LIMA : a two-moment mixed-phase microphysical scheme driven by a multimodal population of cloud condensation and ice freezing nuclei

As stressed by the Aerosols, Clouds, Precipitation, Climate research program (ACPC, 2009), the aerosols, clouds and precipitation form a strongly coupled system. Aerosol particles affect the cloud microstructure through their ability to nucleate droplets or ice crystals. They are also involved in complex processes and in many feedbacks impacting both the cloud physics and the cloud dynamics. Thus, unlike many others, the newly developed LIMA (Liquid Ice Multiple Aerosols) scheme aims at representing at best the diversity of aerosol particles and their different properties regarding the nucleation and cloud system interactions at convective scale.

The 2-moment mixed-phase microphysical scheme LIMA was developed in the MESO-NH (Lafore et al. 1998, http://mesonh.aero.obs-mip.fr/) non-hydrostatic mesoscale research model. Prognostic variables in LIMA include the mixing ratios for 5 water condensate species (droplets, drops, ice crystals, snow and graupel), and number concentrations for three of these five species, namely cloud droplets, rain drops and pristine ice crystals. Aerosols are represented by 3D, prognostic number concentrations for as many modes as deemed necessary, each mode being defined by a chemical composition, a log-normal size distribution, and the property to nucleate cloud droplets or/and ice crystals. The aerosol bulk budgets are carefully evaluated by tracing both free and nucleated/scavenged aerosols.

The CCN activation scheme to form the cloud droplets is an extension of Cohard et al. (1998) to treat the competition between several CCN modes. It is based on an estimate of the local maximum supersaturation, from which the number of activated droplets is derived from calibrated and bounded activation spectra. The heterogeneous IFN nucleation follows the empirical parameterization of Phillips et al. (2008), which is constrained by observations of insoluble aerosol loading in the troposphere and ice nucleation rates in a continuous flow diffusion chamber under controlled temperature and supersaturation conditions. Coated IFN are included separately. LIMA integrates the below-cloud washing out of aerosols by rain (Berthet et al. 2010). Other nucleating processes are considered (droplet and wet aerosol freezing, ice multiplication). LIMA was also interfaced with the radiative scheme for both cloudy particles and aerosols (Aouizerats et al. 2010) in which the radiative properties of a wide range of aerosol types and sizes were introduced.

For use in real case simulations and future operational forecasts, the “near-real-time” MACC analyses (www.gmes-atmosphere.eu/‎) provide the accurate 3D initial and lateral boundary conditions for the multiple aerosol fields. Although the horizontal resolution of these analyses is far from the kilometric scale at which convection is simulated explicitly, they provide an invaluable information about the spatial distribution of the aerosols, especially the presence of rich aerosol layers and sharp vertical gradients. The calibration of this coupling strategy, based on observations from the HyMeX (http://www.hymex.org/) Special Observing Period, is ongoing.

This presentation will illustrate the behavior of LIMA for both 2D idealized cases (cold orographic clouds, tropical squall line), and for 3D real case simulations of heavy precipitation events which occurred during the HyMeX SOP1 in Fall 2012.