Documentation for Calculations of Air-Sea Fluxes

Algorithm for Surface Air-Sea Fluxes

The COARE 3.0b algorithm was used to compute bulk air-sea fluxes, including latent and sensible heat flux, net heat flux, and other associated fluxes including evaporation, evaporation minus precipitation, sensible heat flux due to rain, buoyancy flux, and wind stresses.

The COARE 3.0b algorithm is detailed in:
Fairall, C.W., E. F. Bradley, J. E. Hare, A. A. Grachev, and J. B. Edson, 2003: Bulk Parameterization of Air-Sea Fluxes: Updates and Verification for the COARE Algorithm. J. Climate, 16, pp 571-591.

The fortran software for version 3.0 can be downloaded with FTPS using an FTP tool such as Cyberduck, FileZilla, etc. (ftps://ftp1.esrl.noaa.gov/BLO/Air-Sea/bulkalg/cor3_0/fortran3_0/)

  NOTE: The Matlab software for version 3.6 is also available through FTPS (ftps://ftp1.esrl.noaa.gov/BLO/Air-Sea/bulkalg/cor3_6/).

The COARE algorithm (in Matlab, Fortran, and Python) may alternatively be accessed through the NOAA-PSL GitHub.

Implementation of this algorithm at NOAA/PMEL was guided by:
Cronin, M.F., C. Fairall, and M.J. McPhaden, 2006: An assessment of buoy derived and numerical weather prediction surface heat fluxes in the tropical Pacific. J. Geophys. Res., 111, C06038, doi:10.1029/2005JC003324.

Flux Computation Details

Hourly:
To compute high resolution fluxes, hourly averages are used for all input data, in addition to using the warm-layer and cool-skin corrections which are built into the COARE 3.0b algorithm. To provide the best possible estimates of the high resolution surface fluxes, generally, they are only computed at times when there is a complete set of input data. Fluxes are insensitive to some inputs, so in some cases when these are missing, the missing values are filled in various ways so that fluxes may still be computed.

For sea level pressure, missing data are replaced with the long term mean value for the buoy site, computed from the entire record.

For near surface currents, which are used to compute wind speed relative to the sea surface, missing values are replaced using the OSCAR surface currents from JPL. The OSCAR data have 1/3 degree spatial resolution, and are detailed at http://podaac-ftp.jpl.nasa.gov/dataset/OSCAR_L4_OC_third-deg. When in-situ current observations shallower than 36 meters are not available, OSCAR surface currents from JPL are used instead, interpolated to the buoy location and data time.

Net shortwave radiation is computed using an albedo climatology obtained from the International Satellite Cloud Climate Project at http://isccp.giss.nasa.gov/products/browsesurf1.html. The climatology is interpolated in space and time to the particular OCS mooring location and data time.

If longwave radiation data are missing, they are filled using a parameterization from Clark et al (Clark, N.E., L. Eber, R.M. Laurs, J.A. Renner, and J.F.T. Saur Heat Exchange Between Ocean and Atmosphere in the Eastern North Pacific for 1961-1971, NOAA Tech. Rep. NMFS SSRF-682, U.S. Dept. of Commerc., Washington, D.C., 1974). This parameterization requires an input cloudiness value, and this is computed using another parameterization developed at PMEL. Note that the longwave data available for display and download do not have missing values filled in the way described above. The filling is only done for the purposes of doing the warm layer and cool skin corrections in the COARE bulk algorithm.

Rain rate data are corrected for wind speed, using the method of Serra et al (Serra, Y.L., P.A'Hearn, H.P. Freitag, and M.J. McPhaden, 2001: ATLAS self-siphoning rain gauge error estimates. J. Atmos. Ocean. Tech., 18, 1989-2002). When rain data are missing, the COARE bulk algorithm is still called, using an input rain rate of zero. This allows wind stress and latent and sensible heat fluxes to be computed, but in such cases, computed values which are dependent on rain rate, e.g., the sensible heat flux due to rain and evaporation-precipitation are set to the missing value after the bulk algorithm is called.

Adjustment to standard height using COARE3.0b: Measurement heights of wind on OCS buoys are nominally made at 4.2m, while air temperature and humidity are measured at 2.6m. Derivation of wind speed U, air temperature T, and specific humidity q at standard heights (U_10m,T_2m and q_2m) are based on vertical shear structure of the boundary layer, determined by the logarithmic profiles, according to the Monin-Obukhov similarity theory (MOST):

where U_s is the velocity of the ocean surface or surface ocean current; T_s is the ocean surface temperature with warm layer and cool skin temperature correction; q_s is the saturation humidity at T_s; κ is the von Karman constant; z_o, z_oT, and z_oq are roughness length for wind, temperature and humidity; φ and ϕ are the empirical functions describing the stability dependence of the shear profiles for wind speed and temperature and humidity; L is the Monin-Obukhov length; and U_* , T_* , q_* are MOST scaling factors, determined iteratively in the COARE bulk algorithm.

Daily:
Daily fluxes are computed by averaging the hourly fluxes to daily resolution.

5-Day, Monthly, and Quarterly Averages:
5-Day, monthly, and quarterly averages of surface fluxes are computed from daily fluxes.

Climatologies for Surface Fluxes

The surface heat flux climatologies, which are shown in time series displays and used to compute anomalies, are based on the monthly Objectively Analyzed Air-Sea Fluxes (OAFlux) from Woods Hole Oceanographic Institution. These include latent and sensible heat fluxes, evaporation, net shortwave and longwave radiation, and net heat flux. The climatology for E-P was computed by subtracing the Xie and Arkin rain climatology (1997) from the OAFlux evaporation