High Mountain Asia (HMA) is among the most vulnerable water towers globally and yet future projections of water availability in and from its high‐mountain catchments remain uncertain, as their ...hydrologic response to ongoing environmental changes is complex. Mechanistic modeling approaches incorporating cryospheric, hydrological, and vegetation processes in high spatial, temporal, and physical detail have never been applied for high‐elevation catchments of HMA. We use a land surface model at high spatial and temporal resolution (100 m and hourly) to simulate the coupled dynamics of energy, water, and vegetation for the 350 km2 Langtang catchment (Nepal). We compare our model outputs for one hydrological year against a large set of observations to gain insight into the partitioning of the water balance at the subseasonal scale and across elevation bands. During the simulated hydrological year, we find that evapotranspiration is a key component of the total water balance, as it causes about the equivalent of 20% of all the available precipitation or 154% of the water production from glacier melt in the basin to return directly to the atmosphere. The depletion of the cryospheric water budget is dominated by snow melt, but at high elevations is primarily dictated by snow and ice sublimation. Snow sublimation is the dominant vapor flux (49%) at the catchment scale, accounting for the equivalent of 11% of snowfall, 17% of snowmelt, and 75% of ice melt, respectively. We conclude that simulations should consider sublimation and other evaporative fluxes explicitly, as otherwise water balance estimates can be ill‐quantified.
Plain Language Summary
The Himalayan mountains are a crucial source of water for local communities and the densely populated lowlands. Snow and glaciers are important for the water cycle in mountains, and many studies have focused on them. However, vegetation can play a central role, too. To better understand the water cycle in a high‐elevation Himalayan basin, we use a computer model that considers physical processes of snow and glaciers, soil, and vegetation in high detail. We simulate the catchment water balance and investigate the importance of single components at different elevations and in different seasons. We find that the transfer of water, in vapor form from the ground, snow and ice and the transpiration of plants are important for the water budget in the study basin, causing more water to return to the atmosphere than ice melt contributing to river runoff. Above 6,500 m above sea level, it is the transformation of snow into water vapor and below 4,000 m the transpiration from vegetation that dominate the water budget. We conclude that models simulating the water cycle in high mountain regions should consider vapor fluxes, as otherwise estimates of the water balance can be misleading.
Key Points
Land surface modeling reveals high altitudinal/subseasonal variability in water balance partitioning in a glacierized Himalayan catchment
Water loss through evapotranspiration, dominated by snow sublimation, exceeds water production from glacier melt by 59% at catchment scale
Depletion of cryospheric water budget is dominated by snow melt, but at high elevations is primarily dictated by snow and ice sublimation
Near-surface air temperature is an important determinant of the surface energy balance of glaciers and is often represented by a constant linear temperature gradients (TGs) in models. Spatio-temporal ...variability in 2 m air temperature was measured across the debris-covered Miage Glacier, Italy, over an 89 d period during the 2014 ablation season using a network of 19 stations. Air temperature was found to be strongly dependent upon elevation for most stations, even under varying meteorological conditions and at different times of day, and its spatial variability was well explained by a locally derived mean linear TG (MG–TG) of −0.0088°C m−1. However, local temperature depressions occurred over areas of very thin or patchy debris cover. The MG–TG, together with other air TGs, extrapolated from both on- and off-glacier sites, were applied in a distributed energy-balance model. Compared with piecewise air temperature extrapolation from all on-glacier stations, modelled ablation, using the MG–TG, increased by <1%, increasing to >4% using the environmental ‘lapse rate’. Ice melt under thick debris was relatively insensitive to air temperature, while the effects of different temperature extrapolation methods were strongest at high elevation sites of thin and patchy debris cover.
Melt from supraglacial ice cliffs is an important contributor to the mass
loss of debris-covered glaciers. However, ice cliff contribution is
difficult to quantify as they are highly dynamic ...features, and the paucity
of observations of melt rates and their variability leads to large modelling
uncertainties. We quantify monsoon season melt and 3D evolution of four ice
cliffs over two debris-covered glaciers in High Mountain Asia (Langtang
Glacier, Nepal, and 24K Glacier, China) at very high resolution using
terrestrial photogrammetry applied to imagery captured from time-lapse
cameras installed on lateral moraines. We derive weekly flow-corrected digital elevation models (DEMs)
of the glacier surface with a maximum vertical bias of ±0.2 m for Langtang Glacier and ±0.05 m for 24K Glacier and use change detection
to determine distributed melt rates at the surfaces of the ice cliffs
throughout the study period. We compare the measured melt patterns with
those derived from a 3D energy balance model to derive the contribution of
the main energy fluxes. We find that ice cliff melt varies considerably
throughout the melt season, with maximum melt rates of 5 to
8 cm d−1,
and their average melt rates are 11–14 (Langtang) and 4.5 (24K) times higher
than the surrounding debris-covered ice. Our results highlight the influence
of redistributed supraglacial debris on cliff melt. At both sites, ice cliff
albedo is influenced by the presence of thin debris at the ice cliff
surface, which is largely controlled on 24K Glacier by liquid precipitation
events that wash away this debris. Slightly thicker or patchy debris reduces
melt by 1–3 cm d−1 at all sites. Ultimately, our observations show a
strong spatio-temporal variability in cliff area at each site, which is
controlled by supraglacial streams and ponds and englacial cavities that
promote debris slope destabilisation and the lateral expansion of the
cliffs. These findings highlight the need to better represent processes of
debris redistribution in ice cliff models, to in turn improve estimates of
ice cliff contribution to glacier melt and the long-term geomorphological
evolution of debris-covered glacier surfaces.
The near‐surface boundary layer can mediate the response of mountain glaciers to external climate, cooling the overlying air and promoting a density‐driven glacier wind. The fundamental processes are ...conceptually well understood, though the magnitudes of cooling and presence of glacier winds are poorly quantified in space and time, increasing the forcing uncertainty for melt models. We utilize a new data set of on‐glacier meteorological measurements on three neighboring glaciers in the Swiss Alps to explore their distinct response to regional climate under the extreme 2022 summer. We find that synoptic wind origins and local terrain modifications, not only glacier size, play an important role in the ability of a glacier to cool the near‐surface air. Warm air intrusions from valley or synoptically‐driven winds onto the glacier can occur between ∼19% and 64% of the time and contribute between 3% and 81% of the total sensible heat flux to the surface during warm afternoon hours, depending on the fetch of the glacier flowline and its susceptibility to boundary layer erosion. In the context of extreme summer warmth, indicative of future conditions, the boundary layer cooling (up to 6.5°C cooler than its surroundings) and resultant katabatic wind flow are highly heterogeneous between the study glaciers, highlighting the complex and likely non‐linear response of glaciers to an uncertain future.
Plain Language Summary
The presence of a 0°C ice surface cools the near‐surface air and generates a unique micro‐climate that complicates a glaciers response to future warming. Using a new data series on three glaciers during an extreme summer of 2022, we explore how variable this cooling is in space and time and investigate the factors that can control it. We focus largely on the role of valley and synoptic winds that are found to affect glaciers of varying size and orientation differently, influencing the amount of heat transfer to the ice surface that glaciers receive from outside its own micro‐climate. Moreover, we find that the presence of glacier winds can act to enhance or reduce overall heat transfer to the glacier, depending on the wind strength and degree of boundary layer disruption. We highlight the complexities that are ignored in simpler melt modeling frameworks and demonstrate how, especially under extreme summer heat, indicative of future conditions, static parameters to relate glacier melt to temperature are likely to be inappropriate.
Key Points
Glacier size and alignment with valley/synoptic wind gradients control the magnitude of near‐surface cooling
Valley/synoptic winds can occur between 19% and 64% of the time and contribute between 3% and 81% of total sensible heat to the ice surface
Localized cooling and turbulence in the boundary layer increase the complexity and non‐linearity of glacier response to climate
We explore the robustness and transferability of parameterizations of cloud radiative forcing used in glacier melt models at two sites in the Swiss Alps. We also look at the rationale behind some of ...the most commonly used approaches, and explore the relationship between cloud transmittance and several standard meteorological variables. The 2 m air-temperature diurnal range is the best predictor of variations in cloud transmittance. However, linear and exponential parameterizations can only explain 30–50% of the observed variance in computed cloud transmittance factors. We examine the impact of modelled cloud transmittance factors on both solar radiation and ablation rates computed with an enhanced temperature-index model. The melt model performance decreases when modelled radiation is used, the reduction being due to an underestimation of incoming solar radiation on clear-sky days. The model works well under overcast conditions. We also seek alternatives to the use of in situ ground data. However, outputs from an atmospheric model (2.2 km horizontal resolution) do not seem to provide an alternative to the parameterizations of cloud radiative forcing based on observations of air temperature at glacier automatic weather stations. Conversely, the correct definition of overcast conditions is important.
Hip fractures represent a major challenge for physicians as well as society as a whole. Both poor functional status and delay to surgery are well known risk factors for negative outcomes. We ...hypothesized that the timing of the operation is more important for frail older people than older people without functional limitations before fracture.
We performed a prospective multicenter cohort study on 806 consecutive patients, 75 years of age or older, admitted with a fragility hip fracture to three hospitals in the Emilia-Romagna Region (Italy). All three hospitals had a comanaged care model, and the patients were under the shared responsibility of an orthopedic surgeon and a geriatrician.
Functional status assessed as instrumental activities of daily living was an important predictor of survival after 1 year from fracture. After adjusting for confounders, the hazard ratios per 1 point score of increase from 0 to 8 was 1.30 (95% confidence interval 1.19-1.42, p = .000). Time to surgery increased 1-year mortality in patients with a low instrumental activities of daily living score (hazard ratios per day of surgical delay 1.14, 95% confidence interval 1.06-1.22, p < .001) and intermediate instrumental activities of daily living score (hazard ratios 1.21, 95% confidence interval 1.09-1.34, p < .001) but was an insignificant risk factor in functionally independent patients (hazard ratios 1.05 95% confidence interval 0.79-1.41, p = .706).
Surgery delay is an independent factor for mortality in older patients after hip fracture but only for the frail older people with prefracture functional impairment. If our results are confirmed, a more intensive approach should be adopted for older people with hip fractures who have disabilities.
ABSTRACT
Near-surface air temperature is an important determinant of the surface energy balance of glaciers and is often represented by a constant linear temperature gradients (TGs) in models. ...Spatio-temporal variability in 2 m air temperature was measured across the debris-covered Miage Glacier, Italy, over an 89 d period during the 2014 ablation season using a network of 19 stations. Air temperature was found to be strongly dependent upon elevation for most stations, even under varying meteorological conditions and at different times of day, and its spatial variability was well explained by a locally derived mean linear TG (MG–TG) of −0.0088°C m
−1
. However, local temperature depressions occurred over areas of very thin or patchy debris cover. The MG–TG, together with other air TGs, extrapolated from both on- and off-glacier sites, were applied in a distributed energy-balance model. Compared with piecewise air temperature extrapolation from all on-glacier stations, modelled ablation, using the MG–TG, increased by <1%, increasing to >4% using the environmental ‘lapse rate’. Ice melt under thick debris was relatively insensitive to air temperature, while the effects of different temperature extrapolation methods were strongest at high elevation sites of thin and patchy debris cover.
PDF
