The Fallacy of Drifting Snow |
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Authors: | Edgar L. Andreas |
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Affiliation: | (1) Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA;(2) NOAA Earth System Research Laboratory, Boulder, CO, USA;(3) U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, NH, USA;(4) Present address: NorthWest Research Associates, Inc. (Bellevue Division), 25 Eagle Ridge, Lebanon, NH 03766-1900, USA;(5) Naval Postgraduate School, Monterey, CA, USA |
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Abstract: | A common parametrization over snow-covered surfaces that are undergoing saltation is that the aerodynamic roughness length for wind speed (z 0) scales as au*2/g{alpha u_ast^2/g}, where u * is the friction velocity, g is the acceleration of gravity, and α is an empirical constant. Data analyses seem to support this scaling: many published plots of z 0 measured over snow demonstrate proportionality to u*2{u_ast^2 }. In fact, I show similar plots here that are based on two large eddy-covariance datasets: one collected over snow-covered Arctic sea ice; another collected over snow-covered Antarctic sea ice. But in these and in most such plots from the literature, the independent variable, u *, was used to compute z 0 in the first place; the plots thus suffer from fictitious correlation that causes z 0 to unavoidably increase with u * without any intervening physics. For these two datasets, when I plot z 0 against u * derived from a bulk flux algorithm—and thus minimize the fictitious correlation—z 0 is independent of u * in the drifting snow region, u * ≥ 0.30 ms−1. I conclude that the relation z0 = au*2/g{z_0 = alpha u_ast^2/g} when snow is drifting is a fallacy fostered by analyses that suffer from fictitious correlation. |
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