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.
Highlights
With the advent of remote sensing, some peculiar vortices having characteristics very similar to tropical cyclones have been detected over the Mediterranean Sea [1,2] and are named TropicalLike Cyclones (TLCs), or Medicanes (Mediterranean Hurricanes)
While a thorough investigation of the physical processes and their modeling characterization is left to a dedicated forthcoming paper, here we focus on some key quantities of interest for operational forecasting and emergency management, such as the trajectory and intensity of the pressure minimum, and the wind speed
During the genesis of ROLF, the western Mediterranean sub-basin between the Balearic Islands, Sardinia and the Gulf of Lion was characterized by a sea surface temperatures (SSTs) anomaly of around +3 ◦ C with respect to the climatology 1955–2004 produced by the Mediterranean forecasting System (MFS INGV, [24]) (Figure 3)
Summary
With the advent of remote sensing, some peculiar vortices having characteristics very similar to tropical cyclones have been detected over the Mediterranean Sea [1,2] and are named TropicalLike Cyclones (TLCs), or Medicanes (Mediterranean Hurricanes). TLCs have a typical diameter of 100–300 km [3,4,5,6], while the associated surface wind speeds can occasionally reach hurricane 1 strength according to Saffir-Simpson scale These cyclones may last for several days, the presence of tropical characteristics may be limited to a few hours [4,7,8]. They form predominantly during the end of summer or during autumn, when the latent and sensible heat fluxes are more intense [9] due to the relatively high sea surface temperatures (SSTs) combined with upper-level cold-air intrusions [10]. A typical feature of Medicanes is the presence of a warm core developing over the whole extension of the troposphere, associated with the release of latent heat caused by the convection around the pressure minimum [2,10]
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