Bulk transfer coefficients and dissipation‐derived fluxes in low wind speed conditions over the western equatorial Pacific Ocean

Abstract

Bulk transfer coefficients are derived using latent heat flux, stress magnitude, and mean quantity data obtained aboard a National Oceanic and Atmospheric Administration research aircraft flying over the far western equatorial Pacific Ocean. At wind speeds less than about 4 rn s -• the bulk transfer coefficients increase rapidly with decreasing wind speed. The validity of the inertial dissipation method for deriving turbulent fluxes in low wind speed conditions is tested using the aircraft data. At wind speeds below 4 rn s-• dissipation-derived fluxes tend to be smaller than the corresponding eddy correlation fluxes by about 10-20%.

Inertial dissipation method applied to derive turbulent fluxes over the ocean during the Surface of the Ocean, Fluxes and Interactions with the Atmosphere/Atlantic Stratocumulus Transition Experiment (SOFIA/ASTEX) and Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiments with low to moderate wind speeds

“…This constant value can be explained by the approach to free convection limits where u * is no longer a relevant scale. The magnitude of the minimum friction velocity found here is the same as that obtained by Greenhut and Khalsa [1995]. A similar value was also indicated by the data of YT96, although those authors questioned the validity of their estimates at very low wind speeds, and the effects of instrument noise and other factors cannot be totally discounted.…”

“…For the low wind speed range down to 0.1 m/s our study leads to very high drag coefficients. Ca,, values are found to be higher by a factor of 10-100 compared to some other studies [Cardone, 1969] but are found to be in good agreement with Greenhut and Khalsa [1995] and YT96. The C e,, values for heat fluxes also follow the same tendency, but the increase is not as great as for the Ca,, at low wind speeds.…”

“…When these iterative processes are applied, a problem of convergence occurs in some cases. For example, Greenhut and Khalsa [1995], investigating the turbulent fluxes at low wind speeds in the western equatorial Pacific Ocean (presumably at unstable stratification), mentioned the problem of convergence of the inertial dissipation method. Although they did not discuss the effect of the imbalance term separately, they remarked that In the next sections we shall consider this problem of convergence and see if solutions of systems (2) and (3) can always be found, especially at low wind speeds or unstable stratification.…”

Evaluation of turbulent fluxes at the ocean surface using surface renewal theory

“…When T s is not given, (22) is solved iteratively with (10), (11), (12), (14), (20), and (21). A special area of concern is the reliability of the model at low wind speeds, where fluxes have been shown to have unusual characteristics [e.g., Large and Pond, 1982;Bradley et al, 1991' Greenhut andKhalsa, 1995]. Additionally, a concern exists at the higher wind speeds because of the possible influence of sea spray.…”

Air-sea fluxes: 25 years of progress

“…Based on wind-tunnel experiments a value of /3 = 0.11 has been used (e.g. Smith, 1988), but recent field experiments (Bradley et al, 199 1;Greenhut and Khalsa, 1995) indicate a larger value due to fluxes driven by gusts of convective winds. For wind speeds between 3 and 20 m s-' at a 10 m reference height, decades of field programs have not succeeded in clearly demonstrating an open-ocean neutral transfer coefficient for heat and moisture that differs significantly from 1.1 x 10e3 f 15% (Smith, 1989;Garratt, 1992;DeCosmo et al, 1996).…”