Falling Mixed-Phase Ice Virga and their Liquid Parent Cloud Layers as Observed by Ground-Based Lidars

Chong Cheng,Fan Yi

doi:10.3390/rs12132094

Abstract

Falling mixed-phase virga from a thin supercooled liquid layer cloud base were observed on 20 occasions at altitudes of 2.3–9.4 km with ground-based lidars at Wuhan (30.5 °N, 114.4 °E), China. Polarization lidar profile (3.75-m) analysis reveals some ubiquitous features of both falling mixed-phase virga and their liquid parent cloud layers. Each liquid parent cloud had a well-defined base height where the backscatter ratio R was ~7.0 and the R profile had a clear inflection point. At an altitude of ~34 m above the base height, the depolarization ratio reached its minimum value (~0.04), indicating a liquid-only level therein. The thin parent cloud layers tended to form on the top of a broad preexisting aerosol/liquid water layer. The falling virga below the base height showed firstly a significant depolarization ratio increase, suggesting that most supercooled liquid drops in the virga were rapidly frozen into ice crystals (via contact freezing). After reaching a local maximum value of the depolarization ratio, both the values of the backscatter ratio and depolarization ratio for the virga exhibited an overall decrease with decreasing height, indicating sublimated ice crystals. The diameters of the ice crystals in the virga were estimated based on an ice particle sublimation model along with the lidar and radiosonde observations. It was found that the ice crystal particles in these virga cases tended to have smaller mean diameters and narrower size distributions with increasing altitude. The mean diameter value is 350 ± 111 µm at altitudes of 4–8.5 km.

Highlights

Clouds can exhibit the essential structure of a weather system, sometimes providing a precursor for local precipitation [1]
The evolution of the falling mixed-phase ice virga from their liquid parent cloud base has been observed by ground-based lidars at Wuhan (30.5 ◦N, 114.4 ◦E), China
The thin liquid water cloud layer formation as indicated by high backscatter ratio R and low depolarization δ occurred on the top of a preexisting aerosol/liquid water layer, and the mixed-phase ice virga formation as indicated by high δ occurred continuously below the base height of the cloud layer

Summary

Introduction

Clouds can exhibit the essential structure of a weather system (such as frontal cloud system), sometimes providing a precursor for local precipitation [1]. Clouds are an important part of the Earth’s climate system [2]. A main reason for this situation is a considerable lack of observation-based process-level understanding on the formation, growth and phase transition of atmospheric water particles (liquid droplets and ice crystals) as well as production of precipitation. Since the real cloud processes involve the complex interplay of water vapor/aerosol, atmospheric dynamics and cloud microphysical properties [6], it is a challenge to identify and/or isolate a fundamental process leading to cloud formation and growth from observational data [7]. Some microphysical processes involved in cloud and precipitation are too swift to be resolved by conventional instruments (lidar, radar and airborne in situ measurements). At present, the observational investigations focusing on some condition-simple and detectable cloud phenomena that may facilitate unraveling the practical cloud processes

Methods

Results

Conclusion