Determination of Longwave Irradiance at the Ground

Abstract

<p>Incoming longwave irradiance at the earth's surface under cloudless or near cloudless skies was determined by three instrumental methods. The most reasonable results were obtained using a Eppley pyrgeometer fitted with a silicon dome. Problems associated with an Eppley pyrgeometer equipped with a KRS-5 dome and with a Funk-type Swissteco pyrradiometer are demonstrated. Measured irradiances were compared with those calculated from various physical and empirical models under a variety of atmospheric conditions. The former usually agreed with measured irradiance to within 15%. The latter showed variable agreement. The Swinbank model with only one dependent variable provided as good an estimate as the more complex physical modes. The role of aerosol at infrared wavelengths is examined and is shown to have minimal effect on the longwave flux at the ground.</p>