Thermal properties and thermodynamic limitation of the size of mesospheric ice particles

Physics

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Earth Mesosphere, Ice Particles

Scientific paper

The growth of solid particles by deposition and sublimation of water vapour depends on the rate at which water molecules are desorbed from the particle. While the growth is controlled by the available water supply, the desorption depends only on the temperature of the particle surface. In mesospheric conditions the particle temperature is determined by the balance between absorption of solar and terrestrial radiation, heat transfer by collisions with the ambient gas, and thermal emission which is the Planck radiation function times the particle's absorption cross section. Cooling by collisions is a surface effect while in the limit of dipole scattering, radiation absorption varies as the volume of particle suggesting a size dependence of the equilibrium temperature. Earlier discussions of the heat balance of absorbing particles have indicated that particles exhibiting "metallic" absorption were too warm to retain an ice coating while dielectric particles like ice would absorb in the IR and thus become slightly warmer than the ambient gas. The thermal balance scales as the ratio of particles volume/surface ratio, the air pressure and the accomodation coefficient. The equilibrium temperature of ice grains at the mesopause exceeds the ambient gas temperature, the temperature excess increasing with particle size. The consequence is a narrowing of the height interval allowed for growth and persistence of ice grains which reduces the growth rate and limits to the maximum particle size.

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