the Creative Commons Attribution 4.0 License.

the Creative Commons Attribution 4.0 License.

# A benchmark for testing the accuracy and computational cost of shortwave top-of-atmosphere reflectance calculations in clear-sky aerosol-laden atmospheres

### Jeronimo Escribano

### Alessio Bozzo

### Philippe Dubuisson

### Johannes Flemming

### Robin J. Hogan

### Laurent C.-Labonnote

### Olivier Boucher

Accurate calculations of shortwave reflectances in clear-sky aerosol-laden atmospheres are necessary for various applications in atmospheric sciences. However, computational cost becomes increasingly important for some applications such as data assimilation of top-of-atmosphere reflectances in models of atmospheric composition. This study aims to provide a benchmark that can help in assessing these two requirements in combination. We describe a protocol and input data for 44 080 cases involving various solar and viewing geometries, four different surfaces (one oceanic bidirectional reflectance function and three albedo values for a Lambertian surface), eight aerosol optical depths, five wavelengths, and four aerosol types. We first consider two models relying on the discrete ordinate method: VLIDORT (in vector and scalar configurations) and DISORT (scalar configuration only). We use VLIDORT in its vector configuration as a reference model and quantify the loss of accuracy due to (i) neglecting the effect of polarization in DISORT and VLIDORT (scalar) models and (ii) decreasing the number of streams in DISORT. We further test two other models: the 6SV2 model, relying on the successive orders of scattering method, and Forward-Lobe Two-Stream Radiance Model (FLOTSAM), a new model under development by two of the authors. Typical mean fractional errors of 2.8 % and 2.4 % for 6SV2 and FLOTSAM are found, respectively. Computational cost depends on the input parameters but also on the code implementation and application as some models solve the radiative transfer equations for a range of geometries while others do not. All necessary input and output data are provided as a Supplement as a potential resource for interested developers and users of radiative transfer models.

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Accurate radiative transfer calculations in the Earth's atmosphere are necessary for some applications such as remote sensing of the atmospheric and surface properties, numerical weather prediction, and climate modelling. In this study, we are interested more specifically in the calculation of radiances in the shortwave spectrum in clear-sky aerosol-laden atmospheres of the Earth for reasons explained below. Assuming perfect knowledge of the optical properties of the atmosphere and surface and a plane-parallel atmosphere, solving the radiative transfer equation is a very well posed physical problem that essentially reduces to a (not-so-easy) algorithmic and numerical problem. Accurate methods have existed for a long time, and testing the accuracy of newly developed methods has been standard practice in the radiative transfer community for some time (de Haan et al., 1987; Stamnes et al., 1988; Spurr, 2008; Kotchenova et al., 2006; Kotchenova and Vermote, 2007; Barlakas et al., 2016; Korkin et al., 2017). Well-used state-of-the-art radiative transfer models generally show agreement within 1 % (or better) under most conditions tested. There have also been a number of intercomparison or benchmarking exercises for shortwave radiance calculations, largely motivated by the requirements for accurate calculations in ground-based and satellite aerosol retrievals (Kotchenova et al., 2006; Kokhanovsky et al., 2010a; Emde et al., 2015, 2018). A classical benchmarking exercise consists of comparing the model results to well-known solutions for simple cases of a Rayleigh scattering atmosphere (Coulson et al., 1960; Natraj et al., 2009). However, such cases are very specific and do not represent the variety of conditions met in the real atmosphere. The International Polarized Radiative Transfer (IPRT) working group of the International Radiation Commission has recently defined a set of more realistic cases (Emde et al., 2015, 2018). Another benchmark is to test the numerical convergence of the scheme when the angular and/or vertical resolutions of the model are increased (de Haan et al., 1987; Ganapol, 2017). However, numerical convergence may be difficult to achieve and to assess because the numerical models may experience lower precision when the resolution is increased too much, especially for models coded in single precision. Finally, there have also been benchmarking exercises for aerosol retrievals from observed clear-sky shortwave radiances, which tests simultaneously the accuracy of the radiative transfer model and the aerosol retrieval algorithm (Kokhanovsky et al., 2010b). It should be noted that accounting for the polarization of light is necessary because it affects the accuracy of the computed radiances (e.g. Kotchenova et al., 2006) but also because some aerosol retrieval algorithms rely on the inversion of polarized radiances (e.g. Tanré et al., 2011; Dubovik et al., 2011). However, in cases where the atmosphere contains an important load of non-spherical particles (dust), errors in neglecting polarization have been reported to be less than 1 % (Barlakas, 2016).

Methods for computing shortwave radiances by solving accurately the radiative transfer equation are often computationally expensive. In data assimilation or satellite retrieval applications for which radiances have to be computed many times and on many profiles, the computational cost has always been an issue for pragmatic reasons (a numerical model that is too slow to be run on the fastest computer is virtually useless). However, developing fast or very fast radiative transfer models capable of computing radiances (and not only vertical fluxes) has received little attention so far. Exceptions include early attempts to use neural networks for fast and accurate radiative transfer in the longwave spectrum (Chevallier et al., 1998; Krasnopolsky and Chevallier, 2003; Pfreundschuh et al., 2018). It may also be possible to design fast physical algorithms in the longwave spectrum because of the weaker scattering (as compared to the shortwave spectrum) and isotropic emission (e.g. Wang et al., 2013b). In the shortwave spectrum, one can mention methods based on principal component analysis (Kopparla et al., 2017; Somkuti et al., 2017), two orders of scattering method (Natraj and Spurr, 2007), adding–doubling method (e.g. Wang et al., 2013a), or recent efforts for fast calculations of cloudy-sky visible radiances (Scheck et al., 2016). However, Scheck et al. (2016) rely on tabulated calculations (so-called lookup tables), which only allows them to consider a limited set of possible atmospheric inputs. It is noteworthy in that respect that the performance assessment of the newly released SORD (Successive ORDers of scattering) model (Korkin et al., 2017) shows a runtime analysis for the 52 benchmark cases that were tested (their Fig. 6). There is a mode in the distribution of runtimes that is below 0.01 s but also a fairly long tail with occasional runtimes between 60 and 300 s or even longer than 600 s. Finally the computational cost of algorithms for three-dimensional radiative transfer models have been investigated by Pincus and Evans (2009) and are known to be expensive.

Forecasts and reanalyses of atmospheric aerosols have been produced operationally by combining numerical models and aerosol observations in data assimilation systems (Bocquet et al., 2015; Hollingsworth et al., 2008). Aerosol information provided by satellite retrievals is often assimilated in aerosol models both in research (Collins et al., 2001) and operational (Benedetti et al., 2018) contexts. A widely assimilated quantity is the aerosol optical depth (AOD), a column-integrated quantity that quantifies the interaction of aerosols with atmospheric radiation. So far, the assimilation of AOD retrievals has been successful in constraining the model variables towards the equivalent measured variables, but as the resolution, complexity, and quality requirements of the products vary, possible inconsistencies between the assumptions used for the satellite aerosol retrieval and the model or between the different satellite retrieval algorithms arise. Furthermore, there is some information on aerosol types in the observed reflected solar radiation that may not be included in AOD products and is, therefore, lost for data assimilation.

An alternative pathway to the assimilation of AOD retrieved from passive satellite measurements is the direct assimilation of these measurements (Benedetti et al., 2018). The main challenge of doing so is basically that the satellite retrieval algorithm has to be replaced by a somehow equivalent procedure inside the data assimilation loop. In particular, a shortwave radiative transfer model (RTM) would act as a major piece of the observation operator, and some stringent requirements on accuracy/quality, flexibility, and runtime would have to be met. In the case of variational data assimilation, similar requirements would apply to the Jacobian of the RTM. The purpose of this benchmark paper is to assess the accuracy and computational cost of different RTMs to assimilate top-of-atmosphere reflectances to constrain aerosols. We will exclude the assessment of the Jacobian of the RTMs in this work. The successful implementation of this approach could help to improve the performance of the current aerosol data assimilation systems by providing (i) consistency between modelled aerosol type, aerosol vertical profile, atmospheric conditions, and the radiative transfer calculation, (ii) potentially better characterization of the error covariance matrices for the observations, (iii) flexibility when there is a change in satellite instrument, and (iv) the capability of assimilating simultaneously several different sources of aerosol information. Additionally, developments in this direction could help improve the numerical weather prediction (NWP) models by providing a functional shortwave observation operator to the data assimilation system. The latter also opens the possibility of improvements in the data assimilation of shortwave radiances for the atmospheric and surface properties (Martin et al., 2014), including cloudy atmospheres (Martin and Hasekamp, 2018). Martin and Hasekamp (2018) show that the adjoint framework can be computationally efficient but do not present the computational cost of the SHDOM and FSDOM models that they use.

To our knowledge, only a limited number of studies have been able to
assimilate shortwave radiances in atmospheric composition models.
Weaver et al. (2007) have assimilated the Moderate Resolution Imaging
Spectroradiometer (MODIS) radiances in the Goddard Chemistry Aerosol
Radiation and Transport (GOCART) model coupled to the Arizona Radiation
Transfer Code, a lookup-table-based model, showing that although the
assimilation improves the model AOD, it does not outperform MODIS AOD
products (Remer et al., 2005). Drury et al. (2008) were able to assimilate MODIS
radiances in the GEOS-Chem chemical transport model (coupled with the LIDORT
radiative transfer model) with the resulting aerosol field comparing better
against the Aerosol Robotic Network (AERONET) data than MODIS (collections 4
and 5) products for a 45-day period over North America.
Wang et al. (2010, 2012) have also assimilated MODIS radiances in the
GEOS-Chem model (coupled with the VLIDORT model) showing improvements in the
AOD computations and top–down emission estimates. Xu et al. (2013) also used
GEOS-Chem and VLIDORT to estimate sources of gaseous compounds by
assimilating MODIS radiances and evaluated against OMI (Ozone Monitoring
Instrument) SO_{2} and NO_{2} retrievals and Multiangle Imaging
Spectro-Radiometer (MISR) and AERONET AOD.

Migliorini (2012) discusses some conditions where the assimilation of radiances is equivalent to the assimilation of the retrieval products. For this to happen, the observation operator has to be reasonably (i.e. within the errors of the variable) approximated by a linear function of the control variable and the background information has to be chosen such that the same information content of the observations is propagated in both assimilations. In our case, the first condition is usually assumed true, but the second condition is difficult to implement and to evaluate.

For operational data assimilation purposes, the computational cost of the radiance observation operator is crucial (Weng, 2007). While the computational burden of assimilating satellite radiances is expected to be larger than that of assimilating retrievals, it has not yet been specified, to our knowledge, how fast such an observation operator has to be. A rough estimate could be made using the computing time of the infrared radiance calculation as a guideline. For example, in the European Centre for Medium-range Weather Forecasts (ECMWF) data assimilation system, the infrared radiance observation operator takes around 0.1 ms per profile and channel in a highly parallelized system and roughly 3 times more including the tangent linear and adjoint code calculations. Taking into account the current infrastructure of the Copernicus Atmospheric Monitoring Service (CAMS) at ECMWF, a first estimate of the maximum computing time for shortwave radiance calculation would be approximately 10 ms per profile and channel but ideally of the order of 1 ms (or about 10 times slower than the current RTTOV infrared calculations) in this particular ECMWF/CAMS infrastructure. Likewise, there is a lack of knowledge of the required accuracy of the shortwave radiance calculations needed. A first-order estimate would be an accuracy similar to the measurement or retrieval error, but this could depend on the data assimilation configuration and the measurement characteristics (in terms of multi-spectral and multi-angle viewing capability, quality of the prior including that of the surface reflectance, etc.). There may also be a variety of RTMs for different atmospheric conditions (e.g. clear or cloudy sky) or wavelengths (ultraviolet, visible, or near-infrared).

How computational speed is achieved depends on the desired application. Some methods, such as the discrete ordinate method or the Monte Carlo method (e.g. Emde et al., 2015, 2018), can compute radiances for a set of viewing geometries (given one-sun geometry) in a single simulation. Other methods, such as the successive orders of scattering (e.g. Lenoble et al., 2007), require one simulation for each viewing geometry (again given one-sun geometry). The latter is not necessarily a disadvantage for applications where a different atmospheric profile of optical properties is associated with every viewing geometry. When the problem to be solved requires a lot of tasks to be processed in parallel and frequent thread synchronization, as is the case in many three-dimensional atmospheric models, it is the slowest tasks that matter. Therefore, the tail of the runtime distribution matters as much, if not more, than the average or median runtime across atmospheric conditions and viewing geometries. In the specific case of aerosol radiance assimilation in NWP models, it may nevertheless be possible to leave out the conditions and geometries that take too long, either by eliminating them in advance or by stopping them at runtime during the first iteration if the corresponding tasks exceed a threshold time. In the end, there is likely to be some trade-off to be accepted between accuracy and computational cost of the radiance calculations.

Keeping this introduction in mind, the objective of this study is to provide a benchmarking tool for evaluating shortwave radiances in the clear-sky aerosol-laden atmosphere under a wide range of aerosol conditions. The benchmarking is designed to assess both the accuracy and computational cost of radiative transfer models. Although data assimilation of aerosol radiances is the key motivation for this work, we report computational cost both for single and multiple geometries to make the scope more general. We provide all necessary input and output data for this benchmarking as a Supplement in NetCDF and ACSII format so that it becomes an available resource for model developers and potential model users. The protocol is described in the next section. It is followed by a test of the protocol in different RTMs and some conclusions.

The detailed benchmark protocol is provided as a Supplement. Only the main features are summarized here. We consider five wavelengths spanning the shortwave spectrum: 470, 550, 660, 865, and 1024 nm. These wavelengths correspond to typical wavelengths in shortwave atmospheric windows used by satellite instruments such as MODIS, POLarization and Directionality of the Earth's Reflectances (POLDER)/Polarisation et Anisotropie des Réflectances au sommet de l'Atmosphère, couplées avec un Satellite d'Observation emportant un Lidar (PARASOL), Visible Infrared Imaging Radiometer Suite (VIIRS), Along Track Scanning Radiometer (ATSR), and Advanced Along Track Scanning Radiometer (AATSR). They offer a range of conditions because molecular scattering and the impact of polarization are more important at the shorter wavelengths.

## 2.1 Vertical profiles

The molecular and aerosol profiles are described below. It should be noted that we assume the atmosphere to be plane-parallel; i.e. there is no correction applied for sphericity of the Earth's atmosphere.

### 2.1.1 Molecular profile

We use a midlatitude summer atmospheric profile (McClatchey et al., 1971) and
provide the layer altitude (km), pressure (hPa), temperature (K), relative
humidity (%), water vapour mixing ratio (ppmv), ozone mixing ratio (ppmv),
and humid- and dry-air density (g cm^{−3}) on
50 levels ranging from *z*=0 to ${z}_{max}=\mathrm{120}$ km. The molecular scattering
and absorption optical depths are provided for the 49 corresponding layers
from the surface to the top of the atmosphere using the radiative transfer code
GAME (Dubuisson et al., 2004, 2006). In particular, the absorption
optical depths have been calculated with GAME for the five selected
wavelengths by averaging the gaseous absorption (for H_{2}O and
O_{3}) in each layer from the *k* distribution parameterization used in
the GAME code. Note that gaseous absorption is weak at the selected
wavelengths so this approximation remains valid. The molecular depolarization
ratios are taken from Bodhaine et al. (1999), with values of 0.0288567,
0.0283241, 0.0279199, 0.0275716, and 0.0274591 for the five selected
wavelengths. There is no obligation to use the 49 layers provided in the
protocol. Indeed, it could be interesting to perform the RTM calculations on
a reduced vertical grid to decrease the computational cost. If a
different vertical grid is used, we ask that the vertically integrated
molecular scattering and absorption optical depths be conserved.

### 2.1.2 Aerosol profile

We define the vertical profile of the aerosol extinction coefficient
(*σ*^{aer}) as an exponential function with a height scale *H* of
2 km, normalized to unity when integrated between *z*=0 and
$z={z}_{max}=\mathrm{120}$ km, that is

The aerosol optical depth of a layer *i*, *τ*_{i}, is computed as

with Δ*z*_{i} the layer thickness, that is, the difference between the
altitudes of the layer top ($\stackrel{\mathrm{\u203e}}{{z}_{i}}$) and the layer bottom
($\underset{\mathrm{\xaf}}{{z}_{i}}$). The average aerosol extinction coefficient for layer *i*
then satisfies

We provide ${\mathit{\sigma}}_{i}^{\mathrm{aer}}$ and *τ*_{i} for the 49 layers defined in
our atmospheric profile, but these quantities can be easily recomputed for
any vertical grid. These values are normalized to an AOD of 1, so they have
to be multiplied by the actual AOD of each case.

## 2.2 Aerosol types

Four aerosol types have been defined. Aerosol size distributions are assumed to be log-normal with the parameters shown in Table 1. Refractive indices are taken from the literature (see Table 1) and the aerosols are assumed to be internally mixed, with a volume-weighting rule. The four aerosol types defined in this benchmark have been chosen to represent a variety of size and optical properties observed in the atmosphere. Two fine-mode aerosols are defined: industrial scattering (aer_is) is sulfate aerosol with a relative humidity of 50 %, while industrial absorbing (aer_ia) is a mixture of the Optical Properties of Aerosols and Clouds (OPAC; Hess et al., 1998) aerosol type INSO (insoluble), SOOT (soot), and WASO (water soluble, at 50 % of relative humidity). The industrial absorbing aerosol is similar to an organic-matter aerosol type. We also consider a monomodal dust aerosol and a bimodal sea salt aerosol (aer_ss).

## 2.3 Aerosol optical properties

Aerosol optical properties were computed by the authors using their own Mie
routines, and results were cross-checked for consistency. The Mie routine used
to generate the input data was rewritten in Fortran based on the
implementation of Toon and Ackerman (1981) and was compiled with the Intel Fortran
compiler in quadruple precision. The calculations are monochromatic (i.e. no
integration was performed on the wavelength). The aerosol size distribution
is integrated over the size range reported for each type in
Table 1, with a Gaussian quadrature of 10 000 points in
logarithm of the radius. The number of terms used to compute the sums of the
*a*_{n} and *b*_{n} coefficients is approximated as a function of the Mie
parameter ($x=\mathrm{2}\phantom{\rule{0.125em}{0ex}}\mathit{\pi}\phantom{\rule{0.125em}{0ex}}r/\mathit{\lambda}$) following Wiscombe (1980).

For each aerosol type, we provide the aerosol optical depths at the 440, 670, 865, and 1024 nm wavelengths corresponding to the selected AOD at 550 nm. The aerosol mass extinction coefficient (although not necessary to conduct the RTM calculations), single-scattering albedo, and asymmetry parameter are also given.

The phase matrix is computed and provided at 50 000 points evenly spaced in
scattering angle from 0 to 180^{∘}. As the particles are assumed to be
spherical, only the *S*_{11}, *S*_{12}, *S*_{33}, and *S*_{34} elements of
the phase matrix are non-zero and provided. We also decompose the phase
matrix in generalized spherical functions (de Rooij and van der Stap, 1984; de Haan et al., 1987) as
needed for the vector version of the discrete ordinate method. For this, the
phase matrix is integrated with an 8000-point Gaussian quadrature over the
cosine of the scattering angle, and we provide the first 750 moments of that
decomposition for the six commonly used elements (*α*_{1}, *α*_{2},
*α*_{3}, *α*_{4}, *β*_{1}, *β*_{2}). For completeness, we also
provide the corresponding Legendre coefficients (by definition, the *l*th
Legendre coefficient equals the *l*th Legendre moment multiplied by 2*l*+1).
Thus, we provide all necessary information on aerosol optical properties for the usual RTMs so that no further processing of the aerosol optical properties
should be required. Of course, the RTMs that neglect polarization should only
consider the *S*_{11} term of the phase matrix or the *α*_{1} Legendre
moments.

The accurate reconstruction of the original phase function from the Legendre
decomposition (*S*_{11}) cannot be ensured when a low number of Legendre
moments is used to approximate it. As for the vertical resolution, there is
no obligation for this benchmarking exercise to represent the exact phase
function, and it may be interesting to approximate it to strike a balance
between accuracy and computational cost (see, for example, Ding et al., 2009).
The minimum number of Legendre moments for which the approximated phase
function is positive depends on the aerosol type and the wavelength
(basically, on how large the forward peak is). For the industrial scattering
and industrial absorbing aerosols, a minimum of four to eight moments is required
(depending on the wavelength). For dust aerosol, the minimum is between 14
moments (at 1024 nm) and 40 moments (at 470 nm), while for
sea salt, the minimum is between 144 moments (at 1024 nm) and 348
moments (at 470 nm).

Figure 1 shows the phase function and the linear depolarization ratio for the four aerosol types considered in this benchmark.

## 2.4 Surface reflectance

Two surface reflectance models are selected: a Lambertian and an oceanic bidirectional reflectance distribution function (BRDF) model. For the Lambertian model, three surface albedos were chosen, namely 0, 0.05, and 0.1. The spectral dependence of the surface reflectance is ignored; i.e. the albedo is assumed to be the same for the five wavelengths considered. The second surface reflectance model is the oceanic glitter model from Mishchenko and Travis (1997). This model is suited for both full vector radiative transfer models (i.e. including polarization) and for the scalar approximations.

Wind speed is set to 10 m s^{−1}, but the Mishchenko and Travis (1997) code
assumes an isotropic Gaussian distribution of (oceanic) surface slopes, so
the wind orientation is not taken into account. The salinity is set to
34.3 ‰ and water refractive indices are interpolated, for each
wavelength, from the data provided in Hale and Querry (1973) as inputs to the
Mishchenko and Travis (1997) routine (available at
https://www.giss.nasa.gov/staff/mmishchenko/brf/, last access: 19 February 2019). The effect of shadowing is also taken into
account.

The convention for the azimuthal angle used in the Mishchenko and Travis (1997) routine is the opposite of the convention that we use below to define geometries, so the coupling between the atmospheric RTM and the oceanic code has to be done carefully.

## 2.5 Geometries and configurations

We have selected eight cases for aerosol loadings in the atmosphere, with AOD at 550 nm ranging between 0 (no aerosol) and 2 (high aerosol loading). These values refer to the 550 nm wavelength, and the corresponding AODs can be estimated for the other wavelengths depending on the aerosol types. A table with the equivalences in AOD for each aerosol type and wavelength is also provided in the benchmark protocol.

We have further selected four solar and four viewing zenith angles – *θ*_{s} and *θ*_{v} – from 0^{∘} (zenith) up to
60^{∘}. Given the provided surface reflectance models and the assumption
of spherical (hence randomly oriented) aerosols in the atmosphere, the
radiance at the top of the atmosphere is only dependent on the difference
between the incident and reflected azimuthal angle and not on each value
taken independently. We have chosen five azimuthal angles between
*ϕ*_{r}=0^{∘} (backscatter when solar and viewing zenith
angles are equal) and *ϕ*_{r}=180^{∘} (forward scattering
when solar and viewing zenith angles are equal). The different cases are
summarized in Table 2. Removing the redundancies in
aerosol types for AOD =0 and azimuthal angle for
${\mathit{\theta}}_{\mathrm{s}}={\mathit{\theta}}_{\mathrm{v}}={\mathrm{0}}^{\circ}$, this results in 11 020 cases
for the BRDF surface and 33 060 cases for the Lambertian surfaces (see
Table 3), which gives a grand total of 44 080 cases.

## 2.6 Measured variables

We aim to provide a standardized dataset to evaluate the possibility of assimilating satellite radiances in operational forecasts of atmospheric composition. Thus, two variables are important to estimate for each case: the simulated reflectance (or radiance) that would be observed by the satellite and a “sound” estimate of the radiative transfer code runtime.

We will describe the accuracy of the models in terms of the reflectance,
*ρ*_{I}, of the intensity (i.e. the first Stoke component), which is
related to the radiance, *I*, through the following relationship:

where *I*_{s} is the incident solar flux at the top of the atmosphere
(conveniently taken to equal 1 for reflectance calculations) and
*θ*_{s} is the solar zenith angle.

The runtime may be difficult to estimate and compare between different models. We aim to measure the runtime of the computations excluding input/output operations, as typical applications would embed the RTM code in a larger operational system. The runtime of the RTM should be measured in a single-thread configuration. Ideally, the simulations should be performed on the same core of the computer and with the same system load conditions (except for the RTM) as other threads on the operating system (in particular with a high input/output load) could hamper the correct timing of the model. Multiple repetitions of the computations can be performed to get better estimates.

Given a set of atmospheric and aerosol conditions, some models can compute, almost without extra cost, the radiances for several viewing geometries, while other models require multiple simulations to achieve the same. For this protocol, this would introduce a difference of a factor of circa 80 (the number of viewing geometries) in the reported runtimes. Depending on the application, this feature of some models may or may not be relevant. For this reason, we present here the runtimes of the tested models in two configurations: one taking advantage of the simultaneous output for multiple viewing geometries and another one without this feature. In the second configuration (called “ind” in Sect. 3), each simulation outputs only one geometry while in the first configuration (called “mult” in Sect. 3), it outputs as many values as possible within the specifications of the protocol. The number of multiple outputs per simulation will be detailed in the next section.

## 3.1 Reference model: VLIDORT

The VLIDORT model is an independent implementation of the discrete ordinate
method based in part on earlier work by Siewert (2000a, b) and
other sources. In this work we used the VLIDORT version 2.7, which
includes a BRDF supplement that can be called before the main program. A
detailed user's guide is available (Spurr, 2014). One interesting feature
of this model is that it also provides Jacobians with respect to the model
inputs (not used in this study). In this model, it is possible to choose how
many elements are computed in the Stokes vector. Although VLIDORT is able to
solve the radiative transfer equation for the four-element Stokes vector
$(I,Q,U,V)$
(Spurr, 2008), we will refer to the vector runs when
the 3×3 approximation of the RT problem is used (i.e. neglecting
circular polarization in the atmosphere by solving only for $(I,Q,U)$) and we
will refer to the scalar runs when only the first element of the
Stokes vector is used in the computations (i.e. when polarization of
electromagnetic radiation is not accounted for). VLIDORT is written in double
precision; it has the capability to output, in a single call, radiances for
any number of solar and viewing angles. The protocol vertical resolution with
49 layers is used. It should be noted that the discrete ordinate method shows
a numerical instability for zenith viewing angle *θ*_{v} of
0^{∘} or a relative azimuthal angle *ϕ*_{r} of 90^{∘}.
These angles are therefore prescribed at *θ*_{v}=0.0001^{∘} and *ϕ*_{r}=89.98^{∘} instead to circumvent
this problem. We use the model with 32 streams, and the TMS correction
(Nakajima and Tanaka, 1988) is activated. The TMS correction removes the single-scattering feature computed with a few streams and replaces it by the exact
single-scattering contribution. This correction helps to remove non-physical
features due to the use of a low number of Legendre coefficients in the
computation. The VLIDORT vector is our reference model.

## 3.2 DISORT

The DISORT model (Stamnes et al., 1988) is a flexible and widely used RTM. It
solves the scalar approximation of the radiative transfer equation in a plane-parallel atmosphere by using the discrete ordinate method. In this study, we
use the DISORT code, version 4, available from
http://lllab.phy.stevens.edu/disort/ (last access: 19 February 2019), and we coupled the oceanic BRDF following the code
developer instructions. We use the model with 32 streams, and the TMS
correction (Nakajima and Tanaka, 1988) is activated. It should be noted that most of
this code is written in single precision, but important routines (such as the
computation of the Gaussian nodes) are written in double precision. We use
32 streams in our main configuration, but sensitivity results for 4, 8, 16,
32, 64, 96, and 128 streams are also presented below. The protocol vertical
resolution with 49 layers is used. Similar to VLIDORT, the DISORT code has
numerical instabilities which manifest themselves when the solar and viewing
zenith angles (*θ*_{s} and *θ*_{v}) are equal. In
such cases, we perturb the cosine of the viewing zenith angle by
$\mathrm{8}\times {\mathrm{10}}^{-\mathrm{5}}$.

Given a solar zenith angle, the DISORT model can provide outputs for multiple
viewing geometries. In this study, the number of geometries for the
mult timing configuration is the number of *ϕ*_{r} cases
multiplied by the number of *θ*_{v} cases (i.e. $\mathrm{4}\times \mathrm{5}=\mathrm{20}$).

Although we know the DISORT model to be too slow for online data assimilation of aerosol radiances, we find it useful to document here the trade-off between accuracy and computing time as a function of the number of streams used in the calculations. Figure 2 shows both the mean fractional error and the average computing time for 4, 8, 16, 32, 64, 96, and 128 streams. The mean fractional error is shown relative to our reference VLIDORT (vector) and is of the order of 1 or 1.5 % for the Lambertian and oceanic BRDF surface cases, respectively, as soon as the number of streams exceeds eight. Most of that discrepancy is because DISORT neglects the polarization. When DISORT is compared to VLIDORT (scalar), the mean fractional error is significantly smaller: <0.1 % for the Lambertian surface case and <1 % for the surface BRDF case.

There is a big gain in accuracy when increasing the number of streams from 4 to 8 and little further gain in accuracy beyond 16 streams. A deterioration in accuracy is observed beyond 96 streams in the case of the BRDF surface, which may be due to the DISORT model being coded in single-precision float. While the dependence of model accuracy on the number of streams is well known (Stamnes and Swanson, 1981), we find it useful to confront the loss of accuracy with the gain in computational cost. Computing time increases rapidly with the number of streams (faster than the cube of the number of streams beyond 16 streams). Using 16 streams might, therefore, be an interesting trade-off between accuracy and computational cost.

## 3.3 6SV2

The 6SV2 model is a radiative transfer model based on the successive order of scattering (SOS) method (Vermote et al., 1997). It solves the radiative transfer equation by using the three-element approximation (3×3) of the Stokes vector. The accuracy of the code against previous benchmark and Monte Carlo codes has been reported to be better than 1 % (Kotchenova et al., 2006; Kotchenova and Vermote, 2007). We have used version 2.1 of the model. Two configurations are available: a standard accuracy configuration that consists of 83 Gaussian points for the phase function, 30 vertical levels, 25 zenith angles, and 181 azimuth angles and a high-accuracy configuration that consists of 121 Gaussian points for the phase function, 50 vertical levels, 75 zenith angles, and 181 azimuth angles. We will present results only from the standard accuracy, which is 2 to 70 faster than the high-accuracy configuration, but will comment briefly on the high-accuracy configuration in Sect. 4.2.

Molecular scattering and absorption vertical profiles are embedded in the
code. Both are internally computed according to the atmospheric profile as
part of “core” SOS routines. It is therefore not appropriate to prescribe
our own molecular scattering and absorption profiles, so we use the 6SV2
defaults instead. The difference in the total molecular scattering optical
depth is less than 0.26 % with respect to the files provided in this
protocol. We have coupled the BRDF calculations to the 6SV2 model, but it
should be noted that the atmosphere–surface coupling of the oceanic BRDF
model in 6SV2 does not use the full surface reflection matrix but only the
first column of it (i.e. the column that relates reflected *I* with incoming
*I*, *Q*, and *U*).

The 6SV2 model can only compute one viewing geometry in each model call (i.e. the ind configuration is exactly the same as the mult configuration).

## 3.4 FLOTSAM

We also test in this study a preliminary version of the Forward-Lobe
Two-Stream Radiance Model (FLOTSAM), which is being developed at the European
Centre for Medium-Range Weather Forecasts (ECMWF). Unlike a lookup-table-based
solar RTM, FLOTSAM permits an arbitrary layering of different types of
scatterers, including clouds, aerosols, and molecules. The model has been
designed to be fast enough to be used in iterative assimilation and retrieval
schemes. It exploits the fact that particle phase functions typically contain
a “narrow forward lobe” associated with scattering angles of less than a
few degrees and a “wide forward lobe” associated with scattering angles of
the order of 15^{∘}. This enables coupled ordinary differential
equations to be written down for radiation in the quasi-direct solar beam
(including the narrow forward lobe) and a wider beam of radiation propagating
away from the Sun due to scattering by the wide forward lobe. Additionally,
the upwelling and downwelling diffuse fluxes are computed using the
two-stream approximation. FLOTSAM may, therefore, be thought of as the solar
analogue of the infrared two-stream source function technique of
Toon et al. (1989). To facilitate the use of FLOTSAM in variational assimilation
and retrieval schemes, it has been coded in C++ using version 2.0 of the
Adept library (Hogan, 2014, 2017), which enables the Jacobian to be
easily computed via Adept's fast automatic differentiation capability.
FLOTSAM has been incorporated into the Cloud, Aerosol and Precipitation from
Multiple Instruments using a Variational Technique (CAPTIVATE) retrieval
scheme, which will provide operational products from the forthcoming
EarthCARE satellite (Illingworth et al., 2015).

FLOTSAM is designed to be used in iterative schemes in which multiple calculations are performed with different profiles of optical properties but for the same viewing geometry. This is implemented by allowing the user to perform the set-up calculations once for a particular solar zenith angle, viewing zenith angle, and viewing azimuth angle and then to reuse the information in subsequent radiance calculations for different optical profiles. We assess the computational cost for both the ind and the mult configurations, with the latter consisting of $\mathrm{4}\times \mathrm{4}\times \mathrm{5}\times \mathrm{8}=\mathrm{640}$ cases involving 80 geometry set-up calculations and each separate geometry being used with eight different optical depths.

## 4.1 Methods and statistics

The protocol has been tested for the five model versions described above: VLIDORT (vector), VLIDORT (scalar), DISORT, 6SV2, and FLOTSAM. As explained in Sect. 2.6, the accuracy is quantified in terms of monochromatic reflectances at the top of the atmosphere and the accuracy is computed against the vector version of the VLIDORT model unless stated otherwise.

With as many as 44 080 cases considered in this study, we have to find original ways to visualize the results as explained below. Along with the average reflectance computed for each model, the following statistical measures have also been computed:

where *N* is the number of experiments, *m*_{i} is the reflectance of the
tested model, and *r*_{i} is the reflectance of the reference model for case
*i*. By definition, the mean fractional bias can range between −2 and 2,
with a best score of zero, while the mean fractional errors can range between
0 and 2, with a best score of zero. Both values are multiplied by 100 with
respect to the definitions above in the tables presented below. We also state
the percentages of cases where the relative error of the reflectances is
above defined relative error thresholds. These diagnostics, which could be
detailed for particular subsets of our 44 080 cases, can help to decide if
and under which conditions a particular RTM could be useful, according to the
required accuracy of the reflectance calculations.

Runtimes of the models are presented in an absolute measure (seconds), and no reference is needed. Finally, we will comment on possible trade-offs between accuracy and computing time.

## 4.2 Accuracy

Figure 3 shows histograms of the simulated reflectances of
the VLIDORT (vector) model at the top of the atmosphere for all or various
subsets of the 44 080 cases. These reflectances form the reference
simulations for this study. The width of the histograms (or violins)
in Fig. 3 is proportional to the number of cases in each
bin of the reflectance (*y* axis) discretization. The multiple panels (and
multiple violins) are computed with all the simulations but filtered by the
value indicated on the *x* axis. Therefore, they can be interpreted as analogous to marginal density probability functions of the set of
simulations. Figure 3 shows a wide range of reflectances
(from near 0 to almost 0.5) and known dependencies of the reflectance with
respect to some of the input variables such as the surface albedo (Lambertian
case), the AOD, or the wavelength in this range of the spectrum.

A first comparison of the relative error is shown in
Fig. 4 for two scalar models (with otherwise the same
degree of sophistication as the reference model).
Figure 4 shows the histograms of the relative errors,
$(m-r)/r$, with *m* being the tested model and *r* the reference model (the
vector version of VLIDORT). It can be seen that for both models, DISORT and
the scalar version of VLIDORT, the errors are mostly below 5 %. They are
centred on zero for the Lambertian surface, but a negative bias is observed
for the BRDF case. Comparison of scalar VLIDORT against vector VLIDORT
provides a direct measure of the relative errors due to neglecting
polarization in the model. As expected and because of a larger molecular
scattering contribution, we observe larger relative errors for the shorter
wavelengths (MFE =1.8 % at 470 nm; MFE =0.5 % at
1024 nm). We also observe larger errors for the accumulation-mode
aerosols (industrial scattering and industrial absorption have MFE ≈1.2 %) in contrast to coarse-mode aerosols (dust and sea salt have MFE
≈0.6 %). These errors are associated with the larger polarized phase
function (see Fig. 1) of industrial scattering and industrial
absorption aerosols for large AOD (see Figs. A2
and A4). Although no clear relation is shown
for relative errors as a function of the geometrical parameters in
Fig. 4, Fig. A2 shows a clear
link between the relative errors and the scattering angle for these two
accumulation-mode aerosols at large AOD. The oceanic BRDF function used in
this study also enhances the errors of the scalar versions as these neglect
the polarizing effect of the surface because the glint can increase the
polarized feature of the light for some angles. Larger relative errors are
shown for the dark surface case (Lambertian surface albedo equal to zero has MFE ≈1.2 %) because the reference reflectances are smaller than
for their 0.05 and 0.1 Lambertian counterparts (with MFE ≈0.7 %
and MFE ≈0.5 %, respectively). The error statistics of the VLIDORT
(scalar) and DISORT models are very similar, with noticeable differences only
for the BRDF case (Fig. 4 and Table 3).
The scalar approximation may thus be appropriate for some combinations of
wavelengths, aerosols types, and surface types but not for others. We further
note, that for both types of surfaces, the scalar VLIDORT simulations
outperform DISORT simulations when they are compared against the vector
version of VLIDORT (which is not surprising), but in both cases the overall
mean fractional error is larger than 0.7 % and the mean fractional bias is
negative.

Figure 5 and Table 4 show the relative errors of the other two models used in this study, 6SV2 and FLOTSAM, against VLIDORT (vector). As the FLOTSAM model does not include polarization, it is not expected to outperform the scalar approximations exemplified in the DISORT and VLIDORT (scalar) comparisons. However, this is the case for the 470 nm cases, as can be seen from the corresponding violin plot of Fig. 5. For 6SV2, the relative error shows a mode around zero, while for FLOTSAM, the shapes of the violin plots are more similar to those of the scalar approximations shown in Fig. 4. The reason for the unexpected dependence of the 6SV2 model errors on the wavelength is not completely clear, but it could be related to the technical difficulties in prescribing user-specific molecular scattering and absorption profiles in the 6SV2 code (see Sects. 2.1.1 and 3.3). In contrast to Fig. 4, Fig. 5 shows larger relative errors for sea salt aerosols for both models. While larger errors in Fig. 4 were related to the polarization/depolarization effects from aerosol particles (as shown in the right column of Fig. 1), errors of 6SV2 and FLOTSAM could be related to the more pronounced forward-scattering peak of the sea salt aerosols and their large single-scattering albedo. In fact, errors for large AOD (Fig. A3) are not strongly linked to the amount of polarized radiance (Fig. A4) for sea salt aerosols. In the case of 6SV2, the coarse discretization of the aerosol phase function used in the model configuration (83 Gaussian points, Sect. 3.3) hampers the accurate simulation of the sea salt forward-scattering peak. There is no clear dependence of the relative errors on the viewing and solar geometries but, as for Fig. 4, larger errors are observed for larger AOD and for the surface BRDF case.

Synthetic error statistics are shown in Table 4. Both the 6SV2 and FLOTSAM models have comparable mean fractional errors, but these are larger than those of DISORT and VLIDORT (scalar). Some difference concerning the type of surface can be appreciated, with 6SV2 performing better in the case of Lambertian surfaces. It should be noted that using the high-accuracy configuration of 6SV2 instead of the standard accuracy configuration reduces the mean fractional error from 1.5 % to 1.1 % for the Lambertian surface case but does not improve this score for the BRDF surface case. The mean fractional bias is negative, and it is of similar magnitude for both models. However, 6SV2 presents larger biases over the two types of surfaces (and the sign of the bias differs for the BRDF and Lambertian surfaces). In contrast, FLOTSAM shows virtually no bias for the Lambertian surface cases and a smaller bias than 6SV2 for the BRDF cases. An important difference with the scalar models is the presence of a heavy tail in the relative error distribution, which can be inferred from the last rows of Table 4.

## 4.3 Timing

In this section, we present statistics of computing times for three of the models used in this study. All the computing times presented below were estimated on an Intel i5-4690, 3.5 GHz workstation running Linux (kernel 2.6.32), GNU C++ and Fortran compiler version 4.4.7, on a single processor and with exclusive use of the workstation. We have compiled the models using their default compilation flags, i.e. “-O” for 6SV2, “-O3” and “-march=native” for FLOTSAM, and “-O3” for DISORT.

We start this section with the 6SV2 model which can only be used in
ind configuration. Figure 6 shows the violin
plots of the computing time of the 6SV2 model that spans as much as 2 orders of magnitude. Clearly, the BRDF cases are more expensive than the
Lambertian surface cases, with most computing times ranging between ca. 0.7
and 4 s for the former. This appears to be due to multiple calls to
the BRDF routine. It may be possible to optimize this routine or recode it in
a more efficient way, but we have not attempted to do so. We also note an
increase in computing time with increasing solar zenith angle (*θ*_{s})
and AOD and with decreasing wavelength. This is consistent with a larger
number of scattering orders being necessary in such cases. Cases for coarse
mode aerosols (especially sea salt aerosols) are more expensive than cases
for accumulation-mode aerosols, with some calculations reaching 5 s
for sea salt aerosols. We do not have an explanation as to why the cases for
industrial absorbing aerosols are more expensive than those for industrial
scattering aerosols.

As discussed above, the DISORT and FLOTSAM models offer efficiency savings when multiple profiles are computed together. In the case of DISORT, multiple viewing geometries can be computed for one solar zenith angle. In the case of FLOTSAM, a smaller saving is that the geometry set-up calculations may be computed once for a given solar/viewing geometry and then reused for multiple atmospheric profiles with different optical properties. The runtime of these models is, in general, not sensitive to the viewing geometry, wavelength, aerosol type, or AOD. However, there are some differences between the two types of surface reflectance. Table 5 shows a summary of the measured runtimes for the three tested models. Computing times are indicated for the two measurement procedures (ind and mult) and for the two types of surface reflectance (BRDF and Lambertian). In the case of FLOTSAM, we also report the “one-off set-up” cost, which in the case of the BRDF involves creating a four-dimensional lookup table. We stress that this is invoked only once per call to the executable, so in an operational data assimilation system performing many millions of radiance calculations it would increase the executable runtime by only a very small fraction. The one-off set-up costs are not shown for the 6SV2 and DISORT models as they are not isolated from the rest of the computations.

For the 6SV2 model, values of Table 5 are in concordance with Fig. 6 with runtimes of the order of seconds. The DISORT model average runtime is smaller than that of 6SV2. Similarly to 6SV2, there is an important dependence of the DISORT runtime on the type of surface considered. The extra cost of computing 20 profiles, instead of 1 profile, can be estimated as the difference between the mult and ind cases. For DISORT, this additional cost is about 0.07 s for the Lambertian case and about 0.22 s for the BRDF case, which correspond to 25 % and 18 % of the total mult runtime. This would benefit an application that requires simultaneous calculation of many viewing geometries. The spread of DISORT runtimes indicated in Table 5 arises because DISORT efficiently decreases the number of internal computations for symmetric geometries (solar zenith angle equal to zero, 21 % of the cases), and it is faster for AOD =0 (3.4 % of the cases). For the remaining cases of Table 5, there is almost no spread in the DISORT runtime.

Table 5 shows the runtimes of the FLOTSAM model. Firstly, it is noted that the standard deviation of the reported values is small (around 1 order of magnitude less than the values). This is because FLOTSAM performs the same number of operations for every case; thus, there is no added value of showing histograms of runtimes. For this model, it can be seen that the one-off set-up cost of the BRDF is a little under 3 s, but in the ind case, subsequent radiance calculations are 3–4 orders of magnitude faster than the other two models tested, and the BRDF calculation is only slightly slower. If many profiles have to be computed with the same observation geometry, then the geometry set-up cost is invoked only once and the average cost per profile is reduced. This is illustrated in the mult case in which 640 profiles are computed, which involves 80 different geometries. Excluding the model set-up time, the computation of a single profile takes around 0.3 to 0.4 ms in the ind case and around $\left(\mathrm{99}\phantom{\rule{0.125em}{0ex}}\mathrm{ms}\right)/\mathrm{640}=\mathrm{0.16}$ ms in the mult case.

In this study, we have presented a comprehensive benchmarking methodology and protocol, with an unprecedented number of cases combining different viewing geometries, aerosol optical depths, aerosol types, wavelengths, and surface types. The VLIDORT model in its vector configuration (i.e. considering polarization) is used as the reference model. Preliminary results in terms of accuracy and computing time are presented for three models: the well-known models DISORT and 6SV2, on the one hand, and FLOTSAM, a new fast model under development by two of the authors of this study, on the other. All models perform better when using Lambertian surface reflectance than when using an oceanic BRDF. All the models perform better under low AOD, and the scalar models have lower accuracies at shorter wavelengths. For the Lambertian surface cases, the mean fractional errors of the models are 0.8 %, 1.5 %, and 1.9 % for DISORT, 6SV2, and FLOTSAM, respectively. The BRDF cases show a larger mean fractional error with 1.5 %, 6.6 %, and 4 % for DISORT, 6SV2, and FLOTSAM, respectively.

The DISORT and 6SV2 models show comparable computing time, between tenths of a second and seconds, but DISORT can provide the solution at multiple viewing geometries for each atmospheric condition, while 6SV2 cannot. FLOTSAM is very fast, with computing times of much less than 1 ms per profile. However, it should be noted that this model is still under development and its accuracy could improve further. Moreover, all models present a tail of cases with larger errors and/or computing times. It could be interesting to characterize these better to investigate if screening out those cases can be an option in data assimilation applications. Also, a fast tangent linear and adjoint computation is required for the integration of an RTM in a variational data assimilation system.

The protocol and input and output data are available as a potential resource for interested developers and users of RTMs willing to benchmark their models.

The detailed protocol is available in PDF format. All inputs necessary to the benchmark are available as ASCII text files. The surface BRDF is available as a Fortran routine from the Michael Mishchenko website https://www.giss.nasa.gov/staff/mmishchenko/brf/ (Mishchenko and Zakharova, 2019). The VLIDORT reference results are available in NetCDF format.

The supplement related to this article is available online at: https://doi.org/10.5194/gmd-12-805-2019-supplement.

JE designed the benchmarking exercise, selected the aerosol models, performed the 6SV2 and DISORT simulations, processed the results, and drafted the manuscript. PD prepared the vertical molecular profiles. OB, JE, LL, and AB performed the Mie calculations. LL performed the VLIDORT simulations. RH designed the FLOTSAM model and performed the corresponding simulations, while AB added FLOTSAM's BRDF capability. All authors contributed to the analysis and the manuscript.

The authors do not declare any competing interest.

The authors acknowledge support from the Copernicus Atmosphere Monitoring
Service implemented by ECMWF on behalf of the European Commission. The
authors thank Didier Tanré for his interesting comments on the draft of the
manuscript. The authors would also like to thank the 6SV2 development team
for their help in resolving small issues with the model and Vijay Natraj and
Robert
Spurr and the two anonymous reviewers for their comments in the
discussion phase. Computing was partly performed on the ESPRI data and
computing centre of the Institut Pierre-Simon
Laplace.

Edited by: Andrea Stenke

Reviewed by: two anonymous referees

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