Abstract
Abstract. The mesosphere is one of the most difficult parts of the atmosphere to sample; it is too high for balloon measurements and too low for in situ satellites. Consequently, there is a reliance on remote sensing (either from the ground or from space) to diagnose this region. Ground-based radars have been used since the second half of the 20th century to probe the dynamics of the mesosphere; medium-frequency (MF) radars provide estimates of the horizontal wind fields and are still used to analyse tidal structures and planetary waves that modulate the meridional and zonal winds. The variance of the winds has traditionally been linked qualitatively to the occurrence of gravity waves. In this paper, the method of wind retrieval (full correlation analysis) employed by MF radars is considered with reference to two systems in Antarctica at different latitude (Halley at 76∘ S and Rothera at 67∘ S). It is shown that the width of the velocity distribution and occurrence of “outliers” is related to the measured levels of anisotropy in the received signal pattern. The magnitude of the error distribution, as represented by the wind variance, varies with both insolation levels and geomagnetic activity. Thus, it is demonstrated that for these two radars the influence of gravity waves may not be the primary mechanism that controls the overall variance.
Highlights
Located around 50 to 100 km altitude above the Earth’s surface, the mesosphere is one of the most difficult places to directly study; it is too low for satellites to pass through and too high for meteorological balloons
Winds are derived from the radar signal using the full correlation analysis (FCA) technique outlined by Briggs (1984)
Previous authors have used averaged variance to produce climatologies of gravity waves in the mesosphere and lower thermosphere; Hibbins et al (2007) found an annual climatology of gravity wave activity at Rothera very similar to that displayed in Fig. 9 and noted that this annual trend does not agree with the expected trend of increased activity during the equinoxes; this suggested that some other factor was in play
Summary
Located around 50 to 100 km altitude above the Earth’s surface, the mesosphere is one of the most difficult places to directly study; it is too low for satellites to pass through and too high for meteorological balloons. At sufficiently low altitudes (below ∼ 95 km), the electron density is usually small enough that effects of the refractive index on the signal speed are negligible In this height range, the motion of the plasma is dominated by the background neutral wind such that careful analysis of the returned signal measured on spaced receivers can provide a means of estimating that velocity. This mean flow is part of a large-scale circulation pattern that links the two poles: rising in the summer hemisphere and downwelling in the winter in the polar vortex These waves play an important role in the atmosphere; due to their size, they tend to be unresolved by general circulation models and so their effects are parameterised in global circulation models. The influence of gravity waves is not ruled out but their role in the variance is shown to be somewhat smaller than past work might have shown
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