Investigation of the Marine Boundary Layer Cloud and CCN Properties under Coupled and Decoupled Conditions over the Azores

Abstract:

Six coupled and decoupled marine boundary layer (MBL) clouds were chosen from the 19-month ARM Mobile Facility dataset over the Azores. Thresholds of liquid water potential temperature difference Δθ_L < 0.5 K (>0.5 K) and total water mixing ratio difference Δq_t < 0.5 g/kg (> 0.5 g/kg) below the cloud base were used for selecting the coupled (decoupled) cases. A schematic diagram was given to demonstrate the coupled and decoupled MBL vertical structures and how they associate with non-drizzle, virga and rain drizzle events. Out of a total of 2676 5-min samples, 34.5% were classified as coupled and 65.5% as decoupled; 36.2% as non-drizzle and 63.8% as drizzle (47.7% as virga, 16.1% as rain); 33.4% as daytime and 66.6% as nighttime. The decoupled cloud layer is deeper (0.406 km) than coupled cloud layer (0.304 km), and its liquid water path (LWP) and cloud-droplet effective radius (r_e) values (122.1 gm^-2 and 13.0 µm) are higher than coupled ones (83.7 gm^-2 and 10.4 µm). Conversely, decoupled stratocumuli have lower cloud-droplet number concentration (N_d) and surface cloud condensation nuclei (CCN) concentration (N_CCN) (74.5 cm^-3 and 150.9 cm^-3) than coupled stratocumuli (111.7 cm^-3 and 216.4 cm^-3). The linear regressions between r_e and N_d with N_CCN have demonstrated that coupled r_e and N_d strongly depend on N_CCN and have higher correlations (-0.56 and 0.59) with N_CCN than decoupled results (-0.14 and 0.25). The MBL cloud properties under non-drizzle and virga drizzle conditions are similar to each other, but significantly different to those of rain drizzle.