Climatologically Significant Effects of Some Approximations in the Bulk Parameterizations of Turbulent Air–Sea Fluxes

Abstract

This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulas on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments is examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the period 1982–2014 are computed using SST and surface meteorological state variables from ERA-Interim. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulas. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically on the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%) but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the salt-related reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid- and subpolar latitudes.

Recipes for How to Force Oceanic Model Dynamics

“…In order to run forced ocean simulations with characteristics as close as possible to the coupled model simulations, we had to modify the bulk formula available in NEMO 3.4. To do so, we used the AeroBulk package (Brodeau et al, , now included in NEMO v4), which offers the possibility to use the bulk formula COARE v3.0 (Fairall et al, ), as in our coupled simulations that use the WRF bulk formulae. We also used hourly forcing fields, which corresponds to the coupling frequency of the coupled simulations.…”

Air‐Sea Turbulent Fluxes From a Wave‐Following Platform During Six Experiments at Sea

“…Back to the science issue raised in section 1, our data suggest that the discrepancy of 10% found by Brodeau et al (2017) who compared wind stress from two known bulk algorithms is rather optimistic compared to the achievable accuracy with modern sensors and platforms. Indeed, the 7-10% confidence on friction velocity found with our data has to be doubled to get its equivalent in terms of wind stress.…”

Sensitivity analysis and metamodels for the bulk parametrization of turbulent air–sea fluxes

“…The iterative bulk formulae of reference have been computed using the aerobulk package (https://brodeau.github.io/aerobulk/; Brodeau et al. , ) which gathers CORE, COARE and ECMWF algorithms in a common computational framework. This package is increasingly used for ocean‐only global simulations and has been treated here as a ‘black box’.…”

“…, ; Brodeau et al. , ). If the maximum number of iterations is too small to converge, the turbulent fluxes thus computed may be inconsistent with MO theory.…”

“…It is worth noting at this point that relatively large uncertainties (20-30%) still persist within turbulent air-sea fluxes parametrizations (hereafter bulk formulae; e.g. Brodeau et al, 2017).…”