An alternative procedure to evaluate number concentration rates in two-moment bulk microphysical schemes

A constant width parameter of the species spectra is commonly assumed in bulk microphysical packages that predict mixing ratios and number concentrations. Drastic changes in spectral characteristics are associated with processes that generate significant broadening or narrowing of the spectra in those schemes. This paper presents an alternative procedure to evaluate number concentration rates in two-moment microphysical schemes. The tendency equations depend on changes in mixing ratio and on predominant mass of the particle spectrum, defined as that portion of the particle distribution where most of the mass is concentrated. The approach results in more realistic redistributions of the particle spectra than assumptions made in previous studies using two-moment schemes. A comparison between the direct and indirect evaluation of the number concentration rates for various collection mechanisms is included.