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Fronts
Published in Yeqiao Wang, Atmosphere and Climate, 2020
Jesse Norris, David M. Schultz
Frontogenesis is the process by which fronts form. It is defined mathematically as an increase in the horizontal temperature gradient over time.[1] The opposite is frontolysis, a decrease in the horizontal temperature gradient over time. A front can form from a broad region of initially weak temperature gradient that is increased as a result of the large-scale wind pattern. Two such wind patterns promote frontogenesis: confluence and horizontal shear (Figure 12.1). Confluence is where two or more airstreams originating far from one another meet and merge into a single airstream (for example, airstreams from the southwest and southeast meeting along a north-south-oriented axis and flowing north; Figure 12.1a), similar to the manner in which rivers merge at a confluence. If the two airstreams have different temperatures, then a front can form roughly along the axis of confluence.® In contrast, horizontal wind shear is where wind speed changes in a direction perpendicular to the flow (for example, wind speed changing from southerly to northerly from west to east). If the isotherms are oriented in a way to be rotated by the shear, then a front may form (Figure 12.1b).[3].
Economics of aviation safety and security
Published in Bijan Vasigh, Ken Fleming, Thomas Tacker, Introduction to Air Transport Economics, 2018
Bijan Vasigh, Ken Fleming, Thomas Tacker
Wind shear represents another significant threat to aircraft, since it can cause an aircraft to become uncontrollable. Wind shear was previously undetectable; however, through government and industry research, warning devices have been created to alert pilots of possible wind shear conditions. Based on the wind shear warnings, regulations have been developed to help ensure aircraft do not fly during dangerous wind shear conditions. While the American Eagle ATR-72 de-icing accident highlights the fact that fatal accidents still occur due to ice forming on the wings, advancements in anti-icing have significantly reduced the number of icing accidents. Aircraft manufacturers have designed aircraft with anti-icing boots, while chemical compositions have enabled de-icing to occur on the ground.
Weather radar
Published in Mike Tooley, David Wyatt, Aircraft Communications and Navigation Systems, 2017
Wind shear can occur in both the vertical and horizontal directions; this is particularly hazardous to aircraft during take -off and landing. Specific weather conditions known as microbursts cause short-lived, rapid air movements from clouds towards the ground. When the air from the microburst reaches the ground it spreads in all directions, this has an effect on the aircraft depending on its relative position to the microburst. Referring to figure 20.13, when approaching the microburst, it creates an increase in headwind causing a temporary increase of airspeed and lift for an aircraft approaching the cloud; if the pilot were unaware of the condition creating the increased airspeed, the normal reaction would be to reduce power. When flying through the microburst, the aircraft is subjected to a downdraught. As the aircraft exits the microburst, the downdraught now becomes a tailwind, thereby reducing airspeed and lift. This complete sequence of events happens very quickly, and could lead to a sudden loss of airspeed and altitude. In the takeoff and climb-out phase of flight, an aircraft is flying just above stall speed; wind shear is a severe threat. During approach and landing, engine thrust will be low; if a microburst is encountered, the crew will have to react very quickly to recognise and compensate for these conditions.
A case study of wind turbine loads and performance using steady-state analysis of BEM
Published in International Journal of Sustainable Energy, 2021
Vasishta Bhargava, Sainath Kasuba, Satya Prasad Maddula, Donepudi Jagadish, Md Akhtar Khan, Chinmaya Prasad Padhy, Hari Prasad Chinta, Chandra Sekhar Verma Chekuri, Yagya Dutta Dwivedi
Wind shear is a measure of gradient in wind speed and it can vary in the horizontal and vertical direction in the atmosphere. Since wind speed in the atmosphere changes randomly, it is governed by parameters such as turbulence intensity, roughness height. Further, mean wind speeds are found to increase with height above ground within atmospheric boundary layer. For a given site, the characteristics of atmospheric flow over turbine in practice are the function of several orography factors like obstacles e.g. buildings, trees, vegetation and expressed by reference surface roughness, z0 The turbulence characteristics of wind field seen by a wind turbine rotor can thus be conveniently expressed in terms of roughness height, z, above the ground and wind shear exponent, ε. For a turbine operating at a wind site, the wind shear approximation can be done either by power law or logarithmic law given by Equations (20) and (21), respectively (Göçmen et al. 2016). For the present BEM study, a wind shear exponent of 0.1 for power law in Equation (20) has been assumed which represents the plain or open grass land with few dispersed vegetation.