Gravity wave generation in the lower stratosphere due to passage of the typhoon 9426 (Orchid) observed by the MU radar at Shigaraki (34.85°N, 136.10°E)

Abstract

Intense gravity wave activities were investigated in the lower stratosphere during the typhoon 9426. Strong vertical winds were observed just a few hours before the arrival of the typhoon‐center at the MU radar site. About 30 min to 1 hour after the typhoon‐center had passed, a considerable reduction in vertical wind amplitude was detected. Dominant gravity waves showed time period in the range of 7–8 min, 15 min, and 40–60 min in the upper troposphere and lower stratosphere. In the vicinity of the central region of the typhoon, a gravity wave was observed, which was monochromatic in nature with a vertical wavelength ∼3 km between 1.5 km and 23 km height. In the lower stratosphere, the horizontal wavelength for the prominent period was detected in the range of 10–15 km (for 15 min wave period) and 25–50 km (for 40–60 min wave period). The vertical wavelength of these waves was examined from 2.5 km to 4.0 km. In the horizontal direction, the intrinsic group velocity was estimated between 9 ± 2 and 11 ± 2 m/s. Near the tropopause, the average direction of group velocity was assessed at about 20° ± 3° from the horizontal. The generation of gravity wave like features, in the lower stratosphere, is believed induced by convection, as the low temperature of the clouds indicates a deep penetration over the radar region as seen in the satellite GMS images.