Analysis of fast and ultrafast Kelvin waves simulated by the Kyushu-GCM

Abstract

Properties of fast and ultrafast Kelvin waves from the lower stratosphere to the mesosphere and lower thermosphere (MLT) region simulated by the Kyushu University Middle Atmosphere General Circulation Model (Kyushu-GCM) have been investigated based on a single-year dataset. Eastward-propagating components with zonal wavenumber s=1 have been shown to exhibit Kelvin wave structures. These waves are fast and ultrafast Kelvin waves in the lower mesosphere and the MLT region, respectively. Signatures of Kelvin wave structures are recognized throughout the region from the lower stratosphere to the lower thermosphere. Vertical wavelengths of fast and ultrafast Kelvin waves are identified in the altitude range from 15 to 20km and between 30 and 60km, respectively. Wave amplitudes increase with increasing height, where the growth rate nearly conserves the wave energy density, which is consistent with theoretical predictions and observational results. Eastward-tilting phase lines show upward energy and downward phase propagation. Eastward-tilting vertical structures, vertically growing wave amplitudes, and upward Eliassen–Palm (EP) flux vectors show that the Kelvin waves propagate vertically, implying that they are excited in the troposphere. Dissipation of ultrafast Kelvin waves in the MLT region is highly visible at heights of 90–100km. The zonal accelerations caused by this wave dissipation fluctuate over time and are characterized by values between 8 and −0.5ms−1day−1, while the annual average is approximately 1ms−1day−1 at 100km height.