VLIDORT History

In 2004, the vector code VLIDORT for polarized light was created for Stokes vector output in the pseudo-spherical approximation. Profile linearization was completed in 2005. In 2006, VLIDORT was given the bulk-property linearization, and the BRDF surface capability was installed; thermal emission and exact 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. VLIDORT is currently at Version 2.8.

VLIDORT Capabilities

Similar to LIDORT, VLIDORT Version 2.8 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 down-welling 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 linearly polarized field (V component absent) also exists. Neglecting polarization altogether in VLIDORT gives identical results to 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 field, solar and line-of-sight beams may be computed more accurately for curved spherical media (pseudo-spherical approximation). This "enhanced sphericity" treatment is suitable for important satellite applications involving wide off-nadir viewing geometry, and horizontal variations in the line-of-sight direction.

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.

Unlike LIDORT, VLIDORT generates Stokes vector output [I, Q, U, V]. The option to output the linearly polarized field (V component absent) exists. Neglecting polarization altogether in VLIDORT gives identical results to LIDORT.

Unlike LIDORT, VLIDORT does not use the Green's function method to solve the RTE particular integrals - instead, solutions are obtained using classical substitution methods for dealing with sets of coupled linear differential equations.

VLIDORT also has four supplements: (1) the BRDF supplement is kernel-based (semi-empirical reflectance functions 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); (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 new supplement for generating phase-function inputs to be used directly in single-scatter 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 atmosphere; (3) the option to ingest "observational geometry" angular input, in order to save time on post-processing; and (4) thread-safe code that can be used in parallel computing environments such as OpenMP.

VLIDORT is available in Fortran 90 (Version 2.8) and Fortran 77 (version 2.5).