Optical and geometrical characteristics of cirrus clouds over a Southern European lidar station

Abstract

Abstract. Optical and geometrical characteristics of cirrus clouds over Thessaloniki, Greece (40.6 • N, 22.9 • E) have been determined from the analysis of lidar and radiosonde measurements performed during the period from 2000 to 2006. Cirrus clouds are generally observed in a mid-altitude region ranging from 8.6 to 13 km, with mid-cloud temperatures in the range from −65 • to −38 • C. The cloud thickness generally ranges from 1 to 5 km and 38% of the cases studied have thickness between 2 and 3 km. The retrieval of optical depth and lidar ratio of cirrus clouds is performed using three different methods, taking into account multiple scattering effect. The mean optical depth is found to be 0.31±0.24 and the corresponding mean lidar ratio is 30±17 sr following the scheme of Klett-Fernald method. Sub-visual, thin and opaque cirrus clouds are observed at 3%, 57% and 40% of the measured cases, respectively. A comparison of the results obtained between the three methods shows good agreement. The multiple scattering errors of the measured effective extinction coefficients range from 20 to 60%, depending on cloud optical depth. The temperature and thickness dependencies on optical properties have also been studied in detail. A maximum mid-cloud depth of ∼3.5 km is found at temperatures around ∼−47.5 • C, while there is an indication that optical depth and mean extinction coefficient increases with increasing mid-cloud temperature. A correlation between optical depth and thickness was also found. However, no clear dependence of the lidar ratio values on the cloud temperature and thickness was found.

A decadal cirrus clouds climatology from ground-based and spaceborne lidars above the south of France (43.9° N–5.7° E)

“…Recent studies of cloud optical properties have shown the LR frequency distribution is centred at 25 ± 15 sr for ice clouds (Yorks et al, 2011). This value is consistent with other lidar measurements made using this technique (Giannakaki et al, 2007;Immler and Schrems, 2002). In order to have the same confidence in all cases, including thin or thick clouds, and following the literature, we decided to use a constant LR of 25 sr.…”

“…Similar results have been retrieved regarding the variability of Cloud Mean Height which keeps quite constant from one month to another. A mean monthly variation of ∼ 0.15 km around the total mean value has been observed (Sassen and Comstock, 2001), G07 (Giannakaki et al, 2007) and M11 (Min et al, 2011 9.28 ± 1.77 9.4 ± 1.76 8.85 ± 1.95 9.35 ± 1.47 8.17 ± 1.95 9 ± 1.1 9.16 ± 1.83 CTH [km] 10.94 ± 1.68 11.03 ± 1.84 10.4 ± 1.62 10.91 ± 1.48 10.97 ± 1.92 11.7 ± 0.9 10.77 ± 0.9 CT [km] 1.64 ± 1.1 1.64 ± 1.1 1.56 ± 1 1.56 ± 1 2.79 ± 1.38 2.7 ± 0.9 1.6 ± 1.15 for the class 1 and 3 from both CALIOP and the OHP lidar, and of ∼ 0.3 km for the class 2. Parameters for class 2 cirrus clouds agree well with results from G07.…”

“…Therefore, it is essential to identify and document these different parameters at different geographical locations to improve the parameterisation in numerical models and decrease uncertainties in climate prediction. Lidars designed to monitor cirrus clouds have been deployed at several observatories around the globe for more than a decade, producing regional climatologies from midlatitude (Goldfarb et al, 2001;Giannakaki et al, 2007) and tropical observatories (Comstock et al, 2002;Cadet et al, 2003;Thorsen et al, 2011). Lidar instruments can detect cirrus clouds with optical depths as low as 10 −3 at visible wavelengths, whereas many passive satellite sensors require visible optical depths of ∼ 0.1 for cirrus to be detected and, consequently, may underestimate the actual cirrus occurrence frequency.…”

Optical and Geometrical Properties of Cirrus Clouds over the Tibetan Plateau Measured by Lidar and Radiosonde Sounding at the Summertime in 2014

“…Only rarely do they contain supercooled liquid cloud droplets at their lower altitudes . Cirrus affects the climate of the whole Earth via two opposite effects; an infrared greenhouse effect and a solar albedo effect (Giannakaki et al, 2007). The contribution of each effect depends strongly on cirrus optical properties (Zerefos et al, 10 2003).…”

“…A different technique called 'transmittance method' to determine the optical depth of cirrus clouds from elastic lidar signal has also been developed. It is based on the comparison of the backscattering signals below and above the cloud with assuming that the lidar signals correctly represented the scattering medium (Elouragini and Flamant, 1996;Chen et al, 2002;Giannakaki et al, 2007).…”