The Spectral Radiance Experiment (SPECTRE): Project Description and Sample Results

Abstract

The fundamental climatic role of radiative processes has spurred the development of increasingly sophisticated models of radiative transfer in the earth-atmosphere system. Since the basic physics of radiative transfer is rather well known, this was thought to be an exercise in refinement. Therefore, it came as a great surprise when large differences (30?70 W m?2) were found among longwave infrared fluxes predicted by over 30 radiation models for identical atmospheres during the intercomparison of radiation codes used in climate models (ICRCCM) exercise in the mid-1980s. No amount of further calculation could explain these and other intermodel differences; thus, it became clear that what was needed was a set of accurate atmospheric spectral radiation data measured simultaneously with the important radiative properties of the atmosphere like temperature and humidity. To obtain this dataset, the ICRCCM participants charged the authors to develop an experimental field program. So, the authors developed a program concept for the Spectral Radiance Experiment (SPECTRE), organized a team of scientists with expertise in atmospheric field spectroscopy, remote sensing, and radiative transfer, and secured funding from the Department of Energy and the National Aeronautics and Space Administration. The goal of SPECTRE was to establish a reference standard against which to compare models and also to drastically reduce the uncertainties in humidity, aerosol, etc., which radiation modelers had invoked in the past to excuse disagreements with observations. To avoid the high cost and sampling problems associated with aircraft, SPECTRE was designed to be a surface-based program. The field portion of SPECTRE took place 13 November to 7 December 199 1, in Coffeyville, Kansas, in conjunction with the FRE Cirrus II field program, and most of the data have been calibrated to a usable form and will soon appear on a CD-ROM. This paper provides an overview of the data obtained; it also outlines the plans to use this data to further advance the ICRCCM goal of testing the verisimilitude of radiation parameterizations used in climate models.