A fast and efficient version of the TwO-Moment Aerosol Sectional (TOMAS) global aerosol microphysics model

Abstract

This study develops more computationally efficient versions of the TwO-Moment Aerosol Sectional (TOMAS) microphysics algorithms, collectively called "Fast TOMAS." Several methods for speeding up the algorithm were attempted, but only reducing the number of size sections was adopted. Fast TOMAS models, coupled to the GISS GCM II-prime, require a new coagulation algorithm with less restrictive size resolution assumptions but only minor changes in other processes. Fast TOMAS models have been evaluated in a box model against analytical solutions of coagulation and condensation and in a 3-D model against the original TOMAS (TOMAS-30) model. Condensation and coagulation in the Fast TOMAS models agree well with the analytical solution but show slightly more bias than the TOMAS-30 box model. In the 3-D model, errors resulting from decreased size resolution in each process (i.e., emissions, cloud processing/wet deposition, microphysics) are quantified in a series of model sensitivity simulations. Errors resulting from lower size resolution in condensation and coagulation, defined as the microphysics error, affect number and mass concentrations by only a few percent. The microphysics error in CN70/CN100 (number concentrations of particles larger than 70/100 nm diameter), proxies for cloud condensation nuclei, range from -5% to 5% in most regions. The largest errors are associated with decreasing the size resolution in the cloud processing/wet deposition calculations, defined as cloud-processing error, and range from -20% to 15% in most regions for CN70/CN100 concentrations. Overall, the Fast TOMASmodels increase the computational speed by 2 to 3 times with only small numerical errors stemming from condensation and coagulation calculations when compared to TOMAS-30. The faster versions of the TOMAS model allow for the longer, multi-year simulations required to assess aerosol effects on cloud lifetime and precipitation. Copyright © American Association for Aerosol Research.