4.17
Limit of very strong stability: frictionless concept
A. A. Grachev, CIRES/Univ. of Colorado, NOAA/ETL, Boulder, CO; and E. L. Andreas, C. W. Fairall, P. S. Guest, and P. O. G. Persson
This study focuses on behavior of near-surface atmospheric turbulence in the limit of very stable stratification, which is an end-point that theory must treat. Results are based on data collected at five levels on a 20-m tower over the Arctic pack ice during the yearlong Surface Heat Budget of the Arctic Ocean Experiment (SHEBA). As the Richardson number approaches its critical value, turbulence decays and vertical fluxes vanish. The reduction in the surface friction is responsible for the main features of the atmospheric boundary layer in the limit of very strong stability. First, this regime is associated with the strong influence of the Earth’s rotation. Frictional effects become negligible and the influence of the Coriolis effect comes into play. Observed wind speeds show features of the Ekman spiral even near the surface (surface Ekman layer). Second, the stress falls off faster with increasing stability than the sensible heat flux, and the stress (or friction velocity, U*) ceases to be a relevant scaling parameter in the limit of very strong stability (frictionless or U*-less regime). This leads to new scaling relationships for stability functions and other variables (such as the variances) in this state, which are in good agreement with the SHEBA data. However, even in the supercritical stable regime, some sporadic and intermittent turbulence persists and there is no evidence of a transition to a laminar Ekman layer on average. Overall, a regime in the limit of very stable stratification can be referred to as the intermittently turbulent Ekman layer. .
Session 4, boundary layers: stable, and convective, and transitional (Parallel with Sessions 5 and 6)
Tuesday, 10 August 2004, 8:00 AM-5:15 PM, Vermont Room
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