Direct observation of cloud forcing by ground-based thermal imaging

Instantaneous surface Cloud Radiative Forcing (CRF) in the 7.5-13 mu m region is observed for the first time, using a thermal infrared camera. The sampling of clear sky and cloudy radiances from images of broken cloud fields allows cloud cover, CRF and effective cloud emission to be directly calculated, all within a consistent field of view. Analysis of 1300 images taken over more than two months in Central England shows that surface CRF is a nonlinear function of cloud cover, with daytime forcings larger and less linear than those at night. This nonlinearity is caused both by the increase in cloud optical thickness and the more frequent occurrence of low altitude (warm) cloud as the cloud cover increases. Even for nearly complete cloud cover, effective cloud emission remains significantly less than that of widely assumed homogeneous, optically thick cloud. Possible clear sky sampling errors associated with traditional methods of measuring CRF are also investigated.