All-Sky Camera Upward-looking Thermal Infrared Cloud Characteristics

Satellite data sets are the primary source of observations of cloud characteristics, but downward-looking passive sensors cannot see lower-level clouds obscured by higher clouds nor cloud bases. Observations of low clouds with downward-looking satellite IR are hampered by the small brightness temperature differences between the low cloud top and the underlying surface. In contrast, upward-looking thermal IR can readily distinguish warm clouds against the cold sky. By sampling thermal IR cloud characteristics across the diurnal cycle, upward looking thermal IR observations have the potential to yield improved understanding of transitions in cloudiness at sunrise and sunset and differences in the relative importance of different cloud processes with and without SW fluxes.

Our thermal IR all-sky camera was assembled from commercially available, off-the-shelf parts. The key components are a FLIR Boson thermal IR camera and a FLIR PTU-5 pan-tilt mount. The IR camera has a 50° field of view and a resolution of 640x500 pixels. To obtain imagery of the entire sky, the pan-tilt mount points the camera at 14 different directions, each varying in azimuth and elevation. The volume coverage pattern is executed once per minute, and the entire sky is sampled in less than 30 seconds. The images are then stitched together in software to yield a hemispherical array of IR brightnesses from horizon to horizon.

From the imagery we can infer cloud fraction, cloud coverage characteristics relating to the size and shapes of cloud elements, and estimate the altitude of cloud bases at all times of day. Sequences of images reveal the evolution of individual cloud elements and provide information on the phase space of cloud properties across the diurnal cycle and to related to air mass changes, such as the passage of fronts. Combined with other data from lidar and visible all sky cameras, the upward-looking thermal IR data on cloud outer surface temperature details at small spatial scale (10s of meters) and few minute time scale have high potential to yield new insights on cloud initiation and dissipation.

We will detail the performance of the thermal IR all-sky camera and analyze derived cloud characteristics in the context of data from visible wavelength all-sky imagery and additional atmospheric observations.