| With aircraft observations, ultraclean layers (UCLs) in the marine boundary layer (MBL) are shown to be common features in the stratocumulus to cumulus (Sc-Cu) transition region. The ultraclean layers are defined as layers of either cloud or clear air in which the concen- tration of particles larger than 0.1 mum is below 10 cm--3. Here, idealized parcel modeling shows that in the cumulus regime, collision-coalescence can strongly deplete cloud droplet concentration in Cu updrafts thereby removing cloud condensation nuclei (CCN) from the atmosphere, leading to the formation of UCLs. Furthermore, the model results suggest that the stratocumulus regime is typically not favorable for UCLs formation. A bulk parameteri- zation of the droplet coalescence scavenging rate is derived based from in-situ measurements of droplet size distribution (DSDs). With the bulk parameterization, the fractional droplet loss rate by collision-coalescence is found to be strongly dependent upon liquid water con- tent (LWC), and hence the height above cloud base, indicating that higher cloud top in Cu updraft is a key factor accounting for the observed sharp rise of UCLs coverage in the Sc-Cu transition region compared with the Sc regime. An important implication from this study is that planetary boundary layer (PBL) height, which controls cloud thickness, and therefore LWC in updrafts, could be a crucial factor constraining eciency of coalescence-scavenging and thus the formation of UCLs in Sc-Cu transition regions. |
