On Sedimentation and Advection in Multimoment Bulk Microphysics

In two-moment bulk microphysics schemes, the practice of using different weighted fall velocities for the various moments is known to lead to artificial growth in reflectivity values for fast-falling particles, particularly at the downward leading edge of a precipitation column. Two simple correction schemes that prevent these artifacts while still allowing some effects of size sorting are presented. The corrections are obtained by comparing particle number concentrations that result from two or three different sedimentation calculations. The corrections do not conserve particle number concentrations but do prevent spurious reflectivity growth automatically without the need to place ad hoc limits on mean particle size. Multimoment bulk microphysics schemes often have used inconsistent variables in terms of the appropriate advection equation (e.g., mass mixing ratio and particle number concentration). A brief review of consistent advection and turbulent mixing for such variables is presented to provide clarification.