INTERACTION OF SOLAR-RADIATION WITH A HILL CAP CLOUD

The radiative properties of a hill cap cloud at Great Dun Fell in nort hern England have been studied using broad band solar radiometers. Gro und based measurements of shortwave radiation intensity and cloud liqu id water content were made. An instrumented light aircraft flying abov e cloud top simultaneously measured upward and downward shortwave radi ative fluxes. The cloud albedo has been calculated for two selected ca se studies. The measurements have been compared with a two-dimensional model to calculate airflow over a hill and subsequent formation of ca p cloud. Calculations of Mie scattering were employed to generate the single scattering parameters of cloud droplets as a function of drople t size and wavelength. The transfer of solar radiation through the clo ud was represented using a one-dimensional model. A two stream approxi mation was employed with a spectral resolution of 0.025 mu m. The sola r spectrum at cloud top was calculated using LOWTRAN 7 and the effects of absorption by water vapour were incorporated into the model. For o ne case study very good agreement was found between the model predicti ons and observations of cloud albedo and transmitted radiation. In the other case study strong entrainment substantially reduced the cloud a lbedo near the hill summit but the maximum cloud albedo observed, upwi nd of the hill summit, agreed well with the model predictions. The res ults of the model showed that water vapour made an important contribut ion to in-cloud absorption of radiation.