Errors in Short-Term Ablation Measurements on Melting Ice Surfaces

Abstract

ABSTRACT. R a pid changes in time and space in the micro-relief of an ablating glacier surface and radiation-induced melt within the uppermost ice layer, termed the "weathering crust" , seriously affect the accuracy of the short-term ablation measurements. The various measuring techniques commonly used (stakes, ablatometers, ablatographs) and some new methods (measurement of discharge from a small supraglacial drainage basin, and mass loss directly measured on core samples) are critically reviewed and assessed in the light of these phenomena. The implications for studies of heat a nd mass balance are discussed.It appears that the direct measurement of m ass Aux is the most accurate means of assessing short-term a blation ra tes. The errors in short-term ablation measurements by any m ethod are largely compensatory and consequently do not inAuence long-period m ass-balance estima tes . RESUME. Erreurs des mesures d'ablation cl court terme de la surface de glace enfonte. Des changements rapides dans le temps et l'espace du micro-relief de la surface d'un glacier soumise a l'a blation et la fusion due a la radiation dans la croute superficielle de la glace. affecte serieusement la precision des mesures d'ablation a court terme. Les techniques variees utilisees comm unement (balises, ablatometres, ablatographes) et quelques methodes nouvelles (mesure du debit d'un petit bassin de drainage superficiel , et perte de masse mesuree directement sur des carottes) sont revues d ' une maniere critique a la lumi ere de ces pheno menes.Les implications pour les etudes d'energie et de m asse sont discutees.Il apparait que la mesure directe du Aux d e masse est le moyen le plus prec is pour ev iter l'effet de mesures a court terme de la vitesse d'a blation. Les erreurs de mesures d'ablation a court terme pour chaque methode se compensent largement et en consequence n'i nAuence pas les timations du bilan de masse pour un e longue periode d'observation .

In situ quantification of supraglacial cryoconite morphodynamics using time-lapse imaging: an example from Svalbard

“…Assuming a typical 70 day High Arctic melt season (Hodson and others, 2000), this movement is insignificant at the scale of the glacier, being equivalent to 0.37 m a -1 . Therefore, based on these preliminary data from a typical patch of dispersed cryoconite, and knowledge of distinctly linear relationships between cryoconite area and mass (Cook and others, 2010), we find little evidence for mass transfer of significant quantities of cryoconite across the ice surface by 'washing' by meltwater flow over (or through) the so-called 'weathering crust' (Mü ller and Keeler, 1969). This stability has significant 4.…”

Near‐surface hydraulic conductivity of northern hemisphere glaciers

“…The potential for the water table to be influenced by the melt rate and ingress of surface water into the weathering crust was further considered by using site altitude and the energy balance model melt output (M) for the 1 hr time period preceding the observation of K as explanatory variables. Further, on the basis of Muller and Keeler's () conceptual model of weathering crust development, cumulative SWR in receipt since (a) freezing, (b) the previous rainfall event, and (c) the period of dominant turbulent fluxes was calculated as variables.…”

“…However, it is important to note that such a cold wave propagates downwards (Irvine‐Fynn, Hodson, et al, ; Paterson, ), so any refreezing will occur in the less dense, more porous upper weathering crust and hence may have a greater influence on K than would be expected. Rainfall events and cloudy periods, where turbulent fluxes dominate the energy balance equation (see Hock, ), are identified as crucial for resetting of the weathering crust surface (Müller & Keeler, ). Observations of summer rainfall are limited within our dataset; however, we assume that precipitation, as measured at local automated weather stations, is in the form of rain either supported by in situ observations or as defined by air temperatures in excess of 4 °C.…”

“…Modelled data correlated well with measured values during the period for which directly measured SWR in was available ( r 2 = 0.81). With these data, cumulative energy input (MJ m −2 ) from SWR in since the last freeze event (i.e., temperature < 0 °C) was calculated to explore the qualitative observations of Müller and Keeler () regarding weathering crust development processes. For glaciers with full meteorological data, meltwater production (M) was modelled using a point‐based energy balance model (Brock & Arnold, ) at all auger hole sites for each glacier, with a modification applied to arctic glaciers to account for the high solar azimuth (Irvine‐Fynn et al, ).…”

“…Clear skies lead to glacier surface energy balance dominated by radiative fluxes, which promote weathering crust growth, in some cases of stagnating ice to a depth in excess of 2 m (Fountain & Walder, ; Larson, ). Reduced incident radiation (e.g., due to cloud cover) and high precipitation cause turbulent energy to dominate the glacier surface energy balance, promoting surface lowering, which reduces the thickness of the weathering crust (Müller & Keeler, ; Shumskii, ). Variations in the thickness and porosity of the weathering crust at synoptic and seasonal timescales likely lead to temporal and spatial variability in supraglacial hydraulic permeability, conductivity, and meltwater storage potential.…”