Abstract
Geometrical and optical characteristics of cirrus clouds were studied based on one year of polarization lidar measurements (3969 h on 228 different days between March 2019 and February 2020) at Wuhan (30.5°N, 114.4°E), China. The cirrus clouds showed an overall occurrence frequency of ~48% and occurrence mid-cloud altitude of ~8–16 km over the 30°N plain site. The mean values of their mid-cloud height and temperature were 11.5 ± 2.0 km and −46.5 ± 10.7 ℃, respectively. The cirrus geometrical thickness tended to decrease with decreasing mid-cloud temperature, with a mean value of 2.5 ± 1.1 km. With the decrease of mid-cloud temperature, the cirrus optical depth (COD) tended to decrease, but the depolarization ratio tended to increase. On average, the COD, lidar ratio, and particle depolarization ratio were respectively 0.30 ± 0.36, 21.6 ± 7.5 sr, and 0.30 ± 0.09 after multiple scattering correction. Out of a total of the observed cirrus events, sub-visual, thin, and dense cirrus clouds accounted for 18%, 51%, and 31%, respectively. The cirrus clouds showed seasonal variations with cloud altitude maximizing in a slightly-shifted summertime (July to September) where the southwesterly wind prevailed and minimizing in winter months. Seasonally-averaged lidar ratio and depolarization ratio showed maximum values in spring and summer, respectively. Furthermore, a positive correlation between the cirrus occurrence frequency and dust column mass density was found in other seasons except for summer, suggesting a heterogeneous ice formation therein. The cirrus cloud characteristics over the lidar site were compared with those observed at low and mid latitudes.
Highlights
Cirrus clouds are composed primarily of ice crystals, that form in the cold upper troposphere with a variety of forms and shapes [1]
We present the mean of the optical and geometrical characteristics of cirrus clouds over a 30◦N plain region based on one year of polarization lidar measurements (3969 h on 228 different days between March 2019 and February 2020) at Wuhan (30.5◦N, 114.4◦E), China
For a detailed study of the optical and geometrical properties of cirrus clouds, one measurement case observed by our polarization lidar which started at 1800 local time (LT) on 25 July 2019 and lasted for
Summary
Cirrus clouds are composed primarily of ice crystals, that form in the cold upper troposphere with a variety of forms and shapes [1]. Studies show that cirrus clouds cover on average 30% of the Earth’s surface [1,2], and play a significant role in the Earth-Atmosphere system radiation budget [1,3]. Cirrus clouds can induce relatively high albedos and reflect part of the incoming solar radiation back into space cooling the atmosphere. The contribution of each effect depends strongly on cirrus optical properties, vertical and horizontal coverage [5,6,7]. Significant uncertainties with respect to the radiative and climate effect of cirrus clouds still remain due to our limited knowledge of cirrus geometrical and optical properties [8,9]. A detailed study of cirrus clouds is highly essential to climate modeling studies
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