Abstract
Atmospheric solar tides observed in the middle atmosphere reveal various temporal and spatial variability, and snap-shot observations sometimes deviate from “average” tidal features. To evaluate transient behavior of this global-scale wave, time evolution of solar atmospheric tide has been investigated by means of time-dependent numerical modelling. A spectral method is applied to the primitive equation system, in which horizontal momentum equations, thermodynamic equation and lower boundary condition for the geopotential height are solved by the appropriate time integration scheme, with the continuity and hydrostatic equations solved downward and upward, respectively, in the altitude direction. Boundary conditions assume non-slip hypothesis below and diffusion-dominated spongy layer above. Time evolution of (2, 2) mode for the forcing with rise time of 2 days exhibits set-up time of 20 days or more, and vertical group propagation in the lower altitude region is shown to be comparable to theoretical value of about 0.6 m/sec. For (1, -2) mode, set-up time is short, and oscillation of complex amplitude with period of about 1.2 day is detected which might be due to beats with excited 5∼7-day period normal mode wave. Propagating (1, 1) mode shows smaller vertical group velocity for the rise-up of tidal structure. Semidiurnal tide excited non-linearly by the diurnal thermotidal forcing shows horizontal velocity component of some 10 m/sec at around 90 km which might affect the linear description of semidiurnal tide. These and other findings are informative to delineate tidal variability quantitatively.
