Ultraclean Layers and Optically Thin Clouds in the Stratocumulus-to-Cumulus Transition. Part I: Observations

Abstract

A common feature of the stratocumulus-to-cumulus transition (SCT) is the presence of layers in which the concentration of particles larger than 0.1 μm is below 10 cm−3. These ultraclean layers (UCLs) are explored using aircraft observations from 14 flights of the NSF–NCAR Gulfstream V (G-V) aircraft between California and Hawaii. UCLs are commonly located in the upper part of decoupled boundary layers, with coverage increasing from less than 5% within 500 km of the California coast to ~30%–60% west of 130°W. Most clouds in UCLs are thin, horizontally extensive layers containing drops with median volume radii ranging from 15 to 30 μm. Many UCL clouds are optically thin and do not fully attenuate the G-V lidar and yet are frequently detected with a 94-GHz radar with a sensitivity of around −30 dB Z. Satellite data indicate that UCL clouds have visible reflectances of ~0.1–0.2 and are often quasi laminar, giving them a veil-like appearance. These optically thin veil clouds exist for 1–3 h or more, are associated with mesoscale cumulus clusters, and likely grow by spreading under strong inversions. Active updrafts in cumulus (Cu) clouds have droplet concentrations of ~25–50 cm−3. Collision–coalescence in the Cu and later sedimentation in the thinner UCL clouds are likely the key processes that remove droplets in UCL clouds. UCLs are relatively quiescent, and a lack of mixing with dry air above and below the cloud may help to explain their longevity. The very low and highly variable droplet concentrations in UCL clouds, together with their low geometrical and optical thickness, make these clouds particularly challenging to represent in large-scale models.

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.…”

“…Based on the above formula and the mean LWP of thin veil clouds estimated based on aircraft in situ measurements (≈25 g m −2 ), Wood et al (2018) showed that for a threshold optical depth of 3 below which the lidar can penetrate, droplet concentration must be below approximately 20 cm −3 , indicating that low optical thickness of veil clouds is strongly contingent on their low droplet concentrations. Based on the above formula and the mean LWP of thin veil clouds estimated based on aircraft in situ measurements (≈25 g m −2 ), Wood et al (2018) showed that for a threshold optical depth of 3 below which the lidar can penetrate, droplet concentration must be below approximately 20 cm −3 , indicating that low optical thickness of veil clouds is strongly contingent on their low droplet concentrations.…”

“…Based on the above formula and the mean LWP of thin veil clouds estimated based on aircraft in situ measurements (≈25 g m −2 ), Wood et al (2018) showed that for a threshold optical depth of 3 below which the lidar can penetrate, droplet concentration must be below approximately 20 cm −3 , indicating that low optical thickness of veil clouds is strongly contingent on their low droplet concentrations. 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. In addition, the authors suggested that there is a process of size sorting of droplets occurring in the veil clouds such that the largest cloud droplet (i.e., radius >20 μm) is quickly removed by sedimentation, leaving behind smaller droplets (i.e., radius ≈10 μm).…”

“…By using the aircraft remote sensing during CSET, Wood et al (2018) showed that the fraction of low clouds that are optically thin clouds, defined as the sampled clouds that do not fully attenuate the High Spectral Resolution Lidar (HSRL) beam, can be 50% over the SCT regions. A common feature in the upper part of the decoupled MBL over the SCT is the prevalence of ultra clean layers (UCLs), which have been defined as the presence of layers in which the number concentration of particles larger than 0.1 μm is below 10 cm −3 .…”

The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2

“…In previous versions, the minimum CDNC (CDNCmin) was set to 40 cm −3 . The justification for this choice was twofold: first, while observations of clouds with a lower CDNC exist (e.g, Terai et al, 2014;Wood et al, 2018), these smaller concentrations normally occur in clouds or pockets in clouds that are much smaller than our grid boxes. Second, so far we do not account for nitrate aerosols and our treatment of secondary organic aerosols is rather simplistic and likely underestimates the organic aerosol concentration (Zhang et al, 2012).…”