Efficient computation of vapor and heat diffusion between hydrometeors in a numerical model

We present an algorithm for representing diffusion of sensible heat and vapor between hydrometeors and air that is formulated from implicit numerical equations and is therefore stable for long numerical model timesteps, but is solved explicitly without requiring iteration. Liquid, ice, and mixed-phase hydrometeor categories are incorporated into the system, and latent heat of fusion released or absorbed during the diffusion process is accounted for. The method has been incorporated into the Regional Atmospheric Modeling System (RAMS) which prognoses ice–liquid potential temperature as its principal thermodynamic variable, but may be adapted to other models as well. Simulations with RAMS used as a parcel model are presented and demonstrate the performance of the new algorithm.