TURBULENCE LENGTH SCALES IN STABLY STRATIFIED FREE SHEAR-FLOW ANALYZED FROM SLANT AIRCRAFT PROFILES

The vertical turbulence structure in the marine atmosphere close to a coastline is investigated using airborne measurements. The measurement s are from a field experiment close to the coast in the southeast of S weden, in the Baltic Sea. The Baltic Sea has two main properties that make it particularly interesting to study: significant annual lag in s ea surface temperature compared to inland surface temperatures and the fact that it is surrounded by land in all directions within advection distances of from a few hours up to 10-15 hours in normal meteorologi cal conditions. The present results are mostly from spring or early su mmer with mainly cool water that is, with a stable or neutral marine b oundary layer but with substantial heating of the land area during day time, thus considerable thermal contrasts. When the daytime inland con vective boundary layer is advected out over the cool sea, there is a f rictional decoupling in space analogous to the same nocturnal process in time. This sometimes creates a residual layer, a remnant of the inl and convective boundary layer, that can be advected for considerable d istances over the sea. At the top of this layer, wind shear gives rise to a local increase in turbulent kinetic energy. These layers are use d for an analysis of turbulent scales for free shear flow in stable st ratification. The analysis is based on different length scales used in numerical model closures for turbulence processes and reveals the asy mptotic behavior of different scales in the neutral limit and their fu nctional form, and also illustrates the nonlinear relationship between scales for different properties. The applicability of some often used formulations is also discussed. The profiles from the aircraft are ta ken from 25 slant soundings performed in connection to low-level bound ary-layer flights. The results are calculated from turbulence data ext racted through filtering techniques on instantaneous time (space) seri es (individual profiles). The calculated turbulence parameters from al l profiles are lumped together and finally averaged compositely over a ll profiles.