Abstract
Abstract. The paper describes the LIMA (Liquid Ice Multiple Aerosols) quasi two-moment microphysical scheme, which relies on the prognostic evolution of an aerosol population, and the careful description of the nucleating properties that enable 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 IFN is also introduced. These "aged" IFN act first as CCN and then as IFN by immersion nucleation at low temperatures. All the CCN modes are in competition with each other, as expressed by the single equation of maximum supersaturation. The IFN are insoluble aerosols that nucleate ice in several ways (condensation, deposition and immersion freezing) assuming the singular hypothesis. The scheme also includes the homogeneous freezing of cloud droplets, the Hallett–Mossop ice multiplication process and the freezing of haze at very low temperatures. LIMA assumes that water vapour is in thermodynamic equilibrium with the population of cloud droplets (adjustment to saturation in warm clouds). In ice clouds, the prediction of the number concentration of the pristine ice crystals 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. The initiation of the large crystals and aggregates category is the result of the depositional growth of large crystals beyond a critical size. Aggregation and riming are computed explicitly. Heavily rimed crystals (graupel) can experience a dry or wet growth mode. An advanced version of the scheme includes a separate hail category of particles forming and growing exclusively in the wet growth mode. The sedimentation of all particle types is 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 one highlights the sensitivity of orographic ice clouds to IFN types and IFN concentrations. Then a squall line case discusses the microstructure of a mixed-phase cloud and the impacts of pure CCN and IFN polluting plumes. The experiments show that LIMA responds well to the complex nature of aerosol–cloud interactions, leading to different pathways for cloud and precipitation formation.
Highlights
As stressed by the Aerosols, Clouds, Precipitation and Climate (ACPC) Steering Committee, “the aerosol, clouds and precipitation are a strongly coupled system, but the nature of this coupling and its sensitivity to perturbations in one of the elements is poorly understood”
In LIMA, coated ice freezing nuclei (IFN) are treated as aerosols acting first as cloud condensation nuclei (CCN) to produce tagged cloud droplets which are the reservoir for ice nucleation by immersion
This is the major strength of Phillips et al (2008, 2013)’s ice nucleation scheme allowing a sensitivity of the IFN to the particle size distribution and differentiating a nucleation efficiency according to the chemical nature of the IFN, after calibration
Summary
As stressed by the Aerosols, Clouds, Precipitation and Climate (ACPC) Steering Committee (see ACPC, 2009), “the aerosol, clouds and precipitation are a strongly coupled system, but the nature of this coupling and its sensitivity to perturbations in one of the elements is poorly understood”. The two-moment, mixed-phase scheme proposed by Saleeby and van den Heever (2013) stands out from the others and provides a much better representation of aerosol and cloud interactions It features a prognostic evolution of nine aerosol species, including surface production of dust and sea salt, aerosol processing in cloud and rainfall scavenging, and the aerosol radiative effect. This work presents the elaboration of a new mixed-phase quasi two-moment scheme in Meso-NH, called LIMA (Liquid, Ice, Multiple Aerosols), which includes a detailed representation of aerosol–cloud interactions. This scheme integrates a prognostic representation of the polydisperse, heterogeneously distributed aerosol population (AP), and accounts for the distinct nucleabilities of the different aerosol species, both for cloud droplets and ice crystals.
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