dorsal/arxiv
View SchemaThe capacitance of pristine ice crystals and aggregate snowflakes
| Authors | C. D. Westbrook, R. J. Hogan, A. J. Illingworth |
|---|---|
| Categories | |
| ArXiv ID | physics/0610038 |
| URL | https://arxiv.org/abs/physics/0610038 |
Abstract
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).
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"abstract": "A new method of accurately calculating the capacitance of realistic ice\nparticles is described: such values are key to accurate estimates of deposition\nand evaporation rates in NWP models. The trajectories of diffusing water\nmolecules are directly sampled, using random `walkers\u0027. By counting how many of\nthese trajectories intersect the surface of the ice particle (which may be any\nshape) and how many escape outside a spherical boundary far from the particle,\nthe capacitance of a number of model ice particle habits have been estimated,\nincluding hexagonal columns and plates, `scalene\u0027 columns and plates, bullets,\nbullet-rosettes, dendrites, and realistic aggregate snowflakes. For ice\nparticles with sharp edges and corners this method is an efficient and\nstraightforward way of solving Laplace\u0027s equation for the capacitance. Provided\nthat a large enough number of random walkers are used to sample the particle\ngeometry the authors expect the calculated capacitances to be accurate to\nwithin ~1%. The capacitance for our modelled aggregate snowflakes (C/Dmax=0.25,\nnormalised by the maximum dimension Dmax) is shown to be in close agreement\nwith recent aircraft measurements of snowflake sublimation rates. This result\nshows that the capacitance of a sphere (C/Dmax=0.5) which is commonly used in\nnumerical models, overestimates the evaporation rate by a factor of 2. The\neffect of vapor `screening\u0027 by crystals growing in the vicinity of one another\nhas also been investigated. The results clearly show that neighbouring crystals\ngrowing on a filament in cloud chamber experiments can strongly constrict the\nvapor supply to each other, and the resulting growth rate measurements may\nseverely underestimate the rate for a single crystal in isolation (by a factor\nof 3 in our model setup).",
"arxiv_id": "physics/0610038",
"authors": [
"C. D. Westbrook",
"R. J. Hogan",
"A. J. Illingworth"
],
"categories": [
"physics.ao-ph",
"physics.geo-ph"
],
"title": "The capacitance of pristine ice crystals and aggregate snowflakes",
"url": "https://arxiv.org/abs/physics/0610038"
},
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