Effects of shear sheltering in a stable atmospheric boundary layer with strong shear

Abstract

AbstractData from two marine field experiments in the Baltic Sea with stable stratification have been analysed. The purpose was to test the concept of the ‘detached’ or ‘top‐down’ eddies and the ‘shear‐sheltering’ mechanism in the presence of a low‐level wind speed maximum in the atmosphere. Data used include turbulence and profile measurements on two 30 m towers and concurrent wind profiles throughout the boundary layer obtained from pilot‐balloon soundings.Measurements show that large eddies are being suppressed when there is a low‐level wind speed maximum present somewhere in the layer 40–300 m above the water surface and when the stratification is slightly stable. The effect is seen both in normalized standard deviations of the velocity components and in corresponding component spectra. In previous work it was argued that the relatively large eddies, which dominate the low wave number spectra in measurements in the surface layer, are detached or top‐down eddies generated higher up in the boundary layer, that interact with the surface layer. The low‐level wind maximum introduces a distinct layer with strong vorticity which, according to the shear‐sheltering hypothesis, prevents these eddies from penetrating downwards.In the limit of the wind maximum occurring at a very low height (less than about 100 m), usual turbulence statistics characteristic of the ‘canonical’ boundary layer are found. Combining all the statistics, it is demonstrated that the wavelength of maximum spectral energy is locally related to a turbulence length‐scale, which shows that for values of the Richardson number of unity or less the effect of the local wind gradient is greater than that of static stability. The reduction of length‐scale with the strength of a low‐level wind maximum, explains the observed reduction (by a factor of two) of the turbulent flux of sensible heat at the surface. This result indicates that the shear‐sheltering mechanism is likely to play an important role in the turbulent exchange process at the surface in sea areas where low‐level wind maxima are a frequently occurring phenomenon, such as the Baltic and other large water bodies surrounded by landmasses. Copyright © 2004 Royal Meteorological Society