MBL Cloud and CCN Properties under Coupled and Decoupled Conditions

Science

Marine boundary layer (MBL) stratocumuli are turbulently mixed from the top-downward due to negative buoyancy through a combination of longwave (LW) radiative cooling and evaporative cooling at the cloud top (Wood 2012; Shin and Ha 2009). The vertical and horizontal structures of stratocumulus are strongly tied to the vertical structure of the boundary layer. The low-level cloud fraction is greatest when the stratocumulus-opped boundary layer (STBL) depth (z) is shallow (0.5 km<z<1.0 km). These STBLs are often well mixed with transported moisture from the surface to the STBL and capped by a strong temperature and humidity inversion just above the cloud layer. When the STBL top exceeds 1 km and the cloud layer depth becomes thick, the STBL begins to separate into two layers with the upper layer becoming decoupled from the surface moisture supply [Wood, 2012]. Within a decoupled STBL, the stratocumulus layer is often within a well-mixed layer, but the negatively buoyant eddies created by the LW cooling are not strong enough to mix with the subcloud boundary layer (Burleyson et al. 2013; Jones et al. 2011; Wood 2012).

Impact

Six coupled and decoupled MBL clouds were chosen from the 19-month ARM Mobile Facility data set over the Azores. Thresholds of liquid water potential temperature difference ΔθL < 0.5 K (>0.5 K) and total water mixing ratio difference Δqt < 0.5 g/kg (> 0.5 g/kg) below the cloud base were used for selecting the coupled (decoupled) cases. A schematic diagram seen in the image to demonstrate the coupled and decoupled MBL vertical structures and how they associate with non-drizzle, virga and rain drizzle events. For coupled samples, the liquid water potential temperature θL and total water mixing ratio qt are conserved throughout the STBL when non-drizzle occurs, and both variables change sharply above Zt due to dry air above it. With the drier air above Zt, the decrease of qt results in an increase of stability and decrease of ql further, then θl starts to increase sharply. When rain drizzle occurs, ql is greater than zero below Zb, thus θl decreases and qt increases from the surface to Zb. Within the cloud layer, qt decreases and θl increases from Zb towards Zt due to the depletion of rain drizzles. For virga drizzle, both θl and qt are conserved from the surface to the drizzle base, and ql is greater than zero and increases from the drizzle base to Zb, but θl decreases. Within the cloud layer, the situation is the same as rain drizzle. For decoupled samples, the boundary layer is deepened and separated into two layers (dashed line in the image) with its own circulation in each layer. The surface moisture cannot be transported into the upper layer where the cloud stays, qt decreases but θl increases in the upper layer compared to the surface mixed layer. The profiles of θl and qt in the upper layer should have similar patterns to the coupled samples but change quickly due to the deepened cloud layer and without surface moisture supply.

Summary

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 and cloud-droplet effective radius (re) 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 (Nd) and surface cloud condensation nuclei concentration (NCCN) (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 re and Nd with NCCN have demonstrated that coupled re and Nd strongly depend on NCCN and have higher correlations (-0.56 and 0.59) with NCCN 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.