Study On Formation Mechanism Of Ice Accretion On Wires:Observations And Simulations

The temporal variations of ice thickness, structures of the atmospheric boundary layer, spectra of fog droplets and rain droplets, and meteorological elements (air temperature, pressure, wind speed, wind direction) were observed at the Enshi radar station, which located at the top of Shiban Hill in the southwest Hubei province, in the winter of2008and2009. Meanwhile, we obtained the observation of icing on the500kV high voltage transmission lines, including ice thickness, duration of ice accretion and meteorological elements. The variation of ice thickness and meteorological conditions during ice accretion on wires were analyzed, together with a simulation of ice thickness; The variation of ice thickness and meteorological conditions during ice accretion on the500kV high voltage transmission lines were analyzed, together with a contrast to icing conditions on wires; Both direct and indirect effects of freezing drizzle on ice accretion were analyzed for10freezing drizzle events; Finally, The variations of mass concentration of hydrometeors and median volume diameter of fog/cloud droplets has been simulated using Weather Research and Forecasting (WRF) mesoscale model. The main conclusions are as follows:(1) Icing occurred during the passage of a cold front, with a high-pressure system and a cold temperature trough at850hPa, and a southwesterly at500hPa that provided abundant moisture. Ice formation usually started in the evening or early morning, and ice shed around noon the following day when the temperature was-1℃to0℃. The precipitation was mainly freezing rain when the structures of temperature profile was "cold-warm-cold", while the precipitation was freezing drizzle or snow when the whole temperature profile was lower than0℃. The average liquid water content of the fog droplet was distinctly greater during the growth period than during the other three periods, and there was precipitation during the growth period in each case of ice accretion. The growth rate of the ice thickness was clearly correlated with the liquid water content. Simulations using empirical equations were carried out, and the simulated ice thickness agreed with observations fairly well.(2) Temperature at Zhangen tower#307was positively correlated with the temperature at Enshi radar station. The temperature on the surface of the high voltage transmission line was2-4℃higher than ambient air temperature, although the temperatures were positively correlated. Ice formation temperature was about-2℃and ice shedding temperature was about-2to-1℃on the high voltage transmission line, both of which were lower than the temperature threshold values on the test cable. Ice thickness increased slightly when the ice was thick, while ice thickness was significantly affected by temperature variation when the ice was thin. The calculated liquid water content was correlated with observed LWC, but the calculated value was greater.(3) The rain intensity, liquid water content, and diameter of freezing drizzle were almost at low values, and the number concentration was positively correlated with liquid water content. The microphysical properties of fog were suppressed by freezing drizzle, which were at lower values. The smaller fog droplets were affected by freezing drizzle slightly, but the larger fog droplets were affected by freezing drizzle significantly, with transitory increase at beginning and gradual decrease in persistent precipitation. The threshold diameter dividing the small and large fog droplets was correlated to rain intensity. The relationships among the microphysical properties of fog droplets were also affected by freezing drizzle. The relationships were all positive when effects of freezing drizzle were small, but weak positive correlations or even negative correlations emerged as different variation degrees of microphysical properties appeared due to activation/condensation, deactivate/evaporation (ice-crystal effect), and collision-coalescence (turbulent and raindrops’ effect). The direct contribution of freezing drizzle to ice thickness was calculated. Considering both direct and indirection effects, freezing drizzle was the "catalyst" which causing serious icing condition.(4) The variations of mass concentration of hydrometeors and median volume diameter of fog/cloud droplets has been simulated using WRF mesoscale model coupled with the Thompson’s microphysical scheme. The simulations show the inversion layer occurred frequently during icing period which was agreed with the observations. The variations of mass concentration and median volume diameter were well simulated, especially the occurrence time of maximum value of cloud water mass concentration, and emergence of rainfall and snow. There would be the maximal value center of mass concentration of cloud water at about0.3g m-3over the Enshi radar station. Then the maximal value center would move downward and bring a lot of supercooled water which was in favour of the development of ice accretion at Enshi radar station.