Study Of Local Ice Formation In Humid Aerosol/Liquid Water Layer With Ground-based Lidars And Radiosondes

Ice crystal particles account for only a small fraction of the hydrometeors suspended in global air,then can strongly influence the earth's radiation budget and climate,as well as the cloud electrification and precipitation.Initial ice particles have gone through nucleation.There is not a generally accepted theory,especially for ice heterogeneous nucleation,which are influenced by aerosol and atmospheric ambient conditions.The heterogeneous ice formation processes in the atmosphere are not well understood,in particular the actual/dominant mechanisms of ice nucleation as well as the clear link between meteorological conditions,aerosol properties and growth of ice crystals has not been established yet.There is a lack of well-designed field observations for ice formation in clouds,in particular the initial ice formation lack effective observation methods.As one of the advanced active remote sensing instruments,lidar has been widely used in atmospheric sounding.It has high temporal and spatial resolution,as well as the individual ability in detecting aerosol.The capability of sensing fixed target make ground-based lidar the most adequate instrument for exploring the temporal evolution of initial ice formation.In this paper,the study on ice formation processes was performed based on three Wuhan University ground-based lidars together with conventional radiosondes.The current observations present for the first time(to our knowledge)the process-level pattern from liquid water growth to ice formation as well as location of ice nucleation(on the cloud top),providing new insight into ice nucleation mechanisms in the natural atmosphere.The main contents of this study are as follows in detail:1)Given a brief introduction of ice nucleation processes in natural atmospheric environment and summarized the ice nucleation mechanisms put forward by different researchers,indicating that the field observations contribute to the knowledge of ice nucleation.Base on the comparisons of different techniques in cloud observations,it was shown that ground-based lidar is the most suitable instrument for exploring the initial ice formation processes,which cannot be competent by airborne in-situ and radar measurements.2)Introduced the instruments and data used in this study.The 532-nm polarization lidar with high resolution(1 min and 30 m)can provide extinction and backscatter coefficients and information on cloud phase.High-precision water vapor Raman lidar can provide detailed water vapor information within the layer.In addition,spectrally resolved Raman lidar as the world's second water full-spectrum lidar provides the unique liquid water information.The three ground-based lidars together with the temperature and relative humidity profiles from accompanying radiosonde measurements allows us to illustrate optical properties and meteorological conditions during the processes of the liquid water layer evolution and ice formation.3)The multiple-scattering-induced depolarization ratio within cloud is estimated.The multiple scattering can induce an increase in the depolarization ratio with increasing penetration of laser light into the cloud.Furthermore,the larger the lidar FOV,the stronger the multiple-scattering-induced depolarization.Based on the conventional Stokes vector Monte Carlo model,the calculated depolarization ratio(due to multiple scattering)relevant to the observations in this study shows a monotonic smooth increase with penetration depth of the laser light and has a maximum of?0.20.In addition,multiple-scattering-induced depolarization ratio did not exceed 0.1 generally for a small lidar FOV(?1 mrad)at other lidar sites.Thus,in this study,the lidar-observed depolarization ratios larger than 0.3 are an indication of ice formation rather than multiple scattering effect4)Observations with lidars at Wuhan,China from 2010 to 2013 captured nine cases of slowly-ascending humid aerosol/liquid water layers that occurred at altitudes of?2-4 km in winter.Each of them was almost transparent initially with the backscatter ratio far less than 7.0 and depolarization ratio less than 0.03.With a slow ascent and locally-enhanced liquid water and water vapor,the layer developed into a nearly opaque liquid cloud layer and then ice crystals abruptly formed at the upper edge of the cloud layer with very high liquid water content.The ice crystals likely came from water drop freezing.The freezing temperatures estimated from radiosonde measurements were-3 to-8? and the layer was observed to always lie just below an inversion layer.For two available long-lived(>16 h)cases,the ice development on the layer was followed by rainfall.