Browsing by Autor "Pierre Ribstein"

Now showing 1 - 11 of 11

Analysis of seasonal variations in mass balance and meltwater discharge of the tropical Zongo Glacier by application of a distributed energy balance model
(American Geophysical Union, 2011) Jean Emmanuel Sicart; Regine Hock; Pierre Ribstein; Maxime Litt; Edson Ramírez
A distributed energy balance model was applied to Zongo Glacier, Bolivia (16S, 6000-4900 m above sea level, 2.4 km 2 ), to investigate atmospheric forcing that controls seasonal variations in the mass balance and in meltwater discharge of glaciers in the outer tropics. Surface energy fluxes and melt rates were simulated for each 20 20 m 2 grid cell at an hourly resolution, for the hydrological year 1999-2000, using meteorological measurements in the ablation area. Model outputs were compared to measurements of meltwater discharge, snow cover extent, and albedo at two weather stations set up on the glacier. Changes in melt rate in three distinct seasons were related to snowfall and cloud radiative properties. During the dry season (May to August), the low melt rate was mainly caused by low long-wave emission of the cloudless thin atmosphere found at these high altitudes. From September to December, meltwater discharge increased to its annual maximum caused by an increase in solar radiation, which was close to its summer peak, as well as a decrease in glacier albedo. From January on, melt was reduced by snowfalls in the core wet season via the albedo effect but was maintained thanks to high long-wave emission from convective clouds. The frequent changes in snow cover throughout the long ablation season lead to large vertical mass balance gradients. Annual mass balance depends on the timing and length of the wet season, which interrupts the period of highest melt rates caused by solar radiation.
Détermination de l'épaisseur des glaciers de Sarennes (Alpes) et de Chacaltaya (Bolivie) par prospection radar au sol à 50 MHz
(Centre National de la Recherche Scientifique, 1999) M. Descloitres; R. Guerin; E. Ramirez; R. Gallaire; Pierre Ribstein; F. Valla
The ice thickness measurement with a terrestrial radar (50megaHertz) was first tested on the glacier-laboratory of Sarennes incomparison with of preceding field campaign with a lower frequency radar), in the French Alps.,Later this technic was successfully used on the glacier of Chacaltaya in Bolivia. The results of the two series of results are presented and discussed. / La mesure des épaisseurs de glace avec un radar terrestre à 50 MHz a été d'abord mise au point sur le glacier-laboratoire de Sarennes (comparaison avec de précédentes mesures faites avec un radar à plus basse fréquence), dans les Alpes françaises, puis mise en oeuvre avec succès sur le glacier de Chacaltaya en Bolivie. Les résultats des deux campagnes de mesure sont présentés et discutés.
Données isotopiques (<sup>87</sup>86/Sr) et changements hydrologiques depuis 15 000 ans sur l'Altiplano andin
(Taylor & Francis, 2002) Anne Coudrain; M. Loubet; Thomas Condom; Amal Talbi; Pierre Ribstein; Bernard Pouyaud; Jorge Quintanilla; Claudine Dieulin; Bernard Dupré
Résumé Actuellement, le bassin sud (17°-22°S, 126 000 km2) de l'Altiplano reçoit environ 39.4 × 109 m3 an−1 d'eau, dont 4% proviennent du bassin nord (14°-17°S) par le Rio Desaguadero. Le rapport 87Sr/86Sr des écoulements de surface actuels présentent des valeurs distinctes sur les deux bassins permettant d'utiliser ce traceur pour reconstituer des conditions hydrologiques passées contrastées du bassin sud. Pour la période aride de 10 000 à 2000 ans BP, les données acquises et la modélisation du transport de strontium dans un aquifàre renforcent les Hypothàses d'une valeur des précipitations de 80% de l'actuel et de l'absence d'alimentation significative du bassin sud par le Rio Desaguadero. Pour les hauts niveaux d'eau de la phase lacustre Tauca, vers 15 000 ans BP, les données hydrologiques et isotopiques indiquent que pour une alimentation totale estimée à 110 109 m1 an1 les apports du bassin nord pouvaient être limités à 5%.
El Niño‐Southern Oscillation (ENSO) influence on a Sajama volcano glacier (Bolivia) from 1963 to 1998 as seen from Landsat data and aerial photography
(American Geophysical Union, 2001) Yves Arnaud; Frédéric Muller; Mathias Vuille; Pierre Ribstein
Sajama volcano, located in the Bolivian Altiplano, is the southernmost tropical glacier and, owing to its situation, approximately 100 km east of the Pacific coast, is well suited to study the El Niño‐Southern Oscillation phenomenon. Landsat data from 1972 to 1998 and a 1963 aerial photograph are used to monitor the snow line fluctuations on a selected part of Sajama volcano. We assume that a few months after the rainy season, the snow line is representative of the previous rainy season, if no recent snowfall has occurred. By observing precipitation from the stations surrounding Sajama volcano and by verifying snow presence on surrounding summits, we detect images with recent snowfall likely to disturb the climatic significance of the snow line. A snow line evolution model takes into account the different image acquisition dates and adjusts the snow line elevation accordingly for the middle of the dry season. A progressive rise of the snow line elevation is observed from 1963 to 1998 with a sustained rise from 1984 to 1990. The snow line altitude is related to the Southern Oscillation Index. Even after the high precipitation of the 1996–1997 wet season, the following El Niño 1997–1998 leads to a substantial rise of the snow line. The snow line elevation is related primarily to the total rainy season precipitation and to a lesser degree to the maximum monthly mean temperature of the warmest month, thus confirming a greater snow line sensitivity to precipitation than to temperature.
Etude des précipitations et de la fonte sur un glacier tropical: le glacier du Zongo, Bolivie, 16°S
(Taylor & Francis, 2003) Jean Emmanuel Sicart; Pierre Ribstein; Bernard Francou; Robert Gallaire
Résumé Cet article présente les relations entre les précipitations et la fonte sur le glacier du Zongo en Bolivie où l'ablation et l'accumulation sont maximales en saison des pluies (été austral). Les nuages, provenant du bassin amazonien, donnent des précipitations à midi dans les vallées, puis dans l'après-midi en montagne. Lors de l'installation de la saison des pluies de septembre à décembre, les chutes de neige recouvrent temporairement la glace en fusion. Par effet d'albédo, l'énergie solaire absorbée et la fusion varient beaucoup et atteignent leur maximum lorsque le couvert neigeux est réduit. Au cœur de la saison des pluies de janvier à mars, la neige reste à basse altitude sur le glacier, l'albédo est fort et la fonte diminue. La saison sèche de mai à août est une période de faible ablation. La fonte sur les glaciers tropicaux dépend fortement de la répartition saisonnière des précipitations, par l'augmentation d'albédo due aux chutes de neige d'été.
Glaciers of the Tropical Andes: Indicators of Global Climate Variability
(Springer Nature (Netherlands), 2005) Bernard Francou; Pierre Ribstein; Patrick Wagnon; Edson Ramírez; Bernard Pouyaud
Monthly balance and water discharge of an inter-tropical glacier: Zongo Glacier, Cordillera Real, Bolivia, 16° S
(Cambridge University Press, 1995) Bernard Francou; Pierre Ribstein; Ronald Saravia; Eric Tiriau
Abstract Measurements of mass balance were performed every month on Zongo Glacier. Bolivia. Simultaneously, water-discharge, temperature and precipitation data were obtained. The first year of the survey, 1991–92. was marked by an ENSO (El Niño-Southern Oscillation) event with high temperature and low precipitation, whilst the following year, 1992–93, was normal. Results point to the early and late wet season (October-December and March–May: as playing a critical role in the determination of the annual mass balance. The wet season is the warmest period of the year and consequently the duration of the wet season is a highly relevant variable in determining mass balance. Both glaciological and hydrological methods for the determination of the mass balance provide similar results. Our study confirms dial ENSO events have a major influence on the rapid glacier retreat currently affecting this part of the Andes.
Monthly balance and water discharge of an inter-tropical glacier: Zongo Glacier, Cordillera Real, Bolivia, 16° S
(Cambridge University Press, 1995) Bernard Francou; Pierre Ribstein; Ronald Saravia; Eric Tiriau
Abstract Measurements of mass balance were performed every month on Zongo Glacier. Bolivia. Simultaneously, water-discharge, temperature and precipitation data were obtained. The first year of the survey, 1991–92. was marked by an ENSO (El Niño-Southern Oscillation) event with high temperature and low precipitation, whilst the following year, 1992–93, was normal. Results point to the early and late wet season (October-December and March–May: as playing a critical role in the determination of the annual mass balance. The wet season is the warmest period of the year and consequently the duration of the wet season is a highly relevant variable in determining mass balance. Both glaciological and hydrological methods for the determination of the mass balance provide similar results. Our study confirms dial ENSO events have a major influence on the rapid glacier retreat currently affecting this part of the Andes.
Small glaciers disappearing in the tropical Andes: a case-study in Bolivia: Glaciar Chacaltaya (16<sup>o</sup> S)
(Cambridge University Press, 2001) Edson Ramírez; Bernard Francou; Pierre Ribstein; Marc Descloîtres; Roger Guérin; Javier Mendoza; Robert Gallaire; Bernard Pouyaud; Ekkehard Jordan
Abstract Glaciar Chacaltaya is an easily accessible glacier located close to La Paz, Bolivia. Since 1991, information has been collected about the evolution of this glacier since the Little Ice Age, with a focus on the last six decades. The data considered in this study are monthly mass-balance measurements, yearly mappings of the surface topography and a map of the glacier bed given by ground-penetrating radar survey. A drastic shrinkage of ice has been observed since the early 1980s, with a mean deficit about 1 m a −1 w.e. From 1992 to 1998, the glacier lost 40% of its average thickness and two-thirds of its total volume, and the surface area was reduced by >40%. With a mean estimated equilibrium-line altitude lying above its upper reach, the glacier has been continuously exposed to a dominant ablation on the whole surface area. If the recent climatic conditions continue, a complete extinction of this glacier in the next 15 years can be expected. Glaciar Chacaltaya is representative of the glaciers of the Bolivian eastern cordilleras, 80% of which are small glaciers (<0.5 km 2 ). A probable extinction of these glaciers in the near future could seriously affect the hydrological regime and the water resources of the high-elevation basins.
Solid precipitation on a tropical glacier in Bolivia measured with an ultrasonic depth gauge
(Wiley, 2002) Jean Emmanuel Sicart; Pierre Ribstein; Jean Philippe Chazarin; Étienne Berthier
An ultrasonic depth gauge was used to measure snowfall over a 2‐year period near the equilibrium line of the Zongo glacier (2.4 km 2 ), Bolivia (16°S). Study of the influence of wind, air temperature, and air moisture on the measurements gives a quantification of snowfall at a 3‐hour time step, with a sensitivity of 1 cm of snow. The density of fresh snow is estimated by comparison with rain gauge measurements. The year is marked by a dry season from May to August and a wet season from December to April, during which accumulation and melting coincide on the glacier. Snowfall events are associated with a wind of moderate speed from the valley (less than 4 m s −1 ). Masses of moist air originate in the Amazon basin. The orographic effect produces precipitation at midday in the Andean valleys and in the afternoon in the high mountains. Nighttime snowfall events occur during periods of bad weather related to the regional atmospheric circulation and last several days. The density of fresh snow is high, about 250 kg m −3 , because of the high air temperature during snowfall events (over −3°C). The high snow density and the moderate wind speeds prevent snow drifting conditions, which results in low spatial variability of the accumulation on tropical glaciers. Accurate recording of snowfall at a short time step is important for the study of energy fluxes at the glacier surface because snowfall events greatly increase the albedo and solar radiation is generally the main source of melting energy.
Tropical climate and glacier hydrology: a case study in Bolivia
(Elsevier BV, 1995) Pierre Ribstein; Eric Tiriau; Bernard Francou; Ronald Saravia