A Bi-continuous Model for Snow and Graupel in the Community Radiative Transfer Model

As evidenced by the morphology diagram of Nakaya, water ice crystals may develop a highly complex geometry under certain thermodynamic conditions of the environment. Due to the instability in the crystalline growth process, this may lead to the highly complex shapes commonly known as snowflakes. But even for comparatively simple crystal geometries certain random atmospheric processes, such as riming, re-sublimation and particle collisions may lead to a complex shape.

Some of the most complex forms of precipitation formed in such a way is Graupel. Resulting mass densities for such water ice particles may lie anywhere in the range below the 9.167 grams per cubic centimetre of full ice.

Especially in the Microwave regime, the scattering properties of such kind of particles have as of yet not been adequately represented in the Community Radiative Transfer Model (CRTM). In the presentation at hand we elaborate on our proposed solution to address this issue and improve the accuracy of CRTM radiative transfer calculations in this case. As a necessary prerequisite for this, we have developed new particle models for Graupel and snow based on the so-called bi-continuous medium approach. This approach had first been employed to compute the Bidirectional Reflectance of fluffy snow layers on the ground. Similarly, soft ice particles and Graupel may be represented as a sort of random porous medium. In the next step, the corresponding single scattering properties are aggregated to bulk scattering properties and the corresponding binary coefficient files used as Look-Up tables in the CRTM are then created from them. The validation of the improved model for variable-density snow and Graupel is performed on the basis of AMSU-A data collocated with corresponding databases for atmospheric composition and cloud structure.