Previous: Cinematic analysis Next:References Up: Ext. Abst.
5. Discussion and conclusion
Cirrus observations by lidar over Palaiseau indicated the presence
of a high and persistent layer. This layer contained small particles
at the beginning of the observation period. The cirrus observed
on the 6th October 1994 is associated with the cross of a jet stream branch
over Europe. The cirrus top height follows the ascent of the tropopause
when crossing the jet core from the cyclonic toward the anticyclonic
side.
We describe the divergence field and observe the strong correlation
between the local divergence area and the coldest part of the
two main regions of cirrus formation. Vertical cross section of
ageostrophic circulations shows the motion of air masses in the
exit area of a jet streak and verifies former conceptual patterns
proposed by Shapiro.
The jet stream pattern over Europe contains two wind speed maxima
south and west of Iceland. We can link them with tropopause folds
as shown on figures 8and 9(Namias and Clapp, 1949; Donadille, 2000). Tropopause folds correspond
to stratospheric intrusion into the troposphere. In case of turbulent
mixing in the troposphere and during post volcanic period, a consequent
amount of stratospheric aerosols can be transported into the troposphere
and serve as cloud condensation nuclei. That could explain the
presence of very small particle in the cirrus cloud over Palaiseau.
Further study on this case study is currently led.
Figure 8 : Horizontal cross section of the atmosphere at 300 hPa. This figure shows the potential vorticity field over Europe at 300 hPa.
Figure 9: Vertical cross section of the atmosphere along the longitude 2°E. Dashed lines indicate isotachs from 30 to 70 ms-1. Dashed doted lines indicate isentropes. Plain lines indicates isolines of potential vorticity. We see the limit between stratosphere (gray area) and troposphere (white area). Tropopause folds are present on the northen border of jet cores. The fold falls deep into the troposphere reaching the level 600 hPa