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Modern Meteorology and Atmospheric Icing

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Modern Meteorology and Atmospheric Icing IWAIS XI, Montréal, June 2005 Modern Meteorology and Atmospheric Icing Svein M. Fikke Meteorological consultant – overhead lines, Lindeveien 1, 1470 Lørenskog, Norway [email protected] Abstract—Although the water cycle in the atmosphere is still not according to local experiences. Now we all agree to described well enough to provide reliable input parameters for encompass all phenomena that cause accretions of water in its the liquid water content of the air or precipitation rate, fine-scale frozen stage on any type of object into the term “icing” [1], atmospheric models can now describe wind speed, wind direction, air temperature and vertical stability taking [2]. We differentiate icing according to the process during topographical details into account with a horizontal scale of 100 which the accretion is formed. m or less. This means that these models can provide much more “Atmospheric icing” is therefore now a global term for: detailed information than any observation network can do - in-cloud icing, concerning both weather and icing data. Examples are shown on - precipitation icing, and the application of such models for detailed studies on power lines. - hoar frost. A comprehensive scheme for combining models for weather dependent impacts on overhead lines and weather forecasting In-cloud icing is often also denoted as (soft and hard) rime models is suggested. icing. Precipitation icing may result in I. INTRODUCTION - freezing rain, TMOSPHERIC icing has been on the agenda for - wet snow, and A international collaboration since the very first IWAIS - dry snow. (although it was not called so then) in Hanover, New Hoar frost is the result of a phase transition direct from Hampshire, in 1982. Since then tremendous efforts are made vapor to solid, also called “ice condensation” (contrary to by icing colleagues around the world, in order to advance our sublimation). Hoar frost is very light and has a fluffy knowledge and understanding of the processes that take place appearance. As it mostly does not affect structures, it is not within the fascinating substance of water around its freezing discussed further here. point. And yet there are still a lot more questions to be Dry snow accretions are reported from Japan and Alaska answered and problems to be solved, relating to icing itself [3]. This is a quite rare phenomenon and little information is and its impacts on the various structures that are exposed to it. available. In this paper we will review briefly the occurrence of icing B. Occurrence of icing phenomena around the world and how it is modeled. The main We all know very well that atmospheric icing occurs from purpose of the paper is to focus on how modern meteorology time to time in most northern countries, and many places can be applied to obtain better quantifications of the weather several times per year. Freezing rain is frequent especially in input parameters to models, such as wind speed, wind Canada and northern USA. Wet snow is also an annual direction, temperature, cloud water, precipitation rate, phenomenon in Iceland and Norway, but occurs frequently in condensation, evaporation, etc. Although there are still a long all circumpolar countries, including Russia, Japan, Canada, way to go here too, it is the opinion of the author that US and in practically all European countries. Rime icing probably the greatest potential for improvements of the affects mostly high altitude structures in all these countries, in various practical applications of existing models for selecting northern parts of Scandinavia it may however be found below design values, or for case studies, are to be found along these 400 m above sea level (m asl). lines. The biggest ice load ever recorded on an overhead power line was observed in Norway in April 1961, 1400 m asl, see II. ICING – WHAT IS IT AND WHERE DOES IT HAPPEN? Fig. 1. The accretion had an elliptic shape with the longer diameter of 1.4 m and the shorter diameter 0.95m. 1 m length A. Icing types of this accretion was collected and taken down to the village During many years it was a bit difficult to discuss “icing” and weighed to 305 kg! with colleagues from other countries and parts of the world, The devastating freezing rain event in eastern Canada and because they all had different perceptions of the phenomenon, the US in January 1998 is indeed very well known in this audience. It may however be more surprising that icing can cause IWAIS XI, Montréal, June 2005 severe damage to power lines in countries like Spain, see Fig. Wherever sub-freezing temperatures and snow can possibly 2 and South Africa, Fig. 3. As can be seen from these pictures, occur, engineers should be aware of any impact of ice and the icing in both these countries can be severe, and electric snow accretions that may influence their structure in a overhead lines as well as wind power installations have to be negative way. Remember, they have skiing resorts in both designed for the additional loads such ice accretions will Australia and New Zealand, therefore icing may be expected impose on the structures. there too. In countries like Greece, Saudi Arabia and India icing events have been indicated, however, to my knowledge, not reported. III. ICING PARAMETERS AND MODELS The accretion of ice on structures is a very complex and nonlinear process which includes both conductor parameters and meteorological parameters. In order to fully understand the icing phenomena, and to establish icing models for practical applications, it is important to examine carefully both 1) the physical accretion processes themselves on an electric conductor wire, a telecommunication tower, wind turbine or other structures exposed to the icing, and 2) the meteorological environment that rules the input data to the models. A. Physical Icing Models Over the last decades many universities and researchers have studied the micro-scale processes of icing on both stiff and rotating objects of many kinds. A main reason for this is not only the engineers’ need for models to calculate loads on Fig. 1. Rime icing on a 22 kV line on Lønahorgi, 1 400 m asl, near Voss, their structures, but I think also it is because H O is such a Norway, April 1961. (Photo: O. Whist) 2 fascinating chemical substance, in particular around its freezing point, and hence provides a wide variety of theoretical challenges that appeal to researchers in fundamental physics and applied mathematics. For in-cloud icing the most important parameters are related to: - liquid water content in the air - droplet size - air temperature - wind speed - wind direction - humidity (degree of vapor saturation) For precipitation icing (wet snow and freezing rain) the similar parameters shall describe: Fig. 2. Icing on a power line in Spain January 2004 (Photo: Iberinco). - precipitation rate - surface air temperature - liquid water content of snow flakes - wind speed - wind direction - temperature - humidity These parameters describe the immediate environment of the object. It as likewise important to include parameters of the accreting object itself, such as surface property, shape, linear dimensions, torsional stiffness, etc. The history and status of such models are not topics for this Fig. 3. Icing in South Africa. (Photo: Eskom.) paper. Lozowski and Makkonen are handling them in another The message of these examples is that only few areas in the paper for this Conference [4]. The interested reader may also world can be considered to be totally “icing-free zones”. look at [5] and [2]. IWAIS XI, Montréal, June 2005 B. Models of the Atmosphere There are of course still shortcomings, especially with As mentioned above, all icing models need input data respect to the general water cycle in the atmosphere, and in describing the local weather conditions during the icing. If the particular the liquid water content of clouds. But it is in this models are not completed with good input data, then any icing respect very important to note that water vapor is the most model will not be as useful as it could be for practical important greenhouse gas, more important for the global air applications on structures. The most relevant weather temperature than CO2. According to this very significant parameters are linked to: influence on the global climate, there are tremendous efforts - clouds (cover, base and type) made world-wide in ongoing research and observation - trajectories of the wind (history of the air mass) projects, with focus on water vapor in the air, evaporation, - stability (vertical temperature distribution (inversions, condensation, cloud formation, initiation of precipitation, etc.)) precipitation intensity, etc. The most comprehensive study of - precipitation (type and intensity) this kind is the “Global Energy and Water Cycle Experiment” - topographical influence (regional and local scale) (GEWEX) and its sub-project “Global Water Vapor Project” - turbulence (GVaP). GEWEX is headed by the World Meteorological The recent years of developments within meteorological Organization (WMO) in collaboration with the UNESCO sciences, together with computer technology, remote sensing, World Climate Research Programme. It has been going on for and telecommunication – also with internet, have improved many years and the results are constantly being implemented their methods and tools dramatically. The increase in in dynamic and physical models for the atmosphere, both for computer capacity has made it possible to refine the model climatic studies and indeed also for forecasting purposes. grids, and horizontal resolutions down to a few tens of km More information on this can be found in www.gewex.org. have been adapted in operational weather forecasting models for limited areas for many years. Such models were often IV. APPLICATIONS OF METEOROLOGICAL MODELS FOR ICING called “HIRLAM” (“High Resolution Limited Area Model”). There are many examples on how data from regular This has advanced the weather forecasts significantly for both weather stations are used in order to calculate ice loadings, as the public and for a great variety of industrial applications.
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