Snow cover monitoring in the Northern Patagonia Icefield using MODIS satellite images (2000–2006)

doi:10.1016/j.gloplacha.2007.07.005

Global and Planetary Change

Volume 61, Issues 3–4, April 2008, Pages 103-116

https://doi.org/10.1016/j.gloplacha.2007.07.005 Get rights and content

Abstract

The snow cover of the Northern Patagonia Icefield (NPI) was monitored after applying the Normalized Difference Snow Index (NDSI) and the Red/NIR band ratio to 134 Moderate Resolution Imaging Spectroradiometer (MODIS) images captured between 2000 and 2006. The final results show that the snow cover extent of the NPI fluctuates a lot in winter, in addition to its seasonal behaviour. The minimum snow cover extent of the period (3600 km²) was observed in March 2000 and the maximum (11,623 km²) in August 2001. We found that temperature accounts for approximately 76% of the variation of the snow cover extent over the entire icefield. We also show two different regimes of winter snow cover fluctuations corresponding to the eastern and the western sides of the icefield. The seasonality of the snow cover on the western side was determined by temperature rather than precipitation, while on the east side the seasonality of the snow cover was influenced by the seasonal behaviour of both temperature and precipitation. This difference can be explained by the two distinct climates: coastal and continental. The fluctuations in the winter snow cover extent were more pronounced and less controlled by temperature on the western side than on the eastern side of the icefield. Snow cover extent was correlated with temperature R² = 0.75 and R² = 0.74 for the western and eastern sides, respectively. Since limited meteorological data are available in this region, our investigation confirmed that the change in snow cover is an interesting climatic indicator over the NPI providing important insights in mass balance comprehension. Since snow and ice were distinguished snow cover fluctuations can be associated to fluctuations in the snow accumulation area of the NPI. In addition, days with minimum snow covers of summer season can be associated to the period in which Equilibrium Line Altitude (ELA) is the highest.

Introduction

The Northern Patagonia Icefield (NPI) and the Southern Patagonia Icefield (SPI) are important in evaluating the impact of climate change in the southern hemisphere (Casassa et al., 2002). They represent the largest temperate ice masses of South America (Warren and Sugden, 1993) and provide an important source of fresh water. Rignot et al. (2003) showed that the average ice thinning rates more than doubled during the 1995–2000 period compared to the average rate over 1975–2000, reaching an equivalent sea level rise of 0.105 ± 0.011 mm per year. To improve our understanding of the response of glaciers in South America to climate change, several studies have been conducted in Patagonia, most particularly on the NPI. Some of them consider surface monitoring, front and volume changes with remote sensing data such as aerial photographs or high-resolution satellite images (Lliboutry, 1956, Aniya, 1988, Aniya, 2007). It has been suggested that the origin of the thinning, retreat, and loss of glacier surface could result from a change in the icefield's snow accumulation regime (Aniya, 1988), however in order to confirm this suggestion, mass balance measurements are necessary on the NPI.

To monitor glacier mass balance, either field measurements (ablation stakes and accumulation pits) or differential analysis of the multi-temporal Digital Elevation Model (DEM) are required (Bamber and Payne, 2004, Berthier et al., 2004). Applying both of these methods to the NPI is a complex issue because access to the area is difficult and there is a paucity of DEM. Nevertheless, given the sensitivity of snow cover to precipitation and temperature, its study can provide more insights on the evolution of the icefield accumulation regime. The main problem in monitoring the NPI snow cover is the difficulty in making field observations because of unfavourable meteorological conditions (Peña and Escobar, 1987) as well as the large size of the icefield. Remote sensing offers good tools to evaluate the earth's conditions and therefore is particularly appropriate to analyse such large ice bodies.

Romanov and Tarpley (2003) analysed seasonal changes of the snow covered area of South America between May 2000 and November 2001 using data from Geostationary Operational Environmental Satellite 8 (GOES-8) Imager at 1 km and 4 km of spatial resolution. Foster et al., 2002, Foster et al., 2003 analysed the seasonal snow cover extent and depth (for the 1988–2001 and 1979–2001 periods, respectively) in the middle latitudes of South America using data from the Scanning Multichannel Microwave Radiometer (SSMR) instrument onboard the Nimbus 7 satellite and from the Special Sensor Microwave Imager (SSMI) sensors onboard Defense Meteorological Satellite Program (DMSP) satellites. These studies have provided important information on the seasonal behaviour of the snow extent in South America in a global scale. The spatial resolution of the sensors used on those studies is not refined enough for studying the snow cover extent of the NPI.

Snow cover has a highly dynamic behaviour (Lliboutry, 1956) and therefore needs to be monitored frequently. The NPI is very often covered by clouds, and as a consequence, only very few high-resolution images are available. In order to partially address this issue, the Moderate resolution Imaging Spectroradiometer (MODIS) onboard the Terra spacecraft was used because it provides imagery with a repeat time (2 days) and resolution (250, 500 and 1000 m) that are suitable for monitoring the snow cover extent of the NPI.

The National Snow and Ice Data Center distributes the MOD10A1 Snow Products which are daily snow cover extent maps based in MODIS images. The MODIS MOD10A1 Snow Products make it possible to study the daily changes in snow cover. However, they fail to map some snow because of confusion between clouds and snow due to the MOD35 cloud mask which is an input to the MOD10A1 algorithm (Hall et al., 2002). This may hamper the analysis of the snow cover distribution (Romanov and Tarpley, 2003). The capabilities of MODIS Snow Products to identify snow and ice in the NPI were evaluated by testing the MODIS MOD10A1 daily Snow Products in the area of study. We confirmed the fact that too many pixels were mapped as cloud when they should have been mapped as snow. Therefore, the monitoring of the snow cover extent of the NPI over time was derived from raw cloud-free MODIS-Terra data sets over the 6-year period (2000–2006). Due to complex terrain, a topographical correction had to be applied to the MODIS-Terra images.

The objective of this study was to analyse the changes in the extent of the snow cover on the NPI and its relations with the atmospheric conditions. Meteorological data were analysed to elucidate the source of the snow cover fluctuations. This required the icefield to be divided into a western and an eastern side based on the topographic and climatic differences between the two sides of the NPI.

Section snippets

Study area

The Northern Patagonia Icefield (Fig. 1) is located between 46°30′ and 47°30′ of the southern latitudes. It covers a total surface area of 4197 km² (including rock outcrops) and it extends for nearly 125 km north–south between Grosse and Steffen Glaciers, with a maximum width of 71 km in the west–east direction between the frontal tongues of San Quintín and Soler Glaciers (Rivera et al., 2007).

The highest point is the San Valentin summit, 3910 m above sea level (asl) which is located in the

MODIS images

MODIS images belong to the Earth Observation System (EOS) and they were used for this study, because they offer a suitable temporal, spectral and spatial resolution (Table 1). MODIS has 36 bands and is a cross-track-scanning imaging radiometer. The instrument field of view is ± 55° from the nadir. The sensor observes the earth from a sun-synchronous position near the polar orbit at an altitude of 705 km. MODIS-Terra presents global coverage in 2 days. The datasets used are Level 1 B products

Snow cover extraction method

The first task was to evaluate the accuracy of MODIS Snow Products for snow and ice identification in the NPI. Ten MODIS Snow Products (MOD10A1) of different months and years were compared with their corresponding raw images (Fig. 3). In every MODIS Snow Product tested, the whole NPI was mapped as a cloud area when it is really snow and ice as seen on the Fig. 3A.

It is not the purpose of this study to make a thorough analysis of the performance of MODIS MOD10A1 Snow Products. However, on the

Snow extent changes over the entire NPI

This study established the frequency with which the icefield was totally cloud-covered, partially cloud-covered or free of clouds. On average during the 2000–2006 period, the entire icefield was covered by clouds 273 days per year, partially cloud-covered (at least part of the icefield covered by clouds, very often the western side) 73 days per year, and free of clouds 19 days per year. The cloudiest months of the year were January, September and December. The year least [most] cloud-covered

Concluding remarks

The snow cover extent changes between 2000 and 2006 were successfully monitored applying the NDSI and Red/NIR ratio to cloud-free MODIS images. The results showed a seasonal variation in snow cover extent, with a maximum snow cover extent totalling an area of 11,623 km² (August 2001) and a minimum extension of 3600 km² (March 2000). Over the yearly cycle, the snow cover extent correlated well with the Reanalysis air temperature (R² = 0.76). Large fluctuations in snow cover extent over the entire

Acknowledgments

Paulina Lopez conducted this study within a PhD program sponsored by a IRD (Institut de Recherche pour le Développement) scholarship. Two Chilean institutions were associated with this study: the Centro de Estudios Cientificos de Santiago (CECS), which provided the mosaic based on Landsat ETM+ images and the Direccion General de Aguas (DGA) de Aysén.

The MODIS images and the MODIS Snow Products used in this study were acquired as part of NASA's Earth Science Enterprise. The algorithms were

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Snow cover monitoring in the Northern Patagonia Icefield using MODIS satellite images (2000–2006)

Abstract

Introduction

Section snippets

Study area

MODIS images

Snow cover extraction method

Snow extent changes over the entire NPI

Concluding remarks

Acknowledgments

Remote Sensing of Environment

Remote Sensing of Environment

Remote Sensing of Environment

Remote Sensing of Environment

Remote Sensing of Environment

Global and Planetary Change

Remote Sensing of Environment

Glacier inventory for the Northern Patagonia Icefield, Chile and variations 1944/45 to 1985/86

Arctic and Alpine Research

Glacier variations of Hielo Patagonico Norte, Chile, for 1944/45–2004/05

Bulletin of Glaciological Research

Satellite monitoring of equilibrium lines in Northern Patagonia Icefield: 1979 satellite monitoring of equilibrium lines in Northern Patagonia Icefield: 1979–2003

Recent rapid thinning of the Mer de Glace glacier derived from satellite optical images

Geophysical Research Letters

An assessment of the differences between three satellite snow cover mapping techniques

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Reconstructed North American, Eurasian, and Northern Hemisphere snow cover extent, 1915–1997

The Patagonian Icefields. A Unique Natural Laboratory for Environmental and Climate Change Studies

Satellite discrimination of snow/cloud surfaces

International Journal of Remote Sensing

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