利用扫描式红外亮温仪对天空云量的试验观测

An experiment study of cloud cover detection by ground based sky thermal infrared brightness temperature measurements.

中文关键词:  全天空扫描红外成像仪， 云自动化观测， 云量， 云底高

英文关键词:All sky IR scanning imager, Automatic cloud observation, Cloud cover, Cloud base height

中文摘要:

      利用中国科学院大气物理研究所中层大气与全球环境探测实验室自主开发研制的全自动扫描式天空红外亮温仪，于2007年4—8月在北京市气象局南郊观象台进行了试验观测。利用获得的天空红外亮温数据和南郊观象台（54511站）实时地面气象数据，进行了天空云量的计算。将整点前30 min内观测的云量的平均值作为与观象台观测对应的整点观测云量，分不同情况与观象台目视观测结果进行了对比分析。结果表明：（1）对于中低云，两种观测的云量比较一致，但如果在此期间云消散或增加很快，则差别较大。其主要原因是观测的时间和方式不同所造成的。（2）对于以卷云为主的天空情况，特别是高层薄卷云的情况，两种观测的云量差别较大。原因是受到目前试验所使用的红外传感器低温测量范围的限制（-50℃），以及判定云的阈值算法目前还只对于中低云比较适用，对于卷云的判断能力比较薄弱。（3）对于天空情况比较复杂，以及能见度不好的情况（气象站目视云量记录为10-），两种观测的云量差别也很大。其差别既受到仪器性能的影响，也与观测员视力与经验有关。仪器受到最低测量温度的限制，反演算法也有其不确定性，而观测员在低能见度时的观测局限性亦是一个重要原因。利用SIRIS 1进行云量和云底高度观测的优点是时间分辨率高，且全天时自动化，但对于卷云的判断还较薄弱。

英文摘要:

      Measurements of sky thermal infrared brightness temperature (TB) were conducted during the period of April to August in 2007 at the Beijing Meteorological Observatory, using an automatic scanning infrared thermometer （SIRIS 1） developed by the Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), the Institute of Atmospheric Physics, Chinese Academy of Sciences. Cloud cover in the sky was calculated by using the data of TB and the real time surface weather data provided by the Beijing Meteorological Observatory (No.54511). The comparison was made between the two kinds of cloud cover, one was observed by observer in the Beijing Meteorological Observatory, and another was the averaged cloud cover observed by SIRIS 1 over the half hour before each round hour. The results show that (1) for middle and low cloud situations, if the cloud in the sky does not change very fast during the half hour before round hour, the two kinds of cloud cover are very consistent with each other; if the cloud changes rapidly, the difference is great. The reason is because the method and the time period of observation are different. (2) For the sky situation dominated by cirrus, the difference between the two kinds of cloud cover is great due to the limitation of infrared sensor (-50℃) used for measurements and the poor ability of threshold determination to distinguish thin cirrus from clear sky at present, especially for thin cirrus, very high in the sky. (3) For complex cloud situations with poor visibility (e. g. cloud cover 10- as is recorded by meteorological observer), the difference of the two kinds of cloud cover is also great. The difference was caused either by the characteristic of instrument, or by observer's ability of visual observation and experience. As far as the instrument is concerned, the lowest measurement temperature (-50℃) limits the ability of high cloud sensing, and the uncertainty of the cloud retrieval is also a key factor. In addition, the poor horizontal visibility may restrict observer's ability of visual observation. The cloud cover measurements by the SIRIS 1 demonstrates the distinct superiority of high temporal resolution and all sky automatization but the ability of infrared sensor currently used to sense high cirrus is insufficient.