Work in progress: Fluxes in the tropical Pacific

Work in progress: Flux calculations by Meghan Cronin.

In these calculations, I use:

  • COARE v2.5b, for comparison with ChuanLi Jiang's calculations.

  • high res 10 minute data, 13-point hanning filtered, subsampled to 1 hour as input. Output is then daily averaged with a 24-point boxcar filter and subsampled to once per day.

  • When highres data was not available, I used realtime daily averaged data as input into the flux algorithm.

  • Warm layer/cool skin correction was applied to highres flux calculation when highres shortwave radiation data was available. This correction also requires longwave radiation input. When LWR was not available, a mean value of the measured LWR was used. If no LWR had been measured at that site, the Clark et al. (1974) algorithm was used to estimate LWR. This type of gap filling still needs to be evaluated. However, realize LWR is a secondary variable used only to determine the sst to skin temperature correction.

  • winds are relative to the SOME KIND OF SURFACE CURRENT ESTIMATE. (see below for sensitivity study)

  • Minimum relative wind speed was set as 0.2 m/s to avoid pathelogical flux estimates at very low wind speeds. 0.2 m/s is less than the error of the wind sensor. Because only dws was modified, the vector component of the wind stress TAUX and TAUY may not combine to have a magnitude of TAU for very low wind speeds.

  • Webb correction was applied to the latent heat flux, to account for nonconservative flux properties built into this version of the algorithm.

    Testing the role of surface currents in latent heat flux calculation.

    Bulk latent heat flux is: QE = rhoa Ce L S (qs - qa)

    where S= sqrt(dws^2 + wg^2), and dws^2 = (u-us)^2 + (v-vs)^2 is the relative wind speed, and wg is a gustiness factor.

    In Fig. 1, Latent heat flux is computed with 3 different estimates of surface currents (us, vs):

  • in "fluxNOCUR", us = vs = 0 (absolute winds).
  • in "flux", us = mean zonal currents from Johnson et al 2002 & vs = 0.
  • in "fluxwcur", us, vs = daily averaged ADCP top bin at 35 m. Note, that after the end of 2001, there is no ADCP data so I used the mean from the previous 35 m depth currents. This mean U & V is different than the Johnson et al. value.

    Fig. 1. top panel: "Flux" - "Fluxwcur", i.e. QE(w/Johnson et al. mean current) - QE(w/ ADCP);
    bottom panel: "FluxNOCUR" - "Fluxwcur", i.e. QE(w/o current) - QE(w/ADCP).

    This figure shows that your choice of surface velocities can make upto a 40 W/m2 difference in the latent heat flux, although more typical differences were less than 10 W/m2. The surface currents can induce a ~10 W/m2 seasonal cycle in the latent heat flux.

  • Meghan F. Cronin
    Pacific Marine Environmental Laboratory
    7600 Sand Point Way NE
    Seattle, WA 98115 USA
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