A two-moment cloud microphysics scheme with two process-separated modes of graupel

In most up-to-date cloud and weather prediction models cloud microphysics is represented by a scheme which includes five particle categories: cloud droplets, raindrops, cloud ice, snow and graupel. Sometimes an additional category is assumed, mostly called hail, whose formation almost always proceeds via a transformation of large graupel to 'hail' with graupel originating from riming of ice crystals and snowflakes as well as from freezing raindrops. In these schemes the conversion from graupel to hail is not well defined and corresponding parameterizations are difficult if not questionable.

It is thus reasonable to have a clear-cut distinction between graupel created by rimed ice particles (termed RIME graupel hereafter) or by freezing raindrops (frozen-raindrop-induced or FRI graupel for short). In the present study FRI graupel is considered solely initiated from frozen raindrops and growing by deposition and riming/collection of ice particles. In contrast, RIME graupel is formed by the traditional riming path. Consequently the FRI graupel category is in fact a second graupel mode which has only slightly different properties compared to RIME graupel, e.g. higher particle density. Therefore, already during the initial evolution of a mixed-phase cloud, both graupel types can simultaneously appear depending on their formation process. The advantage is a clear and clean definition of the conversion rates while it allows for two graupel distributions which my combine to a bi-modal spectrum. In the present formulation any interaction between RIME and FRI graupel is neglected.

This new approach has been implemented into the two-moment bulk microphysical scheme of Seifert and Beheng (2006) which is part of a test version of the operational weather forecast model - Lokal-Modell - of the German Weather Service. First results of 3D simulations are presented. The case studies comprise continental and maritime situations. It will be shown that one main effect of introducing the new graupel class is a decrease in the total mass of larger ice particles in the upper part of the cloud and an increase in lower levels. This finally leads to significantly enhanced precipitation at ground.