On 15th January 1998, internal gravity waves in the boundary layer were observed with the operational Doppler Radar for Airport Weather (DRAW) at Narita International Airport in Japan. We researched the structures and environmental conditions of these waves during growth, propagation and breakdown phases. Although internal gravity waves can create strong wind shears near surface (the maximum wind shear was 6-7 m/sec/km in this case), they have not been targets of wind shear alert systems yet. The practical motivation for this study comes from the need to document characteristics of these waves for the purpose of avoidance aviation accidents near surface.
According to the Narita DRAW data, the pattern of these waves lasted about one hour and the area spread 80km-100km horizontally over the Pacific Ocean and the Kanto Plain. Three physical properties of these waves have been examined.
(1) The average wavelength was approximately 6.5km.
(2) The phase speed was 3-4 m/s.
(3) The direction of propagation was 50 degrees (NE).
This propagation direction was rotated 160 degrees anti-clockwise from the surface wind vector. This direction is as same as the direction from the center of a coastal extratropical cyclone to the area of these waves. It is suggested that this extratropical cyclone was a source of these waves.
According to the Narita ACARS data, which were frequent soundings of wind and temperature every 15-30 minutes reported from many commercial aircraft, the most prominent environmental characteristic is a strong inversion layer between 1-2km height at the time of these waves. The air at the bottom of the inversion was colder due to an inland air mass and the air at the top of the inversion was warmer due to a southern easterly airflow over the Pacific Ocean. It seems that this strong inversion provided an effective "duct" for these internal gravity waves generated from the coastal extratropical cyclone.