Rough and Irregular Ice Crystals in Mid-latitude Clouds

1. INTRODUCTION Cloud feedbacks are the largest source of uncertainty in climate models. In particular, uncertainties exist concerning the radiative forcing of clouds containing ice crystals, most notably cirrus. Indeed, whether cirrus clouds warm or cool the Earth's surface depends on ice crystal morphology. Reducing this uncertainty requires detailed in situ characterization of cloud particles. Also, detailed knowledge of the scattering properties of cloud particle types is needed for accurate retrieval of cloud microphysical properties from remote sensing. One of the main barriers to achieving these goals is the inability of cloud probes to determine the contribution of small ice crystals (that is crystals smaller than about 50 µm) to the total distribution. This is partly due to their inability to resolve the geometric structure of small ice crystals because of the conflicting demands of high optical resolution and large sample volume (Ulanowski et al. 2004, Connolly et al. 2007, Kaye et al. 2008). There is also growing evidence, largely indirect, that atmospheric ice crystals have shapes departing from idealized geometries based on perfect hexagonal prisms (Garrett 2008, Baran 2012). Korolev et al. (2000) concluded that the majority of ice particles in mid-latitude stratiform clouds observed during several campaigns were of irregular shape. Best fits to data obtained by Lampert et al. (2009) in Arctic ice cloud were consistent with deeply rough hexagonal ice crystals. Gayet et al. (2011) found prevalent particles with imperfect or complex shapes at trailing edge of mid-latitude frontal cirrus. Measurements using the Polar Nephelometer (Shcherbakov et al. 2006) indicated that the surface of Antarctic ice crystals was deeply rough. It is important in this context that particle roughness can dramatically alter the scattering properties of ice crystals. For example, it can significantly reduce the asymmetry parameter (Yang et al. 2008, Ulanowski et al. 2006). Roughness may also account for the relative rarity of ice halos (Ulanowski 2005). Therefore it is