Laurent Sauvage1, Mario Lavorato2, Pierre H. Flamant1 and Eduardo Quel2

1Laboratoire de Météorologie Dynamique du CNRS, France
2Division Radar Laser - CEILAP (CITEFA - CONICET)

We present dynamical processes leading to cirrus formation in the vicinity of jetstreams. Ageostrophic circulations across the jet axis are induced by mesoscale accelerations of the jetstream. Other circulations along the flow are created by largescale curvature of the flow. Vertical branches of these circulations seem to be the main cause of cirrus formation or decay. We will show several observations by lidar conducted in Europe and Argentina. Using LITE data and ECMWF reanalysis, we show on a case study of 11 september 1994 over Europe the close correlation between upward and downward branches of ageostrophic circulations and cirrus formation. We also indicate the close link between cirrus top height and tropopause height. The mechanisms described in this case study for polar jetstreak event are adapted to the Southern hemisphere and described in a case study of subtropical jetstream event over Buenos Aires. We show three days observations of cirrus over Buenos Aires in June 998 and the influence of Andean cordillera on the curvature of subtropical jetstream over south america.