• Energy Research

Katabatic winds are very frequent but poorly understood or simulated over steep slopes. This study focuses on a katabatic jet above a steep alpine slope. We assess the buoyancy terms in both the turbulence kinetic energy (TKE) and the Reynolds shear-stress budget equations. We specifically focus on the contribution of the slope-normal and along-slope turbulent sensible heat fluxes to these terms. Four levels of measurements below and above the maximum wind-speed height enable analysis of the buoyancy effect along the vertical profile as follow: (i) buoyancy tends to destroy TKE, as expected in stable conditions, and the turbulent momentum flux in the inner-layer region of the jet below the maximum wind-speed height $$z_j$$ ; (ii) results also suggest buoyancy contributes to the production of TKE in the outer-layer shear region of the jet (well above $$z_j$$ ) while consumption of the turbulent momentum flux is observed in the same region; (iii) In the region around the maximum wind speed where mechanical shear production is marginal, buoyancy tends to destroy TKE and our results suggest it tends to increase the momentum flux. The present study also provides an analytical condition for the limit between production and consumption of the turbulent momentum flux due to buoyancy as a function of the slope angle, similar to the condition already proposed for TKE. We reintroduce the stress Richardson number, which is the equivalent of the flux Richardson number for the Reynolds shear-stress budget. We point out that the flux Richardson number and the stress Richardson number are complementary stability parameters for characterizing the katabatic flow apart from the region around the maximum wind-speed height.

Tropical glaciology includes investigation of climate variability in poorly documented regions of large surface‐atmosphere energy exchanges. This study examines the surface energy fluxes of the Bolivian Zongo Glacier (16°S, 68°W, 6000–4900 m asl) in order to identify the atmospheric variables that control melting. Measurements from 1998 to 2000 taken from two meteorological stations in the ablation area are analyzed. During the progressive development of the wet season from September to January, melting energy was high: Solar irradiance was close to its summer solstice peak, clouds were sporadic, and albedo was low. During the core of the wet season from January to April the magnitudes of the net short‐wave (+) and net long‐wave (−) radiation fluxes were reduced by frequent clouds and snowfalls so that melting energy was moderate. In the dry season from May to August, melting energy was small because of the energy losses essentially in long‐wave radiation but also in sublimation. The turbulent sensible heat flux to the ice (+) generally offsets the energy loss in latent heat (−), except in the dry season, when sublimation prevailed because of strong wind and dry air. Solar radiation was the main source of energy, but the seasonal changes of the melting energy were driven by long‐wave radiation. In particular, clouds sharply increased the emittance of the thin high‐altitude atmosphere. Closely linked to clouds and humidity, the main seasonal variables of low‐latitude climates, long‐wave radiation is a key variable in the energy balance of tropical glaciers.

Abstract. The aim of this paper is to provide the community with a comprehensive overview of the studies of glaciers in the tropical Andes conducted in recent decades leading to the current status of the glaciers in the context of climate change. In terms of changes in surface area and length, we show that the glacier retreat in the tropical Andes over the last three decades is unprecedented since the maximum extension of the LIA (mid 17th–early 18th century). In terms of changes in mass balance, although there have been some sporadic gains on several glaciers, we show that the trend has been quite negative over the past 50 yr, with a mean mass balance deficit for glaciers in the tropical Andes that is slightly more negative than the computed global average. A break point in the trend appeared in the late 1970s with mean annual mass balance per year decreasing from −0.2 m w.e. in the period 1964–1975 to −0.76 m w.e. in the period 1976–2010. In addition, even if glaciers are currently retreating everywhere in the tropical Andes, it should be noted that as a percentage, this is much more pronounced on small glaciers at low altitudes that do not have a permanent accumulation zone, and which could disappear in the coming years/decades. Monthly mass balance measurements performed in Bolivia, Ecuador and Colombia showed that variability of the surface temperature of the Pacific Ocean is the main factor governing variability of the mass balance variability at the interannual to decadal time scale. Precipitation did not display a significant trend in the tropical Andes in the 20th century, and consequently cannot explain the glacier recession. On the other hand, temperature increased at a significant rate of 0.10 °C decade−1 in the last 70 yr. The higher frequency of El Niño events and changes in its spatial and temporal occurrence since the late 1970s together with a warming troposphere over the tropical Andes may thus explain much of the recent dramatic shrinkage of glaciers in this part of the world.

The reasons for the accelerated glacier retreat observed since the early 1980s in the tropical Andes are analyzed based on the well‐documented Chacaltaya glacier (Bolivia). Monthly mass balance measurements available over the entire 1991–2001 decade are interpreted in the light of a recent energy balance study performed on nearby Zongo glacier and further put into a larger‐scale context by analyzing the relationship with ocean‐atmosphere dynamics over the tropical Pacific‐South American domain. The strong interannual variability observed in the mass balance is mainly dependent on variations in ablation rates during the austral summer months, in particular during DJF. Since high humidity levels during the summer allow melting to be distinctly predominant over sublimation, net all‐wave radiation, via albedo and incoming long‐wave radiation, is the main factor that governs ablation. Albedo depends on snowfall and a deficit during the transition period and in the core of the wet season (DJF) maintains low albedo surfaces of bare ice, which in turn leads to enhanced absorption of solar radiation and thus to increased melt rates. On a larger spatial scale, interannual glacier evolution is predominantly controlled by sea surface temperature anomalies (SSTA) in the eastern equatorial Pacific (Niño 1+2 region). The glacier mass balance is influenced by tropical Pacific SSTA primarily through changes in precipitation, which is significantly reduced during El Niño events. The more frequent occurrence of El Niño events and changes in the characteristics of its evolution, combined with an increase of near‐surface temperature in the Andes, are identified as the main factors responsible for the accelerated retreat of Chacaltaya glacier.

This study investigates the physical basis of temperature‐index models for three glaciers in contrasting climates: Zongo (16°S, 5050 m, Bolivian Tropics), St Sorlin (45°N, 2760 m, French Alps), and Storglaciären (67°N, 1370 m, northern Sweden). The daily energy fluxes were computed during melt seasons and correlated with each other and with air temperature on and outside the glacier. The relative contribution of each flux to the correlations between temperature and melt energy was assessed. At Zongo, net short‐wave radiation controls the variability of the energy balance and is poorly correlated to temperature. On tropical glaciers, temperature remains low and varies little, melt energy is poorly correlated to temperature, and degree‐day models are not appropriate to simulate daily melting. At the yearly scale, the temperature is better correlated to the mass balance because it integrates the ablation and the accumulation processes over a long time period. At Sorlin, the turbulent sensible heat flux is greater because of higher temperatures, but melt variability is still controlled by short‐wave radiation. Temperature correlates well with melt energy mainly through short‐wave radiation, probably because of diurnal advection of warm air from the valley. At Storglaciären, high correlations between temperature and melt energy result from substantial variability of the sensible and latent heat fluxes (which both supply energy to the glacier), and their good correlations with temperature. In the three climates, long‐wave irradiance is the main source of energy, but its variability is small and poorly correlated to the temperature mainly because cloud emissions dominate its variability.

AbstractThis paper presents some preliminary measurements of snow spectral reflectance on the tropical Bolivian Zongo glacier. Measurements show a correct agreement with theoretical spectral albedo of pure snow in the near infrared region, but lower values in the visible region (by 10–20%) probably due to aerosols contained in snow. Impurity contents ranged from 10 to 100 ppmw in one‐week‐old snow collected from the Zongo glacier, but measurements are scarce. Large amounts of snowfall partly compensate the proximity of dust sources in mid‐latitude glaciers, whereas on outer‐tropical glaciers precipitations are not abundant and are very seasonal, and sources of aerosols are proximate. Copyright © 2007 John Wiley & Sons, Ltd.