Low-level wind shear induced by horizontal roll vortices at Narita international airport, Japan

Abstract

Aircraft landing and taking off at Narita International Airport in Japan frequently report low-level wind shear (LLWS), which is a local variation in the wind vector, and turbulence when the prevailing wind is southwesterly, which is crosswind to the runway. On 20 June 2012, just before touchdown, an arriving aircraft at this airport encountered LLWS that consisted of a sudden change in the wind vector from a headwind component of 5 knots (2.6 m s−1) to a tailwind component of 10 knots (5.1 m s−1). This caused a rather hard landing. As the aircraft approached, none of cumulonimbus clouds, fronts, or wind shear lines were observed around the airport. Further analysis of the data measured by the landing aircraft and observations made by the Doppler lidar revealed that the LLWS was caused by horizontal roll vortices that developed in the atmospheric boundary layer (ABL) over the Shimofusa tableland surrounding the airport. The axes of these horizontal roll vortices were nearly parallel to the mean wind direction, while their horizontal and vertical scales were approximately 800 m and 500 m, respectively. In our present study, we demonstrate that the existence of such horizontal roll vortices that cause LLWS can be effectively detected by a single Doppler lidar that utilizes backscattering from aerosols. Although the LLWS associated with horizontal roll vortices has a smaller magnitude than those caused by a microburst, gust front, or front, landing aircraft often encounter these horizontal roll vortices just before touchdown with a much higher probability than other phenomena since horizontal roll vortices occur at a horizontal spacing of approximately 800 m over a wide area during the daytime hours of a clear day.