Comparative Analysis On Cloud Height Data Derived From A Ground-based Millimeter-wave Radar And Other Instruments

Cloud height is one of the most basic but very important meteorological parameters in the study of cloud physics.At present,the measuring methods of cloud height are various.Due to differences in cloud characteristics and measurment principles,inconsistency in ob-servation data of cloud height may be caused.In order to improve the comparability and con-sistency of multi-source cloud height observation data,it is necessary to systematically and theoretically analyze their accuracy and difference,and correct the cloud height retrieval re-sults based on physical principle.Based on a ground-based millimeter-wave radar,a ceilome-ter,operational L-band radiosondes,FY-2 geostationary satellites,and a ground-based infra-red sensor(IRT)data,this study presented their cloud height retrieval methods,analyzed the consistency and difference on their cloud height retrieval results,and comprehensively con-sidered the characteristics of each data to build a combined cloud-height dataset.The results showed that:(1)The cloud vertical structure characteristics observed by the Ka-band cloud radar at Beijing Nanjiao Observatory in 2016 was that the average cloud base height(CBH)is 4.39 km,and that of cloud top height(CTH)was 7.69 km.The number of clear-sky samples ob-served in Beijing was relatively higher than that of cloudy samples.The proportion of cloud samples in summer was more than that in winter.The number of high-cloud samples was higher than that of low-cloud samples.The distribution of cloud heights showed seasonal var-iations.The average cloud height in spring and summer was higher than that in autumn and winter.(2)The comparison results of CBH between the Ka-band cloud radar retrieved and the collocated ceilometer retrieved showed that the correlation coefficient was as high as 0.95.Due to the strong attenuation,the ceilometer cannot detect the cloud vertical structure except the CBH.The overall detection capability of clouds with the CBH higher than 6 km was weak because of the influence of atmospheric low-level aerosols.Millimeter-wave radar is a pow-erful instrument for cloud vertical structure detection.(3)The radiosonde profile was used to determine the cloud vertical structure and com-pared with the cloud radar.The difference in cloud base/top height between them was less than 200/1400 m on average.The correlation coefficient of the cloud height variation trend was greater than 0.8.As to the CBH of the middle and low clouds,the cloud radar can achieve a good match with the sounding observations.(4)The consistency rate of cloud detection between the cloud radar and the FY-2 datasets was 78.1%.The average difference in CTH between the two techniques was 1.46 km.Higher consistencies were obtained for thicker clouds with larger echo intensity and for more con-tinuous clouds.The difference in CTH between low-and mid-level clouds was less than that for high-level clouds.The FY-2 satellite CTH correction scheme for cirrus clouds with a CBH of 6 km or higher is proposed.After correction,the satellite CTH was more consistent with the cloud radar CTH.(5)Based on a ground-based infared temperature sensor(IRT)observation data,the cloud base height was retrieved.Compared with the CBHs derived from the cloud radar,the consistency rate between the two datasets was 81.6%.The correlation coefficient between the two CBH datasets reached 0.62,and the average difference was 0.1 km.The results were su-perior to the original CBH product directly derived from the IRT instrument and closer to the cloud radar data.The IRT CBH of low(high)-level thin cloud was higher(lower)than the cloud radar CBH.(6)A combined cloud-height dataset(CChD)for Beijing 2016 with 1 h time resolution was initially constructed.Compared with other single observation data,the CChD reduced their missing rate to 1%,which better compensated for the missing and attenuation effects of Ka-band cloud radar.Compared with the Ka-band cloud radar,its Euclidean distance between CChD and "third-party observations"(radiosonde)was smaller and the correlation coefficient was improved.The CChD was closer to the "third-party observation" than the cloud radarsample.