Cancellation of aerosol indirect effects in marine stratocumulus through cloud thinning

Applying perturbation theory within a mixed layer framework, the response of the marine boundary layer (MBL) cloud thickness h to imposed increases of the cloud droplet concentration N-d as a surrogate for increases in cloud condensation nuclei (CCN) concentrations is examined. An analytical formulation is used to quantify the response and demonstrate theoretically that for the range of environmental conditions found over the subtropical eastern oceans, on time scales of less than a day, the cloud thickness feedback response is largely determined by a balance between the moistening/cooling of the MBL resulting from the suppression of surface precipitation, and the drying/warming resulting from enhanced entrainment resulting from increased turbulent kinetic energy. Quantifying the transient cloud response as a ratio of the second to the first indirect effects demonstrates that the nature of the feedback is critically dependent upon the nature of the unperturbed state, with the cloud-base height z(cb), being the single most important determinant. For z(cb) < 400 m, increasing N-d leads to cloud thickening in accordance with the Albrecht hypothesis. However, for z(cb) > 400 m, cloud thinning occurs, which results in a feedback effect that increasingly cancels the Twomey effect as z(cb) increases. The environmental conditions favoring an elevated cloud base are relatively weak lower-tropospheric stability and a dry free troposphere, although the former is probably more important over the subtropical eastern oceans. On longer time scales an invariable thickening response is found, and thus accurate quantification of the aerosol indirect effects will require a good understanding of the processes that control the time scale over which aerosol perturbations are modified.