Abstract
ABSTRACTWe present the first dedicated study into the phenomenon of ice sails. These are clean ice structures that protrude from the surface of a small number of debris-covered glaciers and can grow to heights of over 25 m. We draw together what is known about them from the academic/exploration literature and then analyse imagery. We show here that ice sails can develop by one of two mechanisms, both of which require clean ice to become surrounded by debris-covered ice, where the debris layer is shallow enough for the ice beneath it to melt faster than the clean ice. Once formed, ice sails can persist for decades, in an apparently steady state, before debris layer thickening eventually causes a reversal in the relative melt rates and the ice sails decay to merge back with the surrounding glacier surface. We support our image-based analysis with a surface energy-balance model and show that it compares well with available observations from Baltoro Glacier in the Karakoram. A sensitivity analysis of the model is performed and confirms the results from our empirical study that ice sails require a relatively high evaporative heat flux and/or a relatively low sensible heat flux in order to exist.
Highlights
The purpose of this paper is to present the first dedicated investigation into the existence, life cycle and geographical spread of ice sails
The first requirement is for the glacier to be able to develop patches of clean ice surrounded by thin debris-covered ice, so that the more rapidly melting debris-covered ice enables mounds of clean ice to emerge from the glacier surface
From the image analysis it is evident that these clean ice features melt to form planar faces – the characteristic ice sail shape
Summary
The purpose of this paper is to present the first dedicated investigation into the existence, life cycle and geographical spread of ice sails. Ice sails had been referred to as ice pyramids or ice pinnacles, but more recently the name ‘ice sail’ was suggested in a paper by Mayer and others (2006), in which an analysis of the debris cover of Baltoro Glacier in the central Karakoram region of Pakistan was presented They were so named for, as one views these structures in the distance, their relatively flat-sided faces with clearly defined ridges and often high reliefs (see Fig. 2 for a specimen over 25 m tall) give the appearance of the sails of a boat cruising over the debris-covered surface – a comparison made nearly 100 years earlier by de Filippi (1912). Despite the striking appearance of ice sails, only a handful of published references to these structures have been made, mainly (except for Mayer and others (2006)) from before the mid20th century and are generally in regards to the Baltoro Glacier
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