| Cloud radar is a powerful instrument for cloud detection.However,the parameters of radar transmitter and receiver system can drift during operating,which will cause the overall offset error of the observation data and affect the accuracy of cloud physical property retrieval.Therefore,cloud radar data needs to be periodically calibrated in a proper way.In this study,it is mainly adopt the radar reflectivity correction method proposed by Pavlos et.al[62]to improve the data quality of the Ka-band Zenith Radar（KAZR）deployed at the Semi-arid Climate and Environment Observatory site（35.57°N,104.08°E）of Lanzhou University（SACOL）,and compare the variation of the calibration parameters during the this period.Using original and calibrated radar reflectivity factor to calculate ice and liquid water contents and effective particle radius,and the influence of reflectivity factor calibration on cloud microphysical properties is analyzed.The main results are as follows:（1）Based on the matching of satellite and ground observation data,it is mainly adopt the radar reflectivity correction method proposed by Pavlos et.al[62]to improve the data quality of the Ka-band Zenith Radar（KAZR）deployed at SACOL site.It is establishing a 46-month historical data calibration from August 2013 to May 2017 for the KAZR radar reflectivity.In the calibration process,it is using a new method of bilateral filtering for cloud detection to identify cloud signals,retaining more weak cloud and thin cloud,and at the same time,considering the effect of vertical velocity when removing precipitation,which is benifit to retaining more cloud signals.（2）Based on the above improved calibration method,it is given two specific calibration examples for the cold season（from December 2013 to May 2014）and warm season（feom June2014 to November 2014）:The minimum root mean square error（RMSE）is 1.6 d BZ in the cold season,and the calibration offset is-5.0 d BZ.During warm season,the minimum RMSE is 3.5d BZ and the calibration offset is-3.3 d BZ.Compared with cold season,although more valid cloud samples exist in warm season,the wider vertical distribution range and larger RMSE indicate calibration in the warm season is more difficult.This is mainly because the higher temperature and greater energy can lead to stronger upward movements,which generate more low-level and convective clouds with much more complex structures over SACOL site,which is harmful to reflectivity factor calibration.（3）Analyzed the periodic variation of CloudSat calibration of KAZR cloud radar reflectivity factor,mainly including calibration offset,calibration height range and significant points.It is calibrate the historical data of the KAZR reflectance factor of SACOLsite from August 2013 to May 2017.Every 6 months is a calibration cycle,it is 41 calibration results.The maximum calibration offset is-8.0 d BZ（from October 2015 to March 2016 and from January 2016 to June2016）;the minimum calibration offset is 0.1 d BZ（from July 2016 to December 2016）.In generally,the warm season has a large calibration height range and more significant points;while the cold season has a small range and few significant points.（4）The average reflectivity factor data of CloudSat and the average reflectivity factor data of KAZR cloud radar before and after calibration are compared:After KAZR calibration,the maximum average reflectivity factor within the calibration height range is-10.47 d BZ（from June2016 to November 2016）,during the transition from the warm season to the cold season,convective activities frequently occur,which tends to form larger cloud droplets;the minimum average reflectance factor within the calibration height range is-15.62 d BZ,（From October 2010to March 2015）,the temperature and solar radiation conditions in winter are not conducive to cloud formation,and the reflectivity factor is small.（5）Based on the reflectivity factor calibration,identifing ice clouds and water clouds separately,then further using the original and calibrated radar reflectivity to calculate ice and liquid water contents and effective particle radius.It is found that the calibrated reflectivity will significantly reduce the inversion values of the clouds microphysical properties.Among them,the ice water content decreases most significantly from 14.35 mg/m3to 9.27 mg/m3,the decrease rate reached 35.4%.At the same time,the effective particle radius changed relatively small,ice cloud and water cloud decreased by 13.6%and 11.3%,respectively.The influence of the periodic change of the reflectivity factor calibration on the ice/liquid water content and effective particle radius of ice clouds and water clouds is analyzed.On the whole,the reflectivity factor calibration has a greater impact on cloud water content than the effective particle radius.The greater the calibration offset,the greater the reflectivity factor before calibration,and the greater the difference between cloud microphysical properties before and after calibration.This means calibration of cloud radar reflectivity data is important for obtain accurate cloud macro-and micro-physical properties and will help provide solid foundation for long-term clouds study at SACOL site. |
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