Advances in the Measurement of Air-Sea Fluxes for Marine Boundary Layer Research

Marine physicists have made significant progress in recent decades in our ability to directly measure surface fluxes from research vessels and specialized platforms such as the R/P FLIP. These platforms utilize Direct Covariance Flux Systems (DCFS) to remove platform motion from the measured wind speeds to measure the flux directly. Over the past decade or so, researchers have begun to collect long time series, O(year), of momentum and buoyancy fluxes from surface moorings. The instrumentation on these moorings experience less flow distortion and measure a wider variety of conditions given their longer deployments than typical for oceanographic air-sea field campaigns on research vessels.

The latest generation of the DCFS have been deployed on mid- to high-latitude surface moorings as part of the NSF’s Ocean Observing Initiative (OOI). These include moorings in the Irminger Sea and Southern Ocean. In the recently completed NASA SPURS field program, researchers increased the available power onboard the buoy to allow the addition of fast response hygrometers. Combined with the latest generation DCFS, the hygrometers provide direct measurement of the latent heat flux and the ability to isolate the sensible heat flux from the buoyancy flux measurements. The instrumentation developed for these platforms are now being deployed on more mobile platforms that include autonomous sailing drones and boats, wave-riders and expendable drifting buoys and spars. The DCFS on these platforms have evolved to include real-time processing and telemetry of the data to shore, thereby making them more expendable in combination with lower-cost components. This will provide opportunities for deployment in remote and extreme conditions such as the Southern Ocean, without the need for recovery.

These measurements are extremely useful in process studies that investigate the exchange of momentum, heat and mass across the coupled boundary layers with a key application being improvement of bulk turbulent flux parameterizations; particularly at moderate to high winds. These bulk models find wide use in numerical modeling, in field process studies that rely on bulk fluxes from more readily available means, and in their use in global gridded air-sea flux products that combine model and satellite data. The lack of high quality turbulent flux observations near the air-sea boundary during high wind and sea states is a long-standing and serious impediment to improved understanding of air-sea exchange. This talk will describe some of the advances in measurement technology used to measure air-sea fluxes over the world’s oceans in high wind conditions, and provide examples of how this data is being used to improve our understanding of air-sea interaction under a wide-variety of conditions.