时间：2011-04-18 00:52来源：蓝天飞行翻译 作者：航空 点击：次
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b.Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types. Note, however, that microbursts commonly occur within the heavy rain portion of thunderstorms, and in much weaker, benign appearing convective cells that have little or no precipitation reaching the ground. 7.1.46 Meteorology FIG 7.1.13 Evolution of a Microburst WIND SPEEDWIND SPEED 10-20 knots10-20 knots T-5 MinT-5 Min T-2 MinT-2 Min T T + 5 MinT + 5 Min > 20 knots> 20 knots T + 10 MinT + 10 Min 0 123 SCALE (miles)SCALE (miles)Vertical cross section of the evolution of a microburst wind field. T is the time of initial divergence at the surface. The shading refers to the vector wind speeds. Figure adapted from Wilson et al., 1984, Microburst Wind Structure and Evaluation of Doppler Radar for Wind Shear Detection, DOT/FAA Report No. DOT/FAA/PM-84/29, National Technical Information Service, Springfield, VA 37 pp. c. The life cycle of a microburst as it descends in a convective rain shaft is seen in FIG 7.1.13. An important consideration for pilots is the fact that the microburst intensifies for about 5 minutes after it strikes the ground. d. Characteristics of microbursts include: 1.Size. The microburst downdraft is typically less than 1 mile in diameter as it descends from the cloud base to about 1,000.3,000 feet above the ground. In the transition zone near the ground, the downdraft changes to a horizontal outflow that can extend to approximately 2 1/2 miles in diameter. 2.Intensity. The downdrafts can be as strong as 6,000 feet per minute. Horizontal winds near the surface can be as strong as 45 knots resulting in a 90 knot shear (headwind to tailwind change for a traversing aircraft) across the microburst. These strong horizontal winds occur within a few hundred feet of the ground. 3.Visual Signs. Microbursts can be found almost anywhere that there is convective activity. They may be embedded in heavy rain associated with a thunderstorm or in light rain in benign appearing virga. When there is little or no precipitation at the surface accompanying the microburst, a ring of blowing dust may be the only visual clue of its existence. 4.Duration. An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds lasting approxi-mately 2.4 minutes. Sometimes microbursts are concentrated into a line structure, and under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are not uncommon and should be expected. Meteorology 7.1.47 FIG 7.1.14 Microburst Encounter During Takeoff A microburst encounter during takeoff. The airplane first encounters a headwind and experiences increasing performance (1), this is followed in short succession by a decreasing headwind component (2), a downdraft (3), and finally a strong tailwind (4), where 2 through 5 all result in decreasing performance of the airplane. Position (5) represents an extreme situation just prior to impact. Figure courtesy of Walter Frost, FWG Associates, Inc., Tullahoma, Tennessee. e. Microburst wind shear may create a severe experience of penetrating one is characterized in hazard for aircraft within 1,000 feet of the ground, FIG 7.1.14. The aircraft may encounter a headwind particularly during the approach to landing and (performance increasing) followed by a downdraft landing and take-off phases. The impact of a and tailwind (both performance decreasing), possibly microburst on aircraft which have the unfortunate resulting in terrain impact. 7.1.48 Meteorology FIG 7.1.15 NAS Wind Shear Product Systems f. Detection of Microbursts, Wind Shear and Gust Fronts. 1. FAA’s Integrated Wind Shear Detection Plan. (a) The FAA currently employs an integrated plan for wind shear detection that will significantly improve both the safety and capacity of the majority of the airports currently served by the air carriers. This plan integrates several programs, such as the Integrated Terminal Weather System (ITWS), Terminal Doppler Weather Radar (TDWR), Weather System Processor (WSP), and Low Level Wind Shear Alert Systems (LLWAS) into a single strategic concept that significantly improves the aviation weather information in the terminal area. (See FIG 7.1.15.)(b) The wind shear/microburst information and warnings are displayed on the ribbon display terminals (RBDT) located in the tower cabs. They are identical (and standardized) in the LLWAS, TDWR and WSP systems, and so designed that the controller does not need to interpret the data, but simply read the displayed information to the pilot. The RBDTs are constantly monitored by the controller to ensure the rapid and timely dissemination of any hazardous event(s) to the pilot. Meteorology 7.1.49 FIG 7.1.16 LLWAS Siting Criteria (c) The early detection of a wind shear/ micro.burst event, and the subsequent warning(s) issued to an aircraft on approach or departure, will alert the pilot/crew to the potential of, and to be prepared for, a situation that could become very dangerous! Without these warnings, the aircraft may NOT be able to climb out of, or safely transition, the event, resulting in a catastrophe. The air carriers, working with the FAA, have developed specialized training programs using their simulators to train and prepare their pilots on the demanding aircraft procedures required to escape these very dangerous wind shear and/or microburst encounters. 2. Low Level Wind Shear Alert System (LLWAS). (a) The LLWAS provides wind data and software processes to detect the presence of hazardous wind shear and microbursts in the vicinity of an airport. Wind sensors, mounted on poles sometimes as high as 150 feet, are (ideally) located 2,000 . 3,500 feet, but not more than 5,000 feet, from the centerline of the runway. (See FIG 7.1.16.) |
