Relationship between Obukhov and Ozmidov length scales in the stable atmospheric boundary layer
Dr. Andrey A. Grachev
(NOAA/University of Colorado CIRES)
Date: Tuesday, January 29, 2013
Time: 11:00-12:00 noon
Place: Engineering Dean’s Boardroom, 258 Fitzpatrick Hall
Abstract
Measurements of atmospheric small-scale turbulence made at five levels on a 20-m tower during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) are used to examine different aspects of the stable boundary layer (SBL); e.g., the existence of Kolmogorov’s cascade, critical Richardson number, upper limit of applicability of the local similarity theory, z-less concept, relationship between Obukhov and Ozmidov length scales etc. The applicability of the classical Monin-Obukhov similarity theory (1954) (MOST) has been limited by constant-flux layer assumption, which is valid in a narrow range z/L < 0.1 in the SBL. Nieuwstadt (1984) extended the range of applicability of the original theory using the local scaling (height-dependent) in place of the surface scaling, but the limits of applicability of the local similarity theory in the SBL have been blurred. It was recently shown (http://arxiv.org/abs/1202.5066), that applicability of the local MOST in the SBL is limited by inequalities, when both gradient Richardson number, Ri, and flux Richardson number, Rf, are below a "critical value" about 0.20-0.25 (however, Rf_cr = 0.20-0.25 is a primary threshold). This approach is based on the idea that the region of applicability for MOST is the same as for the Kolmogorov turbulence. Applying this prerequisite shows that the data follow classical Monin-Obukhov’s local z-less predictions after data without Richardson-Kolmogorov cascade have been filtered out. This removes previous controversy associated with the z-less concept. Based on the SHEBA data, it is found that in the MOST frameworks, Ozmidov buoyancy length scale is a function of the Obukhov length. In the z-less limit, they are linearly proportional to each other.