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

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 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 0.25&degree; 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.

A case‐study of the coupled ocean–atmosphere response to an oceanic diurnal warm layer

“…A Lagrangian approach has also been proposed by Sandu and Stevens (2011) or Lemarié et al . (2020), with an air mass advected over a changing SST, mimicking the presence of an SST front. Concerning the upper oceanic layer, most numerical studies focusing on the DWL mechanisms are based on one‐dimensional oceanic models such as Shinoda (2005) and Karagali et al .…”

“…More recently, case-studies of cold air outbreaks have been performed with a prescribed time-varying SST in order to represent the transport of a cold and dry air mass over a warmer sea (e.g., Roode et al, 2019 in the CONSTRAIN case;Brilouet et al, 2020 in a HyMeX 1 case). A Lagrangian approach has also been proposed by Sandu and Stevens (2011) or Lemarié et al (2020), with an air mass advected over a changing SST, mimicking the presence of an SST front. Concerning the upper oceanic layer, most numerical studies focusing on the DWL mechanisms are based on one-dimensional oceanic models such as Shinoda (2005) and Karagali et al (2017) exploring the role of the solar radiation, Giglio et al (2017) focusing on the effect of wind gusts on diurnal variability, and Reffray et al (2015) studying the sensitivity to the turbulent vertical mixing.…”

Jarzynski equality and Crooks relation for local models of air-sea interaction

“…The vertical viscosity in the atmosphere is calculated by a Turbulent Kinetic Energy (TKE) scheme with a shear-based length scale (see Sec. 4.1 in Lemarié et al, 2020) and in the ocean a K-Profile Parameterization (KPP, see Sec. 2c in McWilliams and Huckle, 2006) is used.…”

“…The numerical model to solve the above discussed equations, is a variation of the one used in (Lemarié et al, 2020). There are 20 levels in the atmosphere and 20 in the ocean, with first grid points at δ a = 5m and δ o = 1m, in the atmosphere and the ocean, respectively.…”