Top-of-atmosphere albedo bias from neglecting three-dimensional cloud
radiative effects
Abstract
Clouds cover on average nearly 70% of Earth’s surface and regulate the
global albedo. The magnitude of the shortwave reflection by clouds
depends on their location, optical properties, and three-dimensional
(3D) structure. Due to computational limitations, Earth system models
are unable to perform 3D radiative transfer calculations. Instead they
make assumptions, including the independent column approximation (ICA),
that neglect effects of 3D cloud morphology on albedo. We show how the
resulting radiative flux bias (ICA-3D) depends on cloud morphology and
solar zenith angle. Using large-eddy simulations to produce 3D cloud
fields, a Monte Carlo code for 3D radiative transfer, and observations
of cloud climatology, we estimate the effect of this flux bias on global
climate. The flux bias is largest at small zenith angles and for deeper
clouds, while the negative albedo bias is most prominent for large
zenith angles. In the tropics, the radiative flux bias from neglecting
3D radiative transfer is estimated to be 4.0 +/- 2.4 Wm-2 in the mean
and locally as large as 9 Wm-2.