  A new method of accurately calculating the capacitance of realistic ice
particles is described: such values are key to accurate estimates of deposition
and evaporation rates in NWP models. The trajectories of diffusing water
molecules are directly sampled, using random `walkers'. By counting how many of
these trajectories intersect the surface of the ice particle (which may be any
shape) and how many escape outside a spherical boundary far from the particle,
the capacitance of a number of model ice particle habits have been estimated,
including hexagonal columns and plates, `scalene' columns and plates, bullets,
bullet-rosettes, dendrites, and realistic aggregate snowflakes. For ice
particles with sharp edges and corners this method is an efficient and
straightforward way of solving Laplace's equation for the capacitance. Provided
that a large enough number of random walkers are used to sample the particle
geometry the authors expect the calculated capacitances to be accurate to
within ~1%. The capacitance for our modelled aggregate snowflakes (C/Dmax=0.25,
normalised by the maximum dimension Dmax) is shown to be in close agreement
with recent aircraft measurements of snowflake sublimation rates. This result
shows that the capacitance of a sphere (C/Dmax=0.5) which is commonly used in
numerical models, overestimates the evaporation rate by a factor of 2. The
effect of vapor `screening' by crystals growing in the vicinity of one another
has also been investigated. The results clearly show that neighbouring crystals
growing on a filament in cloud chamber experiments can strongly constrict the
vapor supply to each other, and the resulting growth rate measurements may
severely underestimate the rate for a single crystal in isolation (by a factor
of 3 in our model setup).

Atmospheric and Oceanic Physics;Geophysics