Deriving Winds at Cloud-Base Height With an Infrared Camera

Abstract

An uncooled commercial infrared camera is used to retrieve horizontal winds at cloud-base height. The camera is equipped with a microbolometer array of 320 times 240 pixels covering a field of view of 32deg times 24deg. It operates in the atmospheric window from 7.5 to 14 mum . In this wavelength range, the camera has day and night measurement capabilities, independent of the illumination conditions. Optically thick clouds appear as blackbody radiators and offer a large brightness temperature contrast with the much colder clear-sky background. We develop a method to follow the infrared structures in the clouds as they pass through the field of view of the camera. Taking a picture every 6 s, the angular velocity of the cloud base is calculated as the ratio of the angular displacement of the cloud structures between two pictures over the elapsed time. To transform the angular velocity into a metric velocity, we use the temperature profile retrieved by a microwave radiometer called All-Sky MUlti-WAvelength Radiometer. The case study of September 10, 2007 is shown. The retrieved wind is of 18.0 plusmn 2.4 m/s at an altitude of 4164 plusmn 546 m above the instrument, blowing from northwest. These results agree well with radiosondings launched in the vicinity of the experiment site.