The mixing state of polar stratospheric cloud particles in "sandwich structure" observed by lidar - 2. Numerical simulations of the optical properties of the liquid and solid particles with external mixture of STS and NAT particles

A microphysical box model is used to simulate the optical properties of sandwich structure polar stratospheric cloud (PSC) observed by lidar, and interpreted as an external mixture of liquid and solid particles in the companion paper (part 1). The liquid particles are assumed to consist of a supercooled ternary solution (STS). For the composition of the solid particles, nitric acid tryhidrate (NAT) is employed. The simulations indicate that the observed backscattering coefficients of the liquid and solid particles can be explained by the evolution of an ensemble of externally mixed STS and NAT particles along the air parcels trajectory, if initial mixing ratio of HNO3 in the range 13-16 ppbv is assumed for the lower depolarizing layer of the sandwich structure. In the range 15-19 ppbv for the nondepolarizing layer, and the mixing ratio of H2O is assumed to be 5 ppmv. The total number of NAT particles derived from the simulations agrees with the Optical Particle Counter (OPC) observations for the number of particles larger than 1.8 mum, and is on the order of 10(-3)-10(-4) cm(-3) along the cloud's altitude. The solid particles at the altitude of the upper depolarizing layer could not be explained in terms of NAT particles with the narrow size distributions expected from the simulations, while the liquid particles in that layer are probably background stratospheric sulfuric aerosol.