High‐resolution air–sea coupling impact on two heavy precipitation events in the Western Mediterranean

Abstract

The Mediterranean Sea is an important source of heat and moisture for heavy precipitation events (HPEs). Moreover, the ocean mixed layer (OML) evolves rapidly under such intense events. Whereas short‐term numerical weather prediction systems generally use low‐resolution non‐evolving sea surface temperature (SST), the development of high‐resolution high‐frequency coupled system allows us to fully take into account the fine‐scale interactions between the low‐level atmosphere and the OML which occur during HPEs. The aim of this study is to investigate the impact of fine‐scale air–sea interactions and coupled processes involved during the HPEs which occurred during 12–15 October 2012 (IOP13) and 26–28 October 2012 (IOP16a/b) of the HyMeX first field campaign. For that purpose, the high‐resolution coupled system AROME‐NEMO WMED was developed. This system is based on the 2.5 km‐resolution non‐hydrostatic convection‐permitting atmospheric model AROME‐WMED and the 1/36°‐resolution NEMO‐WMED36 ocean model. The coupling frequency is 1 h. To distinguish the effects due to the change in the initial SST field from that due to the interactive 3D ocean, the coupled run is compared to two AROME‐WMED atmosphere‐only experiments with no SST evolution during the 48 h forecast cycles—one using the AROME‐WMED SST analysis, the second using the SST field of the coupled experiment each day at 0000 UTC. The results of the three experiments re‐assert that the SST initial condition strongly influences the HPE forecast, in terms of intensity and location. With water budget analyses, the significant impact of the ocean interactive evolution on the surface evaporation water supply for HPEs is also highlighted. In cases of strong and intense air–sea exchanges, as in the mistral event of IOP16b, the coupling reproduces the intense and rapid surface cooling and demonstrates the importance of representing the ocean turbulent mixing with entrainment at the OML base.

Role of SST Patterns on Surface Wind Modulation of a Heavy Midlatitude Precipitation Event

“…In their numerical simulations of an explosively developing storm over the eastern Mediterranean, they show that when they change the SST forcing with four different realistic fields, the maxima and the total amount of the precipitation are unaffected. The location of the rain bands and the spatial structure of the local heat fluxes, instead, show a significant dependence on the SST, which is also found in the coupled atmosphere‐ocean numerical simulations of Ricchi et al () and Rainaud et al ().…”

SURFEX v8.0 interface with OASIS3-MCT to couple atmosphere with hydrology, ocean, waves and sea-ice models, from coastal to global scales

“…The low-level wind and temperature are close in the two experiments and in agreement with observations, especially for short-range (0-24 h) forecasts. So, the impact of the interactive coupled ocean can be significant for intense weather situations with abrupt changes in the wind speed and/or in the surface fluxes, especially for longer-term forecasts (24-48 h; Rainaud et al, 2017). Further investigations are ongoing to evaluate the coupled processes' impact on other case studies.…”

“…The first ocean-atmosphere high-resolution coupled systems appeared at the beginning of the 2000s to study these interactions (e.g. Pullen et al, 2003;Ren et al, 2004;Loglisci et al, 2004). Nowadays, the development of highresolution, short-to medium-range coupled prediction systems is still challenging, but several groups have undertaken it, based on coupling methods with different levels of sophistication and targeting a large range of applications depending on their interests (Brassington et al, 2015;Heinzeller et al, 2016;Hewitt et al, 2016;Janssen et al, 2013).…”

Impact of two‐way coupling and sea‐surface temperature on precipitation forecasts in regional atmosphere and ocean models

“…The regional atmosphere and the ocean circulation are often modelled separately (limitations of this approach are discussed by Rainaud et al ()) but can also be fully coupled to describe the interaction at the models' boundaries in a balanced and energy‐consistent way (e.g. Rainaud et al ; Wilkin et al ; Lewis et al ). Whereas the earlier approach typically implies constant (static) SST during the atmospheric model forecast, the latter approach allows for the improved description of fast transient processes in the planetary boundary layer.…”

“…The impact of SST on simulation of heavy precipitation events (HPEs) with maximum precipitation exceeding 100 mm in the Mediterranean and Adriatic Seas has been addressed by few recent studies. Whereas the studies of Ivatek‐Šahdan et al () and Stocchi and Davolio () tested the sensitivity of NWP model on SST by applying various SST sources and introduced artificial changes to SST fields, Rainaud et al () applied two‐way coupling of the ocean and NWP models. In this paper, we use a similar approach to the latter work by applying the recently developed two‐way coupled ocean and atmosphere modelling system developed at the Slovenian Environment Agency (ARSO) in cooperation with the National Institute of Biology (Ličer et al ).…”