On the turbulent structure of the marine atmospheric boundary layer from CBLAST Nantucket measurements

In this work preliminary results on the characteristics of the turbulent structure of the Marine
Atmospheric Boundary Layer (MABL) are presented. Measurements used here were
conducted in the framework of the Coupled Boundary Layers Air-Sea Transfer Experiment in
Low Wind (CBLAST-Low) project. A number of in situ (fast and slow sensors) and remote
sensing (SODAR) instruments were deployed on the coast of Nantucket Island, MA, USA.
Measurements of the mean wind, the variances of the three wind components, the
atmospheric stability and the momentum fluxes from the acoustic radar (SODAR) revealed
the variation of the depth, the turbulent characteristics, and the stability of the MABL in
response to the background flow. More specifically, under light south-southwesterly winds,
which correspond to the MABL wind directions, the atmosphere was very stable and low
values of turbulence were observed. Under moderate to strong southwesterly flow, less stable
and neutral atmospheric conditions appeared and the corresponding turbulent quantities were
characterized by higher values. The SODAR measurements, with high temporal and spatial
resolution, also indicated large magnitude of momentum fluxes at higher levels, presumably
associated with the shear forcing near the developed low-level jet.
The measurements from the in-situ instrumentation confirmed that the MABL typically has
small negative momentum and sensible heat fluxes consistent with stable to neutral
stratification while strong diurnal variations were typical for the land surface Atmospheric
Boundary Layer (ABL). The developed internal ABL at the experimental site was in general
less than 10m during the night and could reach 15m heights during the day, particularly under
low-wind conditions.