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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2017-287
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model description paper 05 Dec 2017

Model description paper | 05 Dec 2017

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Geoscientific Model Development (GMD).

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

Thomas Hoarau1, Jean-Pierre Pinty2, and Christelle Barthe1 Thomas Hoarau et al.
  • 1Laboratoire de l’Atmosphère et Cyclones, UMR 8105, CNRS/Météo-France/Université de La Réunion, St Denis, La Réunion, France
  • 2Laboratoire d’Aérologie, University of Toulouse/CNRS/UPS, 14 avenue Edouard Belin, 31400 Toulouse, France

Abstract. 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.

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A representation of the collisional ice break-up process in the two-moment microphysics scheme LIMA v1.0 of Meso-NH T. Hoarau, J.-P. Pinty, and C. Barthe https://doi.org/10.5281/zenodo.1078527

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Short summary
The break-up of ice crystals in clouds is a possible secondary ice multiplication process to explain observations of very high concentrations of small ice crystals at cold temperature. Here the process is modelled by considering shocks between fragile aggregates (assemblage of pristine crystals) and large densely rimed crystals of selected sizes. The simulation of a storm illustrate the perturbations caused by the break-up effect (precipitation, ice concentration enhancement).
The break-up of ice crystals in clouds is a possible secondary ice multiplication process to...
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