Motivation

1
Motivation To obtain statistics on
inertia-gravity wave parameters, using a 15 year
archive of Aberystwyth MST radar observations.
Data Several thousand days of 5-beam VHF
wind-profiler data, height resolution 300 m and
time resolution 2 min, from the British
Atmospheric Data Centre.
Results Programs are written to process the MST
radar archive, calculating amplitude, wavelength
and alignment of inertia-gravity waves in
relation to jet stream structure. Height-time
plots for 10 days, 21-30 January 2002, show an
example of vertical echo power and tropopause
height horizontal wind speed vertical shear of
north-westerly wind component horizontal wind
vectors hodographs of horizontal wind after
high-pass 5 km vertical filter to show
inertia-gravity wave alignment and charts of 300
mb jet-stream wind.
1. Echo power in the stratosphere is related to
static stability. Note how the waves modulate
static stability. This is mainly through
advection, not vertical motion
2. Wind shear is a precise measure of the
location of inertia-gravity waves. Note the
curved phase fronts with time, following contours
of the jet stream.
3. Horizontal wind speed shows the passage of a
number of frontal systems over the radar during
this period. Inertia-gravity waves may be seen in
these plots.
4. Wind directions were generally westerly during
this period
5. By filtering out the large-scale wind
variation in the vertical and plotting u against
v, a hodograph for the wave results. The more
circular the hodograph, the nearer the wave
period to the inertial period. Here, hodographs
are drawn at various heights covering a height
range of 3 km about that point. The bottom of
each profile is red and the top blue. In the
stratosphere the hodographs turn clockwise,
indicating upward propagation of the wave.
6. 300 mb charts for the period, showing the
presence of the jet streams
Conclusions from the entire archive Inertia-gra
vity wave phase fronts often follow the curved
jet stream - tropopause source region, and are
fragmentary, rather than the usual assumption of
plane waves. Marked inertia-gravity wave events
are sometimes observed in the lower
troposphere. Inertia-gravity waves modulate
static stability in the lower stratosphere by far
more than the expected wave perturbation, because
of advection in baroclinic regions. Study of
wave alignment relative to jet stream structure
is ongoing. Research funded by the Natural
Environment Research Council