Abstract
abstract
The influence of mesoscale atmospheric dynamics on the formation
of polar stratospheric clouds (PSCs) is investigated,
based on lidar (light detection and ranging) measurements performed
in Sodankylä (Finland, 67.25° N, 26.65°E) and
Ny-Ålesund (Spitsbergen, 78.55°N, 11.55°E).
The meteorological situation of the arctic stratosphere during the winters
1996/1997 to 1999/2000 is described. As the
winter 1999/2000 was characterized by a very cold and stable polar
vortex, the period was chosen for a study on different
PSC types. Measurements of PSCs consisting of solid and/or liquid particles
are presented, namely the PSC types I a and I b
as well as type I a/b-mixtures and "sandwich-PSCs". In addition, the
extraordinary PSC events of January 24/25 and
January 26/27, 2000, and their likely composition of very large particles
('NAT-rocks') are pointed out.
For the first time, PSC observations at the edge (Sodankylä) and
in the centre (Ny-Ålesund) of the polar vortex are compared.
It is found that the existence of PSCs is favoured at the vortex edge.
Three approaches are made to explain this result.
First, it is shown that temperature fluctuations in stratospheric lee
waves (induced at the Scandinavian mountain ridge) lead
to the formation of water ice PSCs above Sodankylä although the
synoptic temperatures are above the frost point.
The lee wave approach suggests that stratospheric temperatures are
locally shifted below the PSC existence temperature.
In the following, the submitted thesis tracks a new idea by suggesting
the PSC existence temperature being shifted to higher
temperatures by a fluctuation in trace gases (especially H2O).
In this context, the development of filaments at the vortex edge
is calculated using the contour advection method. However, the analysis
of PSC data together with filamentary structures shows
no enhancement or reduction of the PSC probability in the presence
of extrusions or intrusions.
Finally, the water vapour distribution within the polar vortex gives
an explanation on the different PSC observations. Due to the
strong descent, maximum H2O-mixing ratios occur at the vortex edge,
shifting PSC existing temperatures to higher temperatures.
In the centre of the vortex, PSC existence temperatures are shifted
to lower temperatures due to the scarceness of water vapour.
By combining the experimental lidar results with the meteorological
approaches of atmospheric dynamics, it was possible to point
out a complex picture of PSC existence in the northern hemispheric
polar vortex.
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