High-Resolution Image

This is one slide from a well-received recent talk by Warren Wiscombe entitled "The Brouhaha over Enhanced Absorption of Sunlight by Clouds: What Went Wrong?" This talk attempted to review, from a somewhat philosophical viewpoint, the history of the often impassioned enhanced absorption debate and the good things that came out of it. One of the most important "good things" was a much greater attention to flux radiometry, a measurement field which had languished since its invention in the 1920s (or perhaps earlier). The above slide is symbolic of two of the many improvements that resulted from the "brouhaha".

The upper photo shows arrays of Eppley flux radiometers being cross-calibrated on the roof of the Radiometric Calibration Facility at the main Dept. of Energy Atmospheric Radiation Measurements (ARM) site in northern Oklahoma. ARM took the lead in trying to pin down the enhanced absorption, funding two major field campaigns involving both aircraft and many surface sites measuring radiative fluxes. Flux radiometers like these try to measure the downward flux of solar energy. Outdoor calibration under carefully selected weather conditions had always differed by a mysterious 1.5% from indoor calibrations, yet the indoor calibrations had been used exclusively in the past. ARM pioneered the use of outdoor calibration for climate radiometers, which seemed more logical for instruments that were, after all, going to be used exclusively outdoors. ARM set new standards for accuracy and precision of flux radiometry, especially during the field campaigns in which the surface radiometers, for the first time ever, were used as part of an aircraft campaign to measure cloud absorption of sunlight.

The lower photo shows a rig for measuring the direct and diffuse components of the sunlight separately. The direct solar beam is measured by the cylindrical instrument with flanges at each end and a small hole in the end through which the solar beam enters. It is called a "pyrheliometer", literally something for measuring the "fire of the Sun", and has a field of view of about 5 degrees. To keep it pointed at the Sun, which subtends about half a degree in the sky, it sits on a Sun-tracker that costs in the neighborhood of $20,000! The two white disks on a long arm block the direct beam of the Sun so that the two radiometers, with little glass domes, measure only the diffuse sunlight. By summing the direct and diffuse components, a much better measurement of the total incident solar energy flux is obtained than from unshaded radiometers, as in the upper photo. Unfortunately, the high cost of Sun-trackers, and the necessary maintenance of their mechanical components, makes it expensive to field a direct-diffuse radiometer site accurate enough for climate purposes. Many surface radiometer sites around the world rely on the much less accurate unshaded radiometers. Thus, the state of knowledge of surface radiation around the globe is scandalously bad, and it remains one of the poorest known of all climate variables, even though ARM has shown that this measurements can be much better made.