Influence of Wave State and Sea Spray on the Roughness Length: Feedback on Medicanes

Laura Santiloni,Umberto Rizza,Mauro Morichetti,Franciano Scremin Puhales,Davide Bonaldo,Mario Miglietta,Giorgio Passerini,Elisa Canepa,Sandro Carniel,Antonio Ricchi

doi:10.3390/atmos9080301

Abstract

Occasionally, storms that share many features with tropical cyclones, including the presence of a quasi-circular “eye” a warm core and strong winds, are observed in the Mediterranean. Generally, they are known as Medicanes, or tropical-like cyclones (TLC). Due to the intense wind forcings and the consequent development of high wind waves, a large number of sea spray droplets—both from bubble bursting and spume tearing processes—are likely to be produced at the sea surface. In order to take into account this process, we implemented an additional Sea Spray Source Function (SSSF) in WRF-Chem, model version 3.6.1, using the GOCART (Goddard Chemistry Aerosol Radiation and Transport) aerosol sectional module. Traditionally, air-sea momentum fluxes are computed through the classical Charnock relation that does not consider the wave-state and sea spray effects on the sea surface roughness explicitly. In order to take into account these forcing, we implemented a more recent parameterization of the sea surface aerodynamic roughness within the WRF surface layer model, which may be applicable to both moderate and high wind conditions. The implemented SSSF and sea surface roughness parameterization have been tested using an operative model sequence based on COAWST (Coupled Ocean Atmosphere Wave Sediment Transport) and WRF-Chem. The third-generation wave model SWAN (Simulating Waves Nearshore), two-way coupled with the WRF atmospheric model in the COAWST framework, provided wave field parameters. Numerical simulations have been integrated with the WRF-Chem chemistry package, with the aim of calculating the sea spray generated by the waves and to include its effect in the Charnock roughness parametrization together with the sea state effect. A single case study is performed, considering the Medicane that affected south-eastern Italy on 26 September 2006. Since this Medicane is one of the most deeply analysed in literature, its investigation can easily shed some light on the feedbacks between sea spray and drag coefficients.

Highlights

IntroductionAtmosphere 2018, 9, 301 like the presence of a central “eye” strong rotational winds, a warm core, weak vertical wind shear, a cold upper level trough in the early stages
In the last decades, satellite images have revealed the presence of intense vortices in the Mediterranean that show some visual, dynamic and thermodynamic similarities with tropical cyclones, Atmosphere 2018, 9, 301; doi:10.3390/atmos9080301 www.mdpi.com/journal/atmosphereAtmosphere 2018, 9, 301 like the presence of a central “eye” strong rotational winds, a warm core, weak vertical wind shear, a cold upper level trough in the early stages
A single case study is considered in order to study the effect of both sea spray and wave-state on the sea surface roughness for the simulation of a tropical-like cyclone in the Mediterranean Sea. These two forcings cannot be neglected, taking into account the high wind speed reached during the event; in particular, we propose the inclusion of these two terms in the surface layer parameterization scheme of the WRF model, providing a more realistic simulation of the air-sea interaction processes

Summary

Introduction

Atmosphere 2018, 9, 301 like the presence of a central “eye” strong rotational winds, a warm core, weak vertical wind shear, a cold upper level trough in the early stages These cyclones are generally known as Medicanes (acronym for Mediterranean hurricanes) [1] or Mediterranean tropical-like cyclones (TLC). Baroclinic instability is important in the early phase of their lifetime, when they generally show extra-tropical features [1], while barotropic instability may play a relevant role in their mature stage [2] Their development depends critically on the intensity of sea surface fluxes and latent heat release associated to convection, like for tropical cyclones [5,6]. The evolution of their characteristics, and in particular the occurrence of a tropical transition, can be analysed either using the Hart diagram [7,14,15] or by comparing the advective and diabatic terms in the surface pressure tendency equation [16]

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Results

Conclusion