Turbulence Transitions and Advective Boundary Layers over the Sea

This study examines the dependence of six different turbulent velocity scales on the wind speed and on several stability-dependent quantities using data from the LongEZ aircraft collected during the CBLAST Weak Wind Pilot Experiment off the coast of Martha's Vineyard, USA and several additional aircraft datasets. The behavior of turbulent velocity scales defined in terms of the standard deviations of the horizontal and vertical velocity fluctuations, two versions of the friction velocity, and two velocity scales based on the heat flux are contrasted. The dependence of the turbulent velocities on the wind speed at a fixed height shows a well defined transition between two regimes of roughly constant slope. This transition occurs for all six turbulent velocities and occurs for a wide range of averaging times.

The behavior of this transition is examined in terms of wind direction which determines the fetch from the land with offshore flow directions. Low-level wind maxima above thin stable boundary layers are common with offshore flow of warmer air over cooler water. Such conditions also occur at this site with southerly flow from warmer water over the cooler water of the coastal zone. Because the stability is forced by advection, the boundary layer behaves differently compared to nocturnal stable boundary layers over land that are radiatively driven. These differences are explored. The characteristics of the drag coefficient and surface exchange coefficient for heat are interpreted in terms of these differences and the transition between the two scaling regimes.