Observations of Nighttime Clouds Over Chiba, Japan, Using Digital Cameras and Satellite Images

Abstract

Nighttime cloud cover (CC) is analyzed from the data observed for 34 months over Chiba, Japan, using continuously operated digital cameras. The normal camera works in the visible region, providing quantitative and temporal changes of nighttime CC. The second camera captures sky images without the near‐infrared cut filter, and by applying the thresholding method, we obtain information on the temporal changes of thin (pixel values between 72 and 115) and thick (>115) clouds. Comparing the CC from the visible camera with cloud fraction from Atmospheric Infrared Sounder (AIRS) shows that long‐term data from the camera are valuable for correcting the underestimation of the cloud fraction product from AIRS for low‐level clouds. Furthermore, a comparison between the near‐ground temperature and CC results in estimating the seasonal changes of feedback effects of upward thermal radiation from the ground. The current results reveal that thick clouds contribute to longwave cloud forcing (LWCF) below 100 Wm−2, while thin clouds with CC below 30% contribute to LWCF below 10.2 Wm−2.