Turbulence Length Scales in Stably Stratified Free Shear Flow Analyzed from Slant Aircraft Profiles

Abstract

The vertical turbulence structure in the marine atmosphere close to a coastline is investigated using airborne measurements. The measurements are from a field experiment close to the coast in the southeast of Sweden, in the Baltic Sea. The Baltic Sea has two main properties that make it particularly interesting to study: significant annual lag in sea 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 meteorological conditions. The present results are mostly from spring or early summer with mainly cool water that is, with a stable or neutral marine boundary layer but with substantial heating of the land area during daytime, thus considerable thermal contrasts. When the daytime inland convective boundary layer is advected out over the cool sea, there is a frictional decoupling in space analogous to the same nocturnal process in time. This sometimes creates a residual layer, a remnant of the inland convective boundary layer, that can be advected for considerable distances over the sea. At the top of this layer, wind shear gives rise to a local increase in turbulent kinetic energy. These layers are used for an analysis of turbulent scales for free shear flow in stable stratification. The analysis is based on different length scales used in numerical model closures for turbulence processes and reveals the asymptotic behavior of different scales in the neutral limit and their functional 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 taken from 25 slant soundings performed in connection to low-level boundary-layer flights. The results are calculated from turbulence data extracted through filtering techniques on instantaneous time (space) series (individual profiles). The calculated turbulence parameters from all profiles are lumped together and finally averaged compositely over all profiles.