Satellite and ground-based observation are two important approaches for cloud detection. In recent years, serious atmospheric pollution has led to frequently high aerosol concentrations in North China, leading to differences between satellite and surface cloud observations.

ZHANG Xiao (PhD student), along with Dr. TAN Saichun and Prof. SHI Guangyu, from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences investigated the satellite sensor MODIS and surface cloud cover data and compared their difference over the period from 2002 to 2013.

They found that the differences in cloud cover between MODIS and surface data at nighttime (15.58%–16.64%) were greater than that during daytime (12.74%–14.14%), and the differences in winter (29.53%–31.07%) were evidently greater than those in summer (4.64%–6.07%).

They further investigated many influence factors and found that the difference expanded with an increase in aerosol optical depth (AOD) and satellite viewing zenith angle; moreover, the coverage of snow also led to a larger deviation.

Meanwhile, they found that the influence was related to cloud types. For example, AOD had a more significant effect on cumulus, altocumulus and cirrus clouds, which cannot cover the whole sky, than stratus and nimbostratus clouds, which can cover the whole sky.

"MODIS is a passive sensor that is easily affected by atmospheric pollution. Previous studies have shown that high aerosol plumes can be misclassified as cloud by MODIS, leading to erroneously large cloud cover, and hence a larger deviation with surface data. In future, we intend to focus on how to reduce the effects of these factors on cloud detection over North China." Dr. TAN explained.

The study was published in Advances in Atmospheric Sciences.