The Cloud System Evolution in the Trades (CSET) Study—A Showcase for Aerosol-Cloud-Precipitation Interactions

The Cloud System Evolution in the Trades (CSET) study was designed to describe and explain the evolution of the boundary layer aerosol, cloud, and thermodynamic structures along trajectories within the north-Pacific trade-winds. The observational component of this study centered on 7 round-trips made by the NSF NCAR Gulfstream V (GV) between Sacramento, CA and Kona, Hawaii between 1 July and 15 August 2015. The CSET observing strategy used a Lagrangian approach to sample aerosol, cloud, and boundary layer properties upwind from the transition zone over the North Pacific and to resample these areas two days later. GFS forecast trajectories were used to plan the outbound flight to Hawaii and then updated forecast trajectories helped set the return flight plans two days later. Two key elements of the CSET observing system were the newly developed HIAPER Cloud Radar (HCR) and the HIAPER Spectral Resolution Lidar (HSRL). Together they provided an unprecedented characterizations of aerosol, cloud and precipitation structures. A full suite of probes on the aircraft were used for in situ measurements of aerosol, cloud, precipitation, and turbulence properties during the low-level aircraft profiling portions of the flights. A wide range of boundary layer structures and aerosol, cloud, and precipitation conditions were observed during CSET. The cloud systems sampled included solid stratocumulus infused with smoke from Canadian wildfires, mesoscale (10-20 km) cloud-precipitation complexes, and patches of shallow cumuli in environments with accumulation mode aerosol concentrations of less than 50 cm^-3 Ultra clean layers (UCLs with aerosol concentrations of less than 10 cm^-3) were observed frequently near the top of the boundary layer and were often associated with shallow, gray (optically thin) layered clouds—features that are the subjects of focused investigations by the CSET science team. The extent of aerosol, cloud, drizzle and boundary layer sampling that was made over open areas of the North Pacific along 3-day trajectories during CSET is unprecedented and will enable focused modeling studies of cloud system evolution and the role of aerosol-cloud-precipitation interactions in that evolution.