Interaction of solar radiation with a hill cap cloud

Abstract

The radiative properties of a hill cap cloud at Great Dun Fell in northern England have been studied using broad band solar radiometers. Ground based measurements of shortwave radiation intensity and cloud liquid water content were made. An instrumented light aircraft flying above cloud top simultaneously measured upward and downward shortwave radiative fluxes. The cloud albedo has been calculated for two selected case studies. The measurements have been compared with a two-dimensional model to calculate airflow over a hill and subsequent formation of cap cloud. Calculations of Mie scattering were employed to generate the single scattering parameters of cloud droplets as a function of droplet size and wavelength. The transfer of solar radiation through the cloud was represented using a one-dimensional model. A two stream approximation was employed with a spectral resolution of 0.025 μm. The solar spectrum at cloud top was calculated using LOWTRAN 7 and the effects of absorption by water vapour were incorporated into the model. For one case study very good agreement was found between the model predictions and observations of cloud albedo and transmitted radiation. In the other case study strong entrainment substantially reduced the cloud albedo near the hill summit but the maximum cloud albedo observed, upwind of the hill summit, agreed well with the model predictions. The results of the model showed that water vapour made an important contribution to in-cloud absorption of radiation.