Influence of External Factors on the Turbulence Structure in the Atmospheric Boundary Layer

Abstract: The theory used in today’s weather- and climate models to describe processes near the earth’s surface, i.e. transport of heat, moisture and momentum between the ground and the atmosphere, is based on the idea that only local factors are important, such as temperature and wind speed near the ground. However, from measurements made at two sites, one agricultural and one marine, it has been found that large eddies, which are related to the convective boundary layer height, influence the turbulence structure near the ground during unstable conditions more than previously realized. Especially the momentum transport is affected. The large eddies have similar size over land and over the sea, typically 1000 m. The important difference being that over land diurnal variation plays a fundamental role; over the sea such variations are typically absent. From the marine site it has also been found that the turbulence structure of the temperature field over the Baltic Sea is very different from over land. Instead of having a height dependence as expected from theory, the temperature structure seems to be constant with height within the surface layer. Typically, the heat flux over the sea is smaller than over land during convective conditions. This gives rise to a turbulence transport regime which is fundamentally different from that observed during daytime convective conditions over land.