Sensitivity of Glacier Runoff to Winter Snow Thickness Investigated for Vatnajökull Ice Cap, Iceland, Using Numerical Models and Observations

Abstract

Several simulations of the surface climate and energy balance of Vatnajökull ice cap, Iceland, are used to estimate the glacier runoff for the period 1980–2015 and the sensitivity of runoff to the spring conditions (e.g., snow thickness). The simulations are calculated using the snow pack scheme from the regional climate model HIRHAM5, forced with incoming mass and energy fluxes from the numerical weather prediction model HARMONIE-AROME. The modeled runoff is compared to available observations from two outlet glaciers to assess the quality of the simulations. To test the sensitivity of the runoff to spring conditions, simulations are repeated for the spring conditions of each of the years 1980–2015, followed by the weather of all summers in the same period. We find that for the whole ice cap, the variability in runoff as a function of varying spring conditions was on average 31% of the variability due to changing summer weather. However, some outlet glaciers are very sensitive to the amount of snow in the spring, as e.g., the variation in runoff from Brúarjökull due to changing spring conditions was on average 50% of the variability due to varying summer weather.

Glacier Changes in Iceland From ∼1890 to 2019

“…(2) Simulation of the surface mass balance of Vatnajökull for the years 1980/81 to 1991/92 from the HIRHAM5 snowpack model that uses MODIS albedo (Schmidt et al, 2017) and is forced with a ERA-Interim downscaling using the HARMONIE-AROME model at 2.5 km resolution (Schmidt et al, 2019) (see shaded area in Figure 3A). Data from automatic weather stations and glaciological surface mass balance, and runoff measurements were used to constrain the model (Schmidt et al, 2018). 3The non-surface mass-balance estimates from Jóhannesson et al (2020) are taken into account and added to the glaciological and modeled surface mass balance listed above.…”

Dynamic simulations of Vatnajökull ice cap from 1980 to 2300

“…HARMONIE-AROME is forced by ERA-Interim reanalysis data at the lateral boundaries at 6 h intervals. Since HARMONIE-AROME is non-hydrostatic and calculates precipitation prognostically, it provides a better representation of the accumulation in areas with high orographic forcing than HIRHAM5 (Schmidt and others, 2018).…”

“…The offline model can be forced by incoming energy and mass components from another climate model, although then additional missing feedbacks have to be taken into considerations. Schmidt and others (2018) used the model forced with energy and mass fluxes from the NWP HARMONIE-AROME, and achieved improved simulations of the SMB compared to using HIRHAM5 forcing. A similar approach will be used in this study, as the CORDEX simulations will all be used to force the snow pack scheme.…”

“…Lenaerts and Van Den Broeke, 2012; Agosta and others, 2015) and Iceland (e.g. Schmidt and others, 2017, 2018).…”

“…Vatnajökull ice cap, the largest ice cap in Iceland and the focus of this study, experienced slightly positive SMB in the 1980s, but SMB shifted towards negative after the mid-1990s following a sudden increase in sea-surface temperatures at the South-East coast (Björnsson and others, 2013). The RCM HIRHAM5 has been used for studies of the climate of Vatnajökull by Schmidt and others (2017, 2018). Only reanalysis-driven runs were used for these studies, and therefore only the reconstruction of the past climate was evaluated.…”