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a rather narrow range due to the huge thermal inertia in the system. Above this
height the situation is quite different. In the extreme case of the upper atmosphere
discussed already in this chapter, the temperature can vary by nearly 500K in a
single day. Naturally, huge diurnal tides surge across the system from night to
day, leading to the 200m/s (720 km/hr) winds observed. This type of wind field
is termed an in situ tide, since it is generated and detected where it is created: in
the thermosphere. Propagating tides usually are only easily detectable after they
increase in amplitude by propagating upward into regions of low atmospheric
density and are first evident in the lower thermosphere.
A sample analysis of two years of data from the MF radar on Kauai, Hawaii,
is presented in Fig. 3.32. Here we see many interesting wave modes, including
planetary waves at 2- to 10-day periods, diurnal (S1) and semidiurnal (S2) tides,
and even evidence for S3 and S4 tides. Humphreys et al. (2005) have shown that
S1 through S6 tides are generated in the troposphere using GPS techniques. There
is concern that some contamination occurs in the MF data at high frequencies
and high altitudes (
85 km) due to gravity wave phase velocity detection (rather
than winds). But for tidal frequencies and lower, there is general acceptance of
these results.
We postpone for now a discussion of atmospheric waves in these high-fre-
quency regimes, usually referred to as gravity waves, but we must point out
>
Period (days)
10 3
10 2
10 1
10 0
10 22
10 23
10 10
10 10
10 9
10 9
10 8
10 8
10 7
10 7
10 6
10 6
10 5
10 5
10 4
10 4
10 3
10 3
10 28
10 27
10 26
10 25
10 24
10 23
10 22
Frequency (Hz)
Figure 3.32 Frequency spectral of zonal (solid) and meridional (dashed) velocities in
standard form for a two-year data set obtained with the Hawaii MF radar. [After Fritts
and Isler (1994). Reproduced with permission of the American Meteorological Society.]
 
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