SHDOMPPDA: A radiative transfer tool for cloudy sky data assimilation

Variational assimilation of visible and infrared radiances by numerical models in cloudy skies requires forward and adjoint radiative transfer models capable of handling scattering. SHDOMPPDA, a plane-parallel version of SHDOM for use in data assimilation, has been developed in an ongoing project to assimilate GOES radiances in a numerical cloud model. SHDOMPPDA consists of an unpolarized plane-parallel radiative transfer forward model for collimated solar and thermal emission sources of radiation, and corresponding tangent linear and adjoint models. The inputs to the SHDOMPPDA forward model are profiles of pressure, temperature, water vapor, and mass mixing ratio and number concentration for a number of hydrometeor species. Hydrometeor optical properties, including detailed phase functions, are determined from lookup tables as a function of mass mean radius. An interface to user provided molecular absorption routines is included.

The SHDOMPPDA algorithm and construction of the tangent linear and adjoint models is described. The SHDOMPPDA forward model is validated against DISORT by comparing upwelling radiances in multiple directions from 100 cloud model columns at visible and mid-infrared wavelengths. For this test in optically thick clouds the computational time for SHDOMPPDA is comparable to DISORT for visible reflection, and roughly five times faster for thermal emission. The tangent linear and adjoint models are validated by comparison to finite differencing of the forward model. SHDOMPPDA is written in Fortran 90. SHDOMPPDA, programs to make the single scattering tables, example scripts, and documentation are freely distributed from a web site.