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Lasse Makkonen

Abstract

An objective method is presented for the analysis of rotating multicylinder data in measuring the liquid water content and median volume droplet diameter of icing clouds. The method is based on time-dependent numerical modeling of cylinder icing and requires no data on diameters of the iced cylinders. Developing a fully automatic rotating multicylinder instrument is proposed.

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Lasse Makkonen

Abstract

A time-dependent numerical model of ice accretion on wires, such as overhead conductors, is presented. Simulations of atmospheric icing are made with the model in order to examine the dependence of the accreted ice amount on atmospheric conditions.

The results show that in wet growth (glaze formation) under constant atmospheric conditions, the growth rate increases with time until the process changes to dry growth. In dry growth (rime formation) the growth rate typically increases with time at the beginning of the icing process, but later decreases with time when the ice deposit has grown bigger.

The effect of air temperature on the ice load turns out to be rather small for the first 24 hours of icing in typical dry growth conditions, but it is important for long-term icing. The ultimate ice load may either increase or decrease with decreasing air temperature, depending on the other atmospheric conditions and on the duration of icing. These results largely explain the difficulties encountered in estimating the formation of ice loads by simple methods using the routinely measured meteorological parameters.

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Eva Sundin and Lasse Makkonen

Abstract

Atmospheric ice loads are a major design criterion of tall structures in cold regions. In this paper the possibility to derive the design ice loads using analysis of meteorological observations made routinely at a weather station is studied. Ice loads calculated by extrapolating weather station data and using simplistic ice loading and unloading models are compared with those measured on a 323-m-height lattice TV tower. The comparison is made cumulatively in 3-h intervals over seven winter periods. The results show reasonable agreement in the time of the icing events and in overall loads. In the cases where the cumulative ice loads differ, the discrepancies are mostly due to incorrectly predicted unloading events. This study points out the importance of on-site temperature data for successfully estimating cumulative ice loads over long cold periods.

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Lasse Makkonen and J. R. Stallabrass

Abstract

The theory of Langmuir and Blodgett for the droplet collision efficiency was verified by growing rime ice accretions on rotating cylinders in a wind tunnel. The results show that the theory is in excellent agreement with the experimental data in the studied range of mean cylinder collision efficiency from 0.07 to 0.63.

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Lasse Makkonen, Pertti Lehtonen, and Lauri Helle

Abstract

The accuracy of wind measurements in icing conditions is discussed, and wind tunnel calibrations as well as field comparisons are presented for three heated anemometers that use different measuring principles. It is pointed out that ice-free anemometer calibrations, including those provided by manufacturers, are affected by the blockage effect in wind tunnels that are too small. Some anemometers that measure correctly in a wind tunnel give erroneous results in the field. Overall, measuring mean wind speeds and peak values in icing conditions with the accuracy of about 5% seems possible with the present technology, both with rotational and sonic anemometers, but in the most severe environments only some internally heated rotational anemometers are reliable. Wind measurements in icing conditions without due consideration of anemometer selection, specific instrument problems, calibration inaccuracies, mean vertical velocity component, and anti-icing of the supporting structures may result in very big errors.

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Bjørn Egil Kringlebotn Nygaard, Jón Egill Kristjánsson, and Lasse Makkonen

Abstract

In-cloud icing on aircraft and ground structures can be observed every winter in many countries. In extreme cases ice can cause accidents and damage to infrastructure such as power transmission lines, telecommunication towers, wind turbines, ski lifts, and so on. This study investigates the potential for predicting episodes of in-cloud icing at ground level using a state-of-the-art numerical weather prediction model. The Weather Research and Forecasting (WRF) model is applied, with attention paid to the model’s skill to explicitly predict the amount of supercooled cloud liquid water content (SLWC) at the ground level at different horizontal resolutions and with different cloud microphysics schemes. The paper also discusses how well the median volume droplet diameter (MVD) can be diagnosed from the model output. A unique dataset of direct measurements of SLWC and MVD at ground level on a hilltop in northern Finland is used for validation. A mean absolute error of predicted SLWC as low as 0.08 g m−3 is obtained when the highest model resolution is applied (grid spacing equal to 0.333 km), together with the Thompson microphysics scheme. The quality of the SLWC predictions decreases dramatically with decreasing model resolution, and a systematic difference in predictive skill is found between the cloud microphysics schemes applied. A comparison between measured and predicted MVD shows that when prescribing the droplet concentration equal to 250 cm−3 the model predicts MVDs ranging from 12 to 20 μm, which corresponds well to the measured range. However, the variation from case to case is not captured by the current cloud microphysics schemes.

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