Abstract
Abstract. A simplified model of the atmospheric boundary layer (ABL) of intermediate complexity between a bulk parameterization and a three-dimensional atmospheric model is developed and integrated to the Nucleus for European Modelling of the Ocean (NEMO) general circulation model. An objective in the derivation of such a simplified model, called ABL1d, is to reach an apt representation in ocean-only numerical simulations of some of the key processes associated with air–sea interactions at the characteristic scales of the oceanic mesoscale. In this paper we describe the formulation of the ABL1d model and the strategy to constrain this model with large-scale atmospheric data available from reanalysis or real-time forecasts. A particular emphasis is on the appropriate choice and calibration of a turbulent closure scheme for the atmospheric boundary layer. This is a key ingredient to properly represent the air–sea interaction processes of interest. We also provide a detailed description of the NEMO-ABL1d coupling infrastructure and its computational efficiency. The resulting simplified model is then tested for several boundary-layer regimes relevant to either ocean–atmosphere or sea-ice–atmosphere coupling. The coupled system is also tested with a realistic 0.25∘ resolution global configuration. The numerical results are evaluated using standard metrics from the literature to quantify the wind–sea-surface-temperature (a.k.a. thermal feedback effect), wind–current (a.k.a. current feedback effect), and ABL–sea-ice couplings. With respect to these metrics, our results show very good agreement with observations and fully coupled ocean–atmosphere models for a computational overhead of about 9 % in terms of elapsed time compared to standard uncoupled simulations. This moderate overhead, largely due to I/O operations, leaves room for further improvement to relax the assumption of horizontal homogeneity behind ABL1d and thus to further improve the realism of the coupling while keeping the flexibility of ocean-only modeling.
Highlights
Owing to advances in computational power, global oceanic models used for research or operational purposes are configured with increasingly higher horizontal and vertical resolution, resolving the baroclinic deformation radius in the tropics (e.g., Deshayes et al, 2013; Metzger et al, 2014; von Schuckmann et al, 2018)
A crucial hypothesis is that the dominant process at the characteristic scale of the oceanic mesoscale is the so-called downward mixing process which stems from a modulation of atmospheric turbulence by sea surface temperature (SST) anomalies
Our approach can be seen as an extended bulk approach: instead of prescribing atmospheric quantities at 10 m to compute air–sea fluxes via an atmospheric surface layer (ASL) parameterization, atmospheric quantities in the first few hundred meters are used to constrain an atmospheric boundary layer (ABL) model which provides 10 m atmospheric values to the ASL parameterization
Summary
Owing to advances in computational power, global oceanic models used for research or operational purposes are configured with increasingly higher horizontal and vertical resolution, resolving the baroclinic deformation radius in the tropics (e.g., Deshayes et al, 2013; Metzger et al, 2014; von Schuckmann et al, 2018). Fine-scale local models are routinely used to simulate submesoscales, which occur on scales on the order of 0.1–20 km horizontally, and their impact on larger scales (e.g., Marchesiello et al, 2011; McWilliams et al, 2019). By increasing the oceanic model resolution, small-scale features are explicitly resolved, but an apt representation of the associated processes re-. F. Lemarié et al.: Development of a two-way coupled ocean-wave model quires the relevant scales to be present in the surface forcings including the proper interaction with the low-level atmosphere
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