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MIMOSA Simulations

In order to reproduce filamentary structures that can be responsible to a large extent for the transport extra vortex of the PSCs processed air, it is necessary to simulate the horizontal transport with an adequate resolution. The advection scheme is based on the isentropic domain-filling model MIMOSA (Hauchecorne et al. 1999). The horizontal resolution is 20 km. We have chosen a simplified photochemical radicals parametrisation of radicals activation and deactivation due to the presence of NO2. The activation takes place on PSCs surface. The presence of PSCs is defined with a simple temperature threshold criterion. PSCs form below the temperature where the average volume of Supercooled Ternary Solutions (Tsts) increases, say 194 K for 50 mb level. The choice to simulate only STS instead of NAT and ice is motivated by the experimental finding that STS PSC formation is more usual than type II or NAT PSC during Arctic winter.

So it is necessary to define the highest temperature at which PSCs can be formed. The PSCs formation scheme validated by laboratory studies and measurements assume that STS are the first step in aerosol condensation with decreasing temperature (Peter 1997) and NAT PSCs are formed only from warming of type II PSC.

We will not calculate the concentration of chemical species. Tracer representative of the degree of activation is set to 1 in presence of PSCs and decay exponentially with time constant varying from 10 days to 5 days for an air mass inside the polar vortex and at middle latitudes respectively. The model is driven by temperature and wind fields from the European Center for Medium Range Weather Forecasting (ECMWF) analysis with an horizontal resolution of 2.5° and temporal resolution of 24 hours. The comparison between tracer fields and CTM active species simulations (reported in Fierli et al, 2000) indicates that tracer values greater than 0.5 corresponds remarkably well to ClOx concentrations greater than 1 ppbv.