Global reanalysis data sets cover the globe from the surface to the upper stratosphere at 6(3)-hourly time resolutions that make it possible to study diurnal tides in the lower atmosphere. We compare diurnal tidal component in six different reanalysis data sets (MERRA, ERA-Interim, CFSR, JRA25, NCEP1 and NCEP2) with TIMED/SABER temperature data between 20 km and 65 km altitudes. The horizontal and vertical structures reasonably agree between SABER and reanalyses, although the amplitudes are up to 30-50% smaller in the reanalyses than in the SABER in the upper stratosphere to lower mesosphere. Next, the dynamical processes of migrating diurnal tide (DW1) are investigated using the reanalyses. The DW1 temperature amplitudes have a peak in the tropics at 20-65 km and double peaks in midlatitudes in the upper stratosphere (30-50 km). These structures are well represented by the four lowest-order classical Hough modes. The vertical propagation characteristics for each Hough mode are consistent with the prediction by classical theory. The amplitudes in the tropical stratosphere to lower mesosphere maximize at solstices; this seasonal variation is not consistent with that of diabatic heating. Using a linear model and a perturbation theory, it is found that in addition to primary tides generated by pure diabatic heating, secondary tides generated by meridional advection of background zonal momentum have non-negligible contributions to the DW1, creating the above mentioned seasonal variations.

2011 年度気象学セミナー