Atmos. Chem. Phys., 16, 12287–12303, 2016 www.atmos-chem-phys.net/16/12287/2016/ doi:10.5194/acp-16-12287-2016 © Author(s) 2016. CC Attribution 3.0 License. Investigation of ice particle habits to be used for ice cloud remote sensing for the GCOM-C satellite mission Husi Letu1,2, Hiroshi Ishimoto3, Jerome Riedi4, Takashi Y. Nakajima1, Laurent C.-Labonnote4, Anthony J. Baran5, Takashi M. Nagao6, and Miho Sekiguchi7 1Research and Information Center (TRIC), Tokai University, 4-1-1 Kitakaname Hiratsuka, Kanagawa 259-1292, Japan 2Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences (CAS), DaTun Road No. 20 (North), Beijing 100101, China 3Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan 4Laboratoire d’Optique Atmosphérique, UMR CNRS 8518, Université de Lille 1-Sciences et Technologies, Villeneuve d’Ascq, France 5Met Office, Fitzroy Road, Exeter, EX1 3PB, UK 6Earth Observation Research Center (EORC), Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen Tsukuba, Ibaraki 305-8505, Japan 7Tokyo University of Marine Science and Technology, Tokyo 135-8533, Japan Correspondence to: Takashi Y. Nakajima ([email protected]) Received: 29 September 2015 – Published in Atmos. Chem. Phys. Discuss.: 11 November 2015 Revised: 10 September 2016 – Accepted: 15 September 2016 – Published: 29 September 2016 Abstract. In this study, various ice particle habits are in- phase matrix. The characteristics of calculated extinction ef- vestigated in conjunction with inferring the optical proper- ficiency, single-scattering albedo, and asymmetry factor of ties of ice clouds for use in the Global Change Observa- the five ice particle habits are compared. Furthermore, size- tion Mission-Climate (GCOM-C) satellite programme. We integrated bulk scattering properties for the five ice parti- develop a database of the single-scattering properties of five cle habit models are calculated from the single-scattering ice habit models: plates, columns, droxtals, bullet rosettes, database and microphysical data. Using the five ice particle and Voronoi. The database is based on the specification of the habit models, the optical thickness and spherical albedo of Second Generation Global Imager (SGLI) sensor on board ice clouds are retrieved from the Polarization and Direction- the GCOM-C satellite, which is scheduled to be launched in ality of the Earth’s Reflectances-3 (POLDER-3) measure- 2017 by the Japan Aerospace Exploration Agency. A combi- ments, recorded on board the Polarization and Anisotropy nation of the finite-difference time-domain method, the ge- of Reflectances for Atmospheric Sciences coupled with Ob- ometric optics integral equation technique, and the geomet- servations from a Lidar (PARASOL) satellite. The optimal ric optics method is applied to compute the single-scattering ice particle habit for retrieving the SGLI ice cloud properties properties of the selected ice particle habits at 36 wave- is investigated by adopting the spherical albedo difference lengths, from the visible to the infrared spectral regions. This (SAD) method. It is found that the SAD is distributed sta- covers the SGLI channels for the size parameter, which is bly due to the scattering angle increases for bullet rosettes defined as a single-particle radius of an equivalent volume with an effective diameter (Deff/ of 10 µm and Voronoi par- sphere, ranging between 6 and 9000 µm. The database in- ticles with Deff values of 10, 60, and 100 µm. It is confirmed cludes the extinction efficiency, absorption efficiency, av- that the SAD of small bullet-rosette particles and all sizes of erage geometrical cross section, single-scattering albedo, Voronoi particles has a low angular dependence, indicating asymmetry factor, size parameter of a volume-equivalent that a combination of the bullet-rosette and Voronoi models sphere, maximum distance from the centre of mass, par- is sufficient for retrieval of the ice cloud’s spherical albedo ticle volume, and six nonzero elements of the scattering and optical thickness as effective habit models for the SGLI Published by Copernicus Publications on behalf of the European Geosciences Union. 12288 H. Letu et al.: Investigation of ice particle habits to be used for ice cloud remote sensing sensor. Finally, SAD analysis based on the Voronoi habit and Liou, 1998a; Sun et al., 1999, Ishimoto et al., 2012a), model with moderate particle size (Deff D 60 µm) is com- the T-matrix (Baran et al., 2001; Bi and Yang, 2014a, b), the pared with the conventional general habit mixture model, in- discrete dipole approximation method (Purcell and Penny- homogeneous hexagonal monocrystal model, five-plate ag- packer, 1973; Draine and Flatau, 1994; Yurkin et al., 2007), gregate model, and ensemble ice particle model. The Voronoi the boundary element method (Mano, 2000; Groth et al., habit model is found to have an effect similar to that found 2015), the pseudo-spectral time-domain method (Liu, 1997, in some conventional models for the retrieval of ice cloud 1998; Chen et al., 2008; Liu et al., 2012), the surface-integral properties from space-borne radiometric observations. equation method (Nakajima et al., 2009), the improved geo- metric optics method (IGOM) (Yang and Liou, 1996), ge- ometric optics integral equation (GOIE) (Yang and Liou, 1996; Ishimoto et al., 2012a), and the ray-tracing geomet- 1 Introduction ric optics method (GOM) (Takano and Liou, 1989, 1993; Macke, 1993; Macke et al., 1996a; Yang and Liou, 1998b; Ice clouds play an important role in the radiation balance Masuda et al., 2012). of the Earth’s atmospheric system through interaction with Various ice particle models correspond to different ra- solar radiation and infrared emissions (Liou, 1986). How- diative properties. Quantifying optical properties of these ever, large uncertainties exist in quantifying the radiative ice particle models is computationally expensive, and thus impact of ice clouds. This is because they consist of ice for application purposes it is useful to establish a number particles with various microphysical characteristics, e.g. a of libraries that pre-calculate the single-scattering proper- wide range of habits and sizes (C.-Labonnote et al., 2000; ties of various ice particle habits. Using hexagonal plates Forster et al., 2007; Baran et al., 2007; Cole et al., 2014; and columns with a random orientation, Hess and Wieg- Yang et al., 2015). Different ice particle habits have vary- ner (1994) developed a single-scattering property database ing single-scattering characteristics, resulting in different ra- at wavelengths between 0.35 and 3.7 µm. Additionally, Yang diative properties. Satellite observations are important as a et al. (2000) provided a database at wavelengths between 0.2 means of inferring the ice clouds’ optical properties and and 5 µm for six ice particle habits (plates, solid and hollow monitoring their radiative impact on a global climate sys- columns, planar bullet rosettes, spatial bullet rosettes, and ag- tem. However, retrieved ice cloud properties highly depend gregates); Yang et al. (2005) further included two more ice on the assumed ice particle model. In practice, one chooses habits and extended the database to wavelengths between 3 an ice particle model, which may consist of a single habit and 100 µm. Recently, Yang et al. (2013) released a database or a mixture of habits, and look-up tables (LUTs) for ice of a full set of scattering, absorption, and polarisation prop- cloud reflection and transmission characteristics are com- erties, assuming random orientation for a set of 11 habits at puted for a range of input optical properties such as optical a number of wavelengths, ranging between 0.2 and 100 µm. thickness, cloud temperature, and effective particle size. The This database involved the addition of roughness to the par- LUTs and a fast radiative transfer model are subsequently ticle surfaces. This library is based on the Amsterdam dis- used for global operational retrievals. Thus, the choice of an crete dipole approximation, T-matrix, and IGOM methods. ice particle model for a given satellite mission deserves rig- Using this updated library, Baum et al. (2014) developed a orous investigation. The present study aims to better under- new set of bulk scattering and absorption models, with habit stand the performance of several ice cloud habit models, in mixtures for radiative transfer calculations and remote sens- conjunction with applications to the Global Change Obser- ing retrievals of ice clouds. vation Mission-Climate (GCOM-C) satellite mission. There is increasing evidence that the ice particle model Over the past 2 decades, aircraft and balloon in situ obser- should contain some degree of surface roughness (Foot, vations have contributed greatly to understanding ice cloud 1988; Baran et al., 2001, 2003; Ottaviani et al., 2012; van microphysical characteristics and radiative properties (Baran Diedenhoven et al., 2012, 2013, 2014; Cole et al., 2013, et al., 1998, 1999, 2003; Heymsfield et al., 2002; Heyms- 2014; Holz et al., 2016). In particular, using an ensem- field, 2003; Zhang et al., 2009). A variety of ice particle ble ice particle model, Baran and C.-Labonnote (2007) and models has been developed based on in situ observations of Baran et al. (2014) showed that featureless phase functions ice particle habits and their single-scattering properties (e.g. best fitted their multi-angle satellite measurements at so- Macke et al., 1996a, b; McFarquhar and Heymsfield, 1996; lar wavelengths. Interestingly,