GMDGeoscientific Model DevelopmentGMDGeosci. Model Dev.1991-9603Copernicus PublicationsGöttingen, Germany10.5194/gmd-9-1647-2016The libRadtran software package for radiative transfer calculations (version 2.0.1)EmdeClaudiaclaudia.emde@lmu.deBuras-SchnellRobertKyllingArveMayerBernhardGasteigerJosefhttps://orcid.org/0000-0002-4401-0118HamannUlrichhttps://orcid.org/0000-0001-8091-722XKyllingJonasRichterBettinaPauseChristianDowlingTimothyBugliaroLucahttps://orcid.org/0000-0003-4793-0101Meteorological Institute, Ludwig-Maximilians-University,
Theresienstr. 37, 80333 Munich, GermanyNILU – Norwegian Institute for Air Research, Kjeller, NorwayDepartment of Mathematics, Faculty of Mathematics and
Natural Sciences, University of Oslo, Oslo, NorwayMeteoSwiss, Radar, Satellite and Nowcasting Division, Via ai
Monti 146, Locarno, SwitzerlandSchnell Algorithms, Am Erdäpfelgarten 1, 82205 Gilching, GermanyDept. of Physics & Astronomy, University of Louisville, Louisville, KY 40292, USAInstitut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, 82234 Wessling, GermanyClaudia Emde (claudia.emde@lmu.de)3May2016951647167224August20152December201512April201614April2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://gmd.copernicus.org/articles/9/1647/2016/gmd-9-1647-2016.htmlThe full text article is available as a PDF file from https://gmd.copernicus.org/articles/9/1647/2016/gmd-9-1647-2016.pdf
libRadtran is a widely used software package for radiative transfer
calculations. It allows one to compute (polarized) radiances, irradiance, and
actinic fluxes in the solar and thermal spectral regions. libRadtran
has been used for various applications, including remote sensing of clouds,
aerosols and trace gases in the Earth's atmosphere, climate studies, e.g.,
for the calculation of radiative forcing due to different atmospheric
components, for UV forecasting, the calculation of photolysis frequencies, and
for remote sensing of other planets in our solar system. The package has been
described in . Since then several new features have been
included, for example polarization, Raman scattering, a new molecular gas
absorption parameterization, and several new parameterizations of cloud and aerosol optical properties. Furthermore, a graphical user interface is now available,
which greatly simplifies the usage of the model, especially for new users.
This paper gives an overview of libRadtran version 2.0.1 with a focus
on new features. Applications including these new features are provided as
examples of use. A complete description of libRadtran and all its
input options is given in the user manual included in the libRadtran
software package, which is freely available at
http://www.libradtran.org.
Introduction
Radiative transfer modelling is essential not only for remote sensing of planetary
atmospheres, but also for many other fields in atmospheric physics:
atmospheric chemistry, which is largely influenced by photochemical reactions,
calculation of radiative forcing in climate models, and radiatively driven
dynamics in numerical weather prediction models.
The libRadtran software package is a versatile toolbox, which has
been used for various applications related to atmospheric radiation, a list
of publications that have used the package can be found on the website
http://www.libradtran.org; currently it includes more than 400 entries.
Applications include the following topics (the given references are taken as
examples out of the list of publications):
analysis of UV-radiation measurements, from which parameters,
e.g. ozone concentrations, aerosol optical thickness, UV-index,
are derived. Since the libRadtran package originally was a
radiative transfer code for the UV spectral range (the
main executable is still called uvspec), the model is
well established in this research area and frequently used
e.g.;
cloud and aerosol remote sensing using measurements in solar and thermal
spectral regions. The developed retrieval methods are for
ground-based, satellite and air-borne instruments that measure
(polarized) radiances e.g.;
volcanic ash studies including remote sensing of ash mass
concentrations e.g. and visibility of ash
particles from the pilot's perspective e.g.;
remote sensing of surface properties: a model like libRadtran is particularly important to develop atmospheric correction methods
e.g.;
trace gas remote sensing: libRadtran used as a
forward model for retrievals of O3, NO2, and BrO from DOAS
(Differential Optical Absorption Spectroscopy)
measurements e.g.;
calculation of actinic fluxes in order to quantify photolysis
rates for atmospheric chemistry e.g.;
determination of solar direct irradiance and global
irradiance distributions in order to optimize locations of solar energy platforms e.g. and calculation of
circumsolar irradiance ;
simulation of satellite radiances to be
used for data assimilation in numerical weather prediction models
;
validation of radiation schemes included in climate models, calculation of radiative forcing of clouds and
contrail cirrus , impacts of aviation on
climate e.g.;
simulation of heating rates in three-dimensional (3-D) atmospheres
to develop fast radiation parameterizations for large eddy simulation (LES) models;
simulation of solar radiation during a total eclipse;
rotational Raman scattering explaining the filling-in
of Fraunhofer lines in the solar spectrum ;
Estimation of background radiation affecting lidar measurementse.g.;
Remote sensing of planetary atmospherese.g..
Since the publication of the first libRadtran reference paper
, the model has been further developed. It includes numerous
new features that will be the focus of this paper.
One of the major extensions is the implementation of polarization in the
radiative transfer solver MYSTIC (Monte Carlo
code for the phYSically correct Tracing of photons In Cloudy
atmospheres) , which is important because
an increasing number of polarimetric observations have been performed during
the last years and are planned for the future, from ground, satellite, and
aircraft. These observations include more information about optical and
microphysical properties of atmospheric particles than total radiances alone
. Another important reason for
considering polarization is that in the shortwave spectral region (below
about 500 nm), the neglect of polarization can lead to large errors: more
than 10 % for a molecular atmosphere and up to 5 % for an atmosphere with
aerosol .
Moreover libRadtran now includes a solver to calculate rotational
Raman scattering , which improves the accuracy of trace gas
retrievals. Further the Raman-scattering signal can be used to estimate cloud
top pressure from satellite measurements and aerosol properties from surface
and satellite observations.
Numerous state-of-the-art parameterizations for aerosol and ice cloud optical
properties have been included (see Sects. and
). These new parameterizations provide more accurate radiance
calculations. In particular for polarized radiative transfer, which requires not only the scattering phase function but the full scattering phase matrix, new data
on optical properties were required. In order to improve the accuracy for
highly peaked phase functions – which are typical for ice clouds – an
improved intensity correction method has been developed and included into the
DISORT solver , and new variance reduction methods have
been developed for the Monte Carlo solver MYSTIC .
libRadtran has also been rewritten to allow for simulations with an
arbitrary number of cloud and aerosol types – which can, e.g., be used to take
into account detailed particle size distributions (number densities for
discretized size bins) that can be different in each layer. In earlier
versions it was only possible to take into account parameterized size
distributions such as gamma or log-normal distributions.
A new gas absorption parameterization for the solar and thermal spectral
ranges has been developed . It is available in different
spectral resolutions and can be applied for the simulation of radiances and
irradiance. It is particularly useful for efficient simulations of radiances
measured by satellite instruments (see Sect. ).
The DISORT radiative transfer solver has been translated from FORTRAN77 to
the C programming language. All variables were transferred from single to
double precision. These changes improved the numerical stability of the code
and reduced computational time significantly for details
see.
The paper is organized as follows: Sect. provides an
overview of the uvspec radiative transfer model, which is the core of
the libRadtran package. Section gives a short
description of the radiative transfer solvers included in uvspec.
Section provides a summary of how molecules are handled
and outlines various ways to include molecular absorption. Moreover, Rayleigh-scattering parameterizations are described. Section
summarizes the available parameterizations for aerosol microphysical and
optical properties. Section gives an overview of the
parameterizations for water and ice clouds and also outlines how these were
generated. In Sect. available surface properties are
described, including Lambertian reflection, bi-directional distribution
functions and fluorescent surfaces. In Sect. we
describe code and implementation improvements relevant for users.
Section introduces the graphical user interface for
uvspec. Section provides a short summary of
additional tools that come with the libRadtran package. Finally,
Sect. shows a few applications as examples of the
usage of libRadtran.
The uvspec radiative transfer model
The main tool of the libRadtran package is the uvspec radiative
transfer model, which consists of the following parts:
The atmospheric state (e.g. trace gas profiles, cloud liquid water
content, cloud droplet size, aerosol concentration profiles) needs
to be provided as input to the model.
The user may select between various parameterizations to convert the
atmospheric state into optical properties, e.g. to convert from
cloud liquid water content and effective droplet size to extinction
coefficient, single-scattering albedo, and scattering phase function, or phase
matrix when polarization is considered.
The optical properties are passed to a radiative transfer equation
(RTE) solver, where again it is up to the user to select the most appropriate
one for the given application. Currently, more than a dozen different solvers
are included in uvspec. The six most used and maintained RTE solvers
are listed in Table and briefly described in
Sect. . Among them are relatively simple and fast
two-stream solvers to compute irradiance, the widely used discrete ordinate
solver DISORT and also the Monte Carlo solver MYSTIC to compute (polarized)
radiances or irradiance in 3-D geometry.
The output of the RTE solver are radiation quantities as irradiance,
actinic flux, or (polarized) radiance. The quantities are normalized to the
source function, i.e. the solar irradiance in the solar spectral region. In
order to get physical quantities with corresponding units the output may be
post-processed. The uvspec output then corresponds to calibrated
radiances or brightness temperatures for a given instrumental filter
function. It is also possible to obtain integrated solar or thermal
irradiance.
The overall structure of the uvspec model is shown in
Fig. .
The model was originally designed to compute UV radiation; therefore, its name
is uvspec. As said before it now covers the complete solar and
thermal spectral range.
Structure of the uvspec radiative transfer model.
The usage of the model is described in the user guide, which comes along with
the package. The user guide includes descriptions of the RTE solvers,
examples of use as well as detailed documentation of all options and
respective parameters. Below uvspec input options are put in
teletype-font, for example rte_solver.
The uvspec model may be run either from the command line using
output_file]]>
or from the graphical user
interface (see Sect. ).
Radiative transfer equation solvers
The radiative transfer equation solvers currently implemented in
libRadtran.
a 3-D version not included in the free package; available in joint
projects.
Explanation: PP, plane-parallel; PS, pseudo-spherical; SP, fully spherical; 1-D, one-dimensional; 3-D, three-dimensional; ∗ sslidar: see Sect. ; E, irradiance; F, actinic flux; L, radiance; L(TOA), radiance at top of atmosphere;
I is the Stokes vector (polarized radiance).
The RTE for a macroscopically isotropic medium, i.e. randomly oriented
particles and molecules, may be written as dIβds=-I+J,
where the source function J is
J=ω04π∫P(Ω,Ω′)I(Ω′)dΩ′+(1-ω0)Be(T).
Here I=(I,Q,U,V) is the Stokes vector at location (x,y,z),
β the volume extinction coefficient, ω0 the single-scattering
albedo, P(Ω,Ω′) the
scattering phase matrix, and Be(T)=(B(T),0,0,0) the
emission vector including the Planck function B(T). For most applications
in the Earth's atmosphere, thermal emission can be neglected for wavelengths
below about 3 µm. Polarization is also often neglected, in this
case the Stokes vector in Eqs. () and ()
is replaced by the radiance L, the phase matrix becomes the scalar phase
function p(Ω,Ω′) and the emission
vector is just the Planck function B(T).
The uvspec model includes various methods to solve
Eq. (). The list of solvers, which may be selected using the
option rte_solver, is shown in Table .
DISORT
The solver disort is used by default in libRadtran. DISORT
is based on discrete ordinates and allows one to compute
radiances, irradiance, and actinic fluxes in plane-parallel geometry. The
original FORTRAN77 version of the algorithm exhibited several numerical
instabilities for certain combinations of geometries and optical properties.
The FORTRAN77 code has been translated to C-code and is entirely in double
precision (the FORTRAN77 version is mostly in single precision) and includes
dynamic memory allocation (not possible in FORTRAN77). As such, the C version
is numerically stable and also faster than the original FORTRAN77 version. We
thus use the C version of the DISORT algorithm by default. The original
FORTRAN77 version may still be invoked by fdisort2. Both the C-code
and the FORTRAN77 version include the new intensity correction method for
peaked phase functions by , which is used by default.
For calculations with rotational Raman scattering, the C version has been
generalized so that arbitrary source functions (not only a solar or thermal
source function) can be handled . Rotational
(inelastic) Raman scattering from other wavelengths into the wavelength, for
which the RTE is solved, is included into the source
term.
MYSTIC
The most comprehensive solver in libRadtran is the Monte Carlo model
MYSTIC , which may be used to calculate (polarized)
radiances, irradiance, and actinic fluxes in the solar and thermal spectral
regions. Within MYSTIC photons are traced through the atmosphere from the
source towards the sensor or backwards, from the sensor to the source, which
is much more efficient especially in the thermal wavelength region. One of
the main applications of MYSTIC is to calculate radiances in cloudy
atmospheres. The sharp forward scattering of clouds and aerosols causes
numerical problems in Monte Carlo models. In order to avoid these,
sophisticated variance reduction methods have been developed
. These are enabled using mc_vroom on. Solar
radiation is initially unpolarized and becomes polarized by molecular,
aerosol, or cloud scattering in the atmosphere. With the option
mc_polarisation, the full Stokes vector is
calculated. For 1-D atmospheres, MYSTIC may also be operated in spherical
geometry using the option mc_spherical.
The public version of MYSTIC allows for calculations in 1-D (plane-parallel or
spherical) geometry. A full 3-D version is also available for joint projects.
The non-public version includes several other features: complex 3-D
topography and efficient high-spectral-resolution
calculations using absorption lines importance sampling .
Two-stream solvers
For the calculation of irradiance, two fast two-stream solvers are
available.
The first solver, twostr, is described in detail in
. twostr is optimized for calculating actinic
fluxes, and hence heating rates. It can be run in plane parallel as well as
in pseudo-spherical geometry.
The second two-stream method available in libRadtran is
rodents, which is based on the delta-Eddington two stream described,
e.g., in , Sects. 6.1–6.4
Note that
contains two misprints relevant for the
two-stream solver: first, in Eq. (6.50),
α12,Ed=-α21,Ed and
α22,Ed=-α11,Ed. Second, α22 in
Eq. (6.88) should be α2. Also, the derivation in Sect. 6.5
for thermal radiation does not work, instead the equations need to
be derived in analogy to the solar radiation.
. Based on a different
two-stream approach than twostr, it naturally yields different
results. In contrast to twostr, neither the pseudo-spherical
approximation is implemented nor is rodents capable
of calculating actinic fluxes.
For actinic fluxes and atmospheric heating rates, twostr is the
better choice. However, for calculating solar irradiance, we recommend using
rodents: for cases where the resulting irradiance is not negligible
(larger than 2 % of the extraterrestrial irradiance), the difference between
rodents and exact disort calculations is on average 5 %
(7 %) for down(up)-welling irradiance. For twostr the values are
9 % (11 %). Especially in case the atmosphere is only weakly absorbing,
the average differences at top-of-atmosphere (TOA) and at the surface are
only 2 % (1 %) for rodents, whereas they are 5 % at TOA and
even 13 % (18 %) at surface for twostr.
For the thermal irradiance, rodents also gives better results at TOA
(1.6 %) and at the surface (1 %) than twostr (3 %). For
irradiance within the atmosphere, no real preference can be given.
Lidar and radar simulations
In order to complement the instruments that can be simulated by
libRadtran, a lidar simulator called sslidar has been
implemented. It only takes into account single scattering and reflection and
is based on the lidar equation, which is integrated over each range. Note that
in order to obtain a smooth signal, a fine vertical resolution of the model
atmosphere is required. The vertical resolution should correspond to the
range width of the simulated lidar instrument. For radar simulations a
stand-alone tool is available (see Sect. ).
Other solvers
The solver tzs (see Appendix ) is based on the zero-scattering approximation in the thermal spectral range. It may be used for
clear-sky calculations of radiances at TOA. It also
calculates “black cloud” radiances for the application of the CO2-slicing algorithm , which may
be used for the determination of cloud top temperatures from passive remote
sensing measurements in the thermal spectral range.
Nadir top-of-the-atmosphere radiance in the oxygen-A band around
760 nm (left) and in the IR (infra-red) window region (right) for the midlatitude-summer
atmosphere of . All calculations were performed with the
MYSTIC solver using the “absorption lines importance sampling” method
. (Top) high spectral resolution calculation, based on
line-by-line absorption cross sections calculated using ARTS
; (bottom) pseudo-spectral calculations using the
representative wavelengths band parameterizations (reptran) with
different resolutions and lowtran. For comparison see also Fig. 3 in
, which shows transmittances for genln2 line-by-line
calculations and lowtran for the same spectral regions.
For the solar region a fast single-scattering solver sss is
available. These solvers may be used for fast but approximate simulations of
satellite measurements.
Several other RTE solvers are included in uvspec for compatibility
with earlier releases of the package. These include sdisort
(pseudospherical disort), spsdisort (single precision,
pseudospherical disort), fdisort1 (version 1 of DISORT), and
polradtran. While they may still be used, we do
not recommend their use as the other solvers listed in
Table perform better.
Accuracy of solvers
The MYSTIC model has been validated in many international model
intercomparison studies, for radiance calculations with highly peaked phase
functions , for polarized radiance calculations
, and for radiances and irradiance in 3-D model domains
. In all studies MYSTIC belongs to the core of models
that produce equal results within their uncertainty range. MYSTIC agrees
perfectly with DISORT for radiances and irradiance with only a few exceptions,
e.g. for circum-solar radiation, where the second-order intensity correction
included in DISORT is not accurate enough for highly peaked scattering phase
functions . In , a comparison between
DISORT and MYSTIC for a radiance spectrum in the O2-A band is shown. The
relative difference between the solvers is less than 0.05 % here. All other
solvers are approximations and hence less accurate: as mentioned before the
two-stream solvers are only appropriate for irradiance and the tzs
solver only provides radiances in thermal atmospheres and neglects scattering
completely.
The accuracy of MYSTIC depends only on the number of traced photons. The
standard deviation of MYSTIC is calculated when the option mc_std
is enabled. The user may run MYSTIC with many photons as reference for some
cases in order to check the accuracy of other solvers for specific
applications.
MoleculesMolecular absorption parameterizations
Spectral ranges affected by molecular absorption comprising a complex line
structure require parameterizations to reduce the computational cost.
Molecular absorption parameterizations included in libRadtran are
listed in Table . By default the reptran
parameterization is applied. Using the option mol_abs_param, the
user may select the most appropriate parameterization for the specific
application. As an example Fig. shows radiance
calculations for nadir viewing direction at the top of the atmosphere using
the parameterizations reptran and lowtran and line-by-line
calculations.
Absorption parameterizations in libRadtran.
NameDescriptionApplicationReferencesreptrandefault setting; bands parameterized using repr. wavelengths; fine (1 cm-1), medium (5 cm-1), and coarse (15 cm-1) band resolutions available; based on HITRAN2004, MT_CKD and measured absorption cross section data of O3, O4, and NO2; solar and thermal regioncalculation of radiances, simulation of satellite measurementsreptran_channelsatellite channels parameterized using representative wavelengths;fast and accurate simulations for various satellite instrumentslowtranLOWTRAN band model; solar and thermal region, resolution 20 cm-1pseudo-spectral calculations of radianceskato, kato2 kato2.96, katoandwandjicorrelated_k distributions for solar region; different versions available; based on HITRAN96 or HITRAN2000; 148 or 575 sub-bandscalculation of integrated solar irradiancefucorrelated_k distributions for solar (6 bands) and thermal (12 bands) regions; optimized for climate modelscalculation of integrated solar and thermal irradiance, radiative forcing
The reptran parameterization has recently been
included in libRadtran. In reptran integrals over spectral
intervals, e.g. integrated over a narrow spectral band or an instrument
channel response function, are parameterized as weighted means over
representative wavelengths similar to the method described by
. The selection of an optimum set of representative
wavelengths is based on accurate line-by-line simulations for top-of-atmosphere radiances of a highly variable set of atmospheric states. The
ARTS (Atmospheric Radiative Transfer Simulator)
model including state-of-the-art continuum models and
spectroscopic data from HITRAN 2004 were used to
calculate the gas absorption properties. For wavelengths below 1130 nm
measured absorption cross sections of O3, O4, and NO2 are included, as they
are not covered by HITRAN or the continua (see also Sect. ).
Three-band resolutions (fine: 1 cm-1; medium: 5 cm-1; and coarse:
15 cm-1) are available in the solar and thermal spectral range, as well
as a number of instruments on the following satellites: ADEOS (Advanced Earth Observing Satellite), ALOS (Advanced Land Observing Satellite), EarthCARE (Earth Clouds, Aerosols and Radiation Explorer), Envisat (Environmental Satellite), ERS (European Remote-Sensing Satellite), Landsat, MSG (Meteosat Second Generation), PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from Lidar), Proba (Project for On-Board Autonomy), Sentinel, Seosat (Satélite Espanol de Observación de la Tierra), and SPOT (Satellite Pour l'Observation de la Terre). The
parameterization has been validated by comparison to high spectral resolution
calculations. For solar and thermal radiation at the top of atmosphere, as
well as for solar radiation on the ground, the mean parameterization error is
in the range of 1 %. The mean error is slightly larger than 1 % for
thermal radiation at the surface.
The LOWTRAN (low-resolution transmission)-band model adopted from the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) radiative transfer model
is also included in libRadtran.
For the simulation of radiances and irradiance, we recommend to use
reptran because it is faster and more accurate than
lowtran.
Several correlated-k parameterizations with different numbers of bands, i.e.
different accuracy, are included in libRadtran. For the calculation
of integrated solar and thermal irradiance and heating rates, the
correlated-k parameterizations by and are
recommended. Also for the calculation of heating/cooling rates in the higher
atmosphere (above ∼ 20 km), we
recommend these parameterizations because reptran and
lowtran are affected by large errors.
Molecular absorption cross sections
For the spectral region from 160 to 850 nm, libRadtran includes
measured absorption cross sections of various molecules in the atmosphere
(see Table ). Using the option mol_abs_param crs,
these cross sections are used instead of the default reptran
parameterization. For wavelengths below 500 nm, reptran yields
approximately the same results as mol_abs_param crs because the
cross sections from HITRAN and the continua are very small at these
wavelengths and the same measured cross sections are relevant in both cases.
For O2 for instance the cross section data include the Schumann–Runge
bands between 176 and 192.6 nm and the Herzberg continuum between 205 and
240 nm. Ozone absorption bands are for example the Huggins bands between 320
and 360 nm and the Chappuis bands between 375 and 650 nm. Using the option
crs_model the user may specify which cross section data should be
used in the simulations. Alternatively with crs_file, the users may
specify their own absorption cross section data.
Absorption cross section data included in libRadtran; the
non-default parameterizations are put in parentheses.
In the shortwave infrared, thermal infrared, and microwave region, we find a
huge number of absorption lines that are due to vibrational or rotational
transitions in molecules. A line-by-line model is required in order to
calculate spectrally resolved radiances. Line-by-line models take the
absorption line positions as well as line strength parameters from spectral
databases like HITRAN, calculate line broadening, which depends on pressure
and temperature in the atmosphere, and finally obtain absorption optical
thickness profiles. libRadtran does not include a line-by-line model
but it allows one to specify absorption optical thickness profiles using the
option mol_tau_file abs. It is convenient to use the ARTS model
to generate spectrally resolved molecular absorption
data because it outputs the format required by libRadtran. ARTS
includes a comprehensive line-by-line module, it allows one to use different
spectroscopic databases like HITRAN as input and it also includes various
state-of-the-art absorption continuum models. The toolbox Py4CATS
which can be downloaded from www.libradtran.org,
also includes convenient command line programs to generate spectrally resolved
absorption data. The Py4CATS tools, however, do not include
continuum models; hence, it should only be used for simulations where
the continua are not relevant.
Rayleigh-scattering cross sections
The Rayleigh-scattering cross sections are by default calculated using
Eqs. (22)–(23) of . Using the option crs_model rayleigh, the user may select Eq. (29) of or the
formulas proposed by and ,
respectively. The analytical Rayleigh-scattering phase matrix
PR is
PR(Θ)=Δ34(1+cos2Θ)-34sin2Θ00-34sin2Θ34(1+cos2Θ)000032cosΘ0000Δ′32cosΘ+(1-Δ) 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0,
where
Δ=1-δ1+δ/2,Δ′=1-2δ1-δ,
and δ is the depolarization factor that accounts for the anisotropy of
the molecules; δ is also calculated according to .
The Rayleigh phase matrix for δ=0 is shown in
Fig. . For calculations neglecting polarization only
the (1,1) element of the phase matrix, which corresponds to the scattering
phase function, is required.
Aerosols
Besides the models by , which are described in
, libRadtran now includes additional aerosol
properties based on the OPAC (Optical Properties of Aerosols and Clouds) database . OPAC provides the
required parameters for single-scattering calculations: size distribution
parameters, refractive indices, and the density of the material. Data are
available for the spectral range from 250 nm to 40 µm for the
following basic aerosol types: insoluble (inso), water soluble
(waso), soot (soot), sea salt accumulated (ssam),
sea salt coarse mode (sscm), mineral nucleation mode
(minm), mineral accumulated mode (miam), mineral coarse
mode (micm), mineral transported (mitr), and soluble sulfate
aerosol (suso). For the soluble aerosols the parameters depend on
humidity because the aerosol particles swell in humid air. Relative
humidities of 0, 50, 70, 80, 90, 95, 98, and 99 % are included in OPAC. The
option aerosol_species_file allows one to define arbitrary mixtures of
these basic types or to select pre-defined mixtures from OPAC, such as, e.g.,
continental_average, for which uvspec automatically uses
the optical properties closest to the background humidity profile.
Optical properties of all basic aerosol types were calculated using
libRadtran's Mie tool (see Sect. ). For mineral
aerosols, which are highly aspherical, we additionally provide optical
properties calculated with the T-matrix method ,
assuming an aspect ratio distribution of prolate spheroids as described by
.
Phase matrix elements for the basic OPAC aerosol types
“water soluble” (waso), “sea salt accumulated mode”
(ssam), and soot, for a water cloud with a droplet
effective radius of 10 µm, and for Rayleigh scattering
(with δ=0) at a
wavelength of 350 nm. θ is the scattering angle, i.e. the
angle between incoming
and scattered directions.
Water clouds parameterizations in libRadtran.
NameDescriptionApplicationReferenceshudefault setting. Simple parameterization, uses Henyey–Greenstein phase function to approximate Mie phase functionirradiance, heating ratesecham4very simple two-band parameterization of ECHAM4 climate modelcomparison of irradiance to results from ECHAM4mieoptical properties calculated using Mie theory, include full phase matrices(polarized) radiancesgenerated using Mie code by
As an example Fig. shows the phase matrix elements of
the basic OPAC aerosol types, of liquid cloud droplets with an effective
radius of 10 µm and the Rayleigh-scattering phase matrix. Note that
for spherical particles only four elements of the 4×4 scattering phase
matrix are independent whereas for aspherical particles six elements are
required see, e.g.,. Figure shows the
absorption and the scattering optical thicknesses (integrated from the
surface to the top of the atmosphere) for the standard aerosol mixtures in
the spectral region from 300 to 800 nm. As expected, the optical thickness
of the urban aerosol is the largest and that of the
antarctic aerosol the smallest. In general the continental aerosol
mixtures show a stronger wavelength dependency than the maritime mixtures.
The users may also provide their own optical properties data, which may be
generated using libRadtran's Mie tool or other external programs;
more detailed instructions are provided in the libRadtran user
guide.
CloudsWater clouds
Table summarizes the parameterizations of water
cloud optical properties, which may be selected in libRadtran using
the option wc_properties.
For the simulation of irradiance and heating rates, it is normally sufficient
to use a simple parameterization to convert from cloud liquid-water content
and droplet effective radius to the respective optical properties: extinction
coefficient, single-scattering albedo, and asymmetry parameter. For this
purpose libRadtran includes the parameterization generated by
.
For the simulation of radiances more accurate optical properties are needed
and the phase function should not be approximated by a Henyey–Greenstein
function as it is done in . Therefore, we have pre-calculated
cloud optical properties using libRadtran's Mie tool, assuming that
the cloud droplets are gamma distributed:
n(r)=Nrαexp-rreff⋅veff;α=1veff-3.
Calculations have been performed for effective radii reff from 1 to
25 µm with a step width of 1 µm. The effective variance
was set to a value of veff=0.1 and the constant N was determined
by normalization. The size distributions were cut off at a minimum radius of
0.02⋅reff and a maximum radius of 8⋅reff. The
size distribution bins are sampled on a size parameter (2πrλ) grid with a resolution of 0.003. This fine resolution is
necessary to obtain smooth phase matrices. The pre-calculated data include
the wavelength ranges from 250 to 2200 nm (solar) with a resolution of
10 nm and the range from 2.2 to 100 µm (thermal) in 100 steps of
equal wavenumbers. The refractive index of water has been taken from
. In the solar (thermal) region, the phase matrices are
computed from 5000 (500) Legendre polynomials. In the optical properties
files, 129 of the Legendre polynomials are stored, as well as the phase matrix
elements, which are stored on scattering angle grids θ optimized such
that the error of the phase matrix – when interpolated linearly in
cosθ between the grid points – is smaller than 1 %. As an example
Fig. shows the four phase matrix elements of a cloud
droplet distribution with reff=10µm at 350 nm. Here
the cloudbow at θ≈140∘ is clearly visible in the
P11 and P12/P11 elements of the phase matrix. P12/P11
corresponds to the degree of polarization in the principal plane after single
scattering; it can be seen that the maximum in the cloudbow region is about
80 %. The mystic solver uses the phase matrix stored on the
θ-grid, whereas all other solvers use the Legendre polynomials, except
for the intensity correction in disort, which uses the phase function
see also.
Absorption (left) and scattering (right) optical thickness
for various aerosol mixtures specified using the option aerosol_species_file. The
aerosol optical properties as well as the mixtures have been
generated based on OPAC parameters.
For specific applications, e.g. different size distributions, the user can
easily generate optical properties using libRadtran's Mie tool.
Ice clouds
Ice cloud parameterizations in libRadtran
NameDescriptionApplicationReferencesfudefault setting. Simple parameterization using Henyey–Greenstein phase function.irradiance, heating ratesecham4very simple 2-band parameterization of ECHAM4 climate model.comparison of irradiance to results from ECHAM4keyparameterization using a double-Henyey–Greenstein phase function, covers wavelength range from 0.2 to 5.0 µm. Available for various habits.irradiance, heating ratesyangSimilar to key but based on different single-scattering calculations and extended to wavelengths up to 100 µm. Below 3.4 µm equivalent to key.irradiance, heating rates, baumbulk optical properties including phase functions for a realistic mixture of habits. Covers wavelength range from 0.4 to 2.2 µm and from 3.1 to 100 µm.radiancesbaum_v36bulk optical properties including phase matrices for three microphysical models: general habit mixture, solid columns or rough aggregates. All models include severely rough particles. Covers wavelength range from 0.2 to 99 µm.(polarized) radiancesheybulk optical properties including phase matrices based on single-scattering calculations for smooth crystals, covers wavelength range from 0.2 to 5 µm, includes 6 habits and a habit mixture.(polarized) radiancesSingle-scattering properties generated by Hong Gang using the code by , Appendix yang2013bulk optical properties including phase matrices for 9 habits and 3 degrees of roughness, covers wavelength range from 0.2 to 99 µm.(polarized) radiances, Appendix
For ice clouds libRadtran includes a variety of parameterizations
(see Table ) from which the user may select the most
appropriate one for a specific application by specifying the option
ic_properties. Ice clouds are more complex than water clouds
because they consist of ice crystals of different shapes. Some of the ice
cloud parameterizations allow the crystal habit (ic_habit) to be
specified.
As described in the previous section the exact phase matrix is not needed
when irradiance are calculated. For this purpose the parameterizations by
, , and are included in
libRadtran. and approximate the phase
function by a Henyey–Greenstein function. is slightly more
accurate because it uses a double-Henyey–Greenstein function, which represents
the backscattering of ice crystals much better. The parameterization is based
on single-scattering calculations for various ice crystal habits and on
measured size distributions. It is available in the wavelength range from 0.2
to 5 µm. Based on single-scattering data provided by P. Yang and on
the size distributions from J. R. Key we have extended the original
parameterization by to the thermal wavelength region up to
100 µm.
For accurate radiance calculations the parameterizations by
(baum) and the newer one by
, , and (baum_v36) are
available: baum includes full phase functions for a mixture of
particle shapes, the parameterization is based on single-scattering
properties of smooth ice crystals and on a large number of measured size
distributions. baum_v36 includes full phase matrices and three
different habit models: a general habit mixture similar to baum
but for rough ice crystals, and the single habits solid column and
aggregate, both of them severely roughened.
We have generated two further parameterizations (hey and
yang2013) for individual habits, which also include the full phase
matrices (see Appendix ): hey is available for the
wavelength region from 0.2 to 5 µm for smooth particles in the
effective radius range from 5 to 90 µm. The full wavelength region
from 200 nm to 99 µm is available for yang2013, effective
radii may be in the range from 5 to 90 µm and a roughness parameter
may also be specified, ranging from smooth to severely rough. For the
yang2013 parameterization, the single-scattering properties of nine
individual ice crystal habits, which are commonly observed in ice clouds, have
been taken from the database by . The hey
parameterization was generated before this database existed and it is based
on single-scattering data provided by Hong Gang, who used the improved geometrical optics method (IGOM), the same method as used by
.
Please refer to the libRadtran user guide for a list of available
habits for each parameterization.
Figure shows the phase matrix elements of ice crystal
distributions with an effective radius of 40 µm at 550 nm
wavelength. The red lines correspond to smooth crystals and the blue lines to
severely rough crystals. The individual habits are for the yang2013
parameterization. General habit mixtures, which are available for the
hey parameterization based on smooth crystals and for the
baum_v36 parameterization based on severely rough crystals, are also
shown. For most smooth crystals and also for the general habit mixture
ghm of the hey parameterization-scattering features of
hexagonal ice crystals, the most prominent being the halo at 22∘
scattering angle, are visible in all phase matrix elements. The phase
matrices for severely rough crystals do not show halo features and they are
relatively similar for all habits. In reality ice clouds are highly variable:
There are situations when the halo is visible, in this case obviously there
must be regular smooth ice crystals in the cirrus clouds. When no halo is
visible, the assumption of severely roughened crystals might be more
realistic.
Phase matrix elements of ice crystal distributions with an
effective radius of 40 µm at 550 nm wavelength. The red lines
correspond to smooth and the blue lines to severely rough
crystals. The individual habits (solid-column,
column-8elements and plate) are for the parameterization
yang2013, and the general habit mixtures (ghm) are for
hey including smooth crystals and baum_v36 including
severely rough particles.
SurfaceBi-directional reflectance distribution functions
All solvers included in libRadtran may include Lambertian surfaces,
while DISORT and MYSTIC can also handle bi-directional reflectance
distribution functions. libRadtran provides a variety of BRDFs (bi-directional reflection distribution function),
which are listed in Table .
Two parameterizations for land surfaces are available. The first is the
“RPV (Rahman, Pinty, and Verstraete)” parameterization by with the extension by
for modelling snow-covered surfaces. The second is the
“RossLi” BRDF first presented by . The original RossLi BRDF is used in the AMBRALS (the Algorithm for Model Bi-directional Reflectance Anisotropy of the Land Surface) BRDF modelling framework
, and consists of four different kernel combinations, of
which the RossThickLiSparse-Reciprocal combination was identified in several
studies to be the model best suited for the operational MODIS BRDF/Albedo
algorithm see. An additional factor for simulating the
hot spot in vegetation canopies was added by . The version
implemented in libRadtran is the RossThickLiSparse-Reciprocal model as used in
MODIS data, as presented in . The hot spot correction factor
can be turned on if required.
As already stated in , but repeated here for completeness, a
parameterization of the BRDF of water surfaces is also included, which depends
mainly on wind speed and to a lesser degree on plankton concentration and
salinity. For the MYSTIC solver, also the wind direction can be set. In
contrast to vegetation where the typical hot spot occurs in the 180∘
backscatter direction, the main feature for water is specular reflection. The
parameterization in uvspec was adopted from the 6S code
and is based on the measurements of and the
calculations of . A vector version of the ocean
parameterization, developed by and ,
is available for polarization calculations with MYSTIC. The vector version
uses only wind speed as a parameter and does not take into account plankton
concentration, salinity or wind direction.
The surface reflection models
currently implemented in libRadtran.
Option nameBRDF typeNo. of parametersReferencesSolversalbedoLambertian1Allbrdf_camocean BRDF3 + 1D, Mbpdf_tsangpolarized ocean BRDF1Mbrdf_hapkeplanetary & lunar surfaces3D, Mbrdf_ambralsRoss-Li, MODIS Land Surface, RTLSR3D, Mbrdf_rpvland surfaces3 + 3D, M
D: DISORT; M: MYSTIC; RTLSR: RossThickLiSparse-Reciprocal model, optionally with hot spot
parameterization.
Finally, the parameterization of the surfaces of extraterrestrial solid
bodies such as the Moon, asteroids, or the inner planets by
is available.
Only the ocean BRDF parameterizations depend directly on the wavelength. For
all other BRDF models, the parameterization can either be given as being
constant with wavelength (by using, e.g., the option brdf_rpv), or as
a file containing the parameters for each wavelength (using,
e.g., brdf_rpv_file).
Screenshot of the graphical user interface for a
spectral high-resolution simulation of the O2-B band including
a fluorescence source. Plots of input and output data are
included together with the help information for one option. See
text for further explanation.
Fluorescence
For vegetation covered surfaces, a weak solar-induced chlorophyll
fluorescence signal is emitted in the red and far-red spectral regions.
The contribution of fluorescence to the radiance leaving the bottom
boundary is
LgF(μ,ϕ,λ)=F(λ),
where F(λ) is the fluorescence source in the same units as the
incoming solar flux at the top of the atmosphere (for example
mW (m2 nm sr)-1).
The fluorescence source of radiation is included in the disort
solver. It may either be constant or vary as a function of wavelength.
Additional surface bi-directional reflection of radiation may also be
included. The fluorescence source depends on the solar radiation impinging
the vegetation and the type of vegetation. Output from vegetation
fluorescence canopy models, such as that described by , may
readily be used by uvspec.
The previous versions of libRadtran were restricted to using at most
four types of atmospheric constituents: molecules, aerosols, and water and
ice clouds. Any user defined constituent could only be included by replacing,
e.g., water clouds with them. Also, it was not possible to use several types
of ice cloud habits at the same time.
A recent major internal restructuring of the libRadtran code has now
made it possible to use any number of atmospheric constituents for a
radiative transfer simulation. The number is only limited by computational
memory and time. The new input options needed for loading the additional
constituents are profile_file and profile_properties.
They work very similar to the cloud input options; merely the name of the
constituent needs to be defined.
This option increases the flexibility of libRadtran in many ways;
e.g., it can be used to load the optical properties for each size bin of an
aerosol or water or ice cloud. This way, the size distribution may differ
between the atmospheric layers. An example can be found in
.
Change of nomenclature and backward compatibility
As the number of input options had grown to more than 300 over the years, we
decided to restructure the language of the input options. The input options
now have a largely consistent naming and their usage follows certain rules,
making it more easy to find related input options.
We have included a python script in order to provide backward compatibility
for long-established libRadtran users. The script can be found in
the directory src_py. By invoking the command
new_input_file]]>
input files written in the old nomenclature will be translated to the
new nomenclature automatically. Alternatively, the old input file can
be sent directly to uvspec with the following command:
Graphical user interface
The large number of input options available in the uvspec model may
appear overwhelming. To help the user to create uvspec input files a
graphical user interface (GUI) has been developed. The GUI organizes the
input options in logical groups such as “molecular atmosphere”,
“aerosol”, “surface”, etc.; see also the grey bar at the top in
Fig. . Input options that are set by the user and that will be
written to the given input files are shown in bold face (for example option
rte_solver in Fig. ). Options that may be set are
shown as normal characters, while options that are not compatible with other
set options are greyed (for example in Fig. mc_ipa is
greyed since it is not possible to combine it with rte_solver set
to disort).
(Left) the transmittance from ARTS output and radiance
from uvspec.
(Right) the top of the atmosphere nadir viewing radiance in the
O2-B band with (black line) and without (cyan line with circles) a surface
fluorescence source (red line with triangles).
The radiances have been convolved
with a spectral response function with FWHM of 0.3 nm.
Online documentation of the options are available and this is identical to
the documentation in the libRadtran user manual. In
Fig. the documentation for the option
number_of_streams is shown in the lower left corner. The online
help is activated by pointing the mouse at the requested input variable.
Input options that refer to input data files, such as wavelength-dependent
surface albedo, may be plotted from the GUI. In the example in
Fig. , the extraterrestrial flux (upper left subplot), the
surface fluorescence spectrum (lower left subplot), and surface albedo (lower
right subplot) inputs are plotted. Note that the wavelength coverage
(x axis) differs reflecting the different wavelength regions included in
the input data files.
Once all wanted input options are set, they are saved to a user specified
file, and uvspec is run from within the GUI. The output from the run
may readily be plotted using the GUI. For example, in Fig. , the
calculated nadir radiance at the top of the atmosphere is shown in the upper
right subplot. The GUI includes numerous working examples. Users may add more
examples to the GUI specific to their interests.
Other tools
Several additional tools are included in the libRadtran package. An
overview is given in Table 4. New tools are
ssradar, a single-scattering radar simulator (see below), and
pmom, which calculates Legendre polynomials for a given phase
function.
Mie calculations
The tool for Mie calculations (mie) has been extended considerably. The user
may select between two Mie codes, MIEV0 by or
bhmie by . The tool
allows one to generate input optical properties for uvspec calculations
for arbitrary size distributions. It generates full phase matrices, which
are stored on optimized angular grids for a user-defined accuracy. The
radiative transfer solvers MYSTIC and DISORT with the new intensity
correction method use the phase functions/matrices
rather than Legendre polynomials, which are calculated by the Mie
codes.
Single-scattering radar simulator
Single-scattering radar (ssradar) is a stand-alone 1-D pure
Rayleigh-scattering cloud radar simulator that handles arbitrary cloud layers
and droplet size distributions as well as tilted viewing angles and
supercooled water droplets. The radar reflectivity factor is calculated
directly from the droplet distribution with Z=∑iniDi6 where D is the droplet diameter and ni the
distribution number density for the discrete interval Di,Di+1.
Internally available distributions are gamma and log-normal, arbitrary
distributions can be entered using input files.
Some applications
The libRadtran package has been used for numerous applications. Many
of these are listed under the publications link at
http://www.libradtran.org. The examples directory also
includes a number input files that may be used especially by new users to
create input files. Below some applications of libRadtran are
described.
uvspec and ARTS
The high number of absorption lines in the shortwave infrared and the thermal
infrared requires a line-by-line approach to resolve the spectral structure.
Below it is shown how molecular absorption data from ARTS may be combined with
uvspec to perform line-by-line calculations in both the solar and
thermal parts of the spectrum. For both examples the spectral resolution, the
molecules to be included and the line function properties are specified in
the input to ARTS. It is noted that the same ambient atmospheric profile
should be used in both, ARTS and uvspec.
Solar source
Solar induced chlorophyll fluorescence is emitted in the 660 to 800 nm
spectral region with two broad peaks at about 685 and 740 nm. In this
spectral region are the O2-A and O2-B bands which contain a large
number of absorption lines. Although the fluorescence signal is weak,
especially the O2-B region holds promise for retrieval of vegetation
fluorescence from spectrally high-resolution space-borne instruments
. In this spectral region the surface albedo is typically
low while there is a fluorescence peak around 685 nm (see red line right
plot Fig. ). The optical depths from ARTS are input to
uvspec, which calculates the top of the atmosphere radiance (blue
line, left plot of Fig. ) including the fluorescence signal
(red line, right plot of Fig. ), surface albedo (green line,
right plot of Fig. ), and molecular scattering. Measurements
may be made at a lower spectral resolution. The right plot of
Fig. shows radiance spectra convolved with a triangular
spectral response function with a full width at half maximum (FWHM) of
0.3 nm using the conv tool of libRadtran. The spectral
response function was generated with the make_slitfunction tool.
Spectra with (blue line) and without (purple line) fluorescence are
presented. It is seen that the fluorescence signal is relatively larger when
the surface albedo is low, below about 690 nm, compared to larger
wavelengths.
(Top plot) brightness temperature spectra for different
locations as measured by IASI on 15 February 2014, 02:33 UTC,
during the Mt Kelud, Indonesia, eruption. Tentative classification
of the spectra is given in the legend. See text for details.
(Bottom plot) simulated brightness temperature spectra using
ARTS/uvspec. The atmospheric processes included in the simulations
are given in the legend.
Thermal source
The Infrared Atmospheric Sounding Interferometer (IASI) on board the MetOp
satellite measures the radiance from 645 to 2760 cm-1
(15.50–3.6 µm) with a spectral resolution of 0.25 cm-1. Its
main purpose is high-resolution atmospheric sounding of temperature and
humidity, and trace gas column retrievals . It
may also be used to detect volcanic ash seeand references
therein.
The top panel of Fig. shows IASI spectra from a granule
covering the ash cloud following the eruption of Mt Kelud, Indonesia, in
February 2014. The spectra are classified as cloudless (green), ice cloud
(blue), and volcanic ash (red). To investigate the realism of this
identification the spectra were simulated with ARTS/uvspec. For all
simulated spectra, the surface emissivity was set equal to one which is
representative for water. The simulated spectra are shown in the bottom plot
of Fig. .
The cloudless spectrum has brightness temperatures representative for the
ocean at these latitudes. The main molecular absorption features in this part
of the spectrum are water vapour lines throughout the spectrum, ozone (broad
band feature centred around 1050 cm-1), and CO2 (feature below
800 cm-1). The data from ARTS include absorption lines from these
molecules. In the cloudless spectrum the ozone band around 1050 cm-1
has a lower brightness temperature than the radiation at lower and higher
wavenumber, indicating that the radiation in the ozone band was emitted at a
higher altitude with lower temperature than the surface. Overall the
ARTS/uvspec cloudless spectrum agrees well with the measured
spectrum.
For the simulation with an ice cloud, the ice cloud was located between 12
and 13 km. Ice water content was set to 1 g m-3. The ice particles were assumed to consist of
solid columns with reff=40.0µm. The ice cloud
parameterization ic_properties yang was selected. The spectrum
identified as ice cloud (blue curve in top plot of Fig. )
appears saturated for nearly all wavenumbers except for the ozone band
centred around 1050 cm-1. The rather low brightness temperature and
wavenumber-independent behaviour outside the ozone band, indicates that this
is an ice cloud and that it is opaque. The simulation with an ice cloud (blue
curve in bottom plot of Fig. ) agrees well with the measured
spectrum. The higher temperatures in the ozone band implies that this
radiation was emitted at a higher altitude in the stratosphere where the
temperature is higher than at the altitude of the cloud.
The ash simulation included an ash cloud between 17 and 18 km. The ash
particles were assumed to be made of andesite, spherical and mono-disperse
with a radius of 3 µm. The refractive index of andesite was taken
from and the optical properties were calculated using the
mie tool. The ash density was 1×10-3 g m-3 which
corresponds to a mass loading of 1 g m-2 for a 1 km thick cloud.
The red curve in the top plot of Fig. is classified as ash
using the difference in brightness temperature method described by
. This spectrum has a lower brightness temperature than
the cloudless spectrum indicating a colder effective emitting temperature
overall. The general spectral shape is similar to the cloudless spectrum
below 1000 cm-1. Above about 1200 cm-1 the brightness temperature
of the cloudless spectrum generally decreases with increasing wavenumber,
while the converse is true for the ash spectrum. The simulated ash cloud
spectrum (black curve in bottom plot of Fig. ) differs from
the measured spectrum classified as ash. Both the simulated and measured ash
spectra increase in magnitude with increasing wavelength above
1100 cm-1, but the simulated spectrum increases more. Below about
900 cm-1 the spectral behaviour of the measured and simulated spectra
differs. This may be due to either wrong assumptions about the ash type and
hence refractive index and/or the mixing of ice with ash. Ice clouds have an
opposite effect of ash clouds on the brightness temperature between
800 and 1000 cm-1, whereas above 1075 cm-1 ice clouds have only a
very weak dependence on wavenumber see Fig. 2 of. To
test if the presence of both ash and ice could reproduce the measured
spectrum, simulations were made with both an ash cloud and an ice cloud. The
altitude and thickness of the clouds were as above, but the ash cloud density
was 2×10-4 g m-3 and the ice water content 1.5×10-2 g m-3. The resulting spectrum is shown in maroon in the
bottom
plot of Fig. . The mixed scene with both ash and ice is seen
to well reproduce the measured ash spectrum in the top plot of
Fig. .
Simulated satellite image
Figure shows a simulated satellite image (top) and the
corresponding observation (bottom). Three visible channels of the
SEVIRI (Spinning Enhanced Visible and Infrared Imager)
instrument on the MSG satellite were simulated
based on input data from the operational COSMO-DE forecast
of Deutscher Wetterdienst for the 15 July 2012,
12:00 UTC. The spatial resolution of the simulation is
2.8 km × 2.8 km; the SEVIRI observation is
3 km × 3 km at the sub-satellite point. A false colour composite
was generated using the simulated radiance of the 1.6 µm channel
for red, the 0.8 µm radiance for green, and 0.6 µm
radiance for blue. The simulations were performed using the 1-D
disort solver. The MODIS surface albedo data set was used
to set the Lambertian surface albedo. The effective radii
of liquid clouds were parameterized according to ,
and for the optical properties the mie
parameterization was applied. Ice cloud effective radii were parameterized
according to and for the corresponding optical properties
the parameterization baum_v36 was used with the general habit
mixture. Molecular absorption was included using the reptran
parameterization. In the false colour composite water clouds appear white and
ice clouds appear blueish, because ice absorbs in the region of about
1.6 µm. The simulated image looks very similar to the observation.
A major difference is that the ice clouds in the observation appear more
blueish, the reason is that their real optical thickness is larger than in
the COSMO-DE forecast.
(Top) simulation of MSG-SEVIRI image. False colour composite, where
red corresponds to the 1.6 µm channel, green to 0.8 µm,
and blue to 0.6 µm. The simulation was performed using the
disort solver with input data from the operational COSMO-DE forecast
for the 15 July 2012, 12:00 UTC. The axes correspond to SEVIRI pixel.
(Bottom) corresponding SEVIRI image.
Polarization
The MYSTIC solver can be applied to simulate multi-angle multi-spectral
polarized radiances using the option
mc_polarisation.
Polarized radiative transfer using MYSTIC has been validated in
extensive model intercomparison projects
.
Stokes vector components I and Q at wavelengths of 443 nm (blue
solid lines), 670 nm (green dashed lines), and 865 nm (red dashed-dotted
lines) for various atmospheric setups (see text for details). The radiances
are calculated at the top of the atmosphere for viewing angles from -50 to
50∘, where 0∘ corresponds to the nadir direction.
Figure shows an example for simulations at wavelengths
of 443, 670, and 865 nm; these are measured by the POLDER (Polarization and Directionality of the Earth's Reflectances) instrument onboard PARASOL . All simulations are
for a solar zenith angle of 30∘ and show the reflected radiances
(normalized to incoming solar irradiance) at the top of the atmosphere in the
solar principal plane. The viewing angle of 30∘ corresponds to the
exact backscattering direction. The angular resolution is 2∘. All
simulations are for the US-standard atmosphere. The figure shows the first
and second components of the Stokes vector I and Q; the components U
and V are exactly 0 in the principal plane for symmetry reasons.
The first row shows the results for a clear atmosphere, i.e.
Rayleigh scattering and molecular absorption. Here I is largest for
the shortest wavelength because the Rayleigh-scattering cross section
decreases with λ-4, where λ is the wavelength. The
absolute value of Q also increases with increasing Rayleigh-scattering cross section. A negative Q means that Rayleigh scattering
polarizes perpendicular to the scattering plane, which, for single
scattering, corresponds to the principal plane for this geometry.
The second row of the figure shows the same simulation but with an underlying
ocean surface, which is modelled according to
(bpdf_tsang). The wind speed was set to 2 m s-1. I and Q
clearly show the sun glint, which has a maximum at a viewing angle of about
-30∘ and is highly polarized. The intensity of the sun-glint
increases with increasing wavelength since the incoming radiance at the
surface becomes less diffuse when there is less Rayleigh scattering in the
atmosphere.
The third row shows the result for desert aerosol as defined in the OPAC
database (aerosol_species_file desert), with an underlying
Lambertian surface albedo of 0.3. I shows a backscatter peak at 670 and
865 nm. Q looks similar to Rayleigh scattering; however, there are
differences mainly around the backscatter region. At wavelengths of 670 and
865 nm, Q has a minimum in the exact backscatter direction and becomes
positive for viewing angles around this direction.
The fourth row shows a simulation including a water cloud
(wc_properties mie) in 2–3 km altitude with an optical thickness
of 10 and an effective droplet radius of 10 µm. I and Q show
the glory about the backscatter direction and the rainbow at a viewing angle
of about -10∘ corresponding to a scattering angle of 140∘.
In Q the rainbow is more pronounced than in I because Q is less
affected by multiple scattering. The angular resolution shown here is not
sufficient to separate the glory from the backscattering peak in I. The
sign of Q in the rainbow region is the same as for Rayleigh scattering,
whereas it is opposite in the glory region, which means that the rainbow is
polarized perpendicular to the scattering plane, whereas the glory is
polarized parallel to the scattering plane.
The last two rows show simulations with ice clouds, where we have used the
yang2013 parameterization. An ice cloud layer with an optical thickness of 2 was included at an altitude from 9–10 km. The selected habit
was solid_column and we performed simulations for smooth crystals
and for severely rough crystals. The effective crystal radius in
both simulations is 30 µm. The smooth crystals show a backscatter
peak in I and a positive Q about the backscatter direction. Also there
are some smaller features in I and Q. The radiances (I and Q) for
rough crystals are smooth functions of viewing angle. This different
behaviour has been used to determine the fraction of smooth crystals in ice
clouds from POLDER measurements .
Fully spherical geometry
MYSTIC can be operated in fully spherical geometry (mc_spherical 1D). The implementation of 1-D spherical geometry is described in
where it has been used to simulate radiation in the umbral
shadow of a solar eclipse. A comparison to measurements during the total
eclipse in Greece in March 2006 showed a very good
agreement for modelled and measured UV irradiance, which decreased during
totality by 2 to 3 orders of magnitude depending on wavelength.
Fully spherical geometry has also been used to simulate actinic fluxes at
high solar zenith angles up to 92∘.
Another interesting application is the simulation of polarized radiance at
the surface at twilight, because polarized radiance measurements at twilight
can be used to retrieve aerosol optical properties (e.g.
).
As an example we calculated polarized clear-sky radiances for solar
depression angles up to 9∘ for the US-standard atmosphere and default
Rayleigh-scattering and absorption settings. Figure shows
the result as a function of viewing zenith angle. The relative azimuth angle
between sun and observer is 0∘, which means that the observer looks
into the direction of the sun. We see that the intensity decreases by about
4 orders of magnitude for solar depression angles between 0∘ (sun
at horizon) and 9∘ (sun 9∘ below horizon). The degree of
polarization (not shown) at a viewing angle of 5∘ is more than
90 %. All results agree to published results by ,
which indicates that fully spherical geometry works correctly in MYSTIC.
Twilight radiance at 500 and 700 nm calculated
using fully spherical geometry for the US-standard atmosphere. The
lines are for different solar depression angles. The x axis
corresponds to the viewing zenith angle.
Summary
We have presented the libRadtran software package (version 2.0.1),
which is a comprehensive and powerful collection of tools for radiative
transfer simulations of the Earth's atmosphere. It is user-friendly,
well-documented, and is widely used in the scientific community. We have
described various new features and parameterizations, which have been included
after the first publication of libRadtran in 2005. New features are
for example a vector radiative transfer solver and a solver for rotational
Raman scattering. The package includes state-of-the-art parameterizations for
aerosol and ice cloud optical properties and a newly developed efficient
absorption parameterization.
Code availability
The libRadtran package was initiated about 20 years ago and is still
under continuous development. Regularly updated versions of the package are
available from http://www.libradtran.org.
The website includes all released versions of the package. The latest release
is version 2.0.1 and includes the source code, example input files, several
tests, and the graphical user interface. Additional data packages containing
optical properties of clouds and aerosols and the REPTRAN gas absorption
parameterization are also available. The 1-D version of MYSTIC is part of the
libRadtran public release. Please note that the 3-D version of MYSTIC is not
part of the libRadtran public release, it is available in joint projects.
Ice crystal optical properties parameterizations
The parameterization yang2013 is based on the single-scattering data
by . It is available for nine habits and three roughness
parameters. It includes full phase matrices for the spectral range from
200 nm to 99 µm. The hey (Hong, Emde, Yang)
parameterization is available for six individual smooth habits and includes
the full phase matrices for the wavelength region from 0.2 to 5 µm.
The single-scattering properties for the six ice crystal habits have been
generated by Hong Gang based on the improved geometrical optics method
(IGOM), the same which is applied in .
In order to obtain bulk-scattering properties (required by the RTE solver), the single-scattering
properties need to be integrated over the particle size
distribution. In reality the size distributions are highly variable,
for radiative transfer simulations they are often approximated by simple gamma distributions
e.g. or bi-modal gamma distributions .
We assume a gamma size distribution to compute the bulk-scattering
properties as for the water cloud properties (compare Eq. ):
n(re)=Nre1b-3exp-reab.
Here re is a measure of the particle size (the radius in case of spherical
particles) and N is the normalization constant so that the integral over
the distribution yields the number of particles in a unit volume. For
spherical particles the parameters a and b correspond to the effective
radius reff and to the effective variance veff,
respectively. Typical values of cirrus cloud size distributions for b are
in the range between 0.1 and 0.5 . In the
following we take a fixed value of b=0.25. We define the effective particle
size re(L) for an individual ice crystal as follows :
re(L)=34V(L)A(L).
Here L is the maximum dimension of a nonspherical ice crystal and
A and V are the mean projected area and the volume of the particle,
respectively. 2re(L) corresponds to the “effective distance”,
i.e. the representative distance a photon travels through an ice
crystal without experiencing internal reflections and refraction
.
The effective radius of a size distribution is generally defined as
reff=34∫LminLmaxV(L)n(L)dL∫LminLmaxA(L)n(L)dL.
In order to obtain bulk-scattering properties which can be used for radiative
transfer calculations, we pre-calculate bulk optical properties on a
specified equidistant effective radius grid including values from 5 to
90 µm in steps of 5 µm. Now using Eq. () we
iteratively find the parameter a of the size distribution, which results in
the desired effective radius. The bulk optical properties are then calculated
by integration over the gamma distributions with the parameters b=0.25
and the iteratively obtained a depending on the effective radius.
libRadtran requires the extinction coefficient normalized to
1 g m-3 ice:
〈βext(reff)〉=∫LminLmaxA(L)Qext(L)n(L)dLρ∫LminLmaxV(L)n(L)dL.
Here Qext(L) is the extinction efficiency, ρ is the density of
ice, and n(L) is the gamma size distribution which corresponds to the
effective radius reff. The single-scattering albedo 〈ω0〉 is calculated as follows:
〈ω0(reff)〉=∫LminLmaxA(L)ω0(L)Qext(L)n(L)dL∫LminLmaxA(L)Qext(L)n(L)dL.
Finally, libRadtran requires the phase matrix 〈P(reff)〉, which is computed according to the following
equation for each scattering angle θ and for six matrix elements
(denoted by index i) needed to describe the scattering process by randomly
oriented nonspherical particles see e.g.):
〈P(reff,i,θ)〉=∫LminLmaxA(L)P(L,i,θ)ω0(L)Qext(L)n(L)dLA(L)∫LminLmaxω0(L)Qext(L)n(L)dL.
Optical properties for a general habit mixture ghm have also
been calculated for the hey parameterization following the mixing
“recipe” suggested by .
Description of TZS solver
This solver is based on the zero-scattering approximation and can be used to
calculate clear-sky or “black cloud” radiances at the TOA in the thermal spectral range. Without scattering the formal solution
of the RTE for the upward intensity (radiance) at TOA
Iν(τ=0,μ,ϕ) at a given frequency ν reduces to
Iν(τ=0,μ,ϕ)=Iν(τ∗,μ,ϕ)exp(-τ∗/μ)+∫0τ∗dτμBν(τ)exp(-τ/μ).
Here we used the (vertical) absorption optical thickness τ measured from
top of atmosphere as the vertical coordinate such that τ=0 at TOA and
τ=τ∗ at the surface. Variables μ and ϕ denote the cosine
of the zenith angle and the azimuth angle, respectively. Planck's function at
a given frequency ν is represented by Bν(τ) and its temperature
dependence is contained implicitly in τ.
The first term on the right-hand side in Eq. () represents the
contribution of the surface and the second one the contribution of the
atmosphere. The surface contribution can be written as
Iν(τ∗,μ,ϕ)=ϵsBν(τ∗)+2(1-ϵs)∫01∫0τ∗Bν(τ)exp(-(τ∗-τ)/μ)dτdμ,
with the first term representing the emission of the surface
(ϵs= surface emissivity) and the second one the reflection at
the surface of the radiation emitted by the atmosphere toward the surface.
The factor 2 comes from the integration over the azimuth angle ϕ.
Under the approximation of Planck's function Bν(τ) as a piecewise
linear function in τ between two consecutive levels, both integrals can
be solved as a function of the exponential integral
Ei(x)=∫-∞-xe-y/ydy.
The Supplement related to this article is available online at doi:10.5194/gmd-9-1647-2016-supplement.
Acknowledgements
Numerous colleagues have contributed with software and comments to the
package. We would like to thank K. Stamnes, W. Wiscombe, S. C. Tsay, and
K. Jayaweera (disort), F. Evans (polradtran), S. Kato (correlated-k distribution), J.-M. Vandenberghe, F. Hendrick, and M. V. Roozendael
(sdisort), T. Charlock, Q. Fu, and F. Rose (Fu and Liou code), D. Kratz
(AVHRR routines), B. A. Baum, P. Yang, L. Bi, H. Gang, J. Key, B. Reinhardt,
and A. Gonzales (ice cloud optical properties), P. Ricchiazzi (LOWTRAN/SBDART
gas absorption), M. Hess (OPAC aerosol database), W. Wiscombe, C. F. Bohren,
and D. Huffman (Mie codes), M. Mishchenko (water reflectance matrix),
O. Engelsen (implementation of ozone cross sections), the ARTS community and
Franz Schreier (line-by-line models), and J. Betcke (implementation of King Byrne
equation). Thanks to all users for feedback and contributions, which helped
to improve the software over the years. Thanks also to L. Scheck for
providing the simulated satellite image shown in Sect. .
Finally, we thank two anonymous reviewers and the topical editor K. Gierens
for their useful comments. Part of the libRadtran development was
funded by ESA (ESASLight projects AO/1-5433/07/NL/HE,
AO/1-6607/10/NL/LvH). Edited by: K. Gierens
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