Clouds have large effects on the Earth's radiation budget, both in the longwave and shortwave regions of the spectrum. Lack of knowledge of cloud distributions and cloud properties, and the behaviour of clouds during climatic change, limits the accuracy of climate-prediction models. Cloud distributions and radiative properties are now being monitored by satellite; however, there is also a need for surface measurements of cloud properties. Climate research would benefit if the routine solar radiation measurements made at weather stations worldwide, and on many research ships, using broadband pyranometers, could be used to determine the cloud radiative forcing (CRF) at the surface. Here we investigate what information about clouds can be obtained from broadband measurements, with particular attention to the effect of surface albedo on the measured downward shortwave irradiance. A parameterization is developed and applied to pyranometer measurements from ship observations in the Antarctic sea-ice zone and the Southern Ocean.

A multi-level spectral radiative transfer model is used to develop simple but accurate parameterizations for cloud transmittance as a function of cloud optical depth, solar zenith angle, and surface albedo, for use over snow and ice surfaces. The same functional form is used for broadband and spectral transmittances, but with different coefficients for each spectral interval. When the parameterization is applied to measurements of "raw" cloud transmittance (the ratio of downward irradiance measured under clear sky at the same zenith angle), an "effective" optical depth is inferred for the cloud field, which may be inhomogeneous and even patchy. This effective optical depth can then be used to compute what the transmittance of the same cloud field would be under different conditions of solar illumination and surface albedo. The parameterization faithfully mimics the radiative transfer model, with rms errors of 1-2 %. Lack of knowledge of cloud droplet sizes causes little error in inference of cloud radiative properties; the largest source of error is uncertainty in surface albedo.

The parameterization is applied to broadband-shortwave downward irradiance measurements from ship voyages in the Southern Ocean from 1988 to 2000. The cloud-transmittance measurements will be used together with cloud-type distributions and surface albedo observations to estimate the geographical and seasonal variation of effective optical depth, average cloud transmittance and shortwave cloud forcing, in the ocean around Antarctica.