A Simulation Study of Hydrometeor Sedimentation with Multi-Moment Parameterization Schemes: Comparison with an Explicit Bin Microphysical Model

Microphysical process of drops sedimentation is examined in one-moment, two-moment, and three-moment bulk microphysics parameterizations in the context of a 1D model, with no other physical processes active. The solution from an explicit bin model is used as a reference against which the bulk model simulations are compared. The bulk moment scheme configurations used commonly predicted variables: number concentration, mass, and reflectivity from moment methods, when prognostic variables are interpreted as statistical moments of a drop size distribution. We used Gamma distribution function for this propose. In this study also, we examined the implication of the shape parameter of the Gamma distribution function in three moment bulk microphysics parametrizations and conclude that the shape parameter is restrictive for initial prognostic variables of the three-moment schemes. Three moment schemes show a better agreement with explicit bin model than other schemes.

Size sorting gravitational effect of drops is one of the consequences of the sedimentation process. In this study it can be seen how there is an improvement in three-moment bulk parameterization with respect to the two-moment bulk parameterization. In the case of one-moment bulk parameterization, no size sorting effect can be observed.