An Investigation of the Variability of Tides in the Upper Mesosphere (Meteor Radar)

Abstract

A computer simulation of the Urbana, Illinois, meteor radar is performed to estimate the accuracy of the data analysis procedures in determining upper mesospheric tides. Various wind models such as planetary waves, gravity waves, and turbulence are tested and it is found that for reasonable values of atmospheric variance the two-dimensional standard deviation of the tidal estimator is at most 8 ms('-1). The observed variability of the tides at Urbana during March 1980 exceeds the variability of a modelled steady-state tide, and the nature of the observed variability suggests a non-linear interaction of the tide with gravity and/or planetary waves. The semidiurnal tide also exhibits phase fluctuations indicating the possible presence of a transient S(,2,3) mode.

A post-processing selection algorithm is used to determine the meteor echoes detected on the Poker Flat, Alaska, MST radar from a composite database consisting of meteor and turbulence echoes. The echo distributions depend on the geometry of the antenna beams, the presence of strong turbulence echoes, average power transmitted, and the number of coherent averages used in the signal processing. Winds are derived from the meteor echoes and are used to compute monthly mean winds and tides in the upper mesosphere. These results are consistent with previous high-latitude observations. Tidal amplitudes are largest in the summer and weakest in the winter. The tides exhibit large month-to-month variability, although the seasonal averages in 1983 are in agreement with those of 1984. The semidiurnal tide exhibits a dramatic phase shift of approximately 180(DEGREES) from October to November of both years. Results suggest future enhancement of the MST wind data can be achieved with real-time meteor echo detection.

Finally, a meteor echo detection system that can be attached to many ST or MST radars that operate in the very-high-frequency (VHF) range is developed. The system can be operated in parallel with the normal data collection of turbulence echoes. Wind measurements using the Platteville, Colorado, ST radar during January 1986 are consistent with the general northern hemisphere winter circulation. The height-averaged mean winds show westerlies and southerlies. The semidiurnal tide is circularly polarized and rotates clockwise. The diurnal tide is not well resolved.