Aircraft measurements of the solar absorption by broken cloud fields: A case study

Abstract

AbstractMeasurements of broad‐band radiation at solar (0.3–3.0 μm) and near‐infrared (0.7–3.0 μm) wavelengths are presented for three flights of the Meteorological Research Flight C130 aircraft in broken cloud fields. The solar absorption is determined from observations made above cloud top and below cloud base, using data in the two spectral bands to minimize errors of cloud sampling between different runs and to reduce the scatter due to cloud edge effects. The method assumes that cloud layer absorption in the visible region is negligible compared with that in the near‐infrared. This allows average absorption to be calculated for small sections of the aircraft track, which will generally include both fully clear and totally overcast cases. The smallest absorption measured is about 2%, in agreement with model results for the clear atmosphere. The largest absorption was found to be 8–12% (for a maximum cloud depth of 1 km), which is similar to measurements reported for layer cloud. An additional finding is that absorption by the broken cloud field varies uniformly with albedo, indicating that the parametrization of absorption in numerical models can be adequately described as a simple function of the albedo in partially cloudy areas.