Clear Air Turbulence and Upper Level Meteorological Patterns

1969 ◽  
pp. 337-360
Author(s):  
DeVer Colson
Keyword(s):  
Air Turbulence ◽  
Upper Level

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

Atmosphere ◽  
2020 ◽  
Vol 11 (10) ◽  
pp. 1099
Author(s):  
Sabina Ștefan ◽  
Bogdan Antonescu ◽  
Ana Denisa Urlea ◽  
Livius Buzdugan ◽  
Meda Daniela Andrei ◽  
...  
Keyword(s):  
Large Scale ◽  
Lower Stratosphere ◽  
Meteorological Data ◽  
Horizontal Temperature Gradient ◽  
Jet Streams ◽  
Mountain Waves ◽  
Upper Troposphere ◽  
Air Turbulence ◽  
Upper Level ◽  
Tropopause Folding

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.

scholarly journals Observations and Numerical Simulations of Inertia–Gravity Waves and Shearing Instabilities in the Vicinity of a Jet Stream

2004 ◽  
Vol 61 (22) ◽  
pp. 2692-2706 ◽  
Author(s):  
Todd P. Lane ◽  
James D. Doyle ◽  
Riwal Plougonven ◽  
Melvyn A. Shapiro ◽  
Robert D. Sharman
Keyword(s):  
Numerical Simulations ◽  
Gravity Waves ◽  
Jet Stream ◽  
Sheared Flow ◽  
Mesoscale Structure ◽  
Air Turbulence ◽  
Research Aircraft ◽  
Wave Development ◽  
Upper Level ◽  
Inertia Gravity Waves

Abstract The characteristics and dynamics of inertia–gravity waves generated in the vicinity of an intense jet stream/ upper-level frontal system on 18 February 2001 are investigated using observations from the NOAA Gulfstream-IV research aircraft and numerical simulations. Aircraft dropsonde observations and numerical simulations elucidate the detailed mesoscale structure of this system, including its associated inertia–gravity waves and clear-air turbulence. Results from a multiply nested numerical model show inertia–gravity wave development above the developing jet/front system. These inertia–gravity waves propagate through the highly sheared flow above the jet stream, perturb the background wind shear and stability, and create bands of reduced and increased Richardson numbers. These bands of reduced Richardson numbers are regions of likely Kelvin–Helmholtz instability and a possible source of the clear-air turbulence that was observed.

Recent Advances in the Understanding of Near-Cloud Turbulence

2012 ◽  
Vol 93 (4) ◽  
pp. 499-515 ◽  
Author(s):  
Todd P. Lane ◽  
Robert D. Sharman ◽  
Stanley B. Trier ◽  
Robert G. Fovell ◽  
John K. Williams
Keyword(s):  
Hazard Identification ◽  
Commercial Aircraft ◽  
Convective Clouds ◽  
Air Turbulence ◽  
Reporting Systems ◽  
Deep Convective Clouds ◽  
Airline Passengers ◽  
Research Questions ◽  
Upper Level ◽  
The Cost

Anyone who has flown in a commercial aircraft is familiar with turbulence. Unexpected encounters with turbulence pose a safety risk to airline passengers and crew, can occasionally damage aircraft, and indirectly increase the cost of air travel. Deep convective clouds are one of the most important sources of turbulence. Cloud-induced turbulence can occur both within clouds and in the surrounding clear air. Turbulence associated with but outside of clouds is of particular concern because it is more difficult to discern using standard hazard identification technologies (e.g., satellite and radar) and thus is often the source of unexpected turbulence encounters. Although operational guidelines for avoiding near-cloud turbulence exist, they are in many ways inadequate because they were developed before the governing dynamical processes were understood. Recently, there have been significant advances in the understanding of the dynamics of near-cloud turbulence. Using examples, this article demonstrates how these advances have stemmed from improved turbulence observing and reporting systems, the establishment of archives of turbulence encounters, detailed case studies, and high-resolution numerical simulations. Some of the important phenomena that have recently been identified as contributing to near-cloud turbulence include atmospheric wave breaking, unstable upper-level thunderstorm outflows, shearing instabilities, and cirrus cloud bands. The consequences of these phenomena for developing new en route turbulence avoidance guidelines and forecasting methods are discussed, along with outstanding research questions.

Turbulence and Gravity Waves within an Upper-Level Front

2005 ◽  
Vol 62 (11) ◽  
pp. 3885-3908 ◽  
Author(s):  
Steven E. Koch ◽  
Brian D. Jamison ◽  
Chungu Lu ◽  
Tracy L. Smith ◽  
Edward I. Tollerud ◽  
...  
Keyword(s):  
Structure Function ◽  
Gravity Wave ◽  
Gravity Waves ◽  
Function Analysis ◽  
Numerical Models ◽  
Wide Spectrum ◽  
Potential Temperature ◽  
Scale Model ◽  
Air Turbulence ◽  
Upper Level

Abstract High-resolution dropwindsonde and in-flight measurements collected by a research aircraft during the Severe Clear-Air Turbulence Colliding with Aircraft Traffic (SCATCAT) experiment and simulations from numerical models are analyzed for a clear-air turbulence event associated with an intense upper-level jet/frontal system. Spectral, wavelet, and structure function analyses performed with the 25-Hz in situ data are used to investigate the relationship between gravity waves and turbulence. Mesoscale dynamics are analyzed with the 20-km hydrostatic Rapid Update Cycle (RUC) model and a nested 1-km simulation with the nonhydrostatic Clark–Hall (CH) cloud-scale model. Turbulence occurred in association with a wide spectrum of upward propagating gravity waves above the jet core. Inertia–gravity waves were generated within a region of unbalanced frontogenesis in the vicinity of a complex tropopause fold. Turbulent kinetic energy fields forecast by the RUC and CH models displayed a strongly banded appearance associated with these mesoscale gravity waves (horizontal wavelengths of ∼120–216 km). Smaller-scale gravity wave packets (horizontal wavelengths of 1–20 km) within the mesoscale wave field perturbed the background wind shear and stability, promoting the development of bands of reduced Richardson number conducive to the generation of turbulence. The wavelet analysis revealed that brief episodes of high turbulent energy were closely associated with gravity wave occurrences. Structure function analysis provided evidence that turbulence was most strongly forced at a horizontal scale of 700 m. Fluctuations in ozone measured by the aircraft correlated highly with potential temperature fluctuations and the occurrence of turbulent patches at altitudes just above the jet core, but not at higher flight levels, even though the ozone fluctuations were much larger aloft. These results suggest the existence of remnant “fossil turbulence” from earlier events at higher levels, and that ozone cannot be used as a substitute for more direct measures of turbulence. The findings here do suggest that automated turbulence forecasting algorithms should include some reliable measure of gravity wave activity.

scholarly journals A Numerical Study of Clear-Air Turbulence (CAT) Encounters over South Korea on 2 April 2007

2010 ◽  
Vol 49 (12) ◽  
pp. 2381-2403 ◽  
Author(s):  
Jung-Hoon Kim ◽  
Hye-Yeong Chun
Keyword(s):  
South Korea ◽  
West Coast ◽  
Korean Peninsula ◽  
Numerical Study ◽  
Vertical Wind Shear ◽  
Jeju Island ◽  
Convective System ◽  
The West ◽  
Air Turbulence ◽  
Upper Level

Abstract On 2 April 2007, nine cases of moderate-or-greater-level clear-air turbulence (CAT) were observed from pilot reports over South Korea during the 6.5 h from 0200 to 0830 UTC. Those CAT events occurred in three different regions of South Korea: the west coast, Jeju Island, and the eastern mountain areas. The characteristics and possible mechanisms of the CAT events in the different regions are investigated using the Weather Research and Forecasting model. The simulation consists of six nested domains focused on the Korean Peninsula, with the finest horizontal grid spacing of 0.37 km. The simulated wind and temperature fields in a 30-km coarse domain are in good agreement with those of the Regional Data Assimilation and Prediction System (RDAPS) analysis data of the Korean Meteorological Administration and observed soundings of operational radiosondes over South Korea. In synoptic features, an upper-level front associated with strong meridional temperature gradients is intensified, and the jet stream passing through the central part of the Korean Peninsula exceeds 70 m s−1. Location and timing of the observed CAT events are reproduced in the finest domains of the simulated results in three different regions. Generation mechanisms of the CAT events revealed in the model results are somewhat different in the three regions. In the west coast area, the tropopause is deeply folded down to about z = 4 km because of the strengthening of an upper-level front, and the maximized vertical wind shear below the jet core produces localized turbulence. In the Jeju Island area, localized mixing and turbulence are generated on the anticyclonic shear side of the enhanced jet, where inertial instability and ageostrophic flow are intensified in the lee side of the convective system. In the eastern mountain area, large-amplitude gravity waves induced by complex terrain propagate vertically and subsequently break down over the lee side of topography, causing localized turbulence. For most of the CAT processes considered, except for the mountain-wave breaking, standard NWP resolutions of tens of kilometers are adequate to capture the CAT events.

scholarly journals Influences of Moist Convection on a Cold-Season Outbreak of Clear-Air Turbulence (CAT)

2012 ◽  
Vol 140 (8) ◽  
pp. 2477-2496 ◽  
Author(s):  
Stanley B. Trier ◽  
Robert D. Sharman ◽  
Todd P. Lane
Keyword(s):  
Gravity Waves ◽  
Cold Season ◽  
Vertical Shear ◽  
Moist Convection ◽  
Flow Perturbation ◽  
Air Turbulence ◽  
Central United States ◽  
Upper Level ◽  
Vertically Propagating ◽  
Upper Level Jet

Abstract The 9–10 March 2006 aviation turbulence outbreak over the central United States is examined using observations and numerical simulations. Though the turbulence occurs within a deep synoptic cyclone with widespread precipitation, comparison of reports from commercial aircraft with radar and satellite data reveals the majority of the turbulence to be in clear air. This clear-air turbulence (CAT) is located above a strong upper-level jet, where vertical shear ranged between 20 and 30 m s−1 km−1. Comparison of a moist simulation with a dry simulation reveals that simulated vertical shear and subgrid turbulence kinetic energy is significantly enhanced by the anticyclonic upper-level flow perturbation associated with the organized convection in regions of observed CAT. A higher-resolution simulation is used to examine turbulence mechanisms in two primary clusters of reported moderate and severe turbulence. In the northern cluster where vertical shear is strongest, the simulated turbulence arises from Kelvin–Helmholtz (KH) instability. The turbulence farther south occurs several kilometers above shallow, but vigorous, moist convection. There, the simulated turbulence is influenced by vertically propagating gravity waves initiated when the convection impinges on a lowered tropopause. In some locations these gravity waves amplify and break leading directly to turbulence, while in others they aid turbulence development by helping excite KH instability within the layers of strongest vertical shear above them. Although both clusters of turbulence occur either above or laterally displaced from cloud, a shared characteristic is their owed existence to moist convection within the wintertime cyclone, which distinguishes them from traditional CAT.

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

2019 ◽  
Vol 76 (6) ◽  
pp. 1709-1726 ◽  
Author(s):  
Elisa Spreitzer ◽  
Roman Attinger ◽  
Maxi Boettcher ◽  
Richard Forbes ◽  
Heini Wernli ◽  
...  
Keyword(s):  
Potential Vorticity ◽  
Vertical Section ◽  
Inversion Layer ◽  
Conveyor Belt ◽  
Weather Forecasts ◽  
Air Turbulence ◽  
Flow Features ◽  
Upper Level ◽  
Pv Generation ◽  
Ecmwf Model

Abstract The 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.

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

2019 ◽  
Vol 35 (2) ◽  
pp. 507-521
Author(s):  
Dan-Bi Lee ◽  
Hye-Yeong Chun ◽  
Jung-Hoon Kim
Keyword(s):  
Weather Prediction ◽  
Grid Point ◽  
Probabilistic Forecast ◽  
Ensemble Forecasts ◽  
Air Turbulence ◽  
Probabilistic Forecasts ◽  
Ensemble Mean ◽  
Upper Level ◽  
Nwp Model ◽  
The Individual

Abstract To 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.

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