Comparison between MODIS-derived day and night cloud cover and surface observations over the North China Plain
Satellite and human visual observation are two of the most important observation approaches for cloud cover. In this study, the total cloud cover (TCC) observed by MODIS onboard the Terra and Aqua satellites was compared with Synop meteorological station observations over the North China Plain and its surrounding regions for 11 years during daytime and 7 years during nighttime. The Synop data were recorded eight times a day at 3-h intervals. Linear interpolation was used to interpolate the Synop data to the MODIS overpass time in order to reduce the temporal deviation between the satellite and Synop observations. Results showed that MODIS-derived TCC had good consistency with the Synop observations; the correlation coefficients ranged from 0.56 in winter to 0.73 in summer for Terra MODIS, and from 0.55 in winter to 0.71 in summer for Aqua MODIS. However, they also had certain differences. On average, the MODIS-derived TCC was 15.16% higher than the Synop data, and this value was higher at nighttime (15.58%–16.64%) than daytime (12.74%–14.14%). The deviation between the MODIS and Synop TCC had large seasonal variation, being largest in winter (29.53%–31.07%) and smallest in summer (4.46%–6.07%). Analysis indicated that cloud with low cloud-top height and small cloud optical thickness was more likely to cause observation bias. Besides, an increase in the satellite view zenith angle, aerosol optical depth, or snow cover could lead to positively biased MODIS results, and this affect differed among different cloud types.
Keywordscloud cover MODIS cloud-top height cloud optical thickness aerosol optical depth view zenith angle
地面和卫星观测是目前云观测中两个最重要的观测途径. MODIS 作为被动遥感卫星, 华北地区经常出现的雾霾天气中厚气溶胶团对太阳辐射的强迫作用将进一步影响其对云的观测. 目前的研究中, 对于华北地区卫星和地面观测的对比及影响其观测差异的可能因素, 以及不同云类型下二者的云量观测差异的研究仍旧不足. 本次研究对由 MODIS 卫星和地面观测站观测的华北地区总云量(TCC)进行了对比. 研究表明, MODIS 观测的总云量略大于地面观测. 其中, 夜间二者差别(16.64%)大于日间(14.14%), 而冬季差别(31.07%)明显大于夏季(6.07%). 而云顶高度较低以及光学厚度较小的云更容易出现较大的观测偏差. 影响观测差异的原因还有卫星观测角, 气溶胶光学厚度以及积雪. 对于多数类型的云, 卫星观测角, 气溶胶光学厚度以及积雪量越大, 其云量观测差异也越大. 其中, 垂直发展较为旺盛的积雨云以及破碎状的云受卫星观测角的影响比层状云更大; 而不能覆盖全天空的云相对覆盖全天空的云受到气溶胶光学厚度的影响更大.
关键词云量 MODIS 云顶高度 云光学厚度 气溶胶光学厚度 卫星观测角
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This work was supported by the National Natural Science Foundation of China (Grant Nos. 41590874 and 41590875) and the Ministry of Science and Technology of China (Grant No. 2014CB953703). The MODIS cloud and aerosol properties were provided by the Level 1 and Atmosphere Archive and Distribution System of the NASA Goddard Space Flight Center. We are grateful to the China Meteorological Administration for providing the visual surface cloud cover data.
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