[1] 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.

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Atmospheric controls of the heat balance of Zongo Glacier (16°S, Bolivia)

Forest disturbance has a significant impact on hydrology due to its effect on the forest canopy, which is important for precipitation interception, transpiration, site micrometeorology, and snow accumulation and ablation. This study examines the impact of mountain pine beetle infestation and subsequent forest death on snow ablation. Dead stands experience needle loss and canopy reduction due mainly to the loss of small branches and stems, which has a subsequent impact on micrometeorological conditions. Ablation is driven largely by incoming short-wave radiation, which in dead stands is greater than in alive stands, but does not reach that available in clearcuts. Long-wave radiation emission in dead stands is lower than that in alive stands, reducing its contribution to snowpack warming and ablation. Turbulent flux contributions to snow ablation are limited in forest stands relative to clearcuts, although they are slightly greater in dead than alive stands due to the more open forest structure. Additional studies are required to refine the basic energy balance model and incorporate all processes affecting the snow ablation energy balance.

Snow ablation energy balance in a dead forest stand

Two models that estimate glacier ablation from simple meteorological observations are described. Both the energy-balance and bulk-aerodynamic approaches are used to determine the bulk-exchange coefficient for transfer of sensible- and latent-heat energy to the melting ice surface from 17 summer days of measurement on Ivory Glacier, New Zealand. The behaviour of the coefficient is examined over progressively increasing time intervals and as a function of wind speed and gradient, and of atmospheric stability. The models are verified with 36 d of independent data from a different summer on the same glacier. The more accurate short-term estimates of ablation are obtained using the energy-balance approach with empirically but independently derived values of the exchange coefficient.

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A comparison of the energy-balance and bulk-aerodynamic approaches for estimating glacier melt

Local scale ablation and energy budget measurements are reported for the Ivory glacier in the Southern Alps of New Zealand over 53 days during two consecutive summer periods. Ablation averaged 38 mm day−1, but varied from less than 10 mm day−1 to over 70 mm day−1. Radiation supplied 52% of the energy for glacier melt, with the convective fluxes contributing most of the remainder. The results are related to large scale synoptic weather patterns over the southwest Pacific region. Different synoptic situations generate distinctive energy budgets, with radiation important during southerly circulation patterns and the convective fluxes relatively more important with northerly circulation patterns. These findings are discussed in the context of the retreat of New Zealand glaciers this century and possible circulation changes.

The synoptic climatology of ablation on a New Zealand glacier

Multi-annual records of glacier surface meteorology and energy balance are necessary to resolve glacier–climate interactions but remain sparse, especially in the Southern Hemisphere. To address this, we present a record from the ablation zone of Brewster Glacier, New Zealand, between October 2010 and September 2012. The mean air temperature was 1.2°C at 1760 m a.s.l., with only a moderate temperature difference between the warmest and coldest months (∼8°C). Long-term annual precipitation was estimated to exceed 6000 mm a−1, with the majority of precipitation falling within a few degrees of the freezing level. The main melt season was between November and March (83% of annual ablation), but melt events occurred during all months. Annually, net radiation was positive (a source of energy) and supplied 64% of the melt energy, driven primarily by net shortwave radiation. Net longwave radiation was often positive during cloudy conditions in summer, demonstrating the radiative importance of clouds during melt. Turbulent sensible and latent heat fluxes were directed towards the surface in the summer months, accounting for just over a third of the energy for melt (34%). The energy gain associated with rainfall was small except during heavy events in summer.

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A 22 month record of surface meteorology and energy balance from the ablation zone of Brewster Glacier, New Zealand

The snow surface energy budgets in two alpine basins in New Zealand's South Island were calculated from meteorological observations made during a spring melt period. The sensible heat flux was the most important energy source and precipitation heat flow the least important at both sites. Latent heat was the second greatest source at the more maritime site while net radiation was more important than latent heat at the other site. The greater cloud cover at the more maritime site accounts for the lower net radiation received there. The differences in the sensible and latent heat fluxes are due to differences in the air temperature, humidity and wind speed between the sites. No reliable relationship between the wind speeds at the two sites appears to exist. The differences in temperature and humidity result from the influences of different airmasses and the modification of airmasses as they traverse the mountain ranges.

R. D. Moore

Papers

A comparison of the snowmelt energy budgets in two alpine basins