Studies with a flexible new radiation code. II: Comparisons with aircraft short‐wave observations

Calculated irradiances from a new radiation code are compared with in situ observations of short‐wave irradiances from the UK Meteorological Office's C‐130 aircraft. Three cases of clear skies are studied and four where a liquid‐water boundary‐layer cloud was present. Under clear‐sky conditions the modelled and in situ observations agree to within 3%, which is the estimated accuracy of the observations. In the cloudy‐sky cases the albedo and transmittance agree to within ±0.1 but the absorption in the model is higher than that observed, sometimes by a factor of two; there is no evidence of anomalous absorption in the observations. The observed absorptions do not exceed 6% for the stratocumulus cases considered. The results clearly identify the problems of representing inhomogeneous clouds as plane parallel layers in radiation models. Analysis of the variability of the cloud microphysics provides some insight into the importance of regions of low optical depth within the clouds.