Modeling Wet Snow Accretion on Power Lines Nygaard, Bjørn Egil Kringlebotn; Ágústsson, Hálfdán; Somfalvi-Tóth, Katalin
Journal of applied meteorology and climatology,
10/2013, Letnik:
52, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Methods to model wet snow accretion on structures are developed and improved, based on unique records of wet snow icing events as well as large datasets of observed and simulated weather. Hundreds of ...observed wet snow icing events are logged in detail in an icing database, most of which include an estimate of the mean and maximum diameter of observed icing on overhead power conductors. Observations of weather are furthermore available from a dense network of weather stations. The existing models for wet snow accretion on a standard cylinder are updated with realistic values for the terminal fall speed of wet snowflakes together with a snowflake liquid fraction–based criterion to identify wet snow. The widely used parameterization of the sticking efficiency is found to strongly underestimate the accretion rate. A calibrated parameterization of the sticking efficiency is suggested on the basis of long-term statistics of observed and modeled wet snow loads. Application of the improved method is demonstrated in a high-resolution simulation for a case of observed widespread and intensive wet snow icing in south Iceland. The results form a basis for mapping the climatology of wet snow icing in the complex terrain of Iceland as well as for preparing operational forecasts of wet snow icing and severe weather for overhead power transmission lines in complex terrain.
On 18 November 2008 a commercial aircraft encountered severe turbulence while flying in westerly flow along the southeastern coast of Iceland and descending from 2500 m down to the ground for a safe ...landing. Numerical simulations at horizontal resolutions of 9, 3, and 1 km are compared to the available observations. The simulations reproduce the situation, with an observed severe downslope windstorm at the ground as well as associated amplified lee waves and a rotor aloft, while climate data indicate that all observed westerly windstorms in the region are of the same type and occur in a similarly structured atmosphere. Strong shear turbulence is simulated at the interface of the lee wave and the rotor, as well as inside the rotor. The lee waves and the turbulence patterns are not stationary and as the upstream vertical wind shear increases, the lee wave becomes less steep, but the turbulence increases temporarily while the rotor circulation breaks down. From a forecasting perspective, this event could have been foreseen quite accurately, but not with the NWP tools that were in use for aviation forecasts, as their resolution was simply not adequate for resolving hazardous features of flow in and above complex terrain on the scale of this event. This event underlines the urgency of delivering products from finescale simulations over complex terrain to pilots and forecasters. Such products need to be developed taking into account the transient nature of the flows and the hazards.
This study quantifies the influence of the local topography on sonic anemometer measurements mounted on tall met-masts deployed in Norwegian fjords. This assessment is done by analysing the ...second-order structure of turbulence in the frequency space. The objective is to evaluate how the data collected by these masts can help with the design of future fjord-crossing bridges. Using measurements from eight masts deployed in three different fjords, spectral characteristics common to the different locations were identified. Among the characteristics investigated, the one-point velocity spectra and the vertical coherence of turbulence are of special interest for bridge design. Using only situations with a mean wind speed of 12 m s−1 and above and rather stationary flow conditions, the remaining wind direction sectors could be classified into two main groups: those with a long fetch over smooth topography and those with a rough heterogeneous terrain upstream of the masts. It was found that if the velocity spectra are normalized by an appropriate estimate of the friction velocity, surface-layer scaling may be applicable in Norwegian fjords, at least under near-neutral conditions. The velocity spectra, estimated with long upstream fjord fetches have characteristics consistent with those collected in coastal and offshore environments. The Davenport model showed limited capabilities in describing the co-coherence of turbulence on vertical separations. The dependency of the Davenport decay coefficient with the height and spatial separation is highlighted and may be substantial in mountainous terrain. Although the spectral flow characteristics estimated on the shore of fjords are relevant for the design of fjord-crossing bridge towers, they may not easily be extrapolated for the modelling of the turbulent flow in the middle of the fjord. Nevertheless, they are still valuable to complement computational fluid dynamic simulations, wind tunnel tests, or remote sensing of wind across the fjord.
•The spectral characteristics of wind turbulence are studied in three Norwegian fjords.•The local topography affects mainly low-frequency velocity fluctuations.•Under strong wind conditions, surface-layer scaling may be applicable in fjords.•The flow characteristics in long fjord fetch are similar to those in coastal regions.•Short upstream fjord fetch shows undocumented one-point spectral characteristics.
The paper investigates the local topographic effect on the mean and integral flow characteristics recorded by sonic anemometers mounted on tall masts near the shoreline of three different Norwegian ...fjords. Two years of measurements are analysed, using data from 25 three-dimensional sonic anemometers mounted at heights from 12 m to 95 m above the ground. The goal is to explore the potential and challenges of using wind measurements from the masts located on the shores of the fjords in the design of planned bridge crossings. Therefore, the study explores the deviations of the mean and turbulent flow characteristics from the traditional case of flat and homogeneous terrain. Only records with mean wind speeds of 12 m s−1 and above at all elevations above the ground are considered due to their relevance in buffeting response, which led to the identification of a limited number of sectors representative of strong wind conditions. Mean incidence angles with absolute values above 6° and low mean wind shear are measured in several of the selected sectors. This highlights the major influence of the local terrain and vegetation around the masts on the wind conditions at the mast locations. Nevertheless, non-dimensional variance and covariance estimates of the velocity components are found to be consistent with values previously measured from bridge decks crossing narrow fjords. The paper explores also an alternative approach to compute the friction velocity, the estimation of which is challenging in a fjord-like topography. This first part of the paper focuses on integral flow characteristics, a second follow-on part will investigate in details which eddy wave-numbers are most affected by the local terrain, based on the analysis of the spectra of the velocity fluctuations.
•Integral wind velocity characteristics are analysed in three Norwegian fjords.•Two years of data, from 25 sonic anemometers mounted on tall masts are used.•The study is valuable for future fjord-crossing long-span bridges.•The three-dimensional flow is heavily affected by the local terrain.•The flow characteristics are sometimes consistent with those from other fjord-crossing bridges.
Abstract
In this study, it is demonstrated how temperature, humidity, and wind profile data from the lower troposphere obtained with a lightweight unmanned aerial system (UAS) can be used to improve ...high-resolution numerical weather simulations by four-dimensional data assimilation (FDDA). The combined UAS and FDDA system is applied to two case studies of northeasterly flow situations in southwest Iceland from the international Moso field campaign on 19 and 20 July 2009. Both situations were characterized by high diurnal boundary layer temperature variation leading to thermally driven flow, predominantly in the form of sea-breeze circulation along the coast. The data assimilation leads to an improvement in the simulation of the horizontal and vertical extension of the sea breeze as well as of the local background flow. Erroneously simulated fog over the Reykjanes peninsula on 19 July, which leads to a local temperature underestimation of 8 K, is also corrected by the data assimilation. Sensitivity experiments show that both the assimilation of wind data and temperature and humidity data are important for the assimilation results. UAS represents a novel instrument platform with a large potential within the atmospheric sciences. The presented method of using UAS data for assimilation into high-resolution numerical weather simulations is likely to have a wide range of future applications such as wind energy and improvements of targeted weather forecasts for search and rescue missions.
Dynamical downscaling of atmospheric flow over Iceland has revealed that prominent downslope accelerated flows are not merely extreme events, but rather constitute a strong climatological signal over ...the larger ice caps. Ice caps are characterised by smooth and cold surfaces and both of these properties have previously been found to enhance downslope flows. In this article, we investigate the response of downslope accelerated flow over Hofsjökull in Central Iceland to an increase in surface roughness and a change in surface temperature corresponding to the effect of melting Hofsjökull's ice cap. We do so by exploring the flow over Hofsjökull for a summertime case by means of several numerical sensitivity experiments. In the experiments, we find a stronger downslope flow acceleration with than without the ice cap. While an increased surface roughness distinctly dampens the downslope flow, the effect of changing the surface temperature is minimal. This study is both of general relevance through its exploration of factors affecting downslope acceleration of stably stratified flow and also of interest because glaciers diminish rapidly in a changing climate.
On 12 August 2009, a series of satellite images revealed asymmetric shedding of atmospheric vortices in the lee of Mt. Snæfellsjökull, and their passage a distance of 120 km across Faxaflói Bay and ...over the city of Reykjavík in West Iceland. After landfall, the vortices were detected by a network of surface weather stations. These observations are presented and with the aid of a numerical simulation, they are discussed in view of existing theories of orographic wakes and vortex shedding. In general, the flow is in line with existing knowledge, but there is a remarkable absence of vortices with anticyclonic rotation. Atmospheric conditions for vortices of this kind are most often favorable in late winter and spring and they are a forecasting challenge.
Since 2014, 11 tall meteorological masts have been erected in
coastal areas of mid-Norway in order to provide observational data for a
detailed description of the wind conditions at several potential ...fjord
crossing sites. The planned fjord crossings are part of the Norwegian Public
Roads Administration (NPRA) Coastal Highway E39 project. The meteorological
masts are 50–100 m high and located in complex terrain near the shoreline
in Halsafjorden, Julsundet and Storfjorden in the Møre og Romsdal county
of Norway. Observations of the three-dimensional wind vector are made at 2–4
levels of each mast with a temporal frequency of 10 Hz. The dataset is
corroborated with observed profiles of temperature at two masts, as well as
observations of precipitation, atmospheric pressure, relative humidity and
dew point at one site. The first masts were erected in 2014, and the
measurement campaign will continue until at least 2024. The current paper
describes the observational setup, and observations of key atmospheric
parameters are presented and put in context with observations and
climatological data from a nearby reference weather station. The 10 min
and 10 Hz wind data, as well as other meteorological parameters, are publicly
available through the Arctic Data Centre (https://doi.org/10.21343/z9n1-qw63; Furevik et
al., 2019).
In this article, we present an overview of the HyIV-CNRS-SecORo (Hydralab IV-CNRS-Secondary Orography and Rotors Experiments) laboratory experiments carried out in the CNRM (Centre National de ...Recherches Météorologiques) large stratified water flume. The experiments were designed to systematically study the influence of double obstacles on stably stratified flow. The experimental set-up consists of a two-layer flow in the water tank, with a lower neutral and an upper stable layer separated by a sharp density discontinuity. This type of layering over terrain is known to be conducive to a variety of possible responses in the atmosphere, from hydraulic jumps to lee waves and highly turbulent rotors. In each experiment, obstacles were towed through the tank at a constant speed. The towing speed and the size of the tank allowed high Reynolds-number flow similar to the atmosphere. Here, we present the experimental design, together with an overview of laboratory experiments conducted and their results. We develop a regime diagram for flow over single and double obstacles and examine the parameter space where the secondary obstacle has the largest influence on the flow. Trapped lee waves, rotors, hydraulic jumps, lee-wave interference and flushing of the valley atmosphere are successfully reproduced in the stratified water tank. Obstacle height and ridge separation distance are shown to control lee-wave interference. Results, however, differ partially from previous findings on the flow over double ridges reported in the literature due to the presence of nonlinearities and possible differences in the boundary layer structure. The secondary obstacle also influences the transition between different flow regimes and makes trapped lee waves possible for higher Froude numbers than expected for an isolated obstacle.
Downslope windstorms at Kvísker in Southeast Iceland are explored using a mesoscale model, observations and numerical analysis of the atmosphere. Two different types of gravity-wave induced ...windstorms are identified. At the surface, their main difference is in the horizontal extent of the lee-side accelerated flow. Type S (Short) is a westerly windstorm, which is confined to the lee-slopes of Mount Öræfajökull, while a Type E (Extended) windstorm occurs in the northerly flow and is not confined to the lee-slopes but continues some distance downstream of the mountain. The Type S windstorm may be characterized as a more pure gravity-wave generated windstorm than the Type E windstorm which bears a greater resemblance to local flow acceleration described by hydraulic theory. The low-level flow in the Type E windstorm is of arctic origin and close to neutral with an inversion well above the mountain top level. At middle tropospheric levels there is a reverse vertical windshear. The Type S windstorm occurs in airmasses of southerly origin. It also has a well-mixed, but a shallower boundary-layer than the Type E windstorms. Aloft, the winds increase with height and there is an amplified gravity wave. Climate projections indicate a possible decrease in windstorm frequency up to the year 2050.
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