Seasonal variations of diurnal and semidiurnal tidal-period perturbations in mesopause region temperature and zonal and meridional winds above Fort Collins, Colorado (40 degrees North, 105 degrees West) based on sodium-Lidar observation over full diurnal cycles.

摘要

With continued efforts by all the members (past and present) in the Na-Lidar group at Colorado State University, the lidar system was capable of simultaneous measurement of mesopause region temperature and horizontal wind at night. Since May 2002, the CSU fluorescence lidar system has been able to perform these observations on 24-hour continuous basis for a long-period, weather permitting. The key factor, which makes lidar observation under sunlit conditions possible, is a pair of robust (reliable and stable) Faraday filters that reduces the sky background. To attain such a Faraday Filter, in this thesis we developed and implemented dual temperature control (to +/-0.1K) of sodium cell inside the filters, and of the cell's tip-off region. The dual control allows independent setting of cell temperature and the Na vapor pressure, thus stabilizing Na vapor density in the cell and the transmission function of the Faraday filter.; This 24-hour continuous observation capability provided us with the first yearlong data set with campaigns of full diurnal cycle coverage leading to the first study of diurnal and semidiurnal tides of mesopause region temperature, zonal and meridional winds based on ground based observation. The yearlong data set include a total of 1,491 hours with 659 hours under sunlit conditions, within which there are 29 sets of 24-hour continuous observation. We binned these 29 data sets into bimonthly time series and performed harmonic analysis to deduce diurnal mean, diurnal and semidiurnal tidal-period oscillations of the mesopause region temperature, zonal and meridional winds over Fort Collins, CO.; The resulting bimonthly tidal amplitudes and phases are compared to the predictions of Global Scale Wave Models (GSWM00 and GSWM02) and Thermosphere, Ionosphere, Mesosphere and Electrodynamics - General Circulation Model (TIME-GCM02). Other than diurnal temperatures in Nov--Dec, we found excellent agreement between observation and GSWM00 model for diurnal tides, suggesting that migrating solar tides dominate the observed oscillations. The observed diurnal temperature phase in Nov--Dec suggests the existence of midnight local heating, which requires further study. For non-winter months, all three models considerably underestimate the semidiurnal amplitudes, implicating the existence of considerable local wave perturbations at or near semidiurnal period. At the present, the causes for the discrepancy between observation and model prediction are not understood. For the bimonthly periods with significant discrepancy between observation and model predictions, analysis of more recent data taken during the same bimonthly periods showed general agreement between two data sets. Continued observation for two additional years is planned. At the end of that time, we should have enough data to determine not only tidal climatology but also tidal variability, revealing unexpected interactions between tides and gravity waves and planetary waves. Our results will provide complementary information to satellite observations which provide global but lack local-time coverage. In collaboration with modeling efforts at National Center for Atmospheric Research, our data set hopefully will play a significant role in advancing our understanding of solar tides and their roles in the structure and dynamics of the Mesosphere and Lower Thermosphere.