The objective of my Ph.D. is to investigate the impact of light-absorbing impurities on the cryosphere using optical remote sensing data. Light-absorbing impurities (LAI) are particulate matter, such as mineral dust and black carbon, that can be deposited on snow and ice, reducing their albedo and accelerating the melt. The impact of LAI on the cryosphere has been studied at a global and regional scale, but still few scientific literature focuses on the European Alps. In the first year, I conducted a sensitivity analysis of a radiative transfer model, the SNow, ICe, and Aerosol Radiative model (SNICAR) in order to study the optical properties of snow and ice. In particular, this model allows to simulate spectral reflectance of snow, as a function of different variables, such as snow grain size [μm], mineral dust concentration [ppm] and dimension [μm], black carbon concentration [ppb], solar zenith angle and snow density, using different atmospheric profiles. During the second year of Ph.D., different field campaigns were organized in order to measure spectral reflectance of snow after LAI depositional events, and to compare observed with simulated spectra. During field campaigns, we flew an Unmanned Aerial Vehicle (UAV) over a flat snow-covered area in the European Alps. Data collected from ground, UAV and satellite (Landsat 8 - Operational Land Imager, OLI) were analysed to estimate the impact of mineral dust on snow optical properties. A novel spectral index non-linearly correlated with mineral dust concentration was proposed and tested at different scales. During the third year, I focused on the impact of LAI on ice in the Alps. Mountain glaciers represent an important source of fresh water across the globe. Those reservoirs are seriously threatened by global climate change, and a widespread reduction of glacier extension has been observed in recent years. Surface processes that promote ice melting are driven both by temperature/precipitation and by albedo. The latter is mainly influenced by the growth of snow grain size and by the impurities content (such as dust, soot, ash, algae etc.). The origin of these light-absorbing impurities can be local or distal; often they aggregate on the glacier tongue forming characteristic cryoconites, that decrease ice albedo promoting the melting. During summer 2015, two field campaigns were conducted at the Vadret da Morteratsch glacier (Swiss Alps). The aim of the campaings was to collect ground hyperspectral reflectance data and ice/snow samples at the glacier ablation zone. During August 2015, the Earth Observing One (EO-1) satellite was planned to acquire a series of scene over the Morteratsch glacier. Furthermore, a Landsat 8 Operational Land Imager (OLI) was downloaded from the Earth Explorer portal. Results from spectra analysis showed interesting features in albedo distribution at Morteratsch glacier. In particular, the ablation area showed very low albedo values (circa 0.2), and this is probably due to multiple processes such as accumulation of particulate matters, collapsing of lateral moraine and debris covering. In addition, the presence of surface cryoconites strongly lowers ice albedo, ground measurements showed that these objects have an albedo smaller than 0.1 and that creates melt pond and surface run off that further increase the absorption of incident radiation and accelerate the melting. Hyperion and Landsat data showed that the glacier has areas with different spectral characteristics. In the area across the Equilibrium Line Altitude (ELA), outcropping dust from a Saharan event was also visible, this is highlighted by high Snow Darkening Index (SDI) values.
L’obiettivo del mio dottorato è studiare l’impatto delle light-absorbing impurities (LAI) sulla criosfera tramite l’utilizzo di telerilevamento ottico. Le LAI sono particelle atmosferiche, come polveri minerali e black carbon, che possono depositarsi su neve e ghiaccio riducendone l’albedo e favorendone la fusione. L’impatto delle LAI sulla criosfera è stato studiato a livello globale e locale, ma ancora poca letteratura scientifica è dedicata allo studio del fenomeno nelle Alpi Europee. Durante il primo anno è stata sviluppata un’analisi di sensitività di un modello di trasferimento radiativo (SNow, ICe, and Aerosol Radiative model, SNICAR) con l’obiettivo di studiare le proprietà ottiche di neve e ghiaccio. In particolare, questo modello permette di simulare la riflettanza spettrale della neve in funzione di diverse variabili quali: la dimensione dei cristalli di neve [μm], la concentrazione di polveri minerali [ppm] e la loro distribuzione dimensionale [μm], la concentrazione di black carbon [ppb], l’angolo zenitale solare e la densità della neve, usando differenti profili atmosferici. In seguito a questa analisi di sensitività, sono state organizzate alcune campagne sperimentali, con l’obiettivo di misurare la riflettanza della neve in seguito ad un evento naturale di deposizione di LAI, quindi confrontare i dati osservati con quelli simulati con il modello SNICAR. Durante le campagne di misura, sono stati organizzati dei sorvoli con un Unmanned Aerial Vehicle (UAV) su zone coperte da neve nelle Alpi Europee. I dati acquisiti da terra, da UAV e da satellite (Landsat 8 - Operational Land Imager, OLI) sono stati analizzati con l’obiettivo di stimare l’impatto delle polveri minerali sulle proprietà ottiche della neve. In seguito, è stato sviluppato e testato un nuovo indice spettrale (Snow Darkening Index, SDI), non-linearmente correlato alla concentrazione di polveri minerali nella neve. Durante l’ultimo anno di dottorato, mi sono concentrato sull’impatto delle LAI sul ghiaccio in ambiente Alpino. I ghiacciai alpini rappresentano infatti un’importante fonte di acqua dolce a livello globale. Queste riserve sono seriamente minacciate dai cambiamenti climatici in atto, e in anni recenti è stata osservata una significativa riduzione delle masse glaciali. I processi superficiali che favoriscono la fusione del ghiaccio sono guidati da temperatura, precipitazioni e albedo. Quest’ultima è influenzata principalmente dalla dimensione dei cristalli di neve e dal contenuto di LAI. L’origine di queste impurità può essere prossimale o remota, e spesso queste si possono aggregare sulla lingua glaciale formando le caratteristiche crioconiti, le quali diminuiscono l’albedo del ghiaccio favorendone la fusione. Durante l’estate del 2015 sono state organizzate due campagne di misura sul ghiacciaio del Morteratsch (Alpi Svizzere). L’obiettivo delle campagne era quello di acquisire spettri di riflettanza e campioni di neve e ghiaccio, in modo da caratterizzare radiativamente e geochimicamente le polveri e i materiali depositati sul ghiacciaio. Inoltre, sono stati analizzati dati satellitari dei sensori Hyperion e Landsat, acquisiti a pochi giorni di distanza dalla campagna. I risultati hanno mostrato che le crioconiti possono diminuire la riflettanza del ghiaccio fino a 0.2 nelle lunghezze d’onda del visibile e vicino infrarosso. Questo processo può fortemente alterare i bilanci radiativi del ghiacciaio, siccome provoca la presenza di acqua di scorrimento superficiale, la quale assorbe ulteriore radiazione incidente e favorisce la fusione del ghiaccio sottostante. I dati Hyperion e Landsat hanno mostrato una grande variabilità nelle proprietà spettrali del ghiacciaio, in particolare tra zona di accumulo ed ablazione. Nella zona a cavallo della Equilibrium Line Altitude (ELA), degli strati di polveri sahariane sono inoltre visibili ed evidenziate dalle mappe di SDI.
(2016). Light-absorbing impurities: source, transport and impact on the cryosphere using optical remote sensing. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2016).
Light-absorbing impurities: source, transport and impact on the cryosphere using optical remote sensing
DI MAURO, BIAGIO
2016
Abstract
The objective of my Ph.D. is to investigate the impact of light-absorbing impurities on the cryosphere using optical remote sensing data. Light-absorbing impurities (LAI) are particulate matter, such as mineral dust and black carbon, that can be deposited on snow and ice, reducing their albedo and accelerating the melt. The impact of LAI on the cryosphere has been studied at a global and regional scale, but still few scientific literature focuses on the European Alps. In the first year, I conducted a sensitivity analysis of a radiative transfer model, the SNow, ICe, and Aerosol Radiative model (SNICAR) in order to study the optical properties of snow and ice. In particular, this model allows to simulate spectral reflectance of snow, as a function of different variables, such as snow grain size [μm], mineral dust concentration [ppm] and dimension [μm], black carbon concentration [ppb], solar zenith angle and snow density, using different atmospheric profiles. During the second year of Ph.D., different field campaigns were organized in order to measure spectral reflectance of snow after LAI depositional events, and to compare observed with simulated spectra. During field campaigns, we flew an Unmanned Aerial Vehicle (UAV) over a flat snow-covered area in the European Alps. Data collected from ground, UAV and satellite (Landsat 8 - Operational Land Imager, OLI) were analysed to estimate the impact of mineral dust on snow optical properties. A novel spectral index non-linearly correlated with mineral dust concentration was proposed and tested at different scales. During the third year, I focused on the impact of LAI on ice in the Alps. Mountain glaciers represent an important source of fresh water across the globe. Those reservoirs are seriously threatened by global climate change, and a widespread reduction of glacier extension has been observed in recent years. Surface processes that promote ice melting are driven both by temperature/precipitation and by albedo. The latter is mainly influenced by the growth of snow grain size and by the impurities content (such as dust, soot, ash, algae etc.). The origin of these light-absorbing impurities can be local or distal; often they aggregate on the glacier tongue forming characteristic cryoconites, that decrease ice albedo promoting the melting. During summer 2015, two field campaigns were conducted at the Vadret da Morteratsch glacier (Swiss Alps). The aim of the campaings was to collect ground hyperspectral reflectance data and ice/snow samples at the glacier ablation zone. During August 2015, the Earth Observing One (EO-1) satellite was planned to acquire a series of scene over the Morteratsch glacier. Furthermore, a Landsat 8 Operational Land Imager (OLI) was downloaded from the Earth Explorer portal. Results from spectra analysis showed interesting features in albedo distribution at Morteratsch glacier. In particular, the ablation area showed very low albedo values (circa 0.2), and this is probably due to multiple processes such as accumulation of particulate matters, collapsing of lateral moraine and debris covering. In addition, the presence of surface cryoconites strongly lowers ice albedo, ground measurements showed that these objects have an albedo smaller than 0.1 and that creates melt pond and surface run off that further increase the absorption of incident radiation and accelerate the melting. Hyperion and Landsat data showed that the glacier has areas with different spectral characteristics. In the area across the Equilibrium Line Altitude (ELA), outcropping dust from a Saharan event was also visible, this is highlighted by high Snow Darkening Index (SDI) values.File | Dimensione | Formato | |
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