Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part II: Depletion of Cloud Droplets and Cloud Condensation Nuclei through Collision–Coalescence

Abstract

In Part I, aircraft observations are used to show that ultraclean layers (UCLs) in the marine boundary layer (MBL) are a common feature of the stratocumulus-to-cumulus transition (SCT) region over the northeast Pacific. The ultraclean layers are defined as layers of either cloud or clear air in which the concentration of particles with diameter larger than 0.1 μm is below 10 cm−3. Here, idealized microphysical parcel modeling shows that in the cumulus regime, collision–coalescence can strongly deplete cloud droplet concentration in cumulus (Cu) updrafts, thereby removing cloud condensation nuclei (CCN) from the atmosphere, suggesting that collision scavenging is likely the key process causing the low particle concentration in UCLs. Furthermore, the model results suggest that the stratocumulus regime is typically not favorable for UCL formation, because condensate amounts are generally not large enough to deplete drops in the time it takes to loft air to the upper planetary boundary layer (PBL). A bulk parameterization of the coalescence-scavenging rate is derived based on in situ measurements. The fractional coalescence-scavenging rate is found to be strongly dependent upon liquid water content (LWC) and, hence, the height above cloud base, indicating that a higher cloud top and thus a greater cloud thickness in a Cu updraft is an important factor accounting for the observed sharp rise of UCL coverage in the SCT region. An important implication is that PBL height, which controls maximum cloud thickness, and therefore LWC in updrafts, could be a crucial factor constraining coalescence scavenging and thus the formation of UCLs in the MBL.

Deeper, Precipitating PBLs Associated With Optically Thin Veil Clouds in the Sc‐Cu Transition

“…Figure 3 shows the climatology of the fraction of low clouds that are optically thin veil clouds f thin,veil from CALIOP (i.e., cloud conditional fraction). One possible explanation for such equatorward decrease in f thin,veil is that since Wood et al (2018) andO et al (2018) suggested that the detrainment mechanism from deep Cu underneath the trade inversion is the key process for the formation of optical thin veil clouds, the boundary layer over the tropics may be unfavorable for the formation of optical thin veil clouds because the inversion becomes weaker and may not preferentially concentrate the location in the vertical of detrainment levels (see Figure 2). Over the northeast pacific (NEP), f thin,veil increases westward from below 0.1 east of 135 ∘ to ∼0.3 over the SCT region.…”

“…Recently, Wood et al (2018) andO et al (2018) showed that a common feature associated with outflow of cumulus clusters over the SCT region is optically thin veil clouds, of which low optical thickness (i.e., < 3) is strongly contingent on its observed low-cloud droplet concentration (i.e., N d < 10 cm −3 ), and such low N d is suggested to be caused by strong precipitation scavenging process in the active cumulus. Further, O et al (2018) suggested that PBL height is a critical factor constraining coalescence scavenging process by regulating the maximum condensate amount in a cloud parcel.…”

“…By using a simple cloud microphysical parcel model, O et al (2018) suggested that the low particle concentrations in the optically thin veil clouds are most likely caused by the strong precipitation scavenging process as the air parcel is lifted through the active cumulus towers, where condensate amounts are generally large enough to sufficiently deplete drops through collision-coalescence. Wood (2006) andO et al (2018) both found that cumulus cloud thickness is a crucial factor determining the loss rate of droplet number concentration due to precipitation scavenging by constraining the maximum condensate amounts in the MBL clouds. Wood et al (2018) showed that veil clouds are relatively free of turbulence, which implies suppression of vertical mixing and may help to explain their high-frequency and longevity.…”

“…Wood et al (2018) showed that veil clouds are relatively free of turbulence, which implies suppression of vertical mixing and may help to explain their high-frequency and longevity. Wood (2006) andO et al (2018) both found that cumulus cloud thickness is a crucial factor determining the loss rate of droplet number concentration due to precipitation scavenging by constraining the maximum condensate amounts in the MBL clouds. In the SCT region, the variation of cloud thickness of cumulus is mainly constrained by PBL height because Cu cloud base height tend to be less variable than PBL top heights.…”

Open cells can decrease the mixing of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer

“…There has been a growing interest in clouds that form in the low aerosol environment found in UCLs, especially with regards to the quasi-laminar stratiform cloud layers that are likely to be detrained remnants of more active shallow cumulus O et al , 2018). These more stratiform layers, also termed veil clouds, have been characterised in the stratocumulus to cumulus transition in the north-east Pacifc and are frequently observed in POCs Terai et al , 2014).…”