National Institute for Global Environmental Change. Final report

Over the past decade or so the evolution and equilibria of persistent decks of stratocumulus climatologically clinging to the edge of summertime subtropical highs has been an issue of increased scientific inquiry. The particular interest in the microphysical structure of these clouds stems from a variety of hypotheses which suggest that anthropogenic influences or biogenic feedbacks may alter the structure of these clouds in a manner which may be climatically significant. Most hypotheses regarding boundary layer influences on climate have been formulated by an examination of the solution space of simple models. The earliest hypothesis of this sort (and the one on the most solid footing) is due to Twomey (1974), who posited that enhanced concentrations of CCN could lead to enhanced droplet reflectivity and enhanced albedos in clouds of modest optical depths. In low lying clouds where the albedo effect dominates, the climate sensitivity to a robust perturbation in cloud albedo may be significant. One of the primary objectives of this current research has been to explore the hypothesis of Twomey. The basic approach was to couple radiative calculations with detailed representations of the droplet spectra. The detailed representation of the droplet spectra was generated by the Large Eddy Simulation-Explicit Microphysics (LES-EM) model coupled to a simple mixed emissivity radiation scheme in order to drive the dynamics. Several simulations were carried out and the resultant microphysical fields were taken from the stationary regime of the turbulent simulation and used to drive a two dimensional radiative model. By comparing the radiative properties of the simulated clouds formed in environments with different CCN concentrations we were able to more accurately quantify the albedo susceptibility of stratocumulus taken to be typical of the FIRE experimental area