Cloud Systems Evolution in the Trades
The Cloud Systems 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 using the NSF/NCAR Gulfstream V (HIAPER). This effort included characterization of the cloud, precipitation and aerosol fields in the stratocumulus and the fair-weather cumulus regimes within the subtropical easterlies over the northern Pacific. These characterizations along trajectories were designed to aid in our understanding and simulation of the transition between the two convective regimes—a critical factor in the climate system. LES models have become a robust tool for Lagrangian simulations of subtropical cloudiness transitions, but there are few good datasets for comprehensively testing these simulations. In particular, we lack adequate observations of the coupled evolution of aerosol, cloud droplet number concentration and precipitation during such transitions. Thus, the observing strategy was to sample aerosol, cloud, and boundary layer properties upwind from the transition zone over the North Pacific and to resample these areas one or two days later. This Lagrangian approach was designed to minimize uncertainties in the large-scale forcing due to horizontal advection in the lower troposphere and thus facilitate model simulations and isolate critical physical processes. Two key elements of the observing system were a newly developed HIAPER Cloud Radar (HCR) and the HIAPER Spectral Resolution Lidar (HSRL). The HCR was used to provide cloud and precipitation characteristics. The HSRL points provided cloud boundaries and aerosol characteristics when viewing non-cloudy volumes.
A full suite of probes on the aircraft was used for in situ measurements of aerosol, cloud, precipitation, and turbulence properties. Two modes of operations were made on flights between the west coast of California and Hawaii. One included remote sensing of the clouds and boundary layer from higher flight levels as the aircraft approached and left the StCu to Cu transition zone. The other involved detailed profiling in the sub-cloud and cloud layer in two or three selected areas before and after the transition zone. On the recon legs dropsondes were used to obtain the thermodynamic and wind structure in and above the boundary layer. Several flight sequences were planned for the June-July 2014 timeframe. Historical analyses were used to develop the sampling strategies needed to ensure optimum chances of sampling uniform air masses along trajectories within the sampling volume available. Models of different complexity were used to assist in the development of observing strategies and detailed flight plans.