Implications of Lorenz-Mie scattering by cloud droplets in an absorbing atmosphere for cloud radiative effects

Abstract:

There is still considerable uncertainty as to the magnitude of
cloud feedbacks to anthropogenic climate change. The magnitude is
determined by changes in the bulk cloud radiative effects (CREs)
on both solar and terrestrial radiation in response to changes in
the Earth's average surface temperature. The range of CRE
responses of low-altitude liquid clouds is one of the dominant
sources of uncertainty. To date, the CREs for these clouds have
been computed using variants of classical far-field Mie theory
applied to spherical particles (i.e., water droplets) embedded in
a non-absorbing medium. At many wavelengths where water vapor is
the predominant radiatively active gas, the assumption of a
non-absorbing medium is manifestly violated, for example in the
near-infrared near the primary and overtone absorption bands of
H2O. For this reason, it is important to redo the calculation of
CREs using new, generalized Lorenz-Mie scattering in an absorbing
atmosphere (Mishchenko et al, 2017 and 2018). In this talk, we
quantify the implications of this generalized formalism for the
reflection, transmission, and absorption of sunlight by
low-altitude clouds. We show how the effects can be accurately
represented using bulk formula for mixtures of vapor and droplets.