A new warm-cloud collection and breakup parameterization scheme for weather and climate models

Microphysical process rates in bulk microphysics schemes have been parameterized in simple forms which may inadequately represent their complicated dependencies on hydrometeor quantities. In this study, we develop a bulk warm rain microphysics scheme that includes physically based parameterizations of collection and breakup. The parameterizations are derived from the stochastic collection and breakup equations by approximating the collection and breakup kernels in elaborate forms and analytically evaluating the integrals in those equations. Although the new scheme performs relatively complicated calculations, it is computationally not too expensive to be used in weather and climate models. We first evaluate the raindrop self-collection and collisional breakup (RSCB) parameterizations developed in this study using a box model. Under the intrinsic limitation of bulk schemes, they generally yield the equilibrium raindrop size distributions closest to those from a bin-based solver. We then evaluate the new scheme through the simulations of shallow cumuli field and a single warm convective cloud. In the simulation of shallow cumuli field, the new scheme quite well reproduces the observed cloud properties. Compared to two other bulk schemes, the new scheme yields somewhat different rainwater mass and raindrop number concentration, which is largely attributable to the differences in autoconversion parameterizations. In the simulation of a single warm convective cloud where RSCB plays a major role, the rate of raindrop number depletion by RSCB and the time of precipitation onset predicted by the new scheme are close to those predicted by a bin scheme used as the reference.

Automated summary

In this study, a physically-based microphysics scheme for warm rain is developed and evaluated. The new scheme performs complex calculations but is computationally feasible for weather and climate models. It is found that the scheme reproduces observed cloud properties well in simulations of shallow cumuli field and warm convective clouds.