I stumbled across this pre-print today - Cancellation of aerosol indirect effects in marine stratocumulus through cloud thinning by Robert Wood in press at J. Atmos. Sci.

Abstract:

Applying perturbation theory within a mixed layer framework, we examine 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. 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 timescales of less than a day the cloud thickness feedback response is largely determined by a balance between moistening/cooling of the MBL due to the suppression of surface precipitation, and drying/warming due to enhanced entrainment resulting from increased turbulent kinetic energy. Quantifying the transient cloud response as a ratio of the second and 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 he 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 cloudbase 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 timescales an invariable thickening response is found, and thus accurate quantification of the aerosol in-direct effects will require a good understanding of the processes that control the timescale over which aerosol perturbations are modified.

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