Microphysical and short-wave radiative structure of wintertime stratocumulus clouds over the Southern Ocean

Abstract

Results are presented from an aircraft measurement campaign carried out over the Southern Ocean near the north-west coast of Tasmania, Australia. The microphysical and radiative characteristics of marine stratocumulus cloud sheets were sampled on four days, three of which are considered baseline days when air parcels had traversed long distances over the ocean without having been exposed to anthropogenic sources of pollution. Clouds were depleted by drizzle, with cloud liquid-water often smaller than 50% of the expected adiabatic value. Horizontal variability in liquid-water content associated with non-drizzle droplets was primarily caused by variations in the droplet number concentration, which was found to be among the lowest ever recorded (10 to 40 cm −3 for clouds of up to 300 m deep). Cloud albedos integrated over the solar spectrum varied from 40 to 60%, which roughly agreed with radiative-transfer computations. Data from one non-baseline day were also obtained and compared with the data obtained during baseline conditions. The functional dependence of cloud optical depth on liquid-water path was found to be much stronger on this day than the others, due to the smaller size of the cloud droplets present. Calculations show that on the days when drizzle was intense, the cloud optical depth could have been reduced by as much as 50% by shifts to larger values of the cloud droplet effective radius.