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Atmospheric Channel
Published in Hamid Hemmati, Near-Earth Laser Communications, 2020
Clear air turbulence phenomena affect the propagation of an optical beam because the refractive index randomly varies in space and time. Mainly, random variation of the refractive index of air depends on the air mixing due to temperature variation in the atmosphere. In fact, sunlight incident upon the earth's surface causes heating of the earth's surface and the air in its proximity. This sheet of warmed air becomes less dense and rises to combine with the cooler air of the above layers, which causes air temperature to vary randomly (from point to point). Because the atmospheric refractive index depends on air temperature and density, it varies in a random fashion in space and time, and this variation is the origin of clear air turbulence.
Atmospheric Channel
Published in Hamid Hemmati, Near-Earth Laser Communications, 2018
Clear air turbulence phenomena affect the propagation of an optical beam because the refractive index randomly varies in space and time. Mainly, random variation of the refractive index of air depends on the air mixing due to temperature variation in the atmosphere. Infact, sunlight incident upon the earth’s surface causes heating of the earth’s surface and the air in its proximity. This sheet of warmed air becomes less dense and rises to combine with the cooler air of the above layers, which causes air temperature to vary randomly (from point to point). Because the atmospheric refractive index depends on air temperature and density, it varies in a random fashion in space and time, and this variation is the origin of clear air turbulence.
Remote optical sensing by laser
Published in John P. Dakin, Robert G. W. Brown, Handbook of Optoelectronics, 2017
Although clear air turbulence at high level in the atmosphere is troubling to aircraft, with risk of injury to unbelted passengers and crew, few if any aircraft have been caused to crash. Low-level wind shear, on the other hand, often but not always associated with thunderstorm activity, has caused many serious accidents, notably with aircraft in landing or take-off phase. At this stage, aircraft have a low airspeed, typically ~120 kt (~60 ms−1). In the presence of wind shear, as the aircraft passes into a region of notably different wind speed, its lift may be greatly reduced. The pilot may have insufficient time to accelerate the aircraft up to sufficient airspeed to keep it aloft. Figure 12.21 shows an early LATAS record of passage through a thunderstorm microburst during the Joint Airport Weather Studies (JAWS) trial in Colorado in 1982. In this case, the focal range was set 250–300 m ahead of the aircraft. The sensitivity extended out to 700–800 m and thus strong shear or turbulent structures entering the extended probe at longer range were evident. In Figure 12.21, the sequence of Doppler spectra in the lidar record showed a headwind that changed by over 40 kt (~20 ms−1) in about 5 s. There was an additional down draft of ~6 ms−1. Analysis of these measurements contributed to the development of a descending vortex-ring model for thunderstorm microburst behavior, in contrast to the more usual vertical jet and outflow model. Simulations with the LATAS parameters have shown that the ~5–10 s warning of shear from a probe range of ~300–600 m could be useful if heeded promptly [123–125]. Wind shear and microbursts are dynamic phenomena; indeed, the simulations showed that there was significant advantage in controlling the aircraft (a medium-size passenger jet) using the airspeed measured ~300 m ahead, but increasing the distance to 600 m produced little further improvement. Nevertheless, in terms of general operation, it would be useful to look at a greater range with pulsed systems and a number of airborne equipments were built in NASA-supported programmes. Both CO2 10 μm and solid state 1 and 2 μm lidars were successfully demonstrated at ranges to ~3 km. Comparative performance in terms of range and velocity resolution, and atmospheric factors in different conditions of rain, high humidity, etc., were assessed [126].
A systematic review of adaption to climate change impacts in the aviation sector
Published in Transport Reviews, 2023
Rachel Burbidge, Christopher Paling, Rachel M. Dunk
The World Area Forecast System currently uses deterministic models to forecast turbulence. Expanding this to probabilistic multi-model forecasts could improve accuracy and assist pilot and flight-planner decision-making (Storer et al., 2019, 2020; Williams, 2017). Ultraviolet Light Detection and Ranging (LiDAR) systems could identify clear-air turbulence up to 10–15 km ahead, allowing pilots time to warn crew and passengers or to attempt to avoid the area (Williams, 2017). However, the costs of retrofitting detection systems are currently higher than the avoidance benefits, though this may change as technology costs decrease and the need for avoidance increases (Williams, 2017). More frequent aircraft maintenance to check for turbulence damage and updated aircraft design to better resist turbulence are both required (Pümpel, 2016).
Effects of mesoscale turbulence on the wind-driven circulation in a closed basin with topography
Published in Geophysical & Astrophysical Fluid Dynamics, 2022
The present approach differs from previous studies in some fundamental aspects. Griffa and Castellari (1991) studied the combined effects of a large-scale wind plus a stochastic component, but they used a QG model with a flat bottom. Here, we consider the SW formulation allowing finite-amplitude topography, which contrasts with the gentle slopes in the QG limit (Cummins and Holloway 1994, Wang and Vallis 1994). Roubicek et al. (1995) examined the large-scale forcing in a similar basin with topography, but the effect of small-scale eddies was parametrised with a Neptune term. From a more ample perspective, the present study suggests that changes in the stochastic forcing might alter large-scale circulations, such as boundary transports or the detachment of western-intensified currents. In that vein but for the atmosphere under climate change conditions, Williams (2017) estimated an increase of the average airspace volume over the North Atlantic containing clear-air turbulence. This investigation contributes to addressing similar mechanisms in the oceanic context.
Dynamic stability of a jet near a transition in static stability
Published in Geophysical & Astrophysical Fluid Dynamics, 2021
The strong shears associated with atmospheric jet streams often lead to Kelvin–Helmholtz instabilities (as observed, e.g. by Klostermeyer and Rüster 1980, Luce et al.2008). The jet streams often exist in the vicinity of the tropopause, with its distinct change in lapse rate and corresponding change in static stability N, but the jet stream core may be above or below the tropopause. This can affect the dynamic stability of the tropopause/jet system (as outlined by Vinnichenko et al.1980). Understanding the relation between the tropopause, jet streams and clear-air turbulence continues to be a major research question for improved aviation turbulence prediction (Sharman et al.2016).