LIDORT History

The original LIDORT code (Version 1.0) developed in 1999 generated radiance and profile-Jacobian output for top-of-atmosphere (TOA) reflectances in a plane-parallel multi-layer atmosphere. In 2000/2001, the model was generalized to include a pseudo-spherical treatment of solar beam attenuation, more output options at arbitrary optical thickness and viewing geometry, and the use of Green's function solution methods (Versions 2.1-2.3). A dedicated single-scattering "sphericity correction" to deal with simulations involving wide-angle off-nadir viewing was introduced in 2002. In 2003, a second linearization facility for bulk-atmospheric quantities (such as total ozone column) was introduced. In 2004 LIDORT Version 2.4 was developed for a detailed treatment of BRDFs and surface property Jacobians. Thermal and surface emission was introduced to LIDORT in 2007, including the Jacobian facility. These developments culminated in Version 3.3 (2008).

In recent years, the emphasis has been on upgrading the LIDORT code and improving performance; in 2010, the code was translated to Fortran 90 (Version 3.5). In 2012, a surface-leaving radiance source was introduced, along with the ability for LIDORT to ingest external single scatter (SS) radiance fields, and a facility for generating output at observational geometries only (Version 3.6). In 2014, additional linearization was developed for temperature Jacobians in the thermal regime, and the code was made thread-safe for use in OpenMP environments (Version 3.7).

More recently, supplements for BRDF and (in particular) surface-leaving radiances were given extensive upgrades, and the single-scatter codes completely revised; this resulted in Version 3.8. In the last 2-3 years, a treatment of the "planetary problem" was added to LIDORT, along with more options for output geometries (version 3.8.1), and most recently, a "sphericity correction" for multiple-scatter light has been introduced for satellite applications; this greatly increases the accuracy of the model in high-SZA/VZA geometrical scenarios where spherical curvature effects are important. LIDORT is currently at Version 3.8.3.

LIDORT Capabilities

LIDORT Version 3.8.3 is a multiple-scattering multi-layer discrete ordinate scattering code with a simultaneous linearization facility for the generation of both radiances and analytic Jacobians (intensity partial derivatives with respect to any atmospheric or surface parameter). The model will generate upwelling and/or downwelling output for any number of geometrical configurations and at any atmospheric level. Linearization is available for atmospheric profile Jacobians or atmospheric bulk-property Jacobians such as total column amounts, as well as for surface properties.

LIDORT is a pure scattering RT model; it ingests total optical properties (layer optical depths, single scattering albedos and either phase-function Legendre expansion coefficients or phase function input for specified input geometries) and their linearizations, and total surface properties. It does not distinguish individual atmospheric absorbers and scatterers, and (for surface BRDFs) the type of reflecting surface.

LIDORT treats multiple scattering in plane-parallel geometry, but for the single scattering (or first-order (FO)) field, solar and line-of-sight (LOS) beams may be computed more accurately for curved spherical media. This is the pseudo-spherical approximation for solar-beam attenuation. This "FO sphericity correction" is important for satellite applications involving wide-angle off-nadir viewing geometry, and horizontal variations in the line-of-sight direction. More recently, a multiple scatter sphericity correction has been added to the code, in which the calculation of multiple scatter source terms is allowed to follow changes in geometrical configurations for LOS paths in curved media.

LIDORT has a complete black-body atmospheric and surface thermal emission treatment; a full linearization capability is installed for thermal sources - this includes temperature Jacobians through the use of Planck function derivatives.

LIDORT uses the Green's function infinite-medium method for solving the layer radiative transfer equations (RTEs), both for solar and thermal-emission sources.

LIDORT has four supplements:

1. the BRDF supplement is based on surface reflection kernels (semi-empirical reflectance functions developed for particular types of surfaces) and provides the total BRDFs required for LIDORT to execute; this includes full surface property linearizations;
2. the SL (surface-leaving) supplement provides surface-leaving radiance sources (currently restricted to water-leaving or solar-induced fluorescence);
3. the SS (single-scatter) supplement holds external first-order fields if these are to be ingested by LIDORT as an alternative to an internal calculation of these fields;
4. the PHASFUNC supplement uses external data sets of scattering phase functions and generates the user-required phase-function and expansion-coefficient optical inputs necessary for running LIDORT.

LIDORT has a number of performance enhancements:

1. the delta-M scaling approximation for sharply-peaked phase functions;
2. the "solution-saving" and "boundary-value telescoping" options to avoid unnecessary computation for situations with contiguous cloud or aerosol layers embedded in a Rayleigh- or isotropic-scattering atmosphere;
3. the option to ingest "observational" or "doublet" geometrical configurations of angle input, in order to save time on post-processing;
4. the code is "thread-safe", in that is can be used in parallel-computing environments such as OpenMP.

LIDORT is available in Fortran 90 (Version 3.8.3), with the older 3.5 Version in Fortran 77.