The Decaying Near‐Surface Boundary Layer of a Retreating Alpine Glacier

Abstract

Near-surface air temperature (Ta) is a dominant control on the glacier surface energy balance through its influence on sensible heat fluxes, longwave radiation receipt, refreezing, and precipitation phase (Hock, 2005;Jouberton et al., 2022;Sicart et al., 2008). Accordingly, Ta is common to all types of model that aim to determine glacier surface melt and mass balance, ranging from simple degree-day approaches (e.g., Hock, 2005) and models of intermediate complexity (e.g.,

Local Controls on Near‐Surface Glacier Cooling Under Warm Atmospheric Conditions

“…At Corbassière, upper sites are found to be cooler than lower ones even under down-glacier wind conditions, though the gradient of TS with flowline distance is stronger for the occurrence of up-valley winds on that glacier (Figure 4c). The pattern of TS for Arolla is in contrast to earlier observations (see Shaw, Buri, McCarthy, Miles, Ayala, & Pellicciotti, 2023), whereby the magnitudes of TS in 2001 (Figure 4a) are similar to those at comparable flowline distances on Corbassière in 2022. Representing these data on a normalized flowline further highlights the differences of the along-glacier TS between glaciers (Figure 4b), but demonstrates the clear role of up-valley winds in diminishing the down-glacier cooling toward the terminus of the glacier (Figure 4d).…”

“…While our analysis is certainly not exhaustive on this topic, we demonstrate that under warm conditions, the presence of a well-developed glacier boundary layer may act as either net gain (Otemma) or net reduction (Corbassière) in total sensible heat supply to the glacier, despite hours in which a strong katabatic wind layer clearly augments the heat supply to the ice at both sites (Figures 9d and 9f). This is clearly not the same for the decaying Arolla Glacier, however (Figure 9b) which, due to the diminishing boundary layer (Shaw, Buri, McCarthy, Miles, Ayala, & Pellicciotti, 2023), reduces the overall importance of local forcing uncertainties (Table 1).…”

“…There has been a large body of past research exploring meteorological conditions on Arolla (Ayala et al, 2015;Petersen et al, 2013;Shaw, Buri, McCarthy, Miles, Ayala, & Pellicciotti, 2023;Strasser et al, 2004), identifying potential controls on the Ta variability above the glacier. Otemma and Corbassière have been mostly studied in terms of their mass balance (e.g., , with Otemma Glacier also being the subject of numerous geomorphological and sediment transfer experiments (e.g., Müller et al, 2022).…”

“…As no past meteorological records on Otemma and Corbassière are known to exist, temporal changes of their near-surface boundary layer cannot be directly assessed, though given a space-for-time substitution one could hypothesize that those glaciers might suffer a similar fate as Arolla as they reduce in size, especially when they already appear to be susceptible to synoptic erosion (Otemma-Figures 7 and 8). In general, this re-emphasizes the need to address forcing uncertainties and biases on glaciers and attempt to represent them either in a physical or statistical framework that accounts for the potential non-linearities under future conditions (e.g., Shaw, Buri, McCarthy, Miles, Ayala, & Pellicciotti, 2023).…”

“…However, unlike this previous work, we have deployed specific observational networks to address wind interactions near the glacier terminus and its influence on local temperature fluctuations in the glacier boundary layer. Our observational period covers an anomalously warm summer of 2022 following a dry winter (Cremona et al, 2023) whereby glacier snow cover was almost completely absent (Shaw, Buri, McCarthy, Miles, Ayala, & Pellicciotti, 2023). The European Alps experienced their most negative mass balance year since records began (SCNAT, 2022) under climatic conditions which put into question the role of the glacier micro-climate for modifying glacier response in the future.…”