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Abstract:In November 2011, an Atlantic depression affected the Mediterranean basin, eventually evolving into a Tropical-Like Cyclone (TLC or Mediterranean Hurricane, usually designated as Medicane). In the region affected by the Medicane, mean sea level pressures down to 990 hPa, wind speeds of hurricane intensity close to the eye (around 115 km/h) and intense rainfall in the prefrontal zone were reported. The intensity of this event, together with its long permanence over the sea, suggested its suitability as a paradigmatic case for investigating the sensitivity of a numerical modeling system to different configurations, air-sea interface parameterizations and coupling approaches. Toward this aim, a set of numerical experiments with different parameterization schemes and levels of coupling complexity was carried out within the Coupled Ocean Atmosphere Wave Sediment Transport System (COAWST), which allows the description of air-sea dynamics by coupling an atmospheric model (WRF), an ocean circulation model (ROMS), and a wave model (SWAN). The sensitivity to different initialization times and Planetary Boundary Layer (PBL) parameterizations was firstly investigated by running a set of WRF standalone (atmospheric-only) simulations. In order to better understand the effect of coupling on the TLC formation, intensification and trajectory, different configurations of atmosphere-ocean coupling were subsequently tested, eventually including the full coupling among atmosphere, ocean and waves, also changing the PBL parameterization and the formulation of the surface roughness. Results show a strong sensitivity of both the trajectory and the intensity of this TLC to the initial conditions, while the tracks and intensities provided by the coupled modeling approaches explored in this study do not introduce drastic modifications with respect to those resulting from a fine-tuned standalone atmospheric run, though they provide by definition a better physical and energetic consistency. Nevertheless; the use of different schemes for the calculation of the surface roughness from wave motion, which reflects the description of air-sea interface processes, can significantly affect the results in the fully coupled runs.
Abstract:In November 2011, an Atlantic depression affected the Mediterranean basin, eventually evolving into a Tropical-Like Cyclone (TLC or Mediterranean Hurricane, usually designated as Medicane). In the region affected by the Medicane, mean sea level pressures down to 990 hPa, wind speeds of hurricane intensity close to the eye (around 115 km/h) and intense rainfall in the prefrontal zone were reported. The intensity of this event, together with its long permanence over the sea, suggested its suitability as a paradigmatic case for investigating the sensitivity of a numerical modeling system to different configurations, air-sea interface parameterizations and coupling approaches. Toward this aim, a set of numerical experiments with different parameterization schemes and levels of coupling complexity was carried out within the Coupled Ocean Atmosphere Wave Sediment Transport System (COAWST), which allows the description of air-sea dynamics by coupling an atmospheric model (WRF), an ocean circulation model (ROMS), and a wave model (SWAN). The sensitivity to different initialization times and Planetary Boundary Layer (PBL) parameterizations was firstly investigated by running a set of WRF standalone (atmospheric-only) simulations. In order to better understand the effect of coupling on the TLC formation, intensification and trajectory, different configurations of atmosphere-ocean coupling were subsequently tested, eventually including the full coupling among atmosphere, ocean and waves, also changing the PBL parameterization and the formulation of the surface roughness. Results show a strong sensitivity of both the trajectory and the intensity of this TLC to the initial conditions, while the tracks and intensities provided by the coupled modeling approaches explored in this study do not introduce drastic modifications with respect to those resulting from a fine-tuned standalone atmospheric run, though they provide by definition a better physical and energetic consistency. Nevertheless; the use of different schemes for the calculation of the surface roughness from wave motion, which reflects the description of air-sea interface processes, can significantly affect the results in the fully coupled runs.
The sensitivity to sea surface temperature (SST) of small‐scale, flood‐causing convective systems in Mediterranean coastal areas is analyzed by means of mesoscale numerical simulations. Two different SST initializations are considered: a coarse field provided by a global atmospheric model and a high‐resolution multisatellite analysis. Quantitative precipitation forecasts are evaluated for a number of recent severe rainfall episodes in Liguria (northwestern Italy). In several cases, using a higher‐resolution SST leads to more realistic precipitation estimates in the forecasting range 36–48 h. In the shorter range, the satellite SST has a limited, or even negative, impact, due to the relatively slow adjustment of initial atmospheric fields. In one case, the satellite SST is beneficial for the only run forced with accurate large‐scale initial conditions. The results of this work suggest that a potentially significant improvement in severe precipitation forecasting in the Mediterranean could be achieved using realistic small‐scale SST fields.
The north‐western Mediterranean Sea is a key location for the thermohaline circulation of the basin. The area is characterized by intense air‐sea exchanges favored by the succession of strong northerly and north‐westerly wind situations (mistral and tramontane) in autumn and winter. Such meteorological conditions lead to significant evaporation and ocean heat loss that are well known as the main triggering factor for the Dense Water Formation (DWF) and winter deep convection episodes. During the HyMeX second field campaign (SOP2, 1 February to 15 March 2013), several platforms were deployed in the area in order to document the DWF and the ocean deep convection, as the air‐sea interface conditions. This study investigates the role of the ocean‐atmosphere coupling on DWF during winter 2012–2013. The coupled system, based on the NEMO‐WMED36 ocean model (1/36° resolution) and the AROME‐WMED atmospheric model (2.5 km resolution), was run during 2 months covering the SOP2 and is compared to an ocean‐only simulation forced by AROME‐WMED real‐time forecasts and to observations collected in the north‐western Mediterranean area during the HyMeX SOP2. The comparison shows small differences in terms of net heat, water, and momentum fluxes. On average, DWF is slightly sensitive to air‐sea coupling. However, fine‐scale ocean processes, such as shelf DWF and export or eddies and fronts at the rim of the convective patch, are significantly modified. The wind‐current interactions constitute an efficient coupled process at fine scale, acting as a turbulence propagating vectors, producing large mixing and convection at the rim of the convective patch.
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