1,721,420 research outputs found
Developing, Testing, and Modeling of an Innovative Thermal Stabilization Method for Alpine Permafrost Protection
No full textCRYO
Modelling snow sublimation to improve estimates of sea level rise
No full textCRYOSThis 5-min presentation was part of the Atmospheric Science Day organized by the Center for Climate Impact and Action (CLIMACT) of UNIL and EPFL
Modeling the monthly mean stream temperature dynamics
No full textWater temperature is a hydrological factor which affects the habitat suitability of many aquatic (fish) species, and is therefore of great concern in the actual context of climate change. Two types of models are currently used to simulate stream temperature: physically-based models, which are typically applied only over limited areas, and regression models, which usually lack the ability to make predictions in ungauged areas. As an attempt to bridge the gap between these two types, we propose a hybrid model based on the energy-balance equation, in which the terms are related to catchment physiographic variables via empirical relationships. The physiographic variables are chosen so as to be available over the entire country (Switzerland), enabling the model to be used in ungauged catchments. This approach is on the one hand more physically-based than the usual regression models – hereby limiting the degree of empiricism associated with its derivation – but on the other hand seeks simplicity and applicability over large areas – making it more practical than the usual physically-based models. In order to test this model, we use it to predict the monthly mean stream temperature over 23 selected medium-sized catchments (3–300 km2) in Switzerland. While selecting the catchments, particular attention is given to cover a large range of different geomorphological conditions, especially regarding altitude, slope and aspect. It is shown that the model compares favorably with standard empirical models such as multi-linear regression.CRYOSECH
Measuring snow transport and sublimation near Princess Elisabeth Station, Antarctica
No full textCRYO
Investigating the Monthly Mean Stream Temperature Dynamics
No full textAffecting the habitat suitability of many fish species, water temperature is a hydrological factor of great concern in the actual context of climate change. Despite more than 40 years of research on this topic, the impact of landscape on the dynamics of stream temperature is still not entirely understood. In the present study, we analyzed the monthly mean stream temperature measurements collected in 26 medium-sized catchments (3–300 km2) in Switzerland. While selecting the catchments, particular attention was given to cover a large range of different geomorphological conditions, especially regarding altitude, slope and aspect. Despite these differences, it was surprisingly found that the thermal regimes of almost all the investigated streams followed a same annual trend. Only the amplitude and the minimum value of this trend were observed to differ between the individual catchments. These two factors could be successfully related to geomorphological characteristics of the catchments using multi-linear regression. The shape of the annual trend was found to vary from one year to the other. This inter-annual variability was attributed to climate, based on the significant correlation between the annual trend and air temperature. As a result of the present study, we obtained a regression model to estimate the monthly mean stream temperature in ungauged catchments based on country-wide available geomorphological variables and the average of the monthly mean air temperature over Switzerland.CRYOSECH
How important are snow transport and sublimation for the mass balance of Antarctica?
No full textThis presentation is part of the session 'Contributing to climate science through research in polar and high-altitude regions: the early-career researchers' point of view' in the Geneva Cryosphere HUB of the COP26 conference. We discuss the mass balance of Antarctica and explain how the modelling of snow transport by the wind and associated sublimation can be improved. These efforts may contribute to better estimates of sea level rise.CRYOSWe thank the Swiss Polar Institute, especially Danièle Rod, for initiating and moderating the session. A video recording of the whole session is available online (link below)
Using GIS data and satellite-derived irradiance to optimize siting of PV installations in Switzerland
Full text linkFor a successful distribution strategy of PV installations, it does not suffice to choose the locations with highest annual total irradiance. Attention needs to be given to spatial correlation patterns of insolation to avoid large system-wide variations, which can cause extended deficits in supply or might even damage the electrical network. One alternative goal instead is to seek configurations that provide the smoothest energy production, with the most reliable and predictable supply. Our work investigates several scenarios, each pursuing a different strategy for a future renewable Switzerland without nuclear power. Based on an estimate for necessary installed capacity for solar power we first use heuristics to pre-select realistic placements for PV installations. Then we apply optimization methods to find a subset of locations that provides the best possible combined electricity production. Depending on the initial assumptions and constraints, the resulting distribution schemes for PV installations vary with respect to required surface area, annual total and lowest short-term production, and illustrate how important it is to clearly define priorities and policies for a future renewable Switzerland.CRYOSDESLRA
High-speed imaging of snow saltation: wind tunnel experiments using natural snow
No full textDrifting snow is a multi-scale process. It is composed of particles rolling and sliding along the surface, particles in saltation following short ballistic trajectories in the first 10 cm above the surface and particles in suspension at higher regions of the atmosphere. Drifting snow is currently represented in some regional and mesoscale atmospheric models by taking into account its effect on snow height, snow sublimation and snow densification. Snow saltation is a sub-grid process in these models and is therefore parameterized. However, the current parameterizations are based on limited field and wind tunnel measurements and do not take into account the effect of the bed characteristics, as grain size, inter-particle cohesion and snow density, on the saltation dynamics. In order to improve the current saltation models, we conducted wind tunnel experiments using natural snow at the WSL Institute for Snow and Avalanche Research SLF to measure the kinematics and shape of particles in saltation. The wind tunnel is located at 1670 m above sea level, has a cross section area of 1x1 m2 and a total length of 14 m. Naturally deposited snow is collected in trays after each snowfall and transported to the tunnel without disturbing the snowpack. We used a high speed camera, aquiring images at 5 kHz with backlighting provided by an LED to capture images of saltating snowflakes. We measured wind speed with an array of pitot tubes positioned 2-10 cm above the snowbed. We additionally measured the density and hardness of the snow cover before the experiments using a box density cutter and a Snow Micro Pen (SMP), respectively. We process the images with a 2D Particle Tracking Velocimetry (PTV) algorithm allowing us to obtain Eulerian and Lagrangian statistics of the kinematic quantities as well as estimates of the snowflake characteristics like size, aspect ratio and orientation. In addition, by assuming a constant particle density, we derive particle mass flux profiles. The results show that the particle size distribution in saltation can indeed be characterized by a lognormal or a gamma distribution. From the analysis of the particle streamwise velocity profiles, it is clear that the assumption of a constant particle speed inside the saltation layer (common in simple saltation models) might not be a good approximation even for low friction velocities. We will present in how far we can assess the influence of the snow properties on mass flux and saltation dynamics as a basis to validate recent model results on the influence of inter-particle cohesion for example. Moreover, this data set will contribute to the development of new parameterizations for snow saltation mass flux and streamwise velocity that would take into account the effect of snow density and hardness.CRYO
A theoretical assessment of heat transfer by ventilation in homogeneous snowpacks
No full textThe effects of heat transfer by ventilation in snow are investigated theoretically. We draw together standard analytical results for fluid flow in porous media and apply them to the case of steady flows induced by periodic roughness elements. These solutions are used to estimate the relative magnitude of ventilation heat transfer in snow. We conclude that topography-driven ventilation is unlikely to have a significant impact on the larger scale energy balance of snow-covered regions since the airflow is confined to a shallow penetration depth or just the roughness elements themselves, rather than the bulk snowpack. In particular, for the limiting case of very warm and moisture saturated air flowing over a melting snow cover, we show that latent and sensible heat due to ventilation have about equal contributions and that this contribution is small compared to the overall surface flux as predicted by the Monin-Obukhov theory
Constraining Latent and Sensible Heat Fluxes during Drifting and Blowing Snow Events in Antarctica using in–situ Measurements and Large–Eddy Simulations
No full textReliable predictions of sea level rise require a quantitative understanding of the mass balance of the Antarctic ice sheet. Water vapor exchange between snow and the atmospheric boundary layer may be an important term in the mass balance equation but current estimates for this process are highly uncertain. The exchange of water vapor becomes particularly strong during drifting and blowing snow events, which are frequent in Antarctica. In these conditions, measured turbulent fluxes based on the Eddy–Covariance (EC) method or the Monin–Obukhov (MO) bulk formula are associated with increased uncertainties that are difficult to quantify. The EC raw data can contain artifacts because blowing snow particles temporarily perturb the measurement signals of ultrasonic anemometers and open–path infrared gas analyzers. The MO bulk approach suffers from the fact that sources or sinks of moisture and heat in the drifting and blowing snow layer violate the assumption of height–constant fluxes. Additionally, strong winds typically result in small vertical differences in temperature and humidity, which makes the MO bulk approach particularly sensitive to instrument–specific biases in temperature and humidity. In view of these limitations, it is difficult to validate models, especially the parametrizations used in large-scale models. Nevertheless, detailed small-scale numerical simulations can help to constrain vapor and heat exchange and to disentangle different sources of uncertainty. In this study, we use Large-Eddy Simulations (LES) as a reference to validate and improve parametrizations of latent and sensible heat fluxes in conditions of drifting and blowing snow. The boundary conditions of the LES simulations are based on field measurements at the Japanese Syowa S17 Station, coastal East Antarctica. Consistent with the almost flat and permanently snow-covered terrain, the LES simulations assume a flat snow surface a the lower boundary of the domain (approximately 38 x 19 x 18 m^3). The transport of snow particles and their interaction with the flat surface is represented by a Lagrangian Stochastic Model coupled with the LES. Vapor and heat exchange between snow particles and the air is based on the energy and mass balance of a spherical ice particle in turbulent flow, neglecting radiative heat transfer. In contrast to most other modeling studies on drifting and blowing snow, the LES simulations do not assume a thermal equilibrium with constant particle temperatures but the model explicitly computes the particle temperatures, which increases the accuracy of the vapor and heat exchange. LES-based steady-state vertical profiles of the latent and sensible heat fluxes are compared with parametrized profiles in a simple one-dimensional model, resembling the approach of existing large-scale models. In the simple model, we focus for simplicity on a suitable representation of the vapour and heat exchange between particles of drifting and blowing snow and the atmosphere, not yet the parametrization of the particle concentration. We propose the following three modifications, which improve significantly the parametrization: (i) additional model levels in the lowest few centimeters above the surface, (ii) prognostic computation of air temperature and specific humidity also at the near-surface levels, and (iii) an empirical expression for the change in particle temperature derived from the LES data. This work is an important step towards reliable parametrizations of drifting snow effects.CRYOSPresentation at the American Meteorological Society’s 24th Symposium on Boundary Layers and Turbulence, held as part of the 103rd AMS Annual Meeting
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