Variational optimization method for calculation of cloud drop growth in an Eulerian drop-size framework

A variational optimization (VO) method that requires specification of only one variable in each bin size for condensation and evaporation calculations in an Eulerian drop-size framework is proposed. The method is tested against the exact solution given by the Lagrangian method using more than 15 000 spectra selected from experiments with a three-dimensional large eddy simulation model with explicit microphysics. The results show that the VO method not only conserves the integral parameters of the spectrum, such as drop number, mean radius, liquid water content, and the effective radius, but also provides an accurate calculation of the spectrum itself.