Hendrik Huwald

Distributed fiber‐optic temperature sensing for hydrologic systems

Instruments for distributed fiber-optic measurement of temperature are now available with temperature resolution of 0.01°C and spatial resolution of 1 m with temporal resolution of fractions of a minute along standard fiber-optic cables used for communication with lengths of up to 30,000 m. We discuss the spectrum of fiber-optic tools that may be employed to make these measurements, illuminating the potential and limitations of these methods in hydrologic science. There are trade-offs between precision in temperature, temporal resolution, and spatial resolution, following the square root of the number of measurements made; thus brief, short measurements are less precise than measurements taken over longer spans in time and space. Five illustrative applications demonstrate configurations where the distributed temperature sensing (DTS) approach could be used: (1) lake bottom temperatures using existing communication cables, (2) temperature profile with depth in a 1400 m deep decommissioned mine shaft, (3) air-snow interface temperature profile above a snow-covered glacier, (4) air-water interfacial temperature in a lake, and (5) temperature distribution along a first-order stream. In examples 3 and 4 it is shown that by winding the fiber around a cylinder, vertical spatial resolution of millimeters can be achieved. These tools may be of exceptional utility in observing a broad range of hydrologic processes, including evaporation, infiltration, limnology, and the local and overall energy budget spanning scales from 0.003 to 30,000 m. This range of scales corresponds well with many of the areas of greatest opportunity for discovery in hydrologic science.

Albedo effect on radiative errors in air temperature measurements

Most standard air temperature measurements are subject to significant errors mainly due to sensor heating by solar radiation, even when the measurement principle is accurate and precise. We present various air temperature measurements together with other measurements of meteorological parameters using different sensor systems at a snow-covered and a vegetated site. Measurements from naturally ventilated air temperature sensors in multiplate shields are compared to temperatures measured using sonic anemometers which are unaffected by solar radiation. Over snow, 30 min mean temperature differences can be as large as 10°C. Unshielded thermocouples were also tested and are generally less affected by shortwave radiation. Temperature errors decrease with decreasing solar radiation and increasing wind speed but do not completely disappear at a given solar radiation even in the presence of effective ventilation. We show that temperature errors grow faster for reflected than for incident solar radiation, demonstrating the influence of the surface properties on radiative errors, and we detect the albedo as a variable with major influence on the magnitude of the error as well as a key quantity in possible error correction schemes. An extension is proposed for an existing similarity regression model to correct for radiative errors; thus, surface-reflected shortwave radiation is identified as a principal source of error and the key variable for obtaining a unique nondimensional scaling of radiative errors.

Evolution of superficial lake water temperature profile under diurnal radiative forcing

In lentic water bodies, such as lakes, the water temperature near the surface typicallyincreases during the day, and decreases during the night as a consequence of the diurnalradiative forcing (solar and infrared radiation). These temperature variations penetratevertically into the water, transported mainly by heat conduction enhanced by eddy diffusion,which may vary due to atmospheric conditions, surface wave breaking, and internaldynamics of the water body. These two processes can be described in terms of an effectivethermal diffusivity, which can be experimentally estimated. However, the transparency of thewater (depending on turbidity) also allows solar radiation to penetrate below the surface intothe water body, where it is locally absorbed (either by the water or by the deployed sensors).This process makes the estimation of effective thermal diffusivity from experimental watertemperature profiles more difficult. In this study, we analyze water temperature profiles in alake with the aim of showing that assessment of the role played by radiative forcing isnecessary to estimate the effective thermal diffusivity. To this end we investigate diurnalwater temperature fluctuations with depth. We try to quantify the effect of locally absorbedradiation and assess the impact of atmospheric conditions (wind speed, net radiation) on theestimation of the thermal diffusivity. The whole analysis is based on the results of fiber opticdistributed temperature sensing, which allows unprecedented high spatial resolutionmeasurements ( 4 mm) of the temperature profile in the water and near the water surface.

Field study of the dynamics and modelling of subgrid-scale turbulence in a stable atmospheric surface layer over a glacier

A field experiment – the Snow Horizontal Array Turbulence Study (SnoHATS) – has been performed over an extensive glacier in Switzerland in order to study small-scale turbulence in the stable atmospheric surface layer, and to investigate the role, dynamics and modelling of the subgrid scales (SGSs) in the context of large-eddy simulations. The a priori data analysis aims at comparing the role and behaviour of the SGSs under stable conditions with previous studies under neutral or unstable conditions. It is found that the SGSs in a stable surface layer remain an important sink of temperature variance and turbulent kinetic energy from the resolved scales and carry a significant portion of the fluxes when the filter scale is larger than the distance to the wall. The fraction of SGS fluxes (out of the total fluxes) is found to be independent of stability. In addition, the stress–strain alignment is similar to the alignment under neutral and unstable conditions. The model coefficients vary considerably with stability but in a manner consistent with previous findings, which also showed that scale-dependent dynamic models can capture this variation. Furthermore, the variation of the coefficients for both momentum and heat SGS fluxes can be shown to be better explained by stability parameters based on vertical gradients, rather than vertical fluxes. These findings suggest that small-scale turbulence dynamics and SGS modelling under stable conditions share many important properties with neutral and convective conditions, and that a unified approach is thus possible. This paper concludes with a discussion of some other challenges for stable boundary-layer simulations that are not encountered in the neutral or unstable cases.

Stream Temperature Response to Three Riparian Vegetation Scenarios by Use of a Distributed Temperature Validated Model

Elevated in-stream temperature has led to a surge in the occurrence of parasitic intrusion proliferative kidney disease and has resulted in fish kills throughout Switzerlands waterways. Data from distributed temperature sensing (DTS) in-stream measurements for three cloud-free days in August 2007 over a 1260 m stretch of the Boiron de Merges River in southwest Switzerland were used to calibrate and validate a physically based one-dimensional stream temperature model. Stream temperature response to three distinct riparian conditions were then modeled: open, in-stream reeds, and forest cover. Simulation predicted a mean peak stream temperature increase of 0.7 °C if current vegetation was removed, an increase of 0.1 °C if dense reeds covered the entire stream reach, and a decrease of 1.2 °C if a mature riparian forest covered the entire reach. Understanding that full vegetation canopy cover is the optimal riparian management option for limiting stream temperature, in-stream reeds, which require no riparian set-aside and grow very quickly, appear to provide substantial thermal control, potentially useful for land-use management.

Estimation of wet surface evaporation from sensible heat flux measurements

A new method is proposed to estimate wet surface evaporation by means of measurements of sensible heat flux and of air temperature, relative humidity, and wind speed at one level only. This formulation is made possible by the linearization of the Bowen ratio, a common assumption in other methods, such as Penmans model and its derivatives. The method will be useful in those cases where the sensible heat flux is more reliably acquired at field scales than the net radiation and the ground heat flux, which are needed in many operational methods because of energy budget considerations. Indeed, the ground heat flux is a notoriously difficult variable to measure on wet surfaces, such as lakes or wetlands, especially at the appropriate length scales, whereas sensible heat flux can be obtained from standard temperature variance methods or other instruments such as scintillometers. The proposed method was tested with field experimental data taken over Lake Geneva in Switzerland, where it showed excellent agreement with evaporation rates measured using eddy covariance techniques.

A multilayer sigma-coordinate thermodynamic sea ice model: Validation against Surface Heat Budget of the Arctic Ocean (SHEBA)/Sea Ice Model Intercomparison Project Part 2 (SIMIP2) data

Keywords: sea ice Reference EPFL-ARTICLE-162504doi:10.1029/2004JC002328View record in Web of Science Record created on 2011-01-24, modified on 2016-08-09

Spatial pattern and stability of the cold surface layer of Storglaciären, Sweden

The mechanisms controlling the spatial distribution and temporal fluctuations of the thermal structure in polythermal glaciers have, to date, been poorly investigated and are not fully understood. ...

Adapting Tilt Corrections and the Governing Flow Equations for Steep, Fully Three-Dimensional, Mountainous Terrain

In recent studies of atmospheric turbulent surface exchange in complex terrain, questions arise concerning velocity-sensor tilt corrections and the governing flow equations for coordinate systems aligned with steep slopes. The standard planar-fit method, a popular tilt-correction technique, must be modified when applied to complex mountainous terrain. The ramifications of these adaptations have not previously been fully explored. Here, we carefully evaluate the impacts of the selection of sector size (the range of flow angles admitted for analysis) and planar-fit averaging time. We offer a methodology for determining an optimized sector-wise planar fit (SPF), and evaluate the sensitivity of momentum fluxes to varying these SPF input parameters. Additionally, we clarify discrepancies in the governing flow equations for slope-aligned coordinate systems that arise in the buoyancy terms due to the gravitational vector no longer acting along a coordinate axis. New adaptions to the momentum equations and turbulence kinetic energy budget equation allow for the proper treatment of the buoyancy terms for purely upslope or downslope flows, and for slope flows having a cross-slope component. Field data show that new terms in the slope-aligned forms of the governing flow equations can be significant and should not be omitted. Since the optimized SPF and the proper alignment of buoyancy terms in the governing flow equations both affect turbulent fluxes, these results hold implications for similarity theory or budget analyses for which accurate flux estimates are important.

Carbon monoxide as a tracer of gas transport in snow and other natural porous media

The movement of air in natural porous media is complex and challenging to measure. Yet gas transport has important implications, for instance, for the evolution of the seasonal snow cover and for water vapor transport in soil. A novel in situmulti-sensor measurement system providing high-resolution observation of gas transport in snow is demonstrated. Carbon monoxide was selected as the tracer gas for having essentially the same density as air, low background concentration, low water solubility, and for being detectable to ? 1 ppmv with small, low-cost, low-power sensors. The plume of 1% CO injections 30 cm below the snow surface was monitored using 28 sensors (4 locations, 7 depths). The CO breakthrough curves obtained at distances of 0.5–1 m were in good agreement with a simple analytical advection-diffusion model. The tracer system appears suitable for a wide range of applications in experimental soil science and hydrology addressing moisture transport and evapotranspiration processes.