A new scheme has been developed based on the observation that supersaturations inside clouds are small and bulk condensation rate provides sufficiently accurate prediction of spectral changes. When condensation first occurs (e.g., near the cloud base), activation parameterization inserts activate... Show moreA new scheme has been developed based on the observation that supersaturations inside clouds are small and bulk condensation rate provides sufficiently accurate prediction of spectral changes. When condensation first occurs (e.g., near the cloud base), activation parameterization inserts activated droplets near the low end of the spectral representation. Subsequent growth by diffusion is represented by shifting the spectrum towards larger sizes so that the spectral changes match the bulk condensation rate. Growth by collision/coalescence leads to formation of drizzle and eventually rain as in the bin scheme. The hybrid bulk-bin approach eliminates the need for supersaturation prediction, which is numerically cumbersome, especially near cloud edges, and typically requires high spatial and temporal resolution (e.g., near the cloud base where droplet activation takes place). A simple approach is proposed to deal with numerical artifacts near cloud boundaries. The key point is to recognize the need for both homogeneous and heterogeneous advection-condensation processes. Predicting the "cloud fraction" guides the selection of either homogeneous or heterogeneous numerical representation. Tests in a multidimensional cloud model illustrate consistency of the hybrid bulk-bin approach. Work to include the hybrid scheme into French Meso-NH model is underway. Show less
