Manifestation of Structural Differences Between Bin and Bulk Schemes

Abstract:

Bin and bulk microphysics schemes are two primary methods to model a collection of cloud droplets. Bin schemes are known to have fewer assumptions about the size distribution by specifying the number of droplets in each size bin but more computationally intensive, while bulk schemes predetermine a functional form for the cloud droplet size distribution, thus making it more computationally efficient yet less flexible. This is known to be the structural difference between the two schemes. In practice, however, they are also different at another level: how physics is parameterized, and separating these two levels of difference is difficult. Here, to control the variable, we will use a novel microphysics scheme (Arbitrary Moment Predictor, or AMP) that was specifically designed to have the same physics parameterization as a bin scheme, while modeling the droplet size distribution the same way as a bulk scheme. Using AMP as a proxy for bulk scheme will thus guarantee that any difference shown in AMP and bin simulation results is a consequence of their different structural design (explicit distribution vs. fixed functional form). We will present the results from 1D simulations that test a wide range of physical conditions, such as different initial altitudes, updraft profiles, and aerosol concentrations. Differences in the macrophysical properties (cloud depth, optical depth, precipitation rate, liquid water content, etc.) and microphysical rates (condensation, evaporation, collision-coalescence, sedimentation, etc.) from the two models will be compared. Additionally, we also run models with certain physical processes shut down in both models, which allows us to pinpoint the exact process that causes the manifestation of the structural differences of the two models. This study helps us better understand what physical processes are sensitive to the fixed functional form and thus gives us guidance as to how to effectively improve bulk schemes.