I am trying to understand how the sublimation process can cause a positive mass balance in general, intuitively as highlighted in the text taken from W. S. B. Paterson,The Physics of Glaciers: 3rd Edition(Pergamon, New York, 1994).
Thank you.
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Strictly speaking sublimation is the phase change from solid to gas without an intermediate liquid phase. So the way the term sublimation has been used in the text is partly wrong. Ice and snow on a glacier can sublimate so that mass is lost directly to the atmosphere as water vapour.
Water vapour in the atmosphere an also be deposited on the glacier as a solid in a reverse process called deposition or desublimation. This is where the text example contains an error in terminology.
The term in the equation conerning phase changes from solid to gas or gas to solid is of course correct and it may be either positive or negative depending on which phase change direction dominates. As is stated losses from solid to gas is likely larger in most if not all cases.
answered Mar 6, 2023 at 18:24
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$\begingroup$ Is deposition/desublimation different from solid form of precipitation? $\endgroup$– kc_nulCommented Mar 6, 2023 at 18:27
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$\begingroup$ Solid precipitation is formed by water vapour condensing on particles (dust, salt etc.) in the atmosphere and then falling to the ground. This is counted as snow accumulation on a glacier. Under extreme cold conditions ice crystals can spontaneously form from vapour in the atmosphere as so-called clear sky precipitation. But again this process occurs in the air, not on the glacier surface and is also considered accumulation (a_s in the eq). $\endgroup$ Commented Mar 6, 2023 at 18:35
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1$\begingroup$ Oh, now I understand, the positive sublimation that is being talked about is clear-sky precipitation. This is interesting. Thank you Prof. Jansson $\endgroup$– kc_nulCommented Mar 6, 2023 at 18:50
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2$\begingroup$ Sorry, no, that is not what I mean. All precip is considered snow accumulation. The sublimation term is solid-gas phase change on the surface. $\endgroup$ Commented Mar 6, 2023 at 21:14