Aspect Ratios of Natural Ice Crystals Derived from High-Resolution Images: Dependence on Environmental Conditions

The impacts of ice clouds on the energy budget of the Earth and their representation in climate models have been identified as important and unsolved problems. Ice clouds consist almost exclusively of non-spherical ice crystals with various shapes (i.e., habits) and sizes. To determine the influences of ice clouds on solar and infrared radiation as required for numerical models and remote sensing studies, knowledge of single-scattering properties of ice crystals is required. Hexagonal prisms (i.e., columns and plates) represent the building blocks of the most common ice crystal habits. To calculate their single-scattering properties, an idealized model representing the morphology of hexagonal prisms is therefore required. An aspect ratio (the ratio between the crystal length and width) is the fundamental information needed to build models of columns, plates, and bullet rosettes; further, single-scattering properties exhibit significant dependence on assumed aspect ratios. Although aspect ratios have been determined in previous studies, they were calculated only within limited temperature and humidity ranges, using small numbers of sample crystals, under unrealistic laboratory conditions, and without considering the geophysical location of clouds.

In this study, the aspect ratios of hexagonal prisms (i.e., columns and plates) and of individual branches of bullet rosettes are obtained from high-resolution images of ice crystal silhouettes recorded by a Cloud Particle Imager (CPI) during the 2006 Tropical Warm Pool International Cloud Experiment (TWP-ICE) in the Tropics, the 2008 Indirect and Semi-Direct Aerosol Campaign (ISDAC) in the Arctic, and the 2010 Small Particles in Cirrus (SPARTICUS) campaign in mid-latitudes.

Software developed at the University of Illinois, the "Ice Crystal Ruler", was used to measure the maximum dimension, length, and width of the ice crystals images. However, because the CPI images are silhouettes, projections of randomly oriented three-dimensional shapes on a plane, the measured dimensions do not represent the real crystal dimensions. Therefore, an iterative approach simulating random orientations of crystals is used to quantify the true relationship between the length and width of ice crystals, which is subsequently represented as a power law relationship.

Using the data from the 3 projects, the relationships between length, width, and maximum dimension were characterized as a function of temperature, relative humidity, ice crystal concentration, and ice water content. Further, dependence of growth processes (i.e., size and aspect ratio) on the ambient atmospheric conditions, and on the geophysical locations (i.e., tropical, mid-latitude, and arctic) are quantified.