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Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 10 | Copyright
Geosci. Model Dev., 11, 4269-4289, 2018
https://doi.org/10.5194/gmd-11-4269-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model description paper 18 Oct 2018

Model description paper | 18 Oct 2018

A representation of the collisional ice break-up process in the two-moment microphysics LIMA v1.0 scheme 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, Université de Toulouse, CNRS, UPS, 14 avenue Edouard Belin, 31400 Toulouse, France

Abstract. The paper describes a switchable parameterization of collisional ice break-up (CIBU), an ice multiplication process that fits in with the two-moment microphysical Liquid Ice Multiple Aerosols (LIMA) scheme. The LIMA scheme with three ice types (pristine cloud ice crystals, snow aggregates, and graupel hail) was developed in the cloud-resolving mesoscale model (Meso-NH). Here, the CIBU parameterization assumes that collisional break-up is mostly efficient for the small and fragile snow aggregate class of particles when they are hit by large, dense graupel particles. The increase of cloud ice number concentration depends on a prescribed number (or a random 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 the cloud-resolving mesoscale model (Meso-NH) to simulate a first case of a three-dimensional deep convective event with heavy production of graupel. The consequence of dramatically changing the number of fragments produced per collision is investigated by examining the rainfall rates and the changes in small ice concentrations and mass mixing ratios. Many budgets of the ice phase are shown and the sensitivity of CIBU to the initial concentration of freezing nuclei is explored.

The scheme is then tested for another deep convective case where, additionally, the convective available potential energy (CAPE) is varied. The results confirm the strong impact of CIBU with up to a 1000-fold increase in small ice concentrations, a reduction of the rainfall or precipitating area, and an invigoration of the convection with higher cloud tops.

Finally, it is concluded that the efficiency of the ice crystal fragmentation needs to be tuned carefully. The proposed parameterization of CIBU is easy to implement in any two-moment microphysics scheme. It could be used in this form to simulate deep tropical cloud systems where anomalously high concentrations of small ice crystals are suspected.

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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 modeled by considering shocks between fragile aggregates (assemblage of pristine crystals) and large densely rimed crystals of selected sizes. The simulations of two storms 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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