For the first time, in situ turbulence measurements collected in the vicinity of the Brazil‐Malvinas Confluence are used to determine the influence of the ocean waves and atmospheric stability on the Marine Atmospheric Surface Layer. From the analysis of 187 high‐frequency sampled segments of temperature and wind velocity, carefully selected from three ship campaigns of the Air‐Sea Interaction at Brazil‐Malvinas Confluence project, we found a particular behavior of the drag coefficient, with a negative trend for a calm wind speed up to 10 m/s when the significant wave height was lower than 2.5 m, and a continuous decrease of the drag coefficient with increasing wind speed for significant wave height higher than 2.5 m. The results suggest that waves act as roughness elements during high wave conditions, inducing a zero‐plane displacement in the order of 0.1 to 1 m as an indication for a wave‐induced roughness layer. In addition, the analysis of the turbulent kinetic energy (TKE) budget indicates the occurrence of upward TKE transport mainly during stable conditions, while the general patterns of transport and dissipation of TKE are similar to observations taken over land surfaces.

Direct covariance flux (DCF) measurements taken from floating platforms are contaminated by wave-induced platform motions that need to be removed before computation of the turbulent fluxes. Several correction algorithms have been developed and successfully applied in earlier studies from research vessels and, most recently, by the use of moored buoys. The validation of those correction algorithms has so far been limited to short-duration comparisons against other floating platforms. Although these comparisons show in general a good agreement, there is still a lack of a rigorous validation of the method, required to understand the strengths and weaknesses of the existing motion-correction algorithms. This paper attempts to provide such a validation by a comparison of flux estimates from two DCF systems, one mounted on a moored buoy and one on the Air–Sea Interaction Tower (ASIT) at the Martha’s Vineyard Coastal Observatory, Massachusetts. The ASIT was specifically designed to minimize flow distortion over a wide range of wind directions from the open ocean for flux measurements. The flow measurements from the buoy system are corrected for wave-induced platform motions before computation of the turbulent heat and momentum fluxes. Flux estimates and cospectra of the corrected buoy data are found to be in very good agreement with those obtained from the ASIT. The comparison is also used to optimize the filter constants used in the motion-correction algorithm. The quantitative agreement between the buoy data and the ASIT demonstrates that the DCF method is applicable for turbulence measurements from small moving platforms, such as buoys.

Abstract. The micro-RPAS (remotely piloted aircraft system) SUMO (Small Unmanned Meteorological Observer) equipped with a five-hole-probe (5HP) system for turbulent flow measurements was operated in 49 flight missions during the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign in 2011. Based on data sets from these flights, we investigate the potential and limitations of airborne velocity variance and TKE (turbulent kinetic energy) estimations by an RPAS with a take-off weight below 1 kg.The integration of the turbulence probe in the SUMO system was still in an early prototype stage during this campaign, and therefore extensive post-processing of the data was required. In order to be able to calculate the threedimensional wind vector, flow probe measurements were first synchronized with the autopilot's attitude and velocity data. Clearly visible oscillations were detected in the resulting vertical velocity, w, even after correcting for the aircraft motion. The oscillations in w were identified as the result of an internal time shift between the inertial measurement unit (IMU) and the GPS sensors, leading to insufficient motion correction, especially for the vertical wind component, causing large values of σ w . Shifting the IMU 1-1.5 s forward in time with respect to the GPS yields a minimum for σ w and maximum covariance between the IMU pitch angle and the GPS climb angle.The SUMO data show a good agreement to sonic anemometer data from a 60 m tower for σ u , but show slightly higher values for σ v and σ w . Vertical TKE profiles, obtained from consecutive flight legs at different altitudes, show reasonable results, both with respect to the overall TKE level and the temporal variation. A thorough discussion of the methods used and the identified uncertainties and limitations of the system for turbulence measurements is included and should help the developers and users of other systems with similar problems.

An internally recording, autonomous instrument has been tested for measurements of ocean turbulence from a mooring line. Measurements were made at a single level in the water column, but for an extended period of time, at a predetermined duty cycle. The instrument is designed to measure, independently, in two different parts of the turbulence wavenumber spectrum: eddy correlation measurements in the inertial subrange and smallscale shear and temperature gradient measurements in the dissipation subrange using shear probes and fastresponse thermistors. For the deployment reported here, the instrument is located in the wave-affected layer, and only the dissipation subrange from the shear probes can be confidently utilized for turbulence measurements. The velocity spectra in the inertial subrange are severely contaminated by platform motion and noise, and the dissipation range of the temperature gradient spectrum is not satisfactorily resolved. The shear spectra are found to be relatively free of contamination in the 1-20-Hz frequency range and are used for dissipation rate calculations. The quality of the measurements is constrained by the angle of attack and the magnitude of mean flow relative to the wave oscillatory velocities. Dissipation rates are consistent with a scaling expected from breaking long waves, when background shear is weak, and are elevated when the gradient Richardson number is small, consistent with additional turbulence production by shear. While limited to a single depth, the instrument makes it possible to collect time series for 3 weeks continuously or for 3 months at a 25% duty cycle.