Development and Testing of Direct Covariance Turbulent Flux Measurements for NDBC TAO Buoys

PIs: James Edson and Tom Farrar (WHOI), Meghan Cronin (NOAA PMEL), Chris Fairall (NOAA/PSD)

Description

This project seeks to transition recent advances in buoy-based air-sea flux measurements to operational use in the TAO buoy array (R2X). In this project a low power Direct Covariance Flux System (DCFS) developed at WHOI is being used as the technology base for future deployments on select NDBC TPOS buoys. The low power DCFS uses a 3-axis Gill R3-50 sonic anemometer, Lord Micro-strain motion sensors and a microprocessor. The processor collects the data and computes motion-corrected Direct Covariance (DC) fluxes and wave statistics in near realtime onboard the buoy. The DCFS then telemeters the fluxes and means back to shore every hour. During the course of the project, we joined forces with the “Enhanced Ocean Boundary Layer Observations on NDBC TAO Moorings” effort led by Karen Grissom (NOAA/NDBC) to build a stand-alone enhanced flux mooring. The DCFS was modified to measure all of the variables necessary to compute the fluxes using the bulk method (i.e., Tair, RH, Pair, Precipitation and Tsea) including radiative fluxes (i.e. long and shortwave radiometers) as shown in Figure 1.

This project was funded by the NOAA’s Global Ocean Monitoring and Observing Program (GOMO) as one of its six technology development projects in support NOAA’s contribution to TPOS 2020.

Sensors deployed on the enhanced surface mooring.

Figure 1. Sensors deployed on the enhanced surface mooring.

Accomplishments

The DCFS and a TAO buoy were successfully built by WHOI and PMEL for the project. The DCFS was modified to measure all of the variables necessary for direct covariance and bulk flux estimates. The buoy tower top and bridal were assembled in Guam prior to loading aboard the M/V Bluefin as shown in Figure 2. The enhanced buoy was was then deployed near a TAO mooring on the equator at 165oE on October 4, 2019 as shown in Figure 3. The DCFS successfully computed and telemetered fluxes, means and wave statistics via Iridium every hour over the course of the deployment, which ended on April 21, 2020.

The telemetered variables include:

    • Direct measurements of the momentum and buoyancy fluxes:, , and
    • Long and short-wave radiative fluxes.
    • Air temperature, pressure, humidity, precipitation, wind speed and direction
    • Significant wave height and direction.
    • Peak and average wave period.
    • TKE dissipation
    • Near sub-surface sea temperature and salinity

The telemetered data has been used to accurately measure all of the terms in the surface heat budget to provide net heat fluxes as shown in Figure 4. These state variables produce high quality bulk fluxes. The combination of the DC fluxes and state variables provide a means to improve bulk formulae for TPOS platforms that do not include a DCFS. For example, the drag coefficient required to estimate the bulk surface stress is plotted in Figure 5. The significant wave height, wave direction, peak period and average period provide estimates of wave age and wave slope. These wave statistics provide a means to develop and improve wave-dependent bulk formulae such as those included in COARE 3.5.

The buoy tower top after assembly in Guam prior to loading aboard the M/V Bluefin.

Figure 2. The buoy tower top after assembly in Guam prior to loading aboard the M/V Bluefin.

The TPOS Enhanced Surface mooring after deployment near the equator at 0o, 165oE.

Figure 3. The TPOS Enhanced Surface mooring after deployment near the equator at 0o, 165oE.

Lessons Learned

The enhanced DCFS worked almost flawlessly during its deployment. The telemetered fluxes and means were used to create the results given here before recovery, which is an obvious advantage of real-time delivery of research-quality data. The realtime DCFS has now been deployed on a number of research buoys including our TPOS mooring, the X-Spar, OOI surface moorings and the recently concluded INCOIS deployment in the Bay of Bengal. As a result, the DCFS is reliable and ready for deployment on the Tier-2 moorings.

The enhanced mooring developed for this project can be considered a proto-type for the Tier-2 moorings we expect to deploy for the TPOS array. The main air-side enhancements, beyond the DCFS, include all the variables required to compute bulk and radiative fluxes (i.e., radiometers, Tair, RH, Pair Tsea and precipitation sensors). These variables are expected to be on all of the TPOS moorings, so an obvious strategy is to enhance the planned measurements on Tier-2 moorings with stand-alone DCFS. The means, fluxes and waves provided by the DCFS and can be directly used in process studies to force the ocean on Tier-2 moorings and to improve the wind speed and wave-based parameterization found in the bulk algorithms used on the Tier-1 moorings.

Presentations

The results from this research project has been presented at the 2020 AMS Annual Meeting and the 2020 Ocean Sciences Meeting:

Edson. J. B., C. A. Clayson, J. Toole and J. T. Farrar, 2020: Autonomous Direct Covariance Flux Systems for Use on Enhanced Surface Moorings and Expendable Platforms over the Open Ocean, 20th Symposium on Meteorological Observations and Instrumentation, Ref: 8.3, AMS, Boston MA.

Edson. J. B., J. T. Farrar, M. F. Cronin, W. S. Kessler, C. W, Fairall and K. Grissom, 2020: Autonomous Direct Covariance Atmospheric Flux and Oceanic Current System: An Enhanced Flux Mooring for TPOS, 2020 AGU/ASLO Ocean Sciences Meeting, San Diego, CA, Abstract ID: IS34D-3385.

Data

Preliminary data is available from PI Edson (jedson@whoi.edu).

All terms in the surface heat budget as telemetered from the surface mooring.

Figure 4. All terms in the surface heat budget as telemetered from the surface mooring.

Figure 5. Drag coefficients computed from the telemetered DC fluxes of momentum and buoyancy from the Mooring Enhancement Pilot Project: .