DOI: 10.14768/mistrals-hymex.1503
title: Aerosol indirect effect sensitivities

description: Simulations were done with the version 3.7.1 of the Weather Research and Forecasting Model (WRF, Skamarock et al., 2008) . The model was run with a 50 km (LR, d01), a 16.6 km (MR, d02) and a 3.3 km (HR, d03) horizontal resolution. It is forced by the Global Forecast System (GFS) model (National Centers for Environmental Prediction National Weather Service, 2000) as initial and boundary conditions. Temperature, humidity, geopotential and velocity components are nudged towards GFS analysis data with a Newtonian-type method using a relaxation coefficient of 5 × 10 −5 s −1. The microphysical scheme used is the Thompson and Eidhammer (2014) scheme which explicitly calculates the number concentrations of aerosols. The latter are represented in a simplified way according to their capacity to nucleate cloud water ("water friendly", WFA) or ice water ("ice friendly", IFA). Aerosol number concentrations are initialized and forced at domain boundaries by a climatology based on Goddard Chemistry Aerosol Radiation and Transport (GOCART) model (Ginoux et al., 2001) simulations. While no surface emissions are applied to IFA, surface emission fluxes are applied to WFA in order to approximately equilibrate the loss of WFA due to scavenging and nucleation. The radiation scheme is RRTMG (Rapid Radiative Transfer Model for General circulation models, Iacono et al., 2008) and uses the cloud water droplets, ice and snow effective radii of the Thompson and Eidhammer (2014) microphysical scheme to resolve the radiative transfer equations. Another climatology of aerosols from Tegen et al. (1997) is used in this radiative scheme and therefore is not affected by any changes in the microphysical aerosol climatology, which enables us to perform sensitivity experiments of the indirect effects of aerosols with fixed aerosol direct effect. The Kain (2004) scheme is used to parameterize convection. The microphysical effects of aerosols are not taken into account explicitly in this parameterization although they can affect convection indirectly through modifications in the temperature or moisture profiles. The model was run to make two extreme simulations in terms of WFA and IFA microphysical concentrations. Both simulations start on April 1 st , 2013 (after one month of spin-up) and end on September 17, 2013. A very high aerosol emission level (1.75 × 10^7 kg.s^−1 for the whole domain) is applied in the first simulation, referred as MAX or polluted simulation and a very low aerosol emission level (1.75 × 10^−4 kg.s^−1 for the whole domain) is applied for the other simulation, referred as MIN or pristine simulation. Although these emission rates are extreme, maximal and minimal value permitted by the microphysics scheme reduce the range of variation of the number of WFA (NWFA) between ∼ 10 cm^−3 and ∼ 10, 000 cm^−3 and of the number of IFA (NIFA) between 0.005 cm^−3 and 10, 000 cm^−3 . Therefore these latter extreme emission rates ensure that both NIFA and NWFA in the MIN (resp. MAX) simulation remain close to their minimal (resp. maximal) permitted values, which corresponds to a 2 × 10^6 factor for NIFA and a 10^3 factor for NWFA between the MAX and the MIN simulations. Another set of MIN and MAX simulations has been performed at a resolution where convection is resolved (3.3 km) and on a smaller domain in Central Europe. An intermediate set of simulations was used to perform one-way nesting between the LR and the HR simulations, ensuring that the LR simulations force the HR simulations at their boundaries. These intermediate simulations were done at 16.6 km of resolution in an intermediate domain and with the same configuration as the LR simulations. The HR simulations have been performed without activating any convection scheme, since horizontal resolution (3.3 km) is sufficient to resolve convection processes, which is the only difference in model configuration between the LR simulations and the HR simulations.