A representation of the collisional ice break-up process in the two-moment microphysics scheme LIMA v1.0 of Meso-NH.

The paper describes a switchable parameterization of CIBU (Collisional Ice Break-Up), an ice multiplication process that fits in with the two-moment microphysical scheme LIMA (Liquid Ice Multiple Aerosols). The LIMA scheme with three ice types (pristine cloud ice crystals, snowaggregates and graupel-hail) was developed in the cloud-resolving mesoscale model Meso-NH. Here the CIBU process assumes that collisional break-up is mostly efficient for small snowaggregate class of particles with a fragile structure when hit by large and dense graupel particles. The increase of cloud ice number concentration depends on a prescribed number of fragments being produced per collision. This point is discussed and analytical expressions of the newly contributing CIBU terms in LIMA are given. The scheme is run in Meso-NH for the case of a three-dimension deep convective cloud with a heavy production of graupel. The consequence of dramatically changing the number of fragments produced per collision is explored in particular to estimate an upper bound of the CIBU effect. The case of a random number of fragments is also proposed to illustrate the consequence of the uncertainty of this parameter. Finally it is concluded that the assessment of CIBU certainly needs accurate laboratory experiments to check the conditions and to tune the efficiency of the process of ice crystal fragmentation. However the proposed parameterization which can be easily implemented in many two-moment microphysics schemes, could be used in this form to simulate the case of real deep tropical clouds where anomalously high concentrations of small ice crystals are suspected to occur. [ABSTRACT FROM AUTHOR]