VLIDORT History

In 2004, the vector code VLIDORT for light scattering with polarization was created for Stokes 3- and 4-vector output. The pseudo-spherical approximation was used from the output, with particular integrals established using the classical (Chandrasekhar) exponential substitution method. Profile linearization was completed in 2005. In 2006, VLIDORT was given the bulk-property linearization, and the reflection-matrix BRDF surface capability was installed; thermal emission and accurate single-scatter calculations were introduced in 2007 (Version 2.0). The capabilities for VLIDORT were then brought up to the same level as those for the scalar code LIDORT; this effort culminated in Version 2.4 (2009). In recent years, the VLIDORT code has been upgraded to Fortran 90 (2011, Version 2.5), and the surface-leaving, external SS and observational geometry options introduced (2013, Version 2.6). In 2015, the code was made thread-safe for use in parallel-computing environments.

Recent developments with the LIDORT code have also been installed in VLIDORT. These include the "planetary problem" facility, temperature (Planck function) Jacobians for thermal sources, and the use of the multiple-scattering sphericity correction. In addition, VLIDORT has an alternative treatment of the 3-vector (linear polarization) radiative transfer equation using the Greek's function method. VLIDORT is currently at Version 2.8.3.

VLIDORT Capabilities

Similar to LIDORT 3.8.3, VLIDORT Version 2.8.3 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.

However, unlike LIDORT, VLIDORT can generate output for the entire Stokes vector [I, Q, U, V]. The option to output the Stokes 3-vector linearly polarized field (V component absent) also exists. Neglecting polarization altogether in VLIDORT gives identical results to those calculated with LIDORT.

As with LIDORT, VLIDORT is a pure scattering RT model; it ingests total optical properties appropriate for doing vector calculations: layer optical depths, single scattering albedos, the 4x4 phase matrix of expansion coefficients appropriate for scattering-matrix expansions in terms of generalized spherical functions, and total surface properties. It does not distinguish individual atmospheric absorbers and scatterers, and (for surface BRDFs) the type of surface.

VLIDORT treats multiple scattering in plane-parallel geometry, but for the single scattering (FO) field, solar and line-of-sight beams may be computed more accurately for curved spherical media.

VLIDORT 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.

In VLIDORT, particular integral RTE solutions have always been obtained using classical (Chandrasekhar) substitution methods for dealing with sets of coupled linear differential equations. However, now there is an alternative for VLIDORT - it can also operate using the infinite-medium Green's function method to solve the RTE (for linear polarization only).

VLIDORT also has the same four supplements:

1. the BRDF supplement is based on surface reflection kernels (semi-empirical reflectance matrices or scalar values developed for particular types of surfaces) and provides total BRDFs required for VLIDORT 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, this is scalar-only);
3. the SS (single-scatter) supplement holds the external first-order fields if these are to be ingested by VLIDORT as an alternative to an internal calculation of these fields;
4. a "VFZMAT" supplement based on the use of external F-matrix data sets for generating F-matrix and expansion-coefficient optical inputs required for VLIDORT calculations.

VLIDORT also has a number of performance enhancements:

1. the delta-M scaling approximation for phase matrices indicative of sharply-peaked forward scattering;
2. "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 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" for use in parallel computing environments such as OpenMP.

VLIDORT is available in Fortran 90 (Version 2.8.3) and Fortran 77 (version 2.5).