Analysis of the migrating diurnal tide in the Whole Atmosphere Community Climate Model

In an effort to understand the structure of the migrating diurnal tide in version 3 of the Whole Atmosphere Community Climate Model (WACCM3), we compare model runs of migrating diurnal wind and temperature fields from WACCM3 with similar results from the Global Scale Wave Model (GSWM), which has undergone extensive comparisons with mesospheric and lower thermospheric wind and temperature observations, and the Global Emperical Wind Model (GEWM), an empirical model of mesopheric and lower thermospheric winds derived from satellite and ground based radar data. We find that WACCM3 reproduces the salient features of the migrating diurnal tide, with increased inter-hemispheric asymmetry at the solstices compared to GSWM, but similar to those observed in the GEWM results. The magnitude of the WACCM3 tidal amplitudes below 110 km fluctuate significantly throughout the year relative to the GSWM amplitudes, displaying a smaller amplitude than GSWM at equinox but stronger around the solstices. Additionally, the WACCM3 results are observed to display unrealistic behavior from 110-120 km at high latitudes. The WACCM3 result appears to be in closer agreement with the GEWM, displaying similar spatial structure and zonal wind amplitudes, though the WACCM3 meridional wind amplitudes can be up to 25% larger than those in the GEWM.; Variables from WACCM3 which are known to affect the amplitude and structure of the migrating diurnal tide, such as the zonal mean zonal winds, solar forcing, and eddy diffusion coefficients are compared with values from the GSWM to understand reasons for the differences in the tidal response. It has been found that the (1,1) component of tropospheric solar forcing in WACCM3 is roughly 60% less than the values used in the GSWM throughout the year, while the total eddy diffusion in the tropical mesosphere and lower thermosphere region is between 60 and 80% less than the GSWM values. As a result, much of the observed seasonal change between the tidal amplitudes in WACCM3 and GSWM is related to this change in diffusion. Significant differences are also found between the zonal mean zonal winds generated self-consistantly in WACCM3 and those specified in the GSWM. These zonal mean zonal winds will perturb the latitudinal structure of the tide. Additionally, the GSWM and GEWM have been tuned to more closely resemble measurements based on satellite and ground based radar data respectively, which have been known to display significant differences in measured tidal amplitude. This must be taken into account when considering the relative magnitudes observed between the WACCM3 tidal amplitudes and the tidal amplitudes from the GSWM and GEWM.