THE CHOPPED PYRGEOMETER - A NEW STEP IN PYRGEOMETRY

A new type of pyrgeometer, which uses modulation of the atmospheric ra diation by a mechanical chopper, has been built for the use of ground- based and airborne measurement of broadband infrared atmospheric irrad iances. The instrument basically consists of two radiometers-a target radiometer and a reference radiometer-using a specially developed chop per. The reference radiometer is used to measure the radiation of the chopper that has to be known additionally to calculate the atmospheric radiation from the signal of the target radiometer. The radiometer eq uations take into account the nonflat spectral response. A spectral co rrection factor is introduced to describe the relation between the wei ghted irradiance at the detector and the irradiance coming from the so urce. One important advantage of the chopped pyrgeometer is its nonsen sitivity to the temperature distribution inside the instrument as well as its fast response. The chopped pyrgeometer has been used during a number of airborne missions. During the Pre-EUCREX Intercomparison Cam paign flown in January 1992, two Eppley pyrgeometers, two pyrgeometers of the Foot-type, and a chopped pyrgeometer were flown, in addition t o other instruments on three aircraft. During a rectangular flight pat tern (box) the absolute irradiance values measured by the five instrum ents differ by about +/- 10% from the average value (approximate to 85 W m(-2)) of all instruments, the chopped pyrgeometer indicating the s mallest values throughout. Since a detailed error budget of all instru ments was not available, special emphasis was placed on relative quant ities (ratios) to describe the trends measured by the various instrume nts along the flight tracks. It has been found that the measurements o f the chopped pyrgeometer show a much better correlation to the measur ement of the Foot pyrgeometers than to the Eppley instruments. A compa rison of four different model calculations with the measured data of a ll participating instruments at ground and at three Eight levels is pr esented.