Vol.31,  No.6, 
October  2020
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Tao Fa, Guan Li, Zhang Xuefen, et al. Variation and vertical structure of clear-air echo by Ka-band cloud radar. J Appl Meteor Sci, 2020, 31(6): 719-728. DOI: 10.11898/1001-7313.20200607.
Citation: Tao Fa, Guan Li, Zhang Xuefen, et al. Variation and vertical structure of clear-air echo by Ka-band cloud radar. J Appl Meteor Sci, 2020, 31(6): 719-728. DOI: 10.11898/1001-7313.20200607.
shu

Variation and Vertical Structure of Clear-air Echo by Ka-band Cloud Radar

  • 1.

    Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044

  • 2.

    Meteorological Observation Center of CMA, Beijing 100081

  • 3.

    Chengdu University of Information Technology, Chengdu 610225

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    Abstract
  • Ka-band cloud radar data from 2017 to 2019 in Beijing Atmosphere Observation test-bed of CMA, combining with observations of automatic weather station and ceilometer are used to analyze the variation and vertical structure of low-level clear-air echo from aspects of intensity, velocity, space scale, depolarization ratio and height of clear-air echo.Based on ceilometer and cloud radar detection sensitivity difference of particle radius and density, clouds and clear-air echo are identified, using image processing technology to distinguish the layered turbulence echo and the dot-like insect echo.Based on different scattering mechanisms, scattering characteristics of the layered turbulence echo and the dot-like insect echo are analyzed. Generally, the range of equivalent reflectivity factors in millimeter wavebands of the clear-air echo caused by atmospheric turbulence is -70 dBZ to -30 dBZ, while the dot-like insect echo reflectivity factor is greater than -30 dBZ.Results show that radar clear-air echoes mainly contain the layered turbulence echo and the dot-like insect echo in the boundary layer and the echo height is within 3000 m. The intensity and height of clear-air echoes show obvious seasonal and diurnal variation characteristics. The echo height is lower in winter and higher in summer, which is well correlated with the surface temperature. There is almost no clear-air echo when the surface temperature is below 5℃, so there is almost no clear-air echo in January, February, November and December, while the average echo height is the highest in July and August. The radar reflectivity factor of clear-air echo is within the range of -40 to -15 dBZ with mean value of -28 dBZ. As the height increases, the reflectivity factor intensity decreases gradually, the peak value of the probability density distribution function of the radar reflectivity factor for clear-air layered turbulence echo is -35 dBZ, and that of the dot-like insect echo is -30 dBZ. The vertical movement speed of clear-air echo is mainly within -1.5 to +0.5 m·s-1, and downward movement is dominant. The linear depolarization ratio value of layered turbulent echo is larger than that of dot-like insect echo, which is generally within the range of -10 to -5 dB. Within 1000 meters at the lower level, the range of linear depolarization ratio is wide and gradually narrows with the increase of height. The linear depolarization ratio of dot-like insect echoes is generally within the range of -15 to -8 dB and increases gradually with the height.
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  • Fig  1.   The flow chart of clear-air echo identification and classification

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    Fig  2.   The overlay graph of the reflectivity factor of cloud radar and cloud base height of ceilometer on 27 Jul 2017

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    Fig  3.   Identification of cloud, layered turbulence echo and dot-like insect echo on 27 Jul 2017

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    Fig  4.   Comparison between echo top-height and air temperature under clear-air condition (a)diurnal variation, (b)monthly variation

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    Fig  5.   Frequency of clear-air echo reflectivity and velocity varying with altitude from 2017 to 2019

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    Fig  6.   Probability density function of radar reflectivity of dot-like echo and layered turbulence echo statistics from 2017 to 2019

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    Fig  7.   Distribution of probability density function for linear depolarization ratio(Ldr) of clear-air echo varying with height from 2017 to 2019

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    Fig  8.   Variation of linear depolarization ratio(Ldr) with reflectivity factor probability density funciton for dot-like insect echo(the shaded) and layered turbulent echo(the contour) from 2017 to 2019

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    Table  1   Performance parameters of the cloud radar

    性能名称 云雷达参数
    工作频率 34.8 GHz±200 MHz
    重复频率 4 kHz
    脉冲宽度 1 μs, 10 μs
    天线口径 1.2 m
    天线形式 卡塞格伦
    波束宽度 0.7°
    天线扫描方式 垂直天顶
    极化方式 水平、垂直极化
    距离分辨率 30 m
    时间分辨率 0.3 s
    最大探测高度 20 km
    最小可测功率 1 km处不超过-40 dBZ
    差分反射率精度 ≤0.2 dB
    线性退极化比精度 ≤0.5 dB
    速度范围 -25~+25 m·s-1
    发射峰值功率 230 W
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    • Received : 2020-05-07
    • Accepted : 2020-07-27
    • Published : 2020-10-26
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