Which Processes are Critical for Driving the Non-Precipitating Stratocumulus Response to Aerosol? (Invited)

Increasing the aerosol concentration in non-precipitating stratocumulus typically leads to increased entrainment of warm, dry air at cloud top and a decrease in liquid water path. These changes have been typically attributed to enhanced evaporation rates and decreased droplet sedimentation rates at cloud top. More recently, it has been hypothesized that altered longwave radiative cooling rates at cloud top may also play a role. First, we present some arguments about why longwave radiation cooling may have a larger role to play in aerosol-shallow-cloud interactions than is typically assumed. Next, we systematically test the relative importance of these processes through a series of mechanism suppression tests. In general, we find that radiation and sedimentation are about equally important for the entrainment response to aerosol whereas evaporation and sedimentation are most important for the liquid water path response. Radiation, while important for the enhanced entrainment, appears not to impact the liquid water path as strongly because the additional entrainment heating is partially offset by the additional radiative cooling (that drives the additional entrainment in the first place). The study highlights the complex relationships between liquid water path, entrainment, and aerosol concentration and it ultimately suggests that GCMs need to parameterize aerosol-induced responses to longwave radiation in addition to sedimentation and evaporation in order to accurately capture aerosol-low cloud interactions.