Abstract
To improve high-resolution numerical environmental prediction, it is essential to represent ocean-atmosphere interactions properly, which is not the case in current operational regional forecasting systems used in Western Europe. The objective of this paper is to present a new forecast-oriented coupled ocean-atmosphere system and its evaluation. This system uses the state-of-the-art numerical models AROME (cy43t2) and NEMO (v3.6) with a horizontal resolution of 2.5 km. The OASIS coupler (OASIS3MCT-4.0), implemented in the SurfEX surface scheme and in NEMO, is used to perform the communications between models. The evaluation of this system is carried out using 7-day simulations from 12 to 19 October 2018, characterised by extreme weather events (storms and heavy precipitation event) in the area of interest. Comparisons with in-situ and L3 satellite observations show that the fully coupled simulation reproduces quantitatively well the spatial and temporal evolution of the sea surface temperature and 10 m wind speed. Sensitivity analysis to OA coupling show that the use of an interactive and high resolution SST, in contrast to actual NWP where SST is persistent and at low resolution, modifies the atmospheric circulation and the location of heavy precipitation. When compared to the operational-like ocean forecast, simulated oceanic fields show a large sensitivity to coupling. Forced ocean simulations highlight that this sensitivity is mainly controlled by the change in the atmospheric model used to drive NEMO (AROME vs. ECMWF IFS operational forecast). The oceanic boundary layer depths can vary by more than 40%. This impact is amplified by the interactive coupling and is attributed to positive feedback between sea surface cooling and evaporation.
Highlights
Ocean-atmosphere feedbacks occur over a wide range of spatial and temporal scales
A new domain over Western Europe, including the two domains used for high resolution atmospheric and oceanic forecasts at Météo-France and Mercator 455 Ocean International (MOI) respectively, has been designed
This coupled system was evaluated through 7-day simulations performed around an October 2018 study case
Summary
Ocean-atmosphere feedbacks occur over a wide range of spatial and temporal scales. They play a critical role in the evolution of climate (Intergovernmental Panel on Climate Change, 2014) and in the evolution of smaller spatial and temporal 45 scales phenomena like tropical cyclones (Bender and Ginis, 2000; Smith et al, 2009; Jullien et al, 2014), mid-latitudes storms (Mogensen et al, 2018; Bouin and Lebeaupin Brossier, 2020b), sometimes leading to heavy precipitation events as for instance in the Mediterranean region (Rainaud et al, 2017; Meroni et al, 2018), dense water formation (Carniel et al, 2016; Lebeaupin Brossier et al, 2017), and ocean dynamics in particular in response to strong wind (e.g. Pullen et al, 2006; Small et al, 2012; Renault et al, 2019b; Jullien et al, 2020). This region is characterised by fine-scale ocean structures: estuaries and regions of freshwater influence related to large river plums (e.g. Simpson et al, 1993; Brenon and Le Hir, 1999; Estournel et al, 2001; Bergeron, 2004); thermal fronts notably in the French Atlantic continental shelf area (Yelekçi et al, 2017) and in particular the Ushant front of tidal origin (Chevallier et al, 2014; Redelsperger et al, 75 2019), or the North Balearic Front in the Western Mediterranean Sea (García et al, 1994); slope current, wind-driven circulation and mesoscale eddies in the Bay of Biscay (van Aken, 2002; Le Boyer et al, 2013); gyres in the Alboran Sea (Viúdez et al, 1998); meanders of the Algerian Current and eddies (Millot et al, 1990; Millot and Taupier-Letage, 2005); shelf circulation, cyclonic gyre, ocean deep convective area and Northern Current in the Gulf of Lions (e.g. Millot, 1991; Echevin et al, 2003; Testor et al, 2018; Carret et al, 2019) It is frequently affected by several kinds of 80 natural hazards of weather origin: strong wind related to storm, cyclogenesis (Trigo et al, 2002; Trigo, 2006) with for some cases an explosive development (Liberato et al, 2013) or even tropical-like characteristics (namely medicanes, Miglietta and Rotunno, 2019), sometimes interacting locally with the coast and/or orography (like mistral and tramontane, Bastin et al, 2006; Obermann et al, 2018); thunderstorms (Taszarek et al, 2019) including Mediterranean heavy precipitation events with floods (Ducrocq et al, 2016); heat waves (De Bono et al, 2004; Darmaraki et al, 2019; Ma et al, 2020); on which ocean atmosphere interactions play a significant role. Better representing the air-sea feedback that occurs at fine-scale in this area is relevant and developing a dedicated ocean-atmosphere coupled prediction system appears essential to improve the high-resolution regional forecasts on both sides
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