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Ann. Geophys., 19, 953-963, 2001
www.ann-geophys.net/19/953/2001/
© European Geosciences Union 2001
Mixing processes in a tropopause folding observed by a network of ST radar and lidar
F. Bertin1, B. Campistron2, J. L. Caccia3, and R. Wilson4
1CETP, 4 Avenue de Neptune, 94107 Saint Maur-des-fossés, France
2C.R.A., 65300 Lannemezan, France
3LSEET, BP 132, 83957 La Garde, France
4S. A., BP 102, 4 Place Jussieu, 75004 Paris, France
Abstract. A campaign devoted to
stratosphere-troposphere exchange mechanisms studies has been held on 4–5
March 1995, during a tropopause folding passage over western Europe. The
observational network included 1 UHF and 3 VHF radar, 1 temperature lidar, and
1 ozone lidar, deployed in the south of France. The fold is associated with a
strong quasi meridional jet stream running along the west side of an upper
level trough forming a potential vorticity (PV) anomaly. During this campaign,
the PV anomaly is advected east-wards without major deformations, with an
average velocity of about 30 km per hour. Therefore, a frozen field hypothesis
has been used in order to compare the results obtained in the different sites
of the network. Under this hypothesis, the same structures associated with the
anomaly are observed by each radar or lidar, at the same time relative to this
anomaly. The fold is put into evidence by the ozone lidar of the network and by
the VHF radars when computing the aspect ratio. The turbulent activity observed
by the OHP VHF radar and by the high resolution UHF PROUST radar (30 m altitude
resolution) is concentrated in the wind shear regions generated by the jet
stream above and below its axis. In the lower level wind shear, turbulent
layers are detected within the fold and across its anticyclonic boundary, thus
allowing turbulent exchanges with the tropospheric air masses. A large area of
aspect ratio greater than 3 is observed in the troposphere during more than ten
hours after the passage of the fold by the VHF radars. This particular
signature is tentatively analyzed as the progressive dilution of air masses of
stratospheric origin extruded from the fold by the turbulent processes. In the
upper level wind shears, turbulent layers parallel to the wind isotachs are
observed, whose thickness is often smaller than 100 m and are separated by non
turbulent regions. A good correlation is found between the spatio-temporal
evolution of these turbulent structures observed by the PROUST radar, and the
stability structures observed at the same relative time by the temperature
lidar. In addition the same turbulent structures are observed to be parallel to
the isophase lines of the wind fluctuations, observed at the same relative time
by the Lannemezan radar. These characteristics are compatible with the presence
of unstable inertia-gravity waves, generated by the jet stream while the
turbulent layers could be the signature of their saturation processes.
Key words. Atmospheric composition
and structure (evolution of the atmosphere) – Meteorology and atmospheric
dynamics (middle atmosphere dynamics; turbulence)
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