A Comparison of Measured and Modeled Broadband Fluxes from Aircraft Data during the ICE '89 Field Experiment

Abstract

During the 1989 intensive field campaign of the International Cirrus Experiment (ICE) over the North Sea, broadband radiative fluxes were measured in, above, and below cirrus cloud by a number of European meteorological research aircraft. One mission during the campaign was an intercomparison flight in clear air with no cloud above in order to compare, among other things, radiative flux measurements made by the U.K. C-130, the French Merlin, and the German Falcon aircraft. All three aircraft measured shortwave flux (0.3–3 µm) with standard Eppley pyranometers above and below the fuselage. The intercomparison showed agreement between the three aircraft of within 2% for both the upwelling and downwelling shortwave flux components. Using a coincident temperature and humidity radiosonde profile, the downward clear-sky fluxes at the level of the aircraft were also calculated using a variety of different radiation models. Modeled shortwave fluxes were all higher (between 2% and 4%) than the measured values. In addition to shortwave fluxes the C-130 and Merlin also measured near-infrared fluxes (0.7–3 µm) by having additional Eppley pyranometers mounted with red domes over the thermopiles. The near-infrared fluxes measured by the Merlin and C-130 were different because slightly different red-dome filters were used; model calculations show the difference between the measured fluxes was consistent with the different pass band of the filters. Infrared fluxes (4–40 µm) were measured using standard Eppley pyrgeometers on the Falcon and pyrgeometers developed at the Meteorological Research Flight on the C-130; comparisons show no significant differences for the downwelling fluxes but the Falcon upwelling fluxes were 7% higher than the corresponding C-130 values. This latter difference is higher than would be expected for these instruments. The modeled infrared fluxes were up to 9% lower than the C-130 and Falcon measurements.