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Introduction

Cirrus clouds cover about 30 % of the earth's surface. They impact the radiation budget [Liou 1986], which in turn governs the global climate. Cirrus clouds have also been invoked as a possible surface for heterogeneous reactions that could impact ozone concentrations in the upper troposphere and lower stratosphere (UT/LS) [Borrmann et al., 1996]. Characterizing cirrus occurrences and their optical properties is critical for climate models, but there are very limited observational data. (See Penner et al. [1999] for an overview.)

In particular characterizing optically thin cirrus has been acknowledged [Rosenfield et al., 1998] as an important, albeit difficult task, for calculating heating rates. Most attention has been focused in the tropics where the largest fraction of these thin clouds has been observed [Wang et al., 1998], and the radiative effects of subvisible cirrus (SVC) are expected to be the greatest (+ 0.5 W m^-2) [Wang et al., 1996]. This prevalence of SVC in the topics has been determined using extra-terrestrial global measurements capable of detecting an optical thickness, , less than 0.03.

The objective of this study is to construct a cirrus climatology using ground-based lidar measurements over the Observatoire de Haute Provence (OHP), France. Our measurements have a high altitude resolution (75 m) and confirm the pressence of SVC at northern midlatitudes. Our study is not the first time ground-based instrumentation has observed SVC at midlatitudes. Indeed, the SVC optical thickness threshold of 0.03 was determined using midlatitude (45 ° N, 90 ° W) lidar data [Sassen et al., 1989]. Most ground-based studies of SVC have been limited in scope (e.g. see Sassen and Cho [1992]) and thus extracting climatological data from lidar work has been quite challenging. Our findings are distinctive because they are based on the most exhaustive record of ground-based cirrus observations.