Carsten Braun

Mathias Vuille; Douglas R. Hardy; Carsten Braun; Frank T. Keimig; Raymond S. Bradley

The analysis of atmospheric circulation anomalies related to snowfall events on Sajama volcano (Bolivian Andes) provides important information for the calibration of an ice core, recently recovered from the summit. Seventeen precipitation episodes were recorded on Sajama volcano during the 1996/1997 summer season (November 1996 to March 1997) by snow depth sensors and additional measurements of an automatic weather station located on the summit. The analysis of atmospheric circulation patterns during these events is based on zonal and meridional wind, air temperature, relative humidity, geopotential height and horizontal divergence at three pressure levels (400, 500, and 700 hPa levels), atmospheric thickness (700 hPa-400 hPa), and precipitable water (vertically integrated), all extracted from the National Centers for Environmental Prediction (NCEP) data set. Highly convective situations prevailed through most of December and January, with strong vertical motion over the Bolivian Altiplano. In February and March, increased moisture advection from the east occurred in midtropospheric levels. These results are confirmed by isobaric 5-day back trajectories and transit time analysis at the 400 hPa level. The extremely southern position of the upper air high-pressure system (“Bolivian High”) in February and March is the main reason for the unusually high precipitation amounts on the Altiplano in 1996/1997. Highly variable patterns of atmospheric circulation can lead to snowfall on Sajama during the summer months.

Douglas R. Hardy; Mathias Vuille; Carsten Braun; Frank R. Keimig; Raymond S. Bradley

Abstract An automated weather station was installed in October 1996 at the summit of Nevado Sajama, located in the western Andean Cordillera of Bolivia (6542 m, 18°06´S, 68°53´W). Meteorological conditions on the mountain are being observed to improve the calibration of geochemical variations within tropical ice cores. This article documents the design and operation of the station and presents a discussion of measurements made through the first annual cycle. Variables analyzed include pressure, incoming solar radiation, air temperature, humidity, wind, and snow accumulation. Large diurnal fluctuations were recorded in most variables, which is not unexpected given the location at 18°S; the data also reveal substantial day-to-day variability and rapid seasonal changes in weather and circulation. As a result, snowfall events and periods of evaporation are more episodic in nature than previously believed. Measurement of atmospheric conditions during and between snowfall events will therefore greatly facilitat...

Ted Lewis; Carsten Braun; Douglas R. Hardy; Pierre Francus; Raymond S. Bradley

Abstract Two large suspended sediment concentration (SSC) pulses were recorded in 1998 in a small snowmelt-fed stream on Ellesmere Island in the Canadian High Arctic. The largest pulse occurred from 7 to 8 July, when 32% of the monitored seasonal sediment transport occurred in only four hours. SSC reached 83,760 mg L−1, exceeding all previously recorded values from high arctic glacially-fed and snowmelt-fed rivers by more than one order of magnitude. The event occurred after the majority of snow in the watershed had melted, and was preceded by a long period of relatively high air temperature, and a small rainfall event on 7 July. We consider the most likely cause of the event to be a rapid mass movement.

Carsten Braun; Douglas R. Hardy; Raymond S. Bradley

Abstract Small, stagnating ice caps at high latitudes are particularly sensitive to climatic fluctuations, especially with regard to changes in ablation season temperature. We conducted mass balance measurements and GPS area surveys on four small High Arctic plateau ice caps from 1999–2002. We compared these measurements with topographic maps and aerial photography from 1959, and with previously published data. Net mass balance (bn) of Murray Ice Cap was −0.49 (1999), −0.29 (2000), −0.47 (2001), and −0.29 (2002), all in meters of water equivalent (m w.eq.). The mass balance of nearby Simmons Ice Cap was also negative in 2000 (bn=−0.40 m w.eq.) and in 2001 (bn=−0.52 m w.eq.). All four ice caps experienced substantial marginal recession and area reductions of between 30 and 47% since 1959. Overall, these icecaps lost considerable mass since at least 1959, except for a period between the mid‐1960s and mid‐1970s characterized regionally by reduced summer melt, positive mass balance, and ice cap advance. The regional equilibrium line altitude (ELA) is located, on average, above the summits of the ice caps, indicating that they are remnants of past climatic conditions and out of equilibrium with present climate. The ice caps reached a Holocene maximum and were several times larger during the Little Ice Age (LIA) and their current recession reflects an adjustment to post‐LIA climatic conditions. At current downwasting rates the ice masses on the Hazen Plateau will completely disappear by, or soon after, the mid‐21st century.

Carsten Braun; Douglas R. Hardy; Raymond S. Bradley; Michael Retelle

Abstract Hydrological and meteorological observations at Lake Tuborg, Ellesmere Island, Nunavut, Canada in 1995 are used to investigate contemporary water and sediment transport processes. Here we describe a new environmental data set for the High Arctic, where such data are scarce. The studied watershed (∼460 km2) ranges in elevation between 63 and ∼1900 m asl and is 88% covered by a lobe of the Agassiz Ice Cap. Streamflow and sediment transport were strongly associated with snowmelt runoff, whereas the direct influence of summer precipitation events was negligible. Snowmelt was primarily controlled by synoptic‐scale climatic processes. Two high‐magnitude pulses of meltwater and slush contributed a significant portion of the measured suspended sediment load to Lake Tuborg. Such events may be associated each year with snowmelt along the Agassiz Ice Cap margin. Additional years of data collection are needed to define the annual and inter‐annual variability of the sediment delivery system, particularly with respect to the relative importance of summer rainfall events. Runoff and sediment transport to Lake Tuborg are very likely to increase under climatic warming conditions.

Carsten Braun; Douglas R. Hardy; Raymond S. Bradley; Michael Retelle

Carsten Braun; Douglas R. Hardy; Raymond S. Bradley; Vicky Sahanatien

Carsten Braun; Douglas R. Hardy; Raymond S. Bradley

Mark C. Serreze; Bruce H. Raup; Carsten Braun; Douglas R. Hardy; Raymond S. Bradley

Carsten Braun; Douglas R. Hardy; Raymond S. Bradley; Vicky Sahanatien