A climatological study of low-level internal gravity waves in precipitating environments over the Kanto Plain, Japan

This paper describes the first climatological study of internal gravity waves, which have horizontal wavelengths of 10km-scale, and propagate horizontally in the lower troposphere in Japan. The coherent gravity wave patterns with significant horizontal wind shear have often been observed in near surface regions far away from the suggested source areas. Therefore, such waves can be extremely hazardous for an aircraft in the takeoff and landing stages of flight. Many properties of these waves such as occurrence, detailed structure, environmental conditions, and, hence, the trapping mechanism (i.e., wave-ducting mechanism) are poorly understood due to the lack of comprehensive observations. In this study, the observations with the Doppler radar for Airport Weather (DRAW) and aircraft soundings of wind and temperature (ACARS) provided unique data of high spatial and temporal resolution, that have been used for analyzing the detailed wave structures. From 1998 to 2003, six events of internal gravity waves were observed with the Narita DRAW. All the events occurred during the cold season, i.e. from October to January. Two synoptic surface weather patterns associated with gravity waves, such as coastal cyclone and stationary front, were identified. All but one of the six observed surface weather patterns included coastal cyclones. The vertical profiles obtained with ACARS reveal two distinct air masses. One of them is the cold northerly flow near the surface. Above the cold flow, the weak southerly geostrophic wind from the synoptic system over the Pacific Ocean exists. Between these air masses, there is a strongly stable layer with vertical wind shear where the wave can be trapped and propagate horizontally. These gravity waves propagated toward the northeast quadrant. The horizontal and vertical wavelengths constituted 4.5 - 7.0km and 1.2 - 4.3km, respectively. Furthermore, all the events occurred with a wind shear greater than the warning threshold commonly accepted in aviation safety.