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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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Karen J. Finstad, Edward P. Lozowski, and Lasse Makkonen

Abstract

In this note, we examine a shortcut for calculating the overall collision efficiency of a droplet spectrum, known as the “median volume diameter” (mvd) approximation. By calculating the overall collision efficiency of a circular cylinder for a variety of natural droplet spectra, first precisely using a spectrum weighting approach, and then as approximated using the mvd, as well as several other representative droplet sizes, we show by comparison that the mvd approximation is a good one, with an average absolute error of about 0.02. While trying to give some mathematical justification for why the mvd approximation works, we show that it can be derived from a single-point numerical integration formula, and that extension of this formula to 2, 3 or 4 points should give correspondingly better approximations. Detailed comparisons confirm that use of the 2-point formula reduces the average error by one-half, while the 3- and 4-point formulae can reduce it even more, depending on the type of spectrum.

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Lasse Makkonen, Ross D. Brown, and Paul T. Mitten

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No abstract available

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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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