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

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.