A Comparison of Bin-Emulating and Spectral Bin Microphysics Schemes in RAMS

It is generally thought that of all types of microphysics parameterizations, bin microphysics schemes have the most realistic representation of clouds and can best capture the complex interactions between clouds and their environment. However, bin microphysics schemes are much more computationally expensive than bulk microphysics schemes. This fact makes bulk schemes much more practical and popular for most atmospheric modeling applications. The microphysics scheme in RAMS (Regional Atmospheric Modeling System) tries to bridge the gap between bin and bulk microphysics schemes. While still a bulk scheme, it is considered to be “bin-emulating” because it represents processes such as cloud droplet nucleation and collision-coalescence by using look-up tables that have been generated using a bin scheme. The use of these tables is thought to produce results that more closely resemble those from a full bin scheme than those that would be produced from a standard bulk scheme without these lookup tables. The Hebrew University spectral bin model has recently been implemented in RAMS, which allows for a direct comparison of the standard RAMS bin-emulating microphysics scheme with a bin scheme. Simulations of non-raining and raining ordinary continental cumulus clouds are performed using the bin and bin-emulating schemes. The non-raining simulations allow for a detailed comparison of the cloud droplet nucleation and condensational growth parameterizations. The raining simulations allow for an evaluation of the autoconversion and accretion processes in the two schemes. The strengths and weaknesses of the bin-emulating scheme will be discussed and suggestions for future improvements made.