A comprehensive two-moment warm microphysical bulk scheme. I: Description and tests

A bulk microphysical scheme which predicts the concentrations and mixing ratios of cloud droplets and raindrops is presented. The scheme draws its originality from the use of generalized gamma law as basis functions for the drop size distributions and also from the attention paid to performing analytical integrations of most of the microphysical transfer rates. The numerical representation of each process has been reviewed throughout and specific tests have been made to evaluate separately the accuracy of the scheme compared with a bin-size model. The scheme, which depends on the specification of a few input parameters shaping the activation spectrum, is incorporated in a three-dimensional non-hydrostatic model with some applications given in a companion paper (Part II).