Ultra-clean Layers and Low Albedo Clouds in the Marine Boundary Layer

During the recent Cloud System Evolution in the Trades (CSET) field program, a common feature of the decoupled subtropical marine boundary layer (MBL) is the presence of ultra-clean layers (UCLs), here defined as horizontally extensive layers with extremely low accumulation mode aerosol concentrations (<10 cm^-3) and frequently as low as 0.1-1 cm^-3). UCLs tend to form in the upper part of the decoupled MBL above the transition layer at the top of the surface mixed layer. Clouds within UCLs are frequently horizontally extensive layers with some of the lowest droplet concentrations found in the troposphere (typically 1-10 cm^-3). They typically have low liquid water contents (0.01-0.1 g/kg), and display a range of thicknesses from <100 m to several hundred meters. Geostationary satellites indicate that the albedos of these extensive UCL clouds to be 0.1-0.2, giving them a “grey” appearance when viewed from space. Most of these layers do not fully attenuate the HSRL lidar flown on the NSF/NCAR HIAPER aircraft in CSET.

The range of aerosol and cloud droplet concentrations in UCL clouds is approximately an order of magnitude below levels typically assumed in modeled MBL clouds. The mechanisms for producing and maintaining UCLs and the clouds within them will be explored in this work, and include longwave cooling and mesoscale lifting of moist layers detrained from shallow cumulus convection. Interestingly, because of the low CCN concentrations in UCL clouds, even modest rates of lifting/cooling (~1 cm/s or ~10 K/day) yield modeled peak supersaturations of 0.2-0.4%, similar to values found in more active MBL clouds with higher aerosol loadings. Most accumulation mode aerosols (and probably a significant fraction of the Aitken mode particles) in these layers therefore activate. Size distributions of UCL clouds show a single mode with a modal radius in the range 15-25 microns, and droplets as large as 50 microns.

Simple parcel modeling indicates that droplets in UCL clouds can grow predominantly via condensational growth, with collision-coalescence unlikely to be important for shaping the droplet size distribution in most cases. This challenges the conventional wisdom that collision-coalescence growth is necessary to grow liquid droplets to radii larger than about 20 microns.

UCLs are a near ubiquitous feature of pockets of open cells (POCs), but their common existence in the deeper, cumulus boundary layers occurring over warmer waters is surprising and suggests that UCLs may be a climatologically important aerosol-cloud interaction phenomenon. The large aerial extent of UCLs and the clouds within them suggests that there is a need to focus more closely on how to represent such features in large scale models.