threshold at each grid point. Here, to calibrate the best EKI thresholds for the moderate-or-greater CAT intensity, the individual EKI thresholds, which vary depending on the resolutions and configurations of the NWP models, are selected using the 95th, 98th, and 98th percentiles of the probability density functions for the EKIs derived from the seven NWP models for a 6-month period. Finally, performance skills of both the ensemble mean and probabilistic forecasts are evaluated against the observations of in situ aircraft eddy dissipation rate and pilot reports. As a result, the ensemble mean forecast shows a better performance skill than the individual EKI forecasts. The reliability diagram for the probabilistic forecast gives a better reliability when using high-percentile EKI values as the threshold although it still suffers overestimation of CAT events likely due to the lack of observation and ensemble spreads.

Lidar observations of the structure and evolution of aerosol clusters in the DELICAT project

The main goal of the DELICAT (DEmonstration of LIdar based CAT detection) project was to check the possibility of early detection of clear air turbulence regions by recording the intensity of reverse molecular scattering by means of a specially designed lidar mounted on the aircraft, operating in the ultraviolet range of wavelengths. Flight tests were conducted from July 17 to August 12, 2013 over the western part of Europe and the adjacent maritime territories. Although measurements in a clear sky were preferred, in most cases aerosol clusters of different power were present along the flight paths. Aerosol clusters should be considered as a source of interference, but their observations also provide an opportunity to obtain information on the properties of the aerosol and the spatial-temporal evolution of clusters. In this paper, we give statistics of observations of aerosol clusters in flights, describe the diversity of their types and trace the influence of flight parameters on the observed characteristics of clusters.

Frequency Change of Clear-Air Turbulence over the North Pacific under 2 K Global Warming – Ensemble Projections Using a 60-km Atmospheric General Circulation Model

Frequency Change of Clear-Air Turbulence over the North Pacific under 2 K Global Warming – Ensemble Projections Using a 60-km Atmospheric General Circulation Model

Can turbulence within the field of view cause significant biases in radiative transfer modeling at the 183 GHz band?

Abstract. The hypothesis whether turbulence within the passive microwave sounders field of view can cause significant biases in radiative transfer modeling at the 183 GHz water vapor absorption band is tested. A novel method to calculate the effects of turbulence in radiative transfer modeling is presented. It is shown that the turbulent nature of water vapor in the atmosphere can be a critical component of radiative transfer modeling in this band. Radiative transfer simulations are performed comparing a uniform field with a turbulent one. These comparisons show frequency dependent biases which can be up to several kelvin in brightness temperature. These biases can match experimentally observed biases, such as the ones reported in Brogniez et al. (2016). Our simulations show that those biases could be explained as an effect of high-intensity turbulence in the upper troposphere. These high turbulence phenomena are common in clear air turbulence, storm or cumulus cloud situations.

Improvements in Nonconvective Aviation Turbulence Prediction for the World Area Forecast System

AbstractFor the next generation of the World Area Forecast System (WAFS), the global Graphical Turbulence Guidance (G-GTG) has been developed using global numerical weather prediction (NWP) model outputs as an input to compute a set of turbulence diagnostics, identifying strong spatial gradients of meteorological variables associated with clear-air turbulence (CAT) and mountain-wave turbulence (MWT). The G-GTG provides an atmospheric turbulence intensity metric of energy dissipation rate (EDR) to the 1/3 power (m2/3 s–1), which is the International Civil Aviation Organization (ICAO) standard for aircraft reporting. Deterministic CAT and MWT EDR forecasts are derived from ensembles of calibrated multiple CAT and MWT diagnostics, respectively, with the final forecast provided by the gridpoint-by-gridpoint maximum of the CAT and MWT ensemble means. In addition, a probabilistic EDR forecast is produced by the percentage agreement of the individual CAT and MWT diagnostics that exceed a certain EDR threshold for turbulence (i.e., multidiagnostic ensemble). Objective evaluations of the G-GTG against global in situ EDR measurement data show that both deterministic and probabilistic G-GTG significantly improve the current WAFS CAT product, mainly because the G-GTG takes into account turbulence from various sources related to CAT and MWT. The probabilistic G-GTG forecast is more reliable at predicting light-or-greater (EDR &gt; 0.15)- than moderate-or-greater (EDR &gt; 0.22)-level turbulence, although it suffers from overforecasting. This will be improved in the future when we use this methodology with NWP ensembles and more observation data will be available for calibration. We expect that the new G-GTG forecasts will be beneficial to aviation users globally.

Comparing the CO2 emission by kilogram transported in normal flight and in a clear air turbulence area

Air transport contributes with 2% of global emission to greenhouse effect. This situation aggravates the occurrence of meteorological phenomenon of Clear Air Turbulence (CAT). When an aircraft flies over a CAT area they increase the fuel consumption to adequate the speed and lift. Consuming more fuel aggravates the greenhouse effect. The objective of this paper is to quantify the emission of pollutants in the atmosphere, through the application of a conceptual and dynamic decision model for microscopic analysis of air traffic flow. The method consists of delimiting the CAT area, staggering flight parameters, and calculating CO2 emissions in normal conditions and with CAT. When there is CAT, there are two options: proceed in the same flight plan or make a route deviation to a lower altitude. The comparison between the conditions of flights with the presence of CAT, infers that changes to a lower altitude during the flights is viable, because there is a reduction in fuel consumption and hence in CO2 emissions.

Doppler Profile Tracing Using MPCF on MU Radar and Sodar: Performance Analysis

Wind profilers (WPs) are ground-based pulsed Doppler radars operating in the UHF/very high frequency (VHF) bands. They use backscatter from clear-air turbulence as a tracer of background wind in the troposphere and up to mesosphere. Wind profiling systems range from large research radar systems to smaller operational units. In recent times, multiparameter cost function (MPCF) has emerged as a computationally efficient Doppler profile estimation method. It has been claimed that the MPCF method can be easily migrated to any vertically sounding clear-air wind profiling system that works in Doppler beam swinging mode. In order to investigate this claim, MPCF was applied to the following wind profiling systems: the middle and upper atmosphere (MU) radar, located at Shigaraki, Japan, which is an active phased array VHF radar system with 475 transceivers, and Doppler sodar, located at Pune, India, which uses acoustic frequency to obtain echoes in the planetary boundary layer. Both these systems are complementary in the sense that they cover ranges from ground to about 80 km. During the experimentation, the MPCF algorithm did not need any change of parameters except the matching of the data reading formats. The results of MPCF on the MU radar were validated with Radiosonde data. These results indicate that the MPCF works seamlessly on all types of WP systems irrespective of the carrier, range, and radar type.