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Introduction

Cirrus clouds play a key role in the atmospheric radiative budget. Radiative balance depends strongly on their optical and microphysical properties, as the ice water content, their crystal habit and size (Stephens, 1990). It is of capital importance to understand the processes that control these properties at all scales.

Mid latitude cirrus have very often been observed in the presence of high and strong winds and polar fronts (Conover, 1960). In the tropics, Sassen et al. (1989) have shown a close correlation between the occurrence of cirrus clouds and the presence of a subtropical jet stream. More recently, using lidar data, Mace et al. (1995) described the formation of a cirrus deck linked with an upper troposphere strong wind event over the United States. Cirrus clouds formed at the tropopause height are very cold (less than 60°C) and seem to contain smaller ice particles than lower and warmer cirrus layers. Then they form more efficient absorbers of thermal infrared radiation per unit mass of ice (Winker, 1998).

Observations of cirrus clouds by lidar are conducted since 1993 at Palaiseau, France (48°42N, 2°16E). Several cirrus cases can be linked with the presence of strong jet streams. We describe below the observations made from the 5 to the 11 of October 1994. We observe a high correlation between the increase of tropopause height and the elevation of the top of the cirrus cloud layer when crossing the jet axis from the cyclonic (cold) side to the (warm) anticyclonic side. Using AVHRR infrared data and ECMWF reanalysis, we describe the large scale formation of cirrus clouds. We use the conceptual patterns defined by Keyser and Shapiro (1986) to explain the ageostrophic circulations in the vicinity of the baroclinic areas. Terms of entrance and exit will stand for a confluence (diffluence) area at the beginning (the end) of a jet streak zone. Accelerations of a straight jet stream in absence of any thermic advection are associated with direct ageostrophic transverse circulations (i.e. normal to the jet axis). The opposite effect is produced (deceleration and indirect transverse circulation) in the exit area. As described by Eliassen (1962) and Shapiro (1981) the existence of cold or warm air advection would shift the circulation toward the anticyclonic or cyclonic side depending of the location (entrance or exit) and the sign of the advection (positive for cold air, negative for warm air). For instance, the effect of cold advection in entrance would shift the direct circulation toward the anticyclonic side of the jet stream and then move and increase the subsiding branch of the circulation below the jet core. This would lead to a deepening of the tropopause fold created by the direct circulation in entrance of the jet streak. Furthermore, a synoptic scale curvature of the jet flow induces ageostrophic circulations along the jet axis owing an upward branch upstream from a ridge and a subsiding branch downstream. Other effects in the atmosphere can produce ageostrophic circulations, but we will only take these two main effects into account in this work.

Case study of the 6 of October 1994 shows a large cirrus deck in the vicinity of a strong polar jet stream in an anticyclonic long wave system over Europe. Several jet streaks areas are formed in the jet flow.

Firstly, we will describe the synoptic conditions at the surface and 300hPa level. Then, We will present the local measurements of lidar and radiosoundings over Palaiseau. The third part will be devoted to the study of ageostrophic circulations and the link between vertical motions and cirrus formation.