Occasionally, storm that shares many features with tropical cyclones, including the presence of a warm core, are observed in the Mediterranean. Sometimes, they are called Medicanes. Due to the intense wind forcing and the consequent development of high wind waves, a large number of sea spray droplets are likely to be produced at the sea surface. In this work, sea spray droplets refer to film, jet and spume droplets. The typical radii at formation of bubble-mediated film and jet droplets are typically less than 5 and 20 ?m, respectively. Spume droplets, which torn off from the crest of breaking waves, are larger, with minimum radius at formation generally about 20 ?m, and a maximum one around 500 ?m. The spray droplets with radii between 10 and 300 ?m (Zhao et al., 2006) contribute most to the air-sea exchange processes of momentum and heat. At this purpose we implemented the Sea Spray Source Function (SSSF) of Wan et al. (2017) in WRF-Chem model version 3.6.1 using the GOCART aerosol sectional module. This SSSF simulates the emission of spume droplets in addition to film and jet droplets, only these last ones are taken into account by still commonly used SSSFs. Traditionally, air-sea momentum flux is computed through the classical Charnock relation (1955) that does not explicitly consider the wave-state and sea spray effects on sea surface roughness. In order to take into account both effects, we implemented the Liu et al. (2011) parameterization of sea surface aerodynamic roughness within the WRF surface layer model (sf_sfclay_physics). It is based on the Volkov (2001) and Makin (2005) parametrizations and may be applicable to both moderate and high wind conditions. This work represents an integrated numerical study utilising the third-generation wave model (SWAN) two-way coupled with the Weather Research and Forecasting Model, running in both stand-alone configuration (WRF) and integrated with the chemistry package (WRF-Chem, modified as described above). The latter is configured using GOCART aerosol module, which explicitly consider the emission and transport of sea spray aerosol. The operative sequence is performed considering first the offline coupling SWAN with WRF, and then the two-way coupling SWAN with WRF-Chem. With this sequence, there is a full consistency between the wind field and wave geometry. A test is performed considering the Medicane occurred in South-Eastern Italy on September 26, 2006. This Medicane is one of the most deeply analyzed in literature, so that an intensive investigation of the feedbacks between sea-spray, drag coefficients and latent heat flux may be made considering our integrated approach in comparison with its known features.
Numerical simulation of a Mediterranean Hurricane coupling SWAN and WRF-Chem models: investigation of feedbacks among sea spray, drag coefficients, and roughness length
Umberto RIZZA;Elisa CANEPA;Antonio RICCHI;Sandro CARNIEL;
2018
Abstract
Occasionally, storm that shares many features with tropical cyclones, including the presence of a warm core, are observed in the Mediterranean. Sometimes, they are called Medicanes. Due to the intense wind forcing and the consequent development of high wind waves, a large number of sea spray droplets are likely to be produced at the sea surface. In this work, sea spray droplets refer to film, jet and spume droplets. The typical radii at formation of bubble-mediated film and jet droplets are typically less than 5 and 20 ?m, respectively. Spume droplets, which torn off from the crest of breaking waves, are larger, with minimum radius at formation generally about 20 ?m, and a maximum one around 500 ?m. The spray droplets with radii between 10 and 300 ?m (Zhao et al., 2006) contribute most to the air-sea exchange processes of momentum and heat. At this purpose we implemented the Sea Spray Source Function (SSSF) of Wan et al. (2017) in WRF-Chem model version 3.6.1 using the GOCART aerosol sectional module. This SSSF simulates the emission of spume droplets in addition to film and jet droplets, only these last ones are taken into account by still commonly used SSSFs. Traditionally, air-sea momentum flux is computed through the classical Charnock relation (1955) that does not explicitly consider the wave-state and sea spray effects on sea surface roughness. In order to take into account both effects, we implemented the Liu et al. (2011) parameterization of sea surface aerodynamic roughness within the WRF surface layer model (sf_sfclay_physics). It is based on the Volkov (2001) and Makin (2005) parametrizations and may be applicable to both moderate and high wind conditions. This work represents an integrated numerical study utilising the third-generation wave model (SWAN) two-way coupled with the Weather Research and Forecasting Model, running in both stand-alone configuration (WRF) and integrated with the chemistry package (WRF-Chem, modified as described above). The latter is configured using GOCART aerosol module, which explicitly consider the emission and transport of sea spray aerosol. The operative sequence is performed considering first the offline coupling SWAN with WRF, and then the two-way coupling SWAN with WRF-Chem. With this sequence, there is a full consistency between the wind field and wave geometry. A test is performed considering the Medicane occurred in South-Eastern Italy on September 26, 2006. This Medicane is one of the most deeply analyzed in literature, so that an intensive investigation of the feedbacks between sea-spray, drag coefficients and latent heat flux may be made considering our integrated approach in comparison with its known features.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
