Characteristics And Seasonal Variations Of Cirrus Clouds From Lidar Observations At Wuhan

Cirrus clouds play an important role in the radiation transmission of the earthatmosphere system and in the atmospheric water cycle system,which make effect on global weather and climate changing.In this paper,the characteristics of cirrus clouds over Wuhan(30.5°N,114.4°E)from March 2019 to February 2020 are studied in detail based on one year of polarization lidar measurements,combined with the observations of the self-developed multi-field-of-view polarization Raman lidar,radiosonde and other instruments.The main research contents are summarized as follows:(1)A multi-field-of-view polarization Raman lidar has been successfully developed.The lidar system has three signal receiving channels: elastic parallel channel,elastic vertical channel and nitrogen Raman channel.An frequency-doubled Nd:YAG laser is utilized as the lidar transmitter with an output energy of ?100 m J per pulse at532.1 nm with a repetition rate of 20 Hz.A 5x beam expander is employed to reduce the divergence angle of the laser beam in order to reduce the full field of view height.The backscattered signals are collected by a 304.8 mm Cassegrain telescope.A motorized diaphragm with a repeatability of 0.01 mm is used to change the received field of view of the lidar.By using the combination of two narrow band interference filters,the suppression of the out-of-band energy of each channel reaches a magnitude of 10 orders.By using the combination of two polarization beam splitter prisms,the extinction ratio reaches 10000:1 in each elastic channel.The ?90° method is utilized to calibrate the channel gain ratio.Afer calibration,the volumn depolarization ratio of molecule is 0.00534 detected by the lidar system.By comparing the profile of atmosphere detrived by the lidar and with another calibrated equipment,it shows that the lidar system is accurate in detection.(2)A new method for identifying horizontally oriented ice crystals by using multifield-of-view lidar,based on the principle of multiple scattering,is proposed.Compared with the traditional parameter inversion method and the slant-vertical polarization lidar combined observation or scanning lidar method,this new method is suitable for daytime observation,which can obtain the height information of the horizontally oriented ice crystals with a high time resolution.(3)Geometrical and optical characteristics of cirrus clouds are studied based on one year of polarization lidar measurements(3969 h on 228 different days between March 2019 and February 2020)at Wuhan,China.The cirrus clouds show an overall occurrence frequency of ?48% and occurrence mid-cloud altitude of ?8-16 km over the30°N plain site.The mean values of their mid-cloud height and temperature are 11.5 ±2.0 km and-46.5 ± 10.7 ?,respectively.On average,the cloud optical depth,lidar ratio,and particle depolarization ratio are respectively 0.30 ± 0.36,21.6 ± 7.5 sr,and0.30 ± 0.09 after multiple scattering correction.Out of a total of the observed cirrus events,sub-visual,thin,and dense cirrus clouds account for 18%,51%,and 31%,respectively.The cirrus clouds show seasonal variations with cloud altitude maximizing in a slightly-shifted summertime(July to September)where the southwesterly wind prevail and minimizing in winter months.Seasonally-averaged lidar ratio and depolarization ratio show maximum values in spring and summer,respectively.Furthermore,a positive correlation between the cirrus occurrence frequency and dust column mass density is found in other seasons except for summer,suggesting a heterogeneous ice formation therein.(4)By analyzing the relationship between the characteristic parameters of cirrus clouds,it is found that as the temperature of cirrus clouds decreases,the geometric thickness,optical thickness and layer-averaged extinction coefficient tend to decrease,while the particle depolarization ratio tends to increase.Moreover,the optical thickness and layer-averaged extinction coefficient have a linear relationship with the temperature of cirrus clouds.The particle depolarization ratio is positively correlated with the lidar ratio when the lidar ratio is less than 30 sr.(5)Comparing the cirrus cloud characteristics over Wuhan with other sites around the world,it shows that cirrus cloud characteristics change with latitude.Compared with cirrus clouds over mid-latitudes,cirrus clouds in tropical regions are higher in altitude,with smaller geometric thickness and lower temperature.As the latitude increases,the frequency of thick cirrus clouds tends to increase,resulting in the optical thickness of cirrus clouds in the middle and high latitudes being significantly greater than those in tropical and subtropical regions.As cirrus clouds in tropical and subtropical regions are mainly formed by convective clouds,the lidar ratio of cirrus clouds in these areas are smaller than that of other latitudes as a whole.