Key Technology Research On Test And Evaluation Of Visibility Meters

Ground meteorological observations need to meet three basic requirements,which are "representative,accuracy and comparable".The purpose of testing the visibility meter is to evaluate whether the measurement results of the visibility meters satisfy these requirements.This evaluation is challenging in most cases due to the differences of visibility meters in sample space,measurement principles and results.To test the measurement results of visibility meters and to evaluate whether the measurement results are "representative,accuracy and comparable",this paper first provides a comparative analysis of transmissometers and forward-scatter visibility meters in terms of the measurement characteristics.Then,the meteorological optical range(MOR)of transmissometers and forward-scatter visibility meters located in Yulin Airport is examined to explore the "comparable" feature of different visibility meters under different weathers,such as sand,floating dust,haze weather,as well as fog,mist and snow fog.The measurements of visibility meters,which are located in Hainan,Dalian,Tianjin,Chengdu and Chongqing airports,are recorded two times a day for consecutive 100 days.In addition,the mid-value method,the graphic scale method and the mean method are adopted to process the measurements of multiple sets of visibility meters.The visibility values reported in METAR are compared with the measurements to assess the "representative" feature of visibility meters.Accuracy is key to the evaluation of meteorological instrument measurements.Through the review of existing literature in this field,the reference value of the visibility measurement is considered a critical factor.Based on the measurement principle of transmissometers,this paper puts forward a Multi-point Visibility Measurement(MVM)method to calculate atmospheric transmittance,extinction coefficient and MOR.The error of atmospheric transmittance and MOR measured by the MVM method is also researched and quantified,followed by the comparison between the proposed method and the traditional transmissometer method.The research results show that under the same hardware conditions,the MVM method has a significantly smaller relative error of measurements than the traditional transmissometer method.At the end of this part,the high-precision multi-point mobile measurement system is realized by the applications of laser,chopper,guide rail and other components.To shorten the time used to evaluate visibility meters,this paper builds an atmospheric environment simulation chamber(AESC),which is equipped with a variety of aerosol-generating devices including atomized aerosol generators and black carbon aerosol generators to create a low-visibility environment.The uniformity of the AESC is a common issue.This paper combines the computational fluid dynamics(CFD)simulation method with the experimental method based on the horizontal distribution of the extinction coefficient to evaluate the uniformity of AESC,especially the horizontal distribution of the extinction coefficient in the AESC.The coordinates of the spatial region where the flow field and concentration field in the chamber are relatively uniform are obtained by CFD results.The distribution of the extinction coefficient at the longitudinal horizontal position in the chamber,as well as the stability of the measurement results,is obtained by the experimental method.At the end of the paper,a low-visibility environment is created in the AESC,and comparisons of visibility meters are performed.Firstly,the changes in MOR,temperature and relative humidity during the experiment in AESC are recorded.An optical particle counter(OPC)is used to record and analyze the concentration of particles with different sizes under varying levels of visibility in the AESC.Based on the environmental analysis of the AESC,the MVM method and transmissometer method are used respectively to process the atmospheric transmittance and calculate the extinction coefficient and MOR.Afterward,the measurements of MVMS and Skopograph II are compared.It is found that the measurements of the MVM method and MVMS have fewer systematic errors and less fluctuation,proving their better ability to reflect the change of visibility in the AESC.