Solar thermal tides are global-scale waves that dominate the dynamical motion in the mesosphere and lower thermosphere (MLT) because their amplitude grows with altitude to conserve wave energy. When propagating into the MLT region, the horizontal wind tidal amplitude can reach the magnitude of the mean wind. While the diurnal tide tends to be confined equatorward of 30°, the semidiurnal tide reaches its peak amplitude in the midlatitudes [Forbes, 1982b, 1995; Vincent et al., 1989; Manson et al., 1989; Manson and Meek, 1991]. The location of the Colorado State University Na lidar at Fort Collins, Colorado (40.6°N, 105.1°W), and mesopause region (80–110 km) coverage, along with fluorescence lidar's advantages of high temporal and spatial resolution and the capability of full diurnal cycle simultaneous observation of temperature and horizontal wind, provide a unique opportunity to study tidal perturbations and their variability [She, 2004; She et al., 2004].
 At a single longitude, one may observe local oscillations at harmonics of the diurnal frequency but not be able to determine whether these oscillations are global (migrating or nonmigrating), local, or superposition of the two. The characteristics of solar tidal waves may be deduced either from zonally averaged local time observations at multiple longitudes that cover full diurnal cycles or from a combination of satellite observations, providing partial local time global coverage, and ground-based full diurnal cycle observations at a fixed location. However, both of these opportunities for joint observation are rare. The semidiurnal tidal period perturbations, reported in this paper, are considered to be monthly means, each computed from about 10 days of full diurnal cycle observations. This amount of data is necessary to average out the variability due to interactions with gravity waves (GWs) and modulation by mean state variation and by short-period planetary waves (PWs) (e.g., 2-day and 5-day waves), leaving only the prevailing tidal periods of interest. Such data are the closest observational representation of solar tides at one location, including both migrating and nonmigrating components, both of which are essential for the evaluation of global models. The seasonal variations of the diurnal tidal period perturbations, which are dominated by the migrating solar thermal tide, in temperature and zonal and meridional winds in the mesopause region over Fort Collins were reported on the basis of the data set from 2002 to 2003 [Yuan et al., 2006]. This paper will focus on the semidiurnal tidal perturbation in these three thermodynamic variables, on the basis of an extended observation period from May 2002 to April 2006.
 The factors that contribute to the semidiurnal tidal variations in the MLT include (1) tidal forcing in the troposphere and stratosphere, mainly H2O and O3 absorption of solar radiation [Forbes, 1995], as well as latent heat release due to raindrop formation [Hagan and Forbes, 2003], (2) modification of the propagation of semidiurnal tidal modes by vertical profiles of temperature and zonal wind, that is, so-called refractive effects [Forbes and Vincent, 1989; Riggin et al., 2003], (3) “mode coupling” resulting from the nonlinear effects of background mean zonal wind and the meridional temperature gradient on the tidal vertical propagation [Lindzen and Hong, 1974; Walterscheid et al., 1980], and (4) nonlinear interactions between PWs and migrating tides [Beard et al., 1999]. There is no significant tidal source in the mesopause region, except for in situ forcing due to chemical [Mlynczak and Solomon, 1993] and secondary ozone heating [Thomas, 1990], which is not as significant as the semidiurnal tidal forcing from the troposphere and stratosphere [Hagan, 1996]. Eddy diffusion due to GW saturation and its related energy dissipation should have a much less significant influence on the semidiurnal tide than it has on the diurnal tide, owing to the longer vertical wavelength (fast vertical propagation) of the semidiurnal tide [Forbes, 1982a], with the exception of higher-order modes such as (2, 5) and (2, 6).
 There have been many tidal studies of horizontal winds in the mesopause region based on ground-based radar observations [Tsuda et al., 1988; Vincent et al., 1989; Manson et al., 1989; Avery et al., 1989; Franke and Thorsen, 1993]. Although relative temperature measurements are possible with meteor radar [Tsutsumi et al., 1996; Hocking et al., 1997], studies of tidal temperature variations from radar observations have been rare. Temperature measurements with high spatial resolution in the mesopause region are performed routinely by sodium and potassium lidars, but they are currently generally limited to nighttime conditions at this juncture. Although they suffer from aliasing due to gaps in the data, the amplitude and phase of the semidiurnal temperature tides have been well determined using multinight observations made over the course of many hours each night [Williams et al., 1998]. Daytime temperature measurements with sodium lidar have been possible since 1995 [Chen et al., 1996] and more recently with potassium lidar [Fricke-Begemann et al., 2002], enabling the determination of temperature diurnal tidal period oscillation based on full diurnal cycle observations in the mesopause region. To date, the only two sets of diurnal data with yearlong coverage have led to published studies on seasonal variations of diurnal and semidiurnal temperature tides [States and Gardner, 2000; She et al., 2002]. Since May 2002, regular full diurnal cycle observations of mesopause region temperature and zonal and meridional winds have been conducted at Colorado State University. By April 2006, the combined data set consisted of 120 full diurnal cycles distributed throughout the year, with a minimum of 7 cycles in April and a maximum of 18 cycles in August. The reported seasonal variations of diurnal tides based on 2002–2003 data [Yuan et al., 2006] are in qualitative agreement with those deduced from the full data set. Earlier comparisons of model results with observations [Forbes and Vial, 1989; Pancheva et al., 2002], coupled with diagnostic studies involving thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) [Roble, 2000] and the Global Scale Wave Model (GSWM) [Hagan et al., 1999; Hagan and Roble, 2001], provided new insight into the behavior and impact of tides and PW in the MLT region. Our initial comparisons [She et al., 2004; Yuan, 2004] of lidar-observed tides with GSWM predictions indicated that while there is general agreement in diurnal tides, the model prediction typically underestimated the semidiurnal amplitudes during the nonwinter months. This discrepancy was also reported in an earlier comparison with radar winds [Pancheva et al., 2002] at different longitudes. In this paper, the seasonal variation of semidiurnal tides in temperature and zonal and meridional winds will be compared to the local output at 41.0°N, 105.0°W of the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA) [Schmidt et al., 2006], a chemistry climate model of the atmosphere from the surface to about 250 km. Since this model includes (either explicitly or implicitly) all processes that are supposed to contribute to the tidal variability in the MLT region, and in particular full tropospheric chemistry and dynamics, a realistic prediction of tidal behavior can be expected. Comparisons of observed tide-removed mesopause mean fields (temperature and zonal and meridional winds) with HAMMONIA simulations based on the same data set were performed by Yuan et al. . A detailed assessment of the temporal data distribution and measurement accuracy are also presented therein.