Assessment of microburst models for downdraft estimation

Abstract

NASA has been conducting research in the development of technologies for forward-looking, airborne, wind shear detection systems. Doppler RADAR and LIDAR are two of the technologies being tested to provide this capability. Both measure the Doppler shift from aerosols, raindrops, and other debris in the air to determine the "line-of-sight" relative velocity of the air. An inherent limitation of this type of system is its inability to measure velocities perpendicular to the line-of-sight. This limitation can result in significant underestimation of the magnitude of wind shear hazard. One solution to this line-of-sight limitation of Doppler type sensors, is to use a theoretical or empirical model of a microburst to estimate the perpendicular velocities from the measured line-of-sight values. This article is a summary of an analytical study to assess the effectiveness of three microburst models in estimating the downdraft from horizontal velocity measurements. The article discusses the development of the models and their characteristics. The models are tested at different stages in the life cycle of a microburst. Nomenclatured = diameter of vortex ring viscous core, m d min = minimum distance from a point (/, z) to a vortex ring filament, m F = wind shear hazard index f(r) = empirical model radial shaping function of the horizontal wind velocity, m/s g = gravitational acceleration, m/s 2 g(r 2 ) = empirical model radial shaping function of the vertical wind velocity p(z) = empirical model vertical shaping function of the horizontal wind velocity q(z) = empirical model vertical shaping function of the vertical wind velocity, m/s R v = radius of the primary vortex ring, m r = radial coordinate (distance from microburst center), m r m = radial coordinate of maximum horizontal wind, m t = time in microburst life cycle, min u = wind component in the r-direction (tailwind positive), m/s u = rate of change of horizontal wind component, m/s 2 V = true airspeed, m/ŝ error = QTTOi in vertical wind component estimate, m/s w = vertical wind component (updraft positive), m/s w modei = model derived vertical wind component, m/s Z v = altitude of the primary vortex ring, m z = vertical coordinate (positive up), m z w = altitude of maximum horizontal wind, m a = empirical model shaping function variable F = circulation strength of vortex ring, m 2 /s ( = ring vortex viscous core damping factor A = empirical model scaling factor, s" 1

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