A simplified atmospheric boundary layer model for an improvedrepresentation of air-sea interactions in eddying oceanic models:implementation and first evaluation in NEMO (4.0)

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 simplified model called ABL1d is to reach an apt representation in ocean-only numerical simulations of some of the key processes associated to 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 0.25d 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/currents (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 term 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 modelling.