Tidal perturbations and variability in the mesopause region over Fort Collins, CO (41N, 105W): Continuous multi‐day temperature and wind lidar observations

Abstract

An unusually long data set was acquired at the sodium lidar facility at Colorado State University (41N, 105W), between Sep 18 and Oct 01, 2003, including a 9‐day continuous observation. This time is long enough to average out the perturbations of gravity waves and short‐period planetary waves. As such, it can be used to define tidal‐period perturbations in temperature and horizontal wind. Assuming the sodium mixing ratio is a constant of motion, the observed tidal‐period oscillation in sodium density follows that of vertical wind. Thus, the data set defines tidal‐period perturbations of temperature and wind vector. The observed amplitudes and phases were compared to Global Scale Wave Model predictions (both GSWM00 and GSWM02). We found excellent agreement in diurnal phases and reasonable agreement in semidiurnal phases. However, GSWM02 overestimates diurnal amplitudes and both model versions underestimate observed semidiurnal amplitudes. Since the data period is long enough for the study of planetary waves and of tidal variability, we perform spectral analysis of the data, revealing a strong quasi 3‐day wave in meridional wind, a 14 hour perturbation in zonal wind, and both 14‐hour and 10‐hour periods in meridional wind, likely the result of nonlinear interactions. The observed semidiurnal amplitudes are much larger than the corresponding diurnal amplitudes above 85 km, and over a few days the diurnal and semidiurnal amplitudes vary by factors of 2–3. Causes for the observed tidal variability in terms of planetary wave modulation and tide‐gravity wave interaction are explored qualitatively.

Global distribution and interannual variations of mesospheric and lower thermospheric neutral wind diurnal tide: 1. Migrating tide

“…In the southern hemisphere, the peak amplitude is much more widely distributed into the midlatitude region (50°N). We also see a tendency to have larger amplitudes during the eastward phase of the stratospheric QBO (2002 and2004) (Figure 6b). …”

“…The satellite orbits the Earth 15 times per day. 2004(20 March 2004. The red track goes to the North Pole (northern track) and the blue track (southern track) passes over the South Pole.…”

“…Ground-based observations have established seasonal and interannual variations of the MLT diurnal tide at various latitudes [e.g., Manson et al, 1988Manson et al, , 1989Manson et al, , 1999Vincent et al, 1998;She et al, 2004;Yuan et al, 2006]. By combining observations from various locations, Pancheva et al [2002] were able to assemble a global picture of the diurnal tide, but some large gaps in coverage remain.…”

Global structure, seasonal and interannual variability of the migrating semidiurnal tide seen in the SABER/TIMED temperatures (2002–2007)

“…A major advantage of the ground-based methods is their ability to study in detail seasonal, intraseasonal and interannual variability of the tides in the MLT at various geographic points (Bernard, 1981;Avery et al, 1989;Manson et al, 1990Manson et al, , 1999Clark and Salah, 1991;Vincent et al, 1998;Jacobi et al, 1999;Pancheva et al, 2000Pancheva et al, , 2002, as well as their vertical structure including vertical wavelengths and direction of propagation She et al, 2004;Yuan et al, 2008). A disadvantage of the ground-based methods is their inability to distinguish between global and local signatures and to provide only coarse information on spatial (latitudinal and longitudinal) structures of the tides.…”

Reply to Comment on "Global structure, seasonal and interannual variability of the migrating semidiurnal tide seen in the SABER/TIMED temperatures (2002–2007)" by Manson et al. (2010)

“…We note especially that in PETAL09 the term "migrating" was not used for the semidiurnal tidal characteristics reported by YETAL08. The comparison was done as an illustration of the considerable amount of work that has been done recently by the Na lidar observations on the semidiurnal tidal variations (She, Correspondence to: D. Pancheva (dpancheva@geophys.bas.bg) 2004; She et al, 2002She et al, , 2003YETAL09). In their comment METAL09 noted some seasonal differences in the tidal features, particularly summer vertical wavelength and September amplification of the tidal amplitude, obtained by satellite and Na lidar temperature measurements.…”