LIMA (v1.0): A quasi two-moment microphysical scheme driven by a multimodal population of cloud condensation and ice freezing nuclei

摘要

The paper describes the LIMA (Liquid Ice Multiple Aerosols) quasi two-momentmicrophysical scheme, which relies on the prognostic evolution of an aerosolpopulation, and the careful description of the nucleating properties thatenable cloud droplets and pristine ice crystals to form from aerosols.Several modes of cloud condensation nuclei (CCN) and ice freezing nuclei(IFN) are considered individually. A special class of partially soluble IFNis also introduced. These "aged" IFN act first as CCN and then as IFN byimmersion nucleation at low temperatures.All the CCN modes are in competition with each other, as expressed by thesingle equation of maximum supersaturation. The IFN are insoluble aerosolsthat nucleate ice in several ways (condensation, deposition and immersionfreezing) assuming the singular hypothesis. The scheme also includes thehomogeneous freezing of cloud droplets, the Hallett–Mossop icemultiplication process and the freezing of haze at very low temperatures.LIMA assumes that water vapour is in thermodynamic equilibrium with thepopulation of cloud droplets (adjustment to saturation in warm clouds). Inice clouds, the prediction of the number concentration of the pristine icecrystals is used to compute explicit deposition and sublimation rates(leading to free under/supersaturation over ice). The autoconversion,accretion and self-collection processes shape the raindrop spectra. Theinitiation of the large crystals and aggregates category is the result of thedepositional growth of large crystals beyond a critical size. Aggregation andriming are computed explicitly. Heavily rimed crystals (graupel) canexperience a dry or wet growth mode. An advanced version of the schemeincludes a separate hail category of particles forming and growingexclusively in the wet growth mode. The sedimentation of all particle typesis included.The LIMA scheme is inserted into the Meso-NH cloud-resolving mesoscale model.The flexibility of LIMA is illustrated by two 2-D experiments. The first onehighlights the sensitivity of orographic ice clouds to IFN types and IFNconcentrations. Then a squall line case discusses the microstructure of amixed-phase cloud and the impacts of pure CCN and IFN polluting plumes. Theexperiments show that LIMA responds well to the complex nature ofaerosol–cloud interactions, leading to different pathways for cloud andprecipitation formation.