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

摘要

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 m~(-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 m~(-2).* A relatively homogeneous spatial pattern of maximum S_R values covered most of each regime's domain for a LWP of 90 g m~(-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.