A re-evaluation of the Stokes drift in the polar
summer mesosphere
The Stokes drift argument put forth by Coy et al. [1986] to
explain the monthly mean downward vertical velocities observed in the
summer polar mesosphere by radars is critically re-examined. The size
of the effect is quite sensitive to the choice of gravity wave
wavenumber and phase speed. We reproduce the Coy et al.
[1986] result for a monochromatic gravity wave and then generalize to
a Garrett-Munk type spectrum. This allows us to easily incorporate
experimentally determined wave field parameters in order to predict a
Stokes drift magnitude. The Stokes drift we calculate for mesospheric
spectra reported in the literature is less than 4 cm/s. This is
nearly a factor of 10 smaller than the mean June and July downward
vertical velocities we have verified from 4-year averaging of the
Poker Flat, Alaska, radar data base. In this extended analysis we
also find that the upward winter mean vertical velocity is smaller
than previously reported but still in apparent conflict with the
mesospheric winter circulation theory. We suggest, as an alternative
to the Stokes drift idea, that the observed summer velocity might be
the terminal velocity of charged aerosols. Such an explanation would
have the advantage of relating mean velocity observations to the large
VHF radar cross section exhibited by the polar summer mesosphere.
This possible connection is discussed further in a companion paper.
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