During the CSET campaign, the NSF/NCAR G-V aircraft frequently encountered ultra clean layers (hereafter UCLs) with extremely low accumulation mode aerosol (i.e. diameter da> 100nm) concentration (hereafter Na), and low albedo (~0.2) warm clouds (termed “gray clouds” in our study) with low droplet concentration (hereafter Nd). In our analysis, UCLs are defined as the clear sky samples with Na (accumulation mode aerosol) lower than 10 cm-3 and gray clouds are defined as the cloudy samples with Nd lower than 10 cm-3. The analysis of CSET aircraft data shows that (1) UCLs and gray clouds are mostly commonly found at a height of 1.5-2km, typically close to the top of the MBL, (2) UCLs and gray cloud coverage as high as 40-60% between 135W and 155W (i.e. Sc-Cu transition region) but occur very infrequently east of 130W (i.e. shallow, near-coastal stratocumulus region), and (3) UCLs and gray clouds exhibit remarkably low turbulence compared with non-UCL clear sky and clouds. It should be noted that most previous aircraft sampling of low clouds occurred close to the Californian coast, so the prevalence of UCLs and gray clouds has not been previously noted.
Based on the analysis of aircraft data, we hypothesize that gray clouds (Nd<10 cm-3) result from detrainment of cloud close to the top of precipitating trade cumuli, and UCLs (Na<10 cc-1) are remnants of these layers when gray clouds evaporates. The simulations in our study are performed using 2-D bin spectral cloud parcel model and version 6.9 of the System for Atmospheric Modeling (SAM). Our 2-D bin spectral parcel model uses Chen and Lamb bin condensation/evaporation scheme and Bott 2-D collision coalescence scheme. Microphysical tendencies in SAM are computed using two-moment Morrison microphysics. Our idealized simulations suggest that collision-coalescence plays a crucial role in reducing Nd such that gray clouds can easily form via collision-coalescence in layers detrained from the cloud top at trade cumulus regime (i.e. with low surface Na ~50 cc-1, high PBL ~ 1800m and vigorous updraft ~ 1ms-1), but can not form at stratocumulus regime (i.e. with relatively high surface Na ~ 200 cc-1, low PBL ~ 800m and updraft ~ 0.5 ms-1), which is consistent with the observed UCLs and gray cloud coverage during CSET such that UCLs and gray cloud mainly occurs at Sc-Cu transition region. Upon evaporation of gray clouds, only few accumulation mode aerosols are returned to the clear sky, leaving horizontally-extensive UCLs (i.e. clean clear sky). The long overturning time (~12hr) for the upper trade PBL means gray clouds and UCLs can extend many hundreds of km. Analysis of CSET flight data and idealized model simulations both suggest cloud top/PBL height may play an important role in the formation of UCLs and gray clouds. In our satellite observation study, the comparison between PBL height (derived from COSMIC and MODIS) and fraction of low optical depth cloud (derived from MODIS and CALIPSO) at NEP trade cumulus regime (20-35N, 140-155W) also suggest a strong positive correlation.
These results suggest that representing UCLs in large scale models will be challenging because of the tremendous subgrid-scale variability in droplet concentration. The role of gray clouds and UCLs in the global radiation budget, cloud feedback processes and indirect radiative effect may be unique and need further exploration.