Three Types Of Wire Icing Researches Based On High-resolution Observations: Combining Meteorological Elements And Microstructure

In order to study the evolution characteristics of wire icing in three types of weather（freezing rain,snow and supercooled fog）and the microphysical characteristics of ice-forming water sources（freezing rain,snow and supercooled fog）,and for refining the important parameters of different icing types and improving frozen disaster forecast level,wire icing and fog/rain microphysics field observations were conducted at Lushan Meteorological Bureau（29 Dec2015-2 Feb 2016,and 19 Dec 2016-17 Jan 2017）,and at Enshi Radar Station（7 Dec 2018-18Feb 2019）.The high time resolution ice weight,ice thickness,ice image,meteorological elements and fog/rain microphysics data were obtained.The growth and shedding mechanisms of icing in three types of weather（freezing rain,snow,and supercooled fog）were analyzed by classification,the influences of meteorological elements on three types of icing（glaze,snow and rime）were compared.The key parameters of the three types of icing models were quantified.The differences in meteorological conditions at different heights and the resulting differences in icing were discussed.The ice-forming water environment was studied,that is,the microphysical characteristics of supercooled fog,freezing rain and snow.Formation mechanisms of microphysical conditions liable to ice formation were explored.The main conclusions were:（1）Freezing raindrops converged and flowed along wire surface after adhering to it,a wet growth was observed,and the ice was uniform and transparent and had almost no bubble.Snowflakes and fog drops froze quickly after adhering to wire,a dry growth was observed,and the ice was white with a lower density.The average icing growth rates in freezing rain,snow,and supercooled fog were 0.4,0.3,and 0.2 mm h^-1,respectively.Air temperature drop reduced water loss as ice accumulated,and higher wind speed increased horizontal particle flow,these were beneficial to ice accretion.The temperature range for freezing rain was relatively narrow,freezing raindrop had a larger inertial force during falling and was not easily affected by the wind to change the falling trajectory,the correlations between the icing growth rate and both the temperature and the wind speed were poor in freezing rain.The correlations between the icing growth rate and both the temperature and the wind speed were stronger in the snow and supercooled fog than in freezing rain.With the decreasing temperature,the icing growth rate increased faster with snow,while that in supercooled fog increased faster as the wind speed increased.In freezing rain,snow,and supercooled fog,the icing growth rates were all positively correlated with the ice thickness,with correlation coefficients of 0.55,0.67,and 0.79,respectively.Ice was shed temporarily when the temperature remained below 0°C,the wind speed fell to 2.7 m s^-1,and the fog liquid water content（FLWC）fell to 0.036 g m^-3 in supercooled fog,and when the solar radiation increased and the temperature exceeded 0°C in freezing rain.According to the icing images taken by the automatic monitoring system,icicles were often produced in freezing rain icing.Therefore,compared with the traditional rounded hypothesis of icing cross section,the fan-shaped hypothesis was closer to the actual cross-sectional shape of freezing rain icing.The correlation coefficient between the form factor reflecting the narrow degree of the fan-shaped section and the temperature three hours ago was0.52.After adding the form factor,the simulation results of the freezing rain icing model better reflected the actual ice thickness changes,especially in the case of longer icicle or higher wind speed.According to the localization test of the sticking efficiency in the wet snow icing model,the average sticking efficiency of the wire to snow was 0.03;its variation range was 0.01～0.10.A integrated model was built by integrating freezing rain,snow,and supercooled fog icing models,the ice thickness calculated by the integrated model was in agreement with the actual observed ice thickness.（2）During the cold wave and icing（2016.1.19-25）,the air temperatures(T_1.5,T₁₀),wind speeds(V_1.5,V₁₀)and icing showed significant differences at 1.5 and 10 m heights from the ground.T_1.5 was generally higher than T₁₀ in precipitation,and the gap between T_1.5 and T₁₀increased when the solar radiation enhanced.After the precipitation,T_1.5 was close to T₁₀,and T₁₀ even exceeded T_1.5 when the solar radiation strengthened.V_1.5 was lower than V₁₀ during the overall icing process,V_1.5 and V₁₀ were closer below 3 m s^-1,and the gap between V_1.5 and V₁₀ was larger above 3 m s^-1.Starting and evolution characteristics of icing at high and low places were contrasted.When solar radiation strengthened during the day,icing at low place was affected by multiple factors of direct solar radiation,significantly increased T_1.5,long-wave radiation and reflected solar radiation by ground,resulting in severe ice loss.Ice thickness picked up at night.The maximum ice thickness during the whole process was 1.2 mm.Appropriate low temperature at high place allowed icing to start earlier than the low place 6 h,and subsequently ice thickness reached 18.0 mm after the two growth phases（corresponded to larger wind speed difference zones between the two heights）.And icing at high place had less loss during strong solar radiation.The ways icing ended at high and low places were contrasted.After stopping growth,the ice at the lower place was shed directly.While the ice thickness at the higher place maintained for 14 h,then it gradually decreased under the influences of strong solar radiation and strong wind.（3）The droplet spectra of the three supercooled fog cases in Lushan showed double-peak distributions.The three types of spectra of peak diameters DFP≤5μm,5μm<DFP<10μm,and D_FP≥10μm accounted for 49.0%,25.3%and 25.7%on average.The increase of FLWC in supercooled fog is conducive to ice accumulation.The highest values of the FLWCs of the three fog cases in Lushan were 0.539,0.641 and 1.306 g m^-3.The common features of the environmental conditions when the highest FLWC values occurred were rain stop and reduced wind speed.The increase of the number of large drops in supercooled fog is conducive to ice accumulation.The maximum DFP values of the three fog cases in Lushan were 14.7,13.3,and14.9μm,respectively.When the DFP reached the maximums,the commonality of the environmental conditions was a sudden temperature drop.The temperatures and relative humidities simulated by the WRF model well reproduced the meteorological conditions of freezing rain and snow cases in Lushan.In the case of freezing rain,an approximate isothermal layer slightly below 0°C appeared from the ground to 5 km.In the case of snow,the temperature of the air layer from 2 to 3.5 km was slightly above 0°C,which improved the liquid water content on precipitation particle surfaces.The evolutions of simulated rain and snow mass concentrations were consistent with the actual occurrences of precipitation.High centers of cloud liquid water,ice crystal,and snow mass concentrations appeard in the upper air during rainfall and snowfall.The microphysical parameters such as rainwater and snow mass concentrations output by the WRF model were input into the freezing rain and wet snow icing prediction models,and the simulated ice thickness was in accordance with the measured ice thickness.