全天空成像仪云量计算方法的改进

全天空成像仪（total sky imager 440,TSI-440）可以实现白天全天空云量的持续自动监测,时空分辨率较高,得到的云量计算结果更精确.首先介绍了TSI-440的基本原理和资料格式,并基于太湖地区2008年5-10月的TSI-440资料及无锡站地面观测资料,采用统计方法详细地分析了不同天气情况下图像的成像特征及云量的计算误差.结果发现：图像的成像特征与能见度密切相关,红蓝比值随着能见度的减小而增大.另外,仪器在处理阴天图像及复杂天空（多云）图像时,易造成一定的云量计算误差.针对上述问题,本文通过直方图分析,重新选定了红蓝比阈值（晴空点阈值0.62,云点阈值0.66）,基于新阈值计算的云量结果较仪器自带的处理结果更为准确,减小了因天气状况不同而产生的云量计算误差.

Improvements of computational methods for cloud cover based on the total sky imager

The total sky imager（TSI-440） is designed to automatically monitor cloud conditions at daytime with higher spatial resolutions and is able to provide more accurate cloud cover data.The paper introduced the fundamental mechanism and data format of the TSI in detail and then analyzed the imaging characteristics under different weather conditions and the cloud cover computational errors based on the routine surface observation data from TSI-440 at Wuxi from May to October 2008 by using statistical method.The results show that the imaging characteristic is closely related with the visibility and the red blue ratio increases as the visibility reduces.After analysis of the images,it is also found that the TSI-440 makes certain errors in computing cloud cover when processing cloudy images under overcast or cloudy conditions.In view of this situation,the paper sets new thresholds （sunny points at 0.62;cloudy points at 0.66） to simulate the sky images and count the cloud points.The recalculated results are much more reliable than the raw results from the TSI.Setting new thresholds could reduce the cloud cover errors effectively,especially on cloudy days.