The Optical Features In The Transition Zone Between Cirrus And Clear-sky Over Eastern Mainland And Adjacent Seas Of China

Several recent studies have found that clouds are surrounded by a transition zone of rapidly changing optical properties. The cloud particles’formation and elimination and aerosol particles’hydration and dehydration could change their concentration, phase, shape, size, and change the thermal structure and optical feature of atmosphere consequently. A transition zone should exist near cirrus lateral boundary facing unsaturated sky. The problems, such as macro-and micro-characteristics of cirrus transition zone, and its physic mechanism, the resulting radiation and precipitation, have not been solved adequately.Lidar could provide high resolution profiles, and satellite could gain continuous global detection. A combination of both has become powerful means for analyzing fine structures of aerosol, thin cirrus, cloud boundary and transition zone. In this paper the Cloud-Aerosol Lidar with Orthogonal Polarization(CALIOP) was applied for detecting a crop burning case, and then mainly used on the research on the properties of cirrus. Backscatter coefficient, color ratio and depolarization ratio computed from CALIOP dataset were used on analysis of the optical features of cirrus-sky transition zone over China eastern land, which were consequently compared with eastern sea and southern sea. Transition zone width was selected as indicator of transition zone characteristics, analyzed its dependence on meteorological conditions using GMAO and NCEP-CFSR data to understand the physic mechanism.The main conclusions of this study are listed as following:1) Horizontal profiles of optical properties and changing particles in cirrus-sky layerThe horizontal profiles of optical properties all clearly shown stair-step shape, reflects the change of concentration, size and shape。The horizontal profiles of backscatter coefficient and depolarization ratio, respectively in log-space and linear-space, could be expressed as functions of distance. The quadratic-fitting in cirrus and linear fitting in transition zone and clear sky can be in good agreement with the statistics. The layer shown the three sections of cirrus body, transition zone and clear sky, and the transition zone could be further subdivided into an intra-cloud gently-changing area, a linear abruptly-changing area and an external- cloud gently-changing area. The transition zone was located between-12km and8km, and the linear abruptly-changing area lied between-4km and lkm, where optical characteristics changed most radically.Optical properties shown different relationship in cirrus, transition zone and clear sky,.and the probability density distribution (PDF) of transition zone position presented a structure different from the horizontal profiles of optical properties. It implies that the transition took place within the internal change of micro-physical and optical features, but not the mixing of different particles.Typical features of ice particles presented0.55-1.25of color ratio and>0.12of depolarization ratio. The proportion of typical-feature particles also demonstrated stepped horizontal profile, its relationship with the optical-property profiles was different in the three sections, which reflected the internal change of scattering particles. The transition was not solely caused by the concentration changes of particle.2) The similarities and differences of optical features in transition zone over different regionsThis study compared three adjacent regions, China eastern land (Land), eastern sea (SeaE) and southern sea (SeaS), which take on different aerosol content and composition. High-loading aerosols over China eastern land contain a large number of continental and anthropological aerosols, and affected the eastern sea in the westerly downstream. The atmosphere over southern sea is clearer, and the predominated aerosols are from ocean in sand storm quiescence and under summer monsoon control.The horizontal profiles of backscatter coefficient, color ratio and depolarization ratio, all presented stair-step shape, showing horizontal-homogeneous clear sky, changing transition zone and horizontal-homogeneous cirrus. Transition zone could also be subdivided into an intra-cloud gently-changing area, a linear abruptly-changing area and an external-cloud gently-changing area. The PDF of depolarization ratio and color ratio were largely consistent across the three regions, so as the typical ice feature and its proportion.The mean values of backscatter coefficient and depolarization ratio in clear sky has demonstrated the different aerosol content. The relative order of cirrus backscatter coefficient is Land>SeaE>SeaS, so as the gradient of backscatter. But the relative orders of cirrus depolarization ratio and its gradient are just the reverse from backscatter. Cirrus over southern sea showed larger size.3) The influence of temperature, relative humidity, large-scale dynamics and aerosols on transitionLarge span of transition zone width on different temperature, relative humidity and large scale dynamics conditions implies that the determinism of any unilateral factors was not strong. Transition should be the result of the combined action of a number of factors together.Transition zone width presented a decreasing tendency with increasing temperature and decreasing relative humidity, which implies the mechanism that temperature and saturation ratio control sublimation rate and then transition zone width afterwards。Transition zone width increases with the increase of vertical velocity and horizontal wind speed, but has nothing to do with airflow direction. Atmospheric motion improving material exchange near cirrus lateral boundary is benefit to horizontal diffusion of ice and stretches out transition zone width.Below-30℃, transition zone width variation showed a regular pattern that with the decrease of temperature sample size in high (low) value region increases (decreases), the span of transition zone width increases and maximum probability density increases. It is because sublimation occurs difficultly on lower temperature (higher altitude), and strengthen wind intensifies horizontal diffusion of cloud particles to stretch transition zone out. Different from that below-30℃, the span and average of transition zone width appeared narrow above-30℃, which could be involved in super-cooled liquid water mixed in cirrus.Temperature also affects the micro-physical properties of the scattering particles. Depolarization ratio reduces along with temperature increases in transition zone. The regional difference of depolarization ratio and its gradient could be attributed to temperature. The amplitude of depolarization ratio change with backscatter coefficient, relative humidity and large scale dynamics, were not distinct.Regional difference of the backscatter coefficient gradient, is relevant to the regional differences of temperature and relative humidity, sublimation would be potential physic mechanism. Regional difference of cirrus backscatter coefficient and color ratio may be related to aerosol. Continental aerosols and marine aerosols participating in heterogeneous nucleation and homogeneous nucleation, with the aid of temperature and relative humidity, could affect the macro-and micro-features of cirrus. Aerosol composition could also affect ice shape and sublimation. The content and composition of aerosols could make more ice nuclei, small ice crystal, more dense cirrus and more dramatic transition over China eastern land. The relationship of aerosol content and cirrus backscatter also presented Twomey effect.