Aerosol air pollution is related to the climate changes occurred on planet Earth in recent decades. The aerosol particles modify the global energy balance through the absorption and deviation of solar and terrestrial radiation (direct effects), and influence the formation of clouds and precipitation by acting as condensation nuclei for water vapour (indirect effects). Mineral dust represents more than 75% of the aerosols present in the atmosphere. The 50% of them come from the Sahara desert, 10% of which is transported to the Mediterranean basin. To date, the quantification of the radiative effects of aerosols in the Mediterranean region is at an early stage and the consequent effects on the regional climate and meteorology have yet to be well defined. In order to deepen this topic, the storm "Vaia" was investigated, since it caused serious damage due to strong gusts of wind and abundant rainfall over Italy between 27 and 30 October 2018. The event was also characterized by high humidity and an intense intrusion of Saharan dust. Different simulations were performed with the WRF-Chem model, implementing the same setup for microphysics, but using a different scheme for the emission of dust (GOCART / AFWA and Shao 2004). The fields relating to wind, water vapor and rainfall were investigated, while information on the load of Saharan dust was obtained by analyzing the AOD (Aerosol Optical Depth). The results obtained by the simulations were compared with observational data to assess their consistency, and from this the following information emerged: 1. Underestimation of the wind speed at 10 m; due to an error in the parameterization of marine roughness, which does not consider the presence of sea-spray in strong wind conditions; 2. Slight overestimation of rainfall; since the model did not correctly consider the indirect effect exerted by Saharan dust on atmospheric microphysics; 3. The high values of AOD are better simulated by the AFWA scheme, while Shao (2004) tends to underestimate them; on the contrary, the relatively lower values are better reproduced by the Shao scheme (2004), while they are overestimated by the AFWA scheme. This overestimation is caused by the dependency of the dust vertical flow on the clay content only, while the Shao scheme (2004) adds some corrections, such as the vegetative cover and the non-erodible surface elements, leading to lower estimates of dust emissions. Although these quantitative inaccuracies, WRF-Chem can be considered a valid tool for simulating meteorology and the transport of dust. The existence of an interaction between mineral dust and atmospheric processes has been confirmed, through a simulation performed by adopting the "aerosol-aware Thompson – Eidhammer" microphysics scheme within the WRF-Chem model. This model allowed to quantify the number of dust particles that acted as freezing cores, influencing the development of the meteorological event in question. It is probable that the Mediterranean region has partly contributed to the strength of the storm, having temperatures of 1-2 ° C above the average and a greater availability of energy and water vapour. Investigations by the CNR-ISAC and the UNIVPM research group are in progress to quantify these effects.
L’inquinamento atmosferico da aerosol è correlato ai cambiamenti climatici che si stanno verificando sul pianeta Terra negli ultimi decenni. Le particelle di aerosol modificano il bilancio energetico globale attraverso l’assorbimento e la deviazione delle radiazioni solari e terrestri (effetti diretti), ed influenzano la formazione di nuvole e le precipitazioni agendo come nuclei di condensazione per il vapore d’acqua (effetti indiretti). Le polveri minerali costituiscono più del 75% degli aerosol presenti nell’atmosfera. Il 50% di esse proviene dal deserto del Sahara e di questo il 10% viene trasportato sul bacino del Mediterraneo. Ad oggi, la quantificazione degli effetti radiativi degli aerosol nel Mediterraneo è in una fase iniziale e i conseguenti effetti sul clima regionale e sulla meteorologia devono ancora essere ben definiti. A tal proposito è stato scelto come caso studio la tempesta “Vaia” che tra il 27 e il 30 ottobre 2018 ha imperversato sull’Italia provocando ingenti danni a causa del forte vento e delle abbondanti precipitazioni. L’evento è stato caratterizzato anche da un’elevata umidità e da un’intensa intrusione di polveri Sahariane. Sono state eseguite differenti simulazioni con il modello WRF-Chem, implementando lo stesso setup per la microfisica, ma un diverso schema per l’emissione delle polveri (GOCART/AFWA e quello di Shao del 2004). Sono stati indagati i campi inerenti il vento, il vapore acqueo e le precipitazioni, mentre le informazioni sul carico delle polveri Sahariane sono state ottenute analizzando l’AOD (Aerosol Optical Depth). I risultati ottenuti dalle simulazioni sono stati confrontati con dati sperimentali per valutarne l’attendibilità, e da ciò sono emerse le seguenti informazioni: 1) Sottostima della velocità del vento a 10 m; dovuta ad un errore nella parametrizzazione della rugosità marina che non considera la presenza di sea-spray in condizioni di vento forte; 2) Leggera sovrastima delle precipitazioni; poiché il modello non ha correttamente considerato l’effetto indiretto esercitato dalle polveri Sahariane sulla microfisica atmosferica; 3) I valori elevati di AOD sono meglio simulati dallo schema AFWA, mentre Shao (2004) tende a sottostimarli; al contrario i valori più bassi sono riprodotti in maniera migliore dallo schema Shao (2004), mentre sono sovrastimati dallo schema AFWA. Tale sovrastima è legata al fatto che lo schema AFWA considera il flusso verticale di polveri dipendente solo dal contenuto di argilla, mentre lo schema Shao (2004) aggiunge alcune correzioni, come la copertura vegetativa e gli elementi a superficie non erodibile, portando a stime inferiori delle emissioni di polvere. Seppur con queste imprecisioni quantitative, WRF-Chem può essere ritenuto un valido strumento di simulazione della meteorologia e del trasporto di polveri. È stata confermata l’esistenza di un’interazione tra le mineral dust e i processi atmosferici, attraverso una simulazione eseguita adottando come schema per la microfisica quello di “Thompson–Eidhammer” all’interno del modello WRF-Chem. Ciò ha permesso di quantificare il numero di particelle di polvere che hanno agito come nuclei di ghiacciamento, influenzando lo sviluppo dell’evento meteorologico in esame. È probabile che alla violenza della tempesta abbia contribuito anche la regione Mediterranea, avente temperature di 1-2 °C sopra la media e una maggiore disponibilità di energia e vapore acqueo. Riguardo ciò sono in fase di progresso le indagini da parte del CNR-ISAC ed il gruppo di ricerca dell’UNIVPM.
RUOLO DELLE POLVERI SAHARIANE SUGLI EPISODI METEOROLOGICI ESTREMI NEL MEDITERRANEO
BREGA, ELEONORA
2018/2019
Abstract
Aerosol air pollution is related to the climate changes occurred on planet Earth in recent decades. The aerosol particles modify the global energy balance through the absorption and deviation of solar and terrestrial radiation (direct effects), and influence the formation of clouds and precipitation by acting as condensation nuclei for water vapour (indirect effects). Mineral dust represents more than 75% of the aerosols present in the atmosphere. The 50% of them come from the Sahara desert, 10% of which is transported to the Mediterranean basin. To date, the quantification of the radiative effects of aerosols in the Mediterranean region is at an early stage and the consequent effects on the regional climate and meteorology have yet to be well defined. In order to deepen this topic, the storm "Vaia" was investigated, since it caused serious damage due to strong gusts of wind and abundant rainfall over Italy between 27 and 30 October 2018. The event was also characterized by high humidity and an intense intrusion of Saharan dust. Different simulations were performed with the WRF-Chem model, implementing the same setup for microphysics, but using a different scheme for the emission of dust (GOCART / AFWA and Shao 2004). The fields relating to wind, water vapor and rainfall were investigated, while information on the load of Saharan dust was obtained by analyzing the AOD (Aerosol Optical Depth). The results obtained by the simulations were compared with observational data to assess their consistency, and from this the following information emerged: 1. Underestimation of the wind speed at 10 m; due to an error in the parameterization of marine roughness, which does not consider the presence of sea-spray in strong wind conditions; 2. Slight overestimation of rainfall; since the model did not correctly consider the indirect effect exerted by Saharan dust on atmospheric microphysics; 3. The high values of AOD are better simulated by the AFWA scheme, while Shao (2004) tends to underestimate them; on the contrary, the relatively lower values are better reproduced by the Shao scheme (2004), while they are overestimated by the AFWA scheme. This overestimation is caused by the dependency of the dust vertical flow on the clay content only, while the Shao scheme (2004) adds some corrections, such as the vegetative cover and the non-erodible surface elements, leading to lower estimates of dust emissions. Although these quantitative inaccuracies, WRF-Chem can be considered a valid tool for simulating meteorology and the transport of dust. The existence of an interaction between mineral dust and atmospheric processes has been confirmed, through a simulation performed by adopting the "aerosol-aware Thompson – Eidhammer" microphysics scheme within the WRF-Chem model. This model allowed to quantify the number of dust particles that acted as freezing cores, influencing the development of the meteorological event in question. It is probable that the Mediterranean region has partly contributed to the strength of the storm, having temperatures of 1-2 ° C above the average and a greater availability of energy and water vapour. Investigations by the CNR-ISAC and the UNIVPM research group are in progress to quantify these effects.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12075/6485