Geometrical and optical properties of cirrus clouds in Barcelona, Spain: Analysis with the two-way transmittance method of 5 years of lidar measurements

Abstract. In this paper a statistical study of cirrus geometrical and optical properties based on 5 years of continuous groundbased lidar measurements with the Barcelona (Spain) Micro Pulse Lidar (MPL) is analysed. First, a review of the literature on the two-way transmittance method is presented. This method is a well-known lidar inversion method used to retrieve the optical properties of an aerosol/cloud layer between two molecular (i.e. aerosol/cloud-free) regions below and above, without the need to make any a priori assumptions about their optical and/or microphysical properties. Second, a simple mathematical expression of the two-way transmittance method is proposed for both ground-based and spaceborne lidar systems. This approach of the method allows the retrieval of the cloud optical depth, the columnar cloud lidar ratio and the vertical profile of the cloud backscatter coefficient. The method is illustrated for a cirrus cloud using measurements from a ground-based MPL and from the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Third, the data base is then filtered with a cirrus identification criterion based on (and compared to) the literature using only lidar and radiosonde data. During the period from November 2018 to September 2022, 367 high-altitude cirrus clouds have been identified at 00 and 12 UTC, of which 203 were successfully inverted with the two-way transmittance method. The statistical results of these 203 high-altitude cirrus clouds show that the cloud thickness is 1.8 ± 1.1 km, the mid-cloud temperature is -51 ± 8 ºC and linear cloud depolarization ratio is 0.32 ± 0.13. The application of the transmittance method yields an average cloud optical depth (COD) of 0.36 ± 0.45 and a mean lidar ratio of 30 ± 19 sr. It is observed that the highest occurrence of cirrus is in spring and the majority of cirrus clouds (48 %) are visible (0.03 < COD < 0.3), followed by opaque (COD > 0.3) with a percentage of 38 %. Together with results from other sites, a possible latitudinal dependence of lidar ratio is detected: the lidar ratio increases with increasing latitude. We also note that in Barcelona the COD correlates positively with the cloud base temperature, lidar ratio and linear cloud depolarization ratio and negatively with the cloud base height. On the one hand, the decrease of the cloud base temperature and COD associated to an increase of the cloud base height occurs because clouds located at higher altitudes are formed from air masses with a lower water vapour content and, therefore, their geometric and optical thickness are smaller. On the other hand, the lidar ratio increases with increasing cloud optical depth, as the complexity and diversity of ice crystal shapes increases, due to collisions and turbulence. Lastly, the linear cloud depolarization ratio has a slightly positive tendency with the cloud optical depth, because as the cloud optical depth increases, the number of ice crystals increases and, as a consequence, the randomly aggregation of ice crystals within the cloud occurs more frequently, making ice crystals rougher and thus more depolarizing.