Condensation and activation in sectional cloud microphysical models

Based on the Linear Discrete Method, new spectral cloud microphysical models with one and two dimensional fixed grids are presented. Several configurations of these models were tested. Their results for droplet nucleation in a steady convective updraft were compared to those of a one dimensional moving grid approach and to further model results from the W-MO cloud modeling workshop. Sensitivity studies for different parameters (time step, grid resolution) and model configurations were carried out. As expected, for high grid resolutions and small time steps, results of the different model configurations converge within 1-2% and, therefore, a reference result is obtained. However, low resolution results deviate by -13% to +20% from this reference. Based on the sensitivity study, a fixed bin model configuration could be identified, which depends only weakly (less than or similar to 5%) on time step and grid resolution. One more important result is that fixed bin models can avoid model errors and uncertainties due to the arbitrary choice of the discretization grid almost completely, which is in contrast to the results for moving grid models especially for coarse resolutions. Therefore, the fixed bin models presented here can be applied efficiently in low bin resolutions, which offers possibilities for coupling with time-consuming chemistry codes or implementation in multidimensional models. (c) 2005 Elsevier B.V. All rights reserved.