73 The fallacy of drifting snow

Tuesday, 25 January 2011
Washington State Convention Center
Edgar L. Andreas, NorthWest Research Associates, Inc., Lebanon, NH
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            A common premise used for estimating the turbulent fluxes over snow-covered surfaces is a Charnock-like relation in which the aerodynamic roughness for the wind speed profile (z0) scales as the square of the friction velocity (u*) when snow is drifting and blowing.  Two large datasets obtained over snow-covered winter sea ice—one from the Arctic, one from the Antarctic—corroborate this premise when I plot z0 against the measured u*.  But this result seems to be fictitious correlation because u* is the independent variable in such plots but is also required for computing the dependent variable, z0.  In other words, z0 and the measured u* have built-in correlation because of shared errors.  On the other hand, when I plot z0 against the u* value that comes from a bulk turbulent flux algorithm (denoted u*,B) and, thus, does not have built-in correlation with z0, z0 is independent of u*,B for u*,B in the drifting snow regime, u*,B > 0.3 m/s.  The premise that z0 increases with u* when snow begins to drift is, therefore, a fallacy that ensues from flawed analyses.

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