In this study, various ice particle habits are investigated in conjunction
with inferring the optical properties of ice clouds for use in the Global
Change Observation Mission-Climate (GCOM-C) satellite programme. We develop a database of the single-scattering properties of five ice habit models:
plates, columns, droxtals, bullet rosettes, and Voronoi. The database is
based on the specification of the Second Generation Global Imager (SGLI)
sensor on board the GCOM-C satellite, which is scheduled to be launched in
2017 by the Japan Aerospace Exploration Agency. A combination of the
finite-difference time-domain method, the geometric optics integral equation
technique, and the geometric optics method is applied to compute the
single-scattering properties of the selected ice particle habits at 36
wavelengths, from the visible to the infrared spectral regions. This covers
the SGLI channels for the size parameter, which is defined as a single-particle radius of an equivalent volume sphere, ranging between 6 and
9000
Ice clouds play an important role in the radiation balance of the Earth's atmospheric system through interaction with solar radiation and infrared emissions (Liou, 1986). However, large uncertainties exist in quantifying the radiative impact of ice clouds. This is because they consist of ice particles with various microphysical characteristics, e.g. a wide range of habits and sizes (C.-Labonnote et al., 2000; Forster et al., 2007; Baran et al., 2007; Cole et al., 2014; Yang et al., 2015). Different ice particle habits have varying single-scattering characteristics, resulting in different radiative properties. Satellite observations are important as a means of inferring the ice clouds' optical properties and monitoring their radiative impact on a global climate system. However, retrieved ice cloud properties highly depend on the assumed ice particle model. In practice, one chooses an ice particle model, which may consist of a single habit or a mixture of habits, and look-up tables (LUTs) for ice cloud reflection and transmission characteristics are computed for a range of input optical properties such as optical thickness, cloud temperature, and effective particle size. The LUTs and a fast radiative transfer model are subsequently used for global operational retrievals. Thus, the choice of an ice particle model for a given satellite mission deserves rigorous investigation. The present study aims to better understand the performance of several ice cloud habit models, in conjunction with applications to the Global Change Observation Mission-Climate (GCOM-C) satellite mission.
Over the past 2 decades, aircraft and balloon in situ observations have contributed greatly to understanding ice cloud microphysical characteristics and radiative properties (Baran et al., 1998, 1999, 2003; Heymsfield et al., 2002; Heymsfield, 2003; Zhang et al., 2009). A variety of ice particle models has been developed based on in situ observations of ice particle habits and their single-scattering properties (e.g. Macke et al., 1996a, b; McFarquhar and Heymsfield, 1996; Yang et al., 2000, 2005, 2013; Um and McFarquhar, 2007, 2009, 2011; Nousiainen et al., 2011; Baum et al., 2005, 2011; Baran and C.-Labonnote, 2007; Ishimoto et al., 2012b; Liu et al., 2014a). Numerous light-scattering computation methods have been employed to calculate the single-scattering properties of the various ice particles, including the finite-difference time-domain (FDTD) method (Yee, 1966; Yang and Liou, 1998a; Sun et al., 1999, Ishimoto et al., 2012a), the T-matrix (Baran et al., 2001; Bi and Yang, 2014a, b), the discrete dipole approximation method (Purcell and Pennypacker, 1973; Draine and Flatau, 1994; Yurkin et al., 2007), the boundary element method (Mano, 2000; Groth et al., 2015), the pseudo-spectral time-domain method (Liu, 1997, 1998; Chen et al., 2008; Liu et al., 2012), the surface-integral equation method (Nakajima et al., 2009), the improved geometric optics method (IGOM) (Yang and Liou, 1996), geometric optics integral equation (GOIE) (Yang and Liou, 1996; Ishimoto et al., 2012a), and the ray-tracing geometric optics method (GOM) (Takano and Liou, 1989, 1993; Macke, 1993; Macke et al., 1996a; Yang and Liou, 1998b; Masuda et al., 2012).
Various ice particle models correspond to different radiative properties.
Quantifying optical properties of these ice particle models is
computationally expensive, and thus for application purposes it is useful to
establish a number of libraries that pre-calculate the single-scattering
properties of various ice particle habits. Using hexagonal plates and columns
with a random orientation, Hess and Wiegner (1994) developed a single-scattering
property database at wavelengths between 0.35 and 3.7
There is increasing evidence that the ice particle model should contain some degree of surface roughness (Foot, 1988; Baran et al., 2001, 2003; Ottaviani et al., 2012; van Diedenhoven et al., 2012, 2013, 2014; Cole et al., 2013, 2014; Holz et al., 2016). In particular, using an ensemble ice particle model, Baran and C.-Labonnote (2007) and Baran et al. (2014) showed that featureless phase functions best fitted their multi-angle satellite measurements at solar wavelengths. Interestingly, using particle images of convective ice clouds from in situ measurements, Ishimoto et al. (2012b) developed a new habit of complex and highly irregular shapes, called the Voronoi aggregate. The phase function of the Voronoi habit varies smoothly with the scattering angle, which is similar to behaviour found from assuming severe surface roughness, including bubbles within the particle, or a combination of included bubbles and surface roughness. However, use of the Voronoi habit model for retrieval of the ice cloud's optical thickness has not yet been investigated.
Numerous articles have investigated the use of optimal ice particle habits derived from various ice habit models and remote sensing measurements from multiple angles, for use in cloud parameter retrievals (Baran et al., 1998, 1999, 2003, 2007; Chepfer et al., 1998; C.-Labonnote et al., 2000; Chepfer et al., 2001; Masuda et al., 2002; Knap et al., 2005; Sun et al., 2006; Baran and C.-Labonnote, 2006). C.-Labonnote et al. (2000, 2001) and Doutriaux-Boucher et al. (2000) developed models of randomly oriented hexagonal ice particles containing spherical air bubbles (the inhomogeneous hexagonal monocrystal (IHM) model) for use in the ice cloud retrievals of the POLarization and Directionality of the Earth's Reflectances (POLDER) measurements. Spherical albedo difference (SAD) analysis is employed to investigate the capability of the IHM model for retrieving the optical properties of ice clouds. It is illustrated that POLDER multi-angle measurements are sensitive to ice particle habits and roughness, at least for ice clouds having an optical thickness larger than 5. Chepfer et al. (2002) investigated effective ice particle habits using multi-angle and multi-satellite methods derived from visible reflectance satellite measurements.
The Second Generation Global Imager (SGLI) on board the GCOM-C satellite, scheduled for launch in 2017 by the Japan Aerospace Exploration Agency (JAXA), measures radiation at 19 visible and near-infrared wavelengths in order to understand the global radiation budget, carbon cycle mechanism, and climate change (Imaoka et al., 2010). Since retrieving ice cloud properties on a global scale from satellite observations requires knowledge of ice microphysical models, it is crucial to identify an optimal choice of ice habits for SGLI. The objectives of this study are to better understand the performance of existing ice models used in other satellite missions, investigate the potential of the Voronoi model, and provide a recommendation for the GCOM-C.
The paper is organised as follows. In Sect. 2, in order to develop the ice cloud property products of the GCOM-C satellite, we describe the method for calculating single-scattering properties at the SGLI-operated wavelengths for the five ice particle habits, including the Voronoi model. In Sect. 3, we apply the newly calculated ice cloud properties to SAD analysis, using POLDER measurements as an example. In Sect. 4, we describe the results of the SAD analysis. Section 5 presents our conclusions.
Specification of the SGLI.
Single-scattering properties for the five ice particle habits are calculated for the SGLI observation channels. The single-scattering properties are used to determine the optimal ice particle habits, using the SAD method. The SGLI is the successor sensor to the Global Imager (GLI) aboard ADEOS-II, which takes measurements at wavelengths ranging from the near-ultraviolet to the thermal infrared. The first satellite, GCOM-C1, is scheduled for launch in 2017 by the JAXA. The GCOM-C mission intends to establish a long-term satellite-observation system to measure essential geophysical parameters on the Earth's surface and in the atmosphere on a global scale to facilitate the understanding of the global radiation budget, carbon cycle mechanism, and climate change (Imaoka et al., 2010). As shown in Table 1, SGLI has 19 channels, including two polarisation channels at visible and near-infrared wavelengths. A detailed description of the SGLI is reported by Imaoka et al. (2010), Nakajima et al. (2011), and Letu et al. (2012).
Four of the ice particle habits (hexagonal columns, plates, bullet rosettes, and droxtals) employed in this study were chosen by referring to the MODIS Collection 5 ice particle model (Baum et al., 2005) and ice cloud in situ measurement data. The habits shown in Fig. 1 are defined with the same parameters (semi-width, length, aspect ratio, and maximum dimension) as were employed in the scattering properties database by Yang et al. (2000, 2005). The Voronoi habit was numerically determined by extraction of Wigner–Seitz cells from a 3-D mosaic image of the ice cloud microphysical data (Ishimoto et al., 2012b). This habit is different from the aggregate model used in the scattering database reported by Yang and Liou (1998b) and Yang et al. (2013). Spatial Poisson–Voronoi tessellations were used to determine the complex structure of the ice particles for the 3-D mosaic image. The geometry of each cell in the Voronoi tessellation was defined and based on the method by Ohser and Mücklich (2000).
SGLI cloud particle habits.
Size parameter with various particle size and calculating wavelength on the SGLI channels (FDTD, GOIE, GOM).
Continued.
A combination of the FDTD, GOIE, and GOM methods was employed to calculate
the single-scattering properties of Voronoi ice habits for a wide range of
size parameters (SZP) and is given by
Consideration of the edge effect (Bi et al., 2010; Bi and Yang, 2014a) is
important for calculating the extinction efficiency (
In this study, microphysical data obtained during 11 field campaigns were
used to generate the particle size distributions (PSDs) of ice crystals
using Eq. (3). To ensure the PSDs are unambiguously those of ice,
microphysical data were filtered by limiting the cloud temperature to
Furthermore, spectral bulk scattering properties were calculated from SGLI
single-scattering database, and the derived PSDs were based on the method
described in Baum et al. (2011). The main steps for calculating the bulk
scattering properties are as follows:
Extract the total projected area, total volume, maximum dimension,
scattering cross section, and scattering phase function parameters at a
specific wavelength for five ice particle models from the SGLI
single-scattering property database. Calculate the effective diameter ( Calculate the bulk-averaged single-scattering albedo ( Select the single-scattering albedo, asymmetry factor, extinction
efficiency,
and scattering phase function with small, medium, and large
Since 1996, three POLDER instruments have been flown to study clouds and
aerosols using multiple angles and polarisation capabilities. The POLDER-1
and POLDER-2 instruments aboard JAXA's ADEOS satellite were operated from
November 1996 to June 1997 and December 2002 to October 2003, respectively.
Both of the POLDER instruments observed intensity from 14 viewing directions,
with scattering angles ranging from 60 to 180
Figure 2a shows the distribution of the number of directional samples used
in the SAD analysis. The number of pixels is increased in the scattering
angle range of 60 to 160
Comparison of the phase functions of randomly oriented column and spheroid particle from FDTD, T-matrix, and ADDA methods.
Comparison of the single-scattering property of the various ice
crystals models computed in this study at wavelength of 1.05 and
2.21
Bulk scattering phase functions of the column, droxtal, plate,
bullet rosette, and Voronoi habit employed in this study with various
effective diameters at wavelengths of 1.05
The steps for applying the SAD analysis to POLDER-3 measurements are as
follows:
Calculate spherical albedo from the POLDER-3 measurements with 16 viewing
geometries for each of the ice particle models. Perform the SAD analysis by taking the difference between the directional
and the direction-averaged cloud spherical albedo. Assume that the phase function for each ice particle model adequately
represents the phase function for all ice particles in each pixel of the
satellite measurement and that the retrievals of the optical thickness and
spherical albedo from the POLDER measurements with different viewing
geometries are the same.
When the SAD is 0, the mean spherical albedo and the spherical albedo from
the specific angle of POLDER-3 measurements are the same. Therefore, the
criteria for selecting the optimal particle habit of the ice cloud are
defined as an SAD near 0 in the 16 viewing geometries of POLDER-3 and a
small angular dependence.
SAD analysis as a function of different particle habits and
effective diameters (
To confirm the accuracy of the calculated single-scattering properties, the
phase functions computed in this study are compared with other results.
Figure 3 shows comparisons of the phase function (
The single-scattering albedo, asymmetry factor, and extinction efficiency
among the key parameters of the single-scattering properties of ice
particles. Figure 4 shows the single-scattering properties of various ice
particle habits at wavelengths of 1.05 and 2.2
Bulk scattering phase functions of the column, droxtal, plate,
bullet-rosette, and Voronoi habits, with various effective diameters at
wavelengths of 1.05
Figure 6 shows the SAD analysis as a function of the scattering angle,
effective particle radius, and ice particle models. The SAD of the droxtal,
column, and plate shows substantial variations in both the scattering angle and
effective particle radius. The variation of SAD for the bullet-rosette model
is more smoothly distributed close to 0 value of the SAD (hereafter, “zero
line”) than with the droxtal, plate, and column models for small
(
Figure 7 shows the slope of the regression function (SRF) and total relative
albedo difference (TRAD) of the SAD for the same five ice particle models
with small, medium, and large particles, as shown in Fig. 6. Values of both
the SRF and TRAD for small particles of the bullet rosette, and for medium
and large particles of the bullet rosette and Voronoi, are the smallest of
all the single-particle models considered. However, there is a peak value of
the SAD in the scattering angle range of 140 to 160
The slope of the regression function and total relative albedo difference in Fig. 6.
SAD analysis of the ensemble ice particle model with
Ice crystals in ice clouds are complex. To simulate this complexity, we
assume different values of distortion (as defined by Macke et al., 1996b) and
apply these to the ensemble model. Numerous previous studies have shown that
the degree of distortion is an important property to consider when retrieving
ice cloud optical properties from multiple-view instruments. To investigate
the influence of the distortion of the ice particle model on retrieval of the
ice cloud properties, we performed the SAD analysis using the ensemble ice
particle models with
Comparison of the SAD analysis for various ice particle models with
Several conventional studies have demonstrated that ice particle models such
as the ensemble ice particle model, IHM, and GHM, as well as some aggregated complex
models with rough surfaces, are useful for operational satellite data
processing (C.-Labonnote et al., 2000, 2001; Doutriaux-Boucher et al., 2000; Baum et
al., 2011, 2014; Baran and C.-Labonnote, 2006, 2007; Cole et al., 2013). For
evaluating the accuracy of the Voronoi model, the SAD of the Voronoi model is
compared with that of the conventional IHM, GHM, five-plate aggregate, and
ensemble ice particle models with
Figure 10 shows the slope of the regression function (top panel) and total relative albedo difference (bottom panel) for the selected models in Fig. 9. The SRF for the GHM, Voronoi, and averaged-ensemble models is significantly smaller than for the other three models. The TRAD values for each habit model are not significantly different. However, the TRAD value obtained for the Voronoi model is slightly smaller than the other models, except for the averaged-ensemble ice particle model. The Voronoi and averaged-ensemble models have small values of SRF and TRAD, indicating that the SAD of the Voronoi and averaged-ensemble models have a low angular dependence.
The slope of the regression function and total relative albedo difference for various ice particle models in Fig. 9.
Ice particle single-scattering properties were investigated for potential use in the GCOM-C satellite programme. The single-scattering properties of five different ice particle models (plates, columns, droxtals, bullet rosettes, and Voronoi) were developed using the FDTD, GOIE, and GOM methods. The accuracy of the single-scattering property was investigated by comparing the phase function from the FDTD method used in this study with conventional results from ADDA and T-matrix methods. The FDTD phase functions were also compared with computational results from GOIE. Results indicate that the FDTD-based phase functions are consistent with results from the ADDA, T-matrix, and GOIE methods, which suggests that the single-scattering property database developed in this study is reliable for use in radiative transfer simulations and applications in the remote sensing of ice clouds.
The characteristics of the single-scattering property database for five
different ice particle models were investigated by analysing the
single-scattering albedo, asymmetry factor, and the extinction efficiency.
Bulk scattering phase functions for five different ice particle models at the
wavelength of 1.05
Furthermore, SAD analysis was performed to determine the optimal ice particle
habit for retrieving the optical thickness and cloud spherical albedo using
POLDER-3 multi-angle measurements. Retrievals were performed using
589 The SADs of the droxtal and column habits show significant variations in
scattering angle and effective particle radius. SAD variation for small particles with the bullet-rosette model is more
smoothly distributed along the zero line than that with other habit models. The Voronoi model SAD is closest to the zero line in scattering angle
for all particle sizes. The bullet-rosette habit for small particles and the Voronoi habit for
all particle sizes are most suitable for retrieving the ice cloud spherical
albedo and optical thickness.
In other words, results of the SAD analysis indicate that the Voronoi
particle has scattering characteristics that are useful for retrievals, e.g.
agreement with POLDER/PARASOL polarised reflectances, a low asymmetry
parameter, and a smooth phase function. Furthermore, the results of SAD
analysis from the Voronoi model were compared with results from the
conventional IHM, GHM, five-plate aggregate, and ensemble ice particle models
with moderate ice particle size in order to evaluate the efficiency of the
Voronoi model. It is concluded that the Voronoi habit model is similar to the
conventional models for retrieval of ice cloud properties with thick optical
thickness, using remote sensing instruments. The results of this study should
be useful not only for developing the ice cloud products of the GCOM-C/SGLI
satellite mission but also for determining the optimal ice particle habit
for ice cloud remote sensing. In future work, we will compare the optical
properties derived from the Voronoi model and the severely roughened
aggregated columns model used in MODIS Collection 6 ice cloud algorithm. We
will also investigate how the Voronoi particle behaves at low optical
thickness values, in direct comparison with the retrievals from
CALIPSO/CALIOP polarisation lidar data.
Data are available upon request.
This work was supported by the GCOM-C/SGLI and EarthCARE project of the Japan Aerospace Exploration Agency (JAXA), the Japan Science and Technology Agency (JST), CREST/EMS/TEEDDA, CAS Pioneer Hundred Talents Program (Y6YR0600QM), and National Natural Science Foundation of China (61261030). The authors would like to thanks ICARE and CNES for providing the POLDER data as well as François Thieuleux for his support with POLDER data analysis. The authors gratefully acknowledge Bryan A. Baum (UW-Madison) for providing the GHM ice particle model. Edited by: J.-Y. C. Chiu Reviewed by: B. Baum and one anonymous referee