The impacts of small ice crystal shape and concentration on the bulk scattering properties of tropical cirrus

The 2.3 μm high-resolution images of ice crystals with maximum dimensions (D) less than 100 micrometers obtained by a Cloud Particle Imager (CPI) during the Tropical Warm Pool International Cloud Experiment (TWP-ICE) and other cirrus experiments appear mainly quasi-spherical (Fig. 1.a). Corresponding idealized models (e.g., the droxtal and Gaussian random sphere) have thus been developed to describe small crystal shapes (Fig. 1). However, crystalline particles of sodium fluorosilicate grown from solution on glass substrates also appear quasi-spherical when imaged by a CPI (Fig. 2 and Ulanowski et al. 2004). Unlike the droxtal or Gaussian random sphere, these particles show a complex structure of several columns originating from a common center. Here, a new model for small ice crystals (a budding bucky ball, 3B) is proposed to represent their shapes (Fig. 2). Because the resolution of the CPI is not sufficient to distinguish which idealized model best represents the observed small ice crystals, sensitivity tests are conducted to determine how different small crystal shapes (sphere, Gaussian random sphere, droxtal, and 3B) affect the single-scattering properties of small crystals and the bulk scattering properties of tropical cirrus. Uncertainties in the total number concentrations (N) of small ice crystals also exist. For example, average N of ice crystals with D < 50 micrometers measured by a Cloud and Aerosol Spectrometer (CAS) were 91 times larger than those measured by a Cloud Droplet Probe (CDP) during TWP-ICE (McFarquhar et al. 2007). These differences in N cause a 530% variation in total cross sectional area. The enhanced N measured by the CAS is mainly due to remnants of large ice crystal shattering and hence is treated as an upper bound for small ice crystal concentrations. The difference in N between the CAS and CDP is large enough to significantly change the bulk scattering properties of tropical cirrus from a climate perspective. In this study, the influences of shapes and concentrations of small ice crystals on the bulk scattering properties of tropical cirrus will be quantified. Figure 1. An example ice crystal (maximum dimension of 70 μm) imaged by a CPI (a) and idealized models of (b) a sphere, (c) a droxtal, and (d) a Gaussian random sphere used to represent small ice crystals in radiative transfer models. Figure 2. An example ice analogue imaged by a scanning electron microscope (a) and by a CPI (b) (Ulanowski et al. 2004). A new model designed by the PI, a budding Bucky ball (3B), that resembles an ice analogue (a) has been developed and is shown (C).