No abstract

BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.

No abstract

Based on (i) the observation of spatially differential glacier retreats in the tropical Rwenzori Range (East Africa) during the 20th century, which are most striking on the mountains Baker and Speke, and (ii) the information on an abrupt climate change to drier conditions in East Africa at the end of the 19th century, the following hypothesis is derived: owing to a drier atmosphere than in a previous period, both accumulation (possibly supported by increasing air temperatures) and convective cloud activity have decreased. Consequently, increased incoming shortwave radiation, especially during the morning hours, induced a differentially increased ablation that could not be compensated by mass advection on the mountains Baker and Speke.The results obtained from a combined radiation-terrain model, run for one more humid and one drier climatic scenario, confirm the hypothesis by quantifying the correlation between increased incoming shortwave radiation and glacier surface area loss. In the context of modern climate fluctuations, the results are a further indicator for a drastic climatic dislocation in East Africa at the end of the 19th century, leaving a humid regime behind and leading to a relatively dry regime, which is forcing the recession of glaciers not only by less accumulation but also by less protection against shortwave radiation through clouds.

[1] In this paper, artificially ventilated and unventilated temperature measurements logged by high-elevation automatic weather stations (AWS) in the tropical Cordillera Blanca, Perú, are compared. Both temperatures are measured with the same sensor and radiation shield by periodic artificial ventilation. The analyzed time series range from October 1999 until May 2001. The temperature differences ÁT = T unvent. À T vent. show a persistent daily cycle with a nighttime mean value of 0.3 K and a daytime mean of 1.3 K. The daytime period of ÁT is characterized by a late morning and an early afternoon maximum and a slight secondary noon minimum. The cycle is most pronounced during the dry season, but is only slightly reduced in the wet season. It is shown that the daily cycle is mainly caused by incoming shortwave radiation and that the resulting temperature difference depends on the irradiation angle to the radiation shield. A simple radiation geometry model is proposed for the explanation of ÁT. The modeled ÁT daily cycle strongly correlates with measured monthly mean ÁT daily cycles and can be used for respective corrections. In case of snow cover, the general cycle of ÁT remains, but additional deviations are caused by reflected shortwave radiation. These deviations cannot be modeled by using simple considerations. Therefore adequately ventilated air temperature measurements on glaciers and snow-covered surfaces should be made. Natural ventilation suffices in the case of persistently high wind speeds; otherwise, artificial ventilation should be implemented whenever possible.