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

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

analysis of

calculation of

determination of solar direct irradiance and global
irradiance distributions in order to optimize locations of

simulation of satellite radiances to be
used for data assimilation in

validation of radiation schemes included in

simulation of heating rates in three-dimensional (3-D) atmospheres
to develop fast radiation parameterizations for

simulation of solar radiation during a

rotational

Estimation of

Remote sensing of

Since the publication of the first libRadtran reference 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)

Moreover libRadtran now includes a solver to calculate rotational
Raman scattering

Numerous state-of-the-art parameterizations for aerosol and ice cloud optical
properties have been included (see Sects.

A new gas absorption parameterization for the solar and thermal spectral
ranges has been developed

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

The paper is organized as follows: Sect.

The main tool of the libRadtran package is the

The

The user may select between various

The optical properties are passed to a

The

The model was originally designed to compute UV radiation; therefore, its name
is

Structure of the

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

The

The radiative transfer equation solvers currently implemented in libRadtran.

The RTE for a macroscopically isotropic medium, i.e. randomly oriented
particles and molecules, may be written as

The

The solver

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

The most comprehensive solver in libRadtran is the Monte Carlo model
MYSTIC

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

For the calculation of irradiance, two fast two-stream solvers are available.

The first solver,

The second two-stream method available in libRadtran is

Note that

For actinic fluxes and atmospheric heating rates,

For the thermal irradiance,

In order to complement the instruments that can be simulated by
libRadtran, a lidar simulator called

The solver

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

For the solar region a fast single-scattering solver

Several other RTE solvers are included in

The MYSTIC model has been validated in many international model
intercomparison studies, for radiance calculations with highly peaked phase
functions

The accuracy of MYSTIC depends only on the number of traced photons. The
standard deviation of MYSTIC is calculated when the option

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

Absorption parameterizations in libRadtran.

The

The LOWTRAN (low-resolution transmission)-band model adopted from the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) radiative transfer model

For the simulation of radiances and irradiance, we recommend to use

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

For the spectral region from 160 to 850 nm, libRadtran includes
measured absorption cross sections of various molecules in the atmosphere
(see Table

For O

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

The Rayleigh-scattering cross sections are by default calculated using
Eqs. (22)–(23) of

Besides the models by

Optical properties of all basic aerosol types were calculated using
libRadtran's Mie tool (see Sect.

Phase matrix elements for the basic OPAC aerosol types
“water soluble” (

Water clouds parameterizations in libRadtran.

As an example Fig.

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.

Table

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

Absorption (left) and scattering (right) optical thickness
for various aerosol mixtures specified using the option

For specific applications, e.g. different size distributions, the user can easily generate optical properties using libRadtran's Mie tool.

Ice cloud parameterizations in libRadtran

For ice clouds libRadtran includes a variety of parameterizations
(see Table

As described in the previous section the exact phase matrix is not needed
when irradiance are calculated. For this purpose the parameterizations by

For accurate radiance calculations the parameterizations by

We have generated two further parameterizations (

Please refer to the libRadtran user guide for a list of available habits for each parameterization.

Figure

Phase matrix elements of ice crystal distributions with an
effective radius of 40

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

As already stated in

The surface reflection models currently implemented in libRadtran.

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

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

Screenshot of the graphical user interface for a
spectral high-resolution simulation of the O

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

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

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

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

The large number of input options available in the

(Left) the transmittance from ARTS output and radiance
from

Online documentation of the options are available and this is identical to
the documentation in the libRadtran user manual. In
Fig.

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.

Once all wanted input options are set, they are saved to a user specified
file, and

Several additional tools are included in the libRadtran package. An
overview is given in

The tool for Mie calculations (

Single-scattering radar (

The libRadtran package has been used for numerous applications. Many
of these are listed under the publications link at

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

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 O

(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/

The Infrared Atmospheric Sounding Interferometer (IASI) on board the MetOp
satellite measures the radiance from 645 to 2760 cm

The top panel 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

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

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

The red curve in the top plot of Fig.

Figure

(Top) simulation of MSG-SEVIRI image. False colour composite, where
red corresponds to the 1.6

The MYSTIC solver can be applied to simulate multi-angle multi-spectral
polarized radiances using the option

Stokes vector components

Figure

The first row shows the results for a clear atmosphere, i.e.
Rayleigh scattering and molecular absorption. Here

The second row of the figure shows the same simulation but with an underlying
ocean surface, which is modelled according to

The third row shows the result for desert aerosol as defined in the OPAC
database (

The fourth row shows a simulation including a water cloud
(

The last two rows show simulations with ice clouds, where we have used the

MYSTIC can be operated in fully spherical geometry (

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

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

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.

The libRadtran package was initiated about 20 years ago and is still
under continuous development. Regularly updated versions of the package are
available from

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.

The parameterization

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

Optical properties for a general habit mixture

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

The first term on the right-hand side in Eq. (

Under the approximation of Planck's function

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 (