The Stratocumulus-to-Cumulus Transition As Understood from the Cloud System Evolution in the Trades (CSET) July 17-19 Case Study

The Stratocumulus to Cumulus transition (SCT) is a noticeable feature in the trade wind regions of all the subtropical oceans, in which air masses originating from within a semi-permanent high-albedo stratocumulus region transition to a low-albedo broken cloud field. The transition has a huge impact on the planetary albedo and is intimately linked to changes in the boundary layer moisture and temperature structure that are still not fully understood. The Cloud System Evolution in the Trades (CSET) campaign was conducted over the NE Pacific Ocean in July and August 2015. 15 research flights conducted a Lagrangian sampling of the transitioning air masses using the NCAR/NSF Gulfstream V as the air parcels moved from colder to warmer sea surface temperatures (SST). The present study is based on two flights (RF06 and RF07) flown on 17^th and 19^th July 2015, respectively. The changes in the boundary layer structure are documented through two measures of decoupling. A comparison is made to the composite assessment of Sandu et. al. (2010); in our case, the SST is warmer, lower-tropospheric stabilities are higher, and the specific humidities at 700 hPa are less, while the large-scale surface divergence seems to be comparable, primarily based on ERA-Interim Reanalysis. Overall, the July 17-19 case study is approximately representative of a typical SCT, adding meaning to a further in-situ and remote sensing analysis. New measurements from the HIAPER Cloud Radar and the High Spectral Resolution Lidar characerize the vertical and horizontal cloud and precipitation structures at the beginning and end of the trajectories. These document an evolution from a mostly well-mixed, overcast stratocumulus layer with low cloud droplet number concentrations (<20/cc) and a mean liquid water path of 125 g/m² to convective turrets reaching two km with remarkably high precipitation rates for shallow convection (>20 mm/day). These are accompanied by thin cloud layers of remarkably low cloud droplet number concentrations. The contribution of the precipitation and its evaporation to the marine BL energetics is evaluated, along with the convection’s relationship to the thin cloud layers. The optical properties of the thin cloud layers are further characterized using the HIAPER Airborne Radiation Package and a millimeter-wavelength radiometer.