Simulation of Deep Convective Clouds from the HyMeX Campaign using a Realistic Population of CCN and IFN

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, the LIMA (Liquid Ice Multiple Aerosols, Vié et al. 2016) 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. 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. CCN activation (after Cohard et al. 1998) and IFN nucleation (following Phillips et al. 2008,2013) explicitly depend on available aerosols properties and concentrations. A companion presentation shows the performance of LIMA for mesoscale convective systems (MCSs) simulations in comparison to detailed microphysical observations from the HyMeX campaign (Ducrocq et al. 2014).

LIMA is used to study the impact of the aerosol population on two MCSs that produced heavy rainfall during HyMeX. In a first step, a simple configuration is chosen: one single mode of CCN particles, and one single mode of IFN aerosols, with an initially homogeneous number concentration. Several simulations are performed varying the aerosol concentration to highlight the changes in the MCS characteristics in different environments.

However, the main point of LIMA is to represent at best the aerosol-cloud interactions. Therefore, to make the most of it, it is essential to provide it with a realistic aerosol population. Two sources were chosen to provide realistic aerosols. Global MACC analyses and forecasts, performed at the ECMWF, are available in near-real-time. Over Europe, MOCAGE simulations, performed at CNRM with a finer representation of aerosols, can be used instead. Both systems provide mass mixing ratios for different types of aerosols. Assumptions about their size distribution and nucleating abilities are necessary to use them as cloud condensation nuclei (CCN) / ice freezing nuclei (IFN) in LIMA.

Airborne aerosols observations were carried out during the HyMeX campaign before each event. Different instruments (SMPS, FMPS, CPC, OPC, CCNC) provided information on the aerosol loading and particle size distribution (PSD). These observations are used together with the aerosol analyses from MACC and MOCAGE to calibrate the mass to number concentration conversion. This work provides a validated method to initialize realistic aerosols to the microphysical scheme LIMA, and enables the use of LIMA for both case studies and real-time forecasting. This is illustrated with simulations of the two considered HyMeX cases using MACC aerosols.