Initial full-diurnal-cycle mesopause region lidar observations: diurnal-means and tidal perturbations of temperature and winds over Fort Collins, CO (41°N,105°W)

摘要

The Colorado State Sodium lidar has been upgraded to a two-beam system capable of simultaneous measurements of mesopause region temperature and winds, over full diurnal-cycles, weather permitting. Though our lidar is a modest system with a power-aperture product of only 0.06 W m~2, good data quality is demonstrated by means of contour plots depicting a 80-h continuous observation between August 9th and 12th, showing the existence of atmospheric waves with different periods along with their coherence and interactions. The salient feature of data with full-diurnal-cycle coverage lies in its ability to describe the vertical profiles of dynamical fields (temperature, zonal and meridional winds) as a unique linear superposition of diurnal-mean and oscillations with different tidal periods, plus a residual term. In this manner, we investigate diurnal-means and oscillations with diurnal and semidiurnal periods. Using 6 data sets between July 17 and August 12, each covering a full-diurnal-cycle as a case study, we found considerable day-to-day variability, as much as 20 K, 35 and 75 m/s for diurnal-mean temperature, zonal wind and meridional wind, respectively, and as 15 K, and 50 m/s, for the diurnal and semidiurnal tidal temperature and wind amplitudes, respectively. While a minimum of 3 full diurnal cycles appears to be adequate in the case studied here, the 6-day composite yields diurnal-means and diurnal tides in agreement with model predictions very well. Since the resulting amplitudes and phases of the observed diurnal oscillations agree well with the global scale wave model, GSWM00 predictions, we conclude that the migrating diurnal tide contributes significantly to the observed oscillations with diurnal period over Fort Collins, CO (41°N, 105°W). Unlike the diurnal perturbations, the observed semidiurnal amplitudes and phases differ from the GSWM00 predictions with considerably smaller model amplitudes. The coherence of solar forcing is found to prevail over the variability for diurnal-mean as well as both tidal oscillations, leading to expected convergence to climatology in a multi-day composite observation. The possible causes for the observed variability, and the role of planetary waves and nonlinear interactions are discussed, along with future studies to assess the extent of local perturbations that may exist in the observed data.