10:45 AM - 12:15 PM
[AAS09-P10] A study on climatology of normal modes with periods of several days using long-term reanalysis data for the whole neutral atmosphere
Keywords:Normal mode, Middle atmosphere, Atmospheric dynamics
Wave solutions derived from Laplace’s tidal equation include normal modes of the Earth atmosphere. In a uniform background field, it is known that the normal modes have the vertical structure of an external wave and a meridional structure represented by the Hough function, which is an eigenfunction of the tidal equation. In the real atmosphere, observational and theoretical studies reported that the spatial structure of the normal modes changes seasonally because the background field is not uniform and has seasonal variation. The seasonal change of the wave structure is generally considered to be small for high-frequency modes such as gravity modes and large for low-frequency modes such as Rossby modes and Rossby-gravity modes, likely due to the degree of the Doppler effect and others. Most previous studies of the global structure of the normal modes are limited to case studies and/or studies focusing only on specific altitudes using satellite data. So far, little research has been carried out on its climatological characteristics in the whole middle atmosphere. Moreover, the structure of the horizontal wind component of the normal modes in the middle atmosphere, which is difficult to observe directly with satellites, is also not well understood. In this study, we investigate the climatological characteristics of the global structure of the normal modes with periods of several days using long-term reanalysis data for the whole neutral atmosphere produced by the data assimilation system, JAGUAR-DAS.
We used the data over 16 years from September 2004 to August 2020. Previous studies indicated that the quasi-2-day wave (QTDW) and 4-day wave predominate in the upper mesosphere. Thus, we performed an analysis of zonal wavenumber-frequency spectra for the geopotential at a height of 0.01 hPa, which corresponds to the upper mesosphere. Based on the results, we extracted the QTDW, 4-day wave, 5-day wave, and 16-day wave.
The seasonal variations of the geopotential component of each wave are generally consistent with those reported in previous studies. The amplitude of the QTDW is particularly large in the summer mesosphere for all the components of geopotential and zonal and meridional winds. It is confirmed that phase structure of each component is consistent with Rossby-gravity waves. The amplitudes of the 4-day wave and 5-day wave are particularly large in the mesosphere and lower thermosphere in May and August in the Northern Hemisphere and in November and February in the Southern Hemisphere, and geopotential and horizontal wind components satisfy a geostrophic balance. It is shown that critical levels for these three waves are observed in the summer mesosphere. The amplitudes are maximized above these critical levels. This fact suggests that the QTDW, 4-day wave and 5-day wave are excited in the summer mesosphere. The amplitude of 16-day wave is large around the stratopause of winter hemisphere, which is larger in the Northern Hemisphere than that in the Southern Hemisphere. For 16-day wave, the relation between the locations of the amplitude maxima and its critical level was not clear compared with other waves.
We used the data over 16 years from September 2004 to August 2020. Previous studies indicated that the quasi-2-day wave (QTDW) and 4-day wave predominate in the upper mesosphere. Thus, we performed an analysis of zonal wavenumber-frequency spectra for the geopotential at a height of 0.01 hPa, which corresponds to the upper mesosphere. Based on the results, we extracted the QTDW, 4-day wave, 5-day wave, and 16-day wave.
The seasonal variations of the geopotential component of each wave are generally consistent with those reported in previous studies. The amplitude of the QTDW is particularly large in the summer mesosphere for all the components of geopotential and zonal and meridional winds. It is confirmed that phase structure of each component is consistent with Rossby-gravity waves. The amplitudes of the 4-day wave and 5-day wave are particularly large in the mesosphere and lower thermosphere in May and August in the Northern Hemisphere and in November and February in the Southern Hemisphere, and geopotential and horizontal wind components satisfy a geostrophic balance. It is shown that critical levels for these three waves are observed in the summer mesosphere. The amplitudes are maximized above these critical levels. This fact suggests that the QTDW, 4-day wave and 5-day wave are excited in the summer mesosphere. The amplitude of 16-day wave is large around the stratopause of winter hemisphere, which is larger in the Northern Hemisphere than that in the Southern Hemisphere. For 16-day wave, the relation between the locations of the amplitude maxima and its critical level was not clear compared with other waves.