Measurements of Cloud Susceptibility

The sensitivity of warm stratocumulus cloud albedo to changes in droplet concentration, termed 'cloud susceptibility', is calculated using data from the UKMO Meteorological Research Flight. Stratocumulus clouds ; in the Eastern Pacific (FIRE 1987), South Atlantic, sub-tropical regions of the North Atlantic (ASTEX) and around the British Isles are studied. The range of susceptibility measured is large and maritime clouds are shown to have the largest susceptibility. Three parametrisations of effective radius are used to simulate the observations of cloud susceptibility and the merits of each discussed. It is shown that effective radius needs to be variable within a model and variation of effective radius with aerosol concentration is an important component of any parametrisation scheme if the sensitivity of clouds optical properties to changes in droplet concentration are to be accurately modelled. Numerical simulations of the changes in cloud radiative and microphysical properties with increasing droplet concentration are carried out. These highlight the high sensitivity of maritime clouds and the rapid reduction in sensitivity as the cloud droplet concentration increases.