| 08 Jul 2025
Quantification and parameterization of snowflake fall speeds in the atmospheric surface-layer
Abstract. The modeled settling speed of frozen hydrometeors has important implications for the prediction of weather and climate. However, it is usually assumed, erroneously, that they fall in still air. Here, we present novel field measurements of individual snowflake microphysical properties and their settling velocities in atmospheric surface-layer turbulence. Individual snowflake motions are tracked in a laser light sheet using particle streak velocimetry (PSV). A hotplate device, the Differential Emissivity Imaging Disdrometer (DEID), is used to obtain precise estimates of snowflake mass, density, and size. Relative to calculated terminal velocities in still air, we present enhancements and reductions of snowflake settling speeds in turbulent air for a broad range of Reynolds and Stokes numbers. Functional forms describing actual snowflake fall speeds are presented and explored. In particular, a promising non-dimensional functional form for the ratio of actual particle fall speed to terminal velocity is presented in terms of turbulence intensity and a new variable called the shape density index or SDI, which is related to an individual hydrometeor's microphysical structure.
Competing interests: Conflict of interest: The DEID technology is protected through patent US20210172855A1 co-authored with D.K.S., E.R.P., and T.J.G. and is commercially available through Particle Flux Analytics, Inc. T.J.G. is a co-owner of Particle Flux Analytics, Inc., which has a license from the University of Utah to commercialize the DEID. Some authors are members of the editorial board of Atmospheric Chemistry and Physics.
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This is a well written manuscript which presents valuable measurements of snowflake fall speeds and I agree with the authors that the proposed parameterization is potentially useful to modelers. I have no major concerns about the methods or analysis and have only minor clarification questions.
1. The MASC is mentioned as being present at the site, but is its data used in the analysis? I don't think it was mentioned in sections 2.2 or 2.3 and my apologies if I missed it. If not used, perhaps say so explicitly or remove mention of this instrument.
2. In equation 2, is pi*a*b the same as the previously defined circumscribed projected area A? If so, it may be useful to draw that connection explicitly.
3. Line 241: I believe the authors meant to refer to Figure 7b rather than 7a.