The Fallacy of Drifting Snow

摘要

A common parametrization over snow-covered surfaces that are undergoing saltation is that the aerodynamic roughness length for wind speed (zo) scales as au_*~2/g, where is the friction velocity, g is the acceleration of gravity, and a is an empiricalconstant. Data analyses seem to support this scaling: many published plots of zo measured over snow demonstrate proportionality to u_*~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 zo in the first place; the plots thus suffer from fictitious correlation that causes zo to unavoidably increase with u_* without any intervening physics. For these two datasets, when I plot zo against u_* derived from a bulk flux algorithm—and thus minimize the fictitious correlation—zo is independent of u_* in the drifting snow region, u_* > 0.30 ms~(_l). I conclude that the relation zo = when snow is drifting is a fallacy fostered by analyses that suffer from fictitious correlation.