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3. Lidar measurements
Lidar measurements have been conducted at Palaiseau from the 5th to 11th of October 1994 (Figure 4). We observed the presence of cirrus clouds during the all period
except from the 9th where the sky stayed clear during all day allowing us to observe
the stratospheric aerosol layer.
Figure 4: Isentropes (broken line) and wind speed (solid line) versus time and height interpolated from radiosondes data above Palaiseau. We ploted cirrus bases and tops retrieved from lidar data collected from the 5th to the 11th october 1994 (black rings and triangles). Thick solid line represents the tropopause height calculated from radiosoundings following the WMO definition.
Measurements from the 5th to the 7th allow us to present a good description of the cirrus cloud linked
with the cross of a polar jet streak event over Palaiseau. The
cross of a branch of a polar jet stream permits the drawing of
a vertical cross section of the jet and the warm frontal area.
We see on figure 4 the elevation of the tropopause the 6th October when crossing through the jet core from the cold cyclonic
side to the anticyclonic side. We show that cyclonic side is clear
of cirrus cloud and that cirrus form in the jet core. During the
6th, cirrus top follows the elevation of the tropopause height from
11.8 km up to 13.75 km.
We receive a very low depolarisation ratio (Figure 5, calculated as the ratio of cross polarized to parallel signal)
but no high backscatter peak from the cirrus layer forming in
the morning of 6th October that might indicate the presence of oriented crystals.
The temperature at the top is -65°C. No supercooled droplets can
exist at this temperature. Then we suppose that cloud is formed
of spheroid and very small ice particles or sulphate aerosols
in solution from volcanic origin. Depolarisation ratio increase
in this layer after a tens of minutes and reach 20 to 40% indicating
the presence of non-spherical ice crystals.
Figure 5 : Vertical profile of depolarization ratio versus time and height observed above Palaiseau on the 6 october 1994. Very low value are found at the top of this cirrus cloud around 0836.