The Role of Quantitative Infrared Imagery in Investigations of the Nocturnal Boundary Layer

Abstract

During the Cooperative Atmospheric?Surface Exchange Study in 1999 (CASES99), an intensive investigation of the stable nocturnal boundary layer, a versatile and sensitive cryogenically cooled thermal imaging radiometer, the Inframetrics PM 380 Thermal Camera, was deployed. The 60-m-high instrumented tower at the central CASES99 site provided the perch from which to survey the instrumented research field. The field of view of 16° (0.28 sr) and an angular resolution of 1/16° (0.0011 sr) enabled a segment of landscape 500 m distant, of approximately 150-m width, to be viewed with a resolution of approximately 0.5 m. Or, looking down from the 50-m level of the tower, a section of the ground surface 15 m on a side could be viewed with a resolution of 5 cm. The surface brightness temperature of any area could be surveyed with a temperature resolution of 0.1°C and a time resolution of 30 Hz. The information obtained from analysis of these thermal images uniquely complemented the data acquired by the more conventional radiometric and meteorological instrumentation. The thermal imager provided valuable information on the landscape-scale changes of surface temperature after sunset and yielded insights into the development of surface drainage flow and its initiation. Also during periods of nocturnal stability the movement of thermal features across the standing vegetation could be traced, allowing the propagation of eddies to be investigated. The examination of apparently uniform land surfaces enabled a quantified analysis of the inhomogeneities of longwave emission. This information is critically important for the understanding of errors in the surface energy balance. The nocturnal thermal images of mature deciduous trees illustrated the extent to which trees modulate local airflow patterns. Finally, the tantalizing prospect of utilizing the thermal image of the 60-m tower itself to achieve a surrogate air temperature profile was examined.