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Optimum Use of Weather Radar

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Optimum Use of Weather Radar OPERATIONS Optimum use of weather radar Optimum use of weather radar In recent years, there have been a number of flights where passengers or crew suffered injuries due to severe turbulence. In some other instances, the aircraft structure was substantially damaged following a hailstorm encounter. Clearly adverse weather can pose a threat to the safe and comfortable completion of a flight, thus it needs to be detected and avoided in a timely manner. DAVID CHRISTIAN LAURENT MARCONNET NORDEN VIDAL Flight Operations Director Flight Surveillance Systems Support & Training Operations & Manager Standards Safety Training Policy Enhancement Safety First #22 | July 2016 023 The airborne weather radar system is an essential tool for pilots to assess the intensity of convective weather ahead of the aircraft. In this respect, it enables the strategic and tactical planning of a safe flight trajectory. Weather radar technology has evolved significantly in the last few years and a range of enhanced products is now available. If properly used, they permit pilots workload to be significantly reduced while substantially reducing encounters with adverse weather. This article offers an overview of the existing weather radar technologies, and provides information and tips on how to tune the system and correctly interpret the available displays. WEATHER RADAR PRINCIPLE AND OPERATION The weather radar system installed the crew to carefully optimize its Prevention on-board aircraft provides the pilot with use. This relies primarily on a good the necessary information to avoid - not meteorological knowledge of weather through anticipation penetrate - adverse weather. phenomena, along with a good is essential. To obtain the maximum benefit from understanding of the available radar the weather radar system requires functions. Flying in adverse weather: lessons learned The aviation industry experience shows aircraft damage (fig.1). that although aircraft are equipped with These events have led us to wonder airborne weather radars, incursions why such encounters happen, and into very active convective cells still clearly show that prevention through occur, resulting in injuries or substantial anticipation is essential. (fig.1) Radome and windshield after hail encounter Weather When it comes to understanding is the active monitoring of the overall radar is of help, why aircraft fitted with technologically meteorological situation by the crew, but the crew overall advanced weather radars can end in addition to the optimum use of the up flying in such unfavorable weather weather radar and correct understanding assessment patterns, we have to consider that getting of the information displayed. We must of the weather the best out of technology onboard is not forget that weather radar is of help, just a part of the answer. A key element but the crew overall assessment of the situation plays of adverse weather avoidance strategies weather situation plays the central role. the central role. OPERATIONS Optimum use of weather radar HOW IS A CUMULONIMBUS, OFTEN CALLED “THUNDERSTORM”, STRUCTURED? Three common threats to aircraft are turbulence (which Understanding how a cumulonimbus cloud is structured is caused when two masses of air collide at different and evolves is key in dodging the associated weather speeds), hail and windshear. All three of these are disturbances. by-products of thunderstorms. High concentration of Ice Crystals FL 350 -40°C 23 000 ft -15°C 16 000 ft Freezing Level WIND SHEAR TURBULENCE WIND SHEAR TURBULENCE DUST Super-cooled liquid droplet Ice crystals Turbulence: Turbulence associated with a cumulonimbus is not it is necessary to apply recommendations for weather limited to inside the cloud. Therefore, when flying in avoidance as summarized in this article. an area where cumulonimbus clouds have developed, Hail: The presence of hail within a Risk of encountering hail relative to cumulonimbus cloud position cumulonimbus varies with altitude and wind: - Below FL 100, hail is equally likely to be encountered under the storm, in the cloud or around it (up to 2 NM). - Between FL 100 and FL 200, approximately 60 percent of hail is encountered in the cumulonimbus and 40 percent is encountered outside the cloud, under the anvil. - Above FL 200, hail is most likely to be encountered inside the cloud. When hail is encountered outside driven upward within the cloud by is less risk of hail in humid air than the cloud, usually the threat of strong drafts. It then freezes and is in dry air. In fact, moisture in the air hail is greater downwind of the transformed into hail before being behaves as a heat conductor, and cumulonimbus because moisture is blown downwind. Paradoxically, there helps to melt the hail. Safety First #22 | July 2016 025 Weather radar principle A knowledge of the radar principle tune this system and interpret the On the Airbus is paramount in order to accurately weather radar display correctly. fleet, all weather Reflectivity radars have full capability to allow Weather detection is based on the The weather radar echo returns vary reflectivity of water droplets(fig.2) . The in intensity as a function of the drop- wet turbulence weather echo appears on the Naviga- let size, composition and quantity. For detection. tion Display (ND) with a color scale that example, a water particle is five times goes from red (high reflectivity) to green more reflective than an ice particle of (low reflectivity). the same size. High Reectivity (fig.2) Wet Hail Weather radar principle Good Reectivity Liquid Rain water Wet Snow Dry Hail Dry Snow Low Reectivity Fog Drizzle Low Reectivity Some weather radars are fitted of light rainfall, depicted in green in with a turbulence display mode. normal mode, is shown in magenta This function (the TURB function) is when there is high turbulence activity. based on the Doppler effect and is The TURB function is on most weather sensitive to precipitation movement. radars only active within a range of 40 Like the weather radar, the TURB NM (Doppler measurement capability) function needs a minimum amount of and should only be used in wet precipitation to be effective. An area turbulence. DID YOU Weather radar operation KNOW Each type of weather radar has its own The flight crew uses four features to has an essential influence on the particularities. To get all the information on operate the radar: optimum tilt setting. the characteristics, limitations and opera- - Antenna tilt: this is the angle between - Gain control: this adjusts the tional recommendations of each weather the centre of the beam and the sensitivity of the receiver. radar model, the user guide of the radar horizon (fig.3). - Radar modes: weather (WX) or manufacturer needs to be studied. - Range control of the ND: this weather + turbulence (WX + T). (fig.3) Definition of the TILT OPERATIONS Optimum use of weather radar Weather radar limitations Weather radar detection capability Reflectivity One of the weather radar limitations is winds produce large scale uplifts of that it indicates only the presence of dry air. The resulting weather cells have is not directly liquid water. The consequence is that much less reflectivity than mid-latitude proportional to a thunderstorm does not have the convective cells. However, turbulence same reflectivity over its altitude range in or above such clouds may have a the level of risk because the quantity of liquid water in higher intensity than indicated by the that may be the atmosphere decreases with the image on the weather radar display. encountered. altitude (fig.4). Yet, the convective On the other hand, air close to the cloud and associated threats may sea can be very humid. In this case, extend significantly above the upper thermal convection will produce clouds detection limit of the weather radar that are full of water: these clouds will (called ‘radar top’). This means that have a high reflectivity, but may not reflectivity is not directly proportional to necessarily be a high threat. the level of risk that may be encountered: a convective cloud may be dangerous, Consequently, limitations of weather even if the radar echo is weak. radars must be well understood and complemented by basic meteorological This is particularly true for equatorial knowledge of the crew and, where overland regions where converging possible, visual observation. Turbulence Area Turbulent area, not detected Reectivity by the weather radar Visible Top Radar Top 0°C (fig.4) Reflective image of a cumulonimbus The weather radar detects: The weather radar does not detect: - Rainfall - Ice crystals, dry hail * and snow - Wet hail and wet turbulence - Clear air turbulence - Windshear - Sandstorms (solid particles are almost transparent to the radar beam) - Lightning * * The latest generations of weather radars offer hail and lightning prediction functions (see the following sections). Safety First #22 | July 2016 027 The beam attenuation phenomenon Another limitation of the weather radar and intensity of that weather may not be A black hole is called ‘shadowing’ or ‘attenuation’. accurately displayed to the pilot. What The weather radar display depends appears to be a thin or inexistent band behind a red area on signal returns: the more intense the of precipitation (fig.5) could in fact be on a weather radar precipitation, the less distance the radar the leading edge of a much larger area can see through. Therefore when the of precipitation. Secondly, any weather display should always radar echo is unable to make the two behind such
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