Medium-Scale Travelling Waves in the Extra-Tropical Upper Troposphere

Abstract

Distinct medium-scale, eastward-travelling waves in the middle and upper troposphere have been detected by using both the MU radar wind data with fine time resolution and the data provided by the Japan Spectral Model (JSM) with fine horizontal resolution. The medium-scale waves appear to exist ubiquitously in a latitudinal band of 30-40°N during the spring season, when travelling long waves with a period of 3-4 days are observed. Through a detailed analysis with spectral and correlation methods, some of the wave characteristics are studied. The maximum amplitude of 4 ms^-1 for the geopotential-height wave component is observed at 250 hPa. The meridional and vertical phase tilts are small. The phase differences of the geopotential height with the horizontal wind and temperature are geostrophic and hydrostatic, respectively. The zonal wavelength and wave period are about 2100 km and 26 h, respectively. The zonal phase velocity of 22 ms^-1 is about twice as large as that of the long waves, indicating that the medium-scale waves are not due to the higher harmonics of the long waves travelling together but due to a unique mechanism of generation.