On the dependence of albedo on cloud microphysics over marine stratocumulus clouds regimes determined from Clouds and the Earth's Radiant Energy System (CERES) data

Abstract

The dependence of the top‐of‐the‐atmosphere (TOA) albedo A on cloud microphysical properties was investigated for the three largest maritime stratocumulus clouds regimes: off California, Southeast Pacific (Chile‐Peru), and southwest Africa (Namibia‐Angola). Absolute S and relative S R albedo susceptibilities to perturbations in cloud droplet number concentrations N d , defined as dA/dN d and dA/dln ( N d ) respectively, were calculated for the season having maximum cloud cover during the period 2006–2010. Satellite‐based susceptibilities were computed by combining an adiabatically based N d estimate and liquid water path (LWP) derived from Terra Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals matched with TOA A from the Clouds and the Earth's Radiant Energy System. Empirical susceptibility maps were calculated for three constant LWP intervals at 25, 50, and 90 g −2 . It was found that S increases with LWP, with small and spatially homogeneous values for low LWP, and a contrasting increase far offshore for larger LWP values. An overall increase of S R with LWP was also observed, with larger values near the coast for LWP = 25 and 50 g −2 . A relatively homogeneous spatial pattern of maximum S R values covered most of each regime's domain for a LWP of 90 g −2 . These results highlight the importance of LWP in modulating the albedo susceptibility. The dependencies of S and S R on LWP are mostly explained by variations in the mean N d and cloud optical thickness ( τ ), with an increase of S with LWP linked to a decrease in N d , whereas S R increased with τ and A , until reaching a maximum for A and τ near 0.36–0.4 and 12–14 respectively, and decreasing thereafter, consistent with expectations based on two‐stream estimates. Larger S R values in the Southeast Pacific are thought to be the consequence of a drier and more pristine atmosphere. Radiative transfer simulations with realistic values of above‐cloud water vapor path and aerosol optical thickness showed that differing atmospheric compositions could explain why the Chile‐Peru regime was the marine stratocumulus cloud deck most susceptible to change its TOA albedo due to fractional changes in N d . Albedo susceptibility increases with liquid water path Susceptibilities mainly controlled by number of droplets and optical thickness Differences among regimes are also explained by the atmospheric composition