Entrainment rate and droplet sedimentation in stratocumulus-capped mixed layers

Recently, A. Ackerman showed that sedimentation of cloud droplets in stratocumulus-capped mixed layers, a process neglected in most prior LES simulations, appreciably reduces the simulated entrainment rate into such layers. If the overlying air is much warmer and drier, the reduced entrainment causes the simulated cloud to appreciably thicken. Higher CCN concentrations can decrease the typical cloud droplet size and fall speed. Ackerman also pointed out that this can decrease sedimentation and increase entrainment, resulting in a thinner cloud layer for more polluted conditions, for conditions typical of the subtropical marine stratocumulus regions. This cloud thinning partly counteracts the first indirect (Twomey) effect of cloud droplet size on aerosol concentration, with important consequences for climate change simulation.

Ackerman regarded the impact of sedimentation on entrainment as a consequence of reduced buoyancy production of TKE. In this argument, sedimentation creates net condensational heating near cloud top (where there is a net loss of cloud water by sedimentation), and net evaporational cooling through the rest of the cloud (due to evaporation of sedimenting cloud droplets). This heating over cooling couplet stabilizes the boundary layer, reduces turbulence, and hence reduces entrainment. Similar arguments have been successfully used to understand feedbacks of drizzle on stratocumulus entrainment.

By careful analysis of LES simulations of a nocturnal nondrizzling stratocumulus cloud layer (the GCSS DYCOMS RF01 case study), we find that addition of droplet sedimentation reduces the entrainment rate, in agreement with Ackerman's findings. However, the boundary layer TKE actually increases, counter to the above argument. This implies that droplet sedimentation is reducing the 'entrainment efficiency', a nondimensional measure of the entrainment rate for a given turbulence level and inversion strength. We argue that in the entrainment zone, sedimentation is significantly depleting the cloud liquid water, so that mixing between cloudy and warmer, drier above-cloud air is accompanied by less evaporation, resulting in mixtures that are more buoyant and less easily entrained into the boundary layer. A formulation for entrainment rate that accounts for this sedimentation effect is proposed and tested in a mixed layer model.