| Cirrus clouds play an important part in modulating regional and global climate via two complex processes, albedo effect and greenhouse effect. Additionally, cirrus clouds are crucial to the hydrological cycle of the atmosphere and to stratosphere-troposphere exchange (STE) processes for water vapor. However, the semi-arid area of northwest China is still lacking in lidar observations of cirrus clouds. Therefore, to understand the morphology and effects of cirrus clouds in this region, this study was designed to determine the statistical characteristics of cirrus clouds by using ground-based micro pulse lidar (MPL-4B) measurements at the Semi-Arid Climate Observatory and Laboratory (SACOL,35.95°N,104.1°E) of Lanzhou University in northwest China. And the radiation effect of cirrus was simulated by radiative transfer model.(1) Macrophysical and optical characteristics of cirrus clouds were investigated at SACOL using micro pulse lidar data and profiling radiometer measurements. Analysis of the measurements allowed the determination of macrophysical properties such as cirrus cloud height, ambient temperature, and geometrical depth, and optical characteristics were determined in terms of optical depth, extinction coefficient, and lidar ratio. Cirrus clouds were generally observed at heights ranging from5.8to12.7km, with a mean of9.0±1.0km. The mean cloud geometrical depth and optical depth were found to be2.0±0.6km and0.350±0.311, respectively. Optical depth increased linearly with increasing geometrical depth. The results derived from lidar signals showed that cirrus over SACOL consisted of thin cirrus and opaque cirrus which occurred frequently in the height of8-10-km. The lidar ratio varied from5to70sr, with a mean value of26±16sr, after taking into account multiple scattering effects. The mean lidar ratio of thin cirrus was greater than that of opaque cirrus. The maximum lidar ratio appeared between0.058and0.3when plotted against optical depth. The lidar ratio increased exponentially as the optical depth increased. The maximum lidar ratio fell between11and12km when plotted against cloud mid-height. The lidar ratio first increased and then decreased with increasing mid-height.(2) In order to understand the spatial and temporal variations of cirrus. The structures and optical properties of the cirrus clouds as well as their spatial and temporal variations are discussed and analyzed. Our results show that cirrus clouds change from thin to thick, observed ranging from7to10km, with a mean thickness of2.0±0.5km. During this period, the samples have temperature between -51and-39℃. The cloud optical depth increases and then decreases with increasing geometrical depth, ranging from0.084to1.649, with a mean value of0.651±0.403. Lidar ratio of cirrus clouds is17±17sr and we have found that lidar ratio of optically thin cirrus is more than that of thick cirrus. Thin cirrus clouds with ambient temperature below-45℃occurr above8.6km and its thickness is lower than1.5km. The lidar ratio of thin cirrus is between5and69sr.The cirrus shortwave radiative forcing is negative while the long-wave radiative forcing is positive no matter in TOA or on the ground, which is cloud shortwave radiative forcing has a cooling effect and long-wave radiative forcing has a warming effect to earth-atmosphere system. Both shortwave radiative forcing and long-wave radiative forcing are consistent with the changes of optical thickness of the cirrus process, when the optical thickness increases, the greater the radiative forcing, cooling./warming is stronger. When the optical thickness reaches its maximum at16:00, the absolute value of the maximum cloud shortwave and longwave radiative forcing at TOA are51.9and59.8W/m2, on the ground, the values are53.8and12.8W/n2.The cooling effect is mainly due to the cirrus weakened short-wave radiation, weaken degree increases with increasing optical thickness. Warming effect is due to the greater optical thickness, the more long-wave radiation emitted by the cirrus. When change the optical thickness of the cloud in model only, the cloud shortwave and longwave radiative forcing was linearly correlated with the optical thickness, the radiative forcing increases as the optical thickness increases linearly. When the optical thickness increases, the warming speed of the earth-atmosphere system is faster than ground. For the ice cloud with the same optical thickness and the height of the cloud base is higher than8.0km, cloud shortwave radiative forcing appears "mutation" This is mainly caused by the shape of the ice particles, distribution and orientation of ice crystals. |
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