The impact of wind shear on observed and simulated trajectories during theACE-1 Lagrangian experiments

As part of the first Aerosol Characterization Experiment (ACE-I), two Lagra ngian experiments were conducted with the intent of observing the evolution of boundary layer air over the course of 24 hours. Smart tetroons, which a djust their internal pressure to maintain a fixed elevation, were used to m ark an ideal air mass. During both Lagrangians, however, the smart tetroons were observed to artificially lose elevation overnight; they dropped to ne ar the ocean surface as a result of liquid-water loading: from condensation . First we use the tetroon trajectories to assess the accuracy of numerical trajectories based on the analysis of four global numerical weather predic tion models. Comparable to other studies, the computed trajectory errors ra nge from 12-34% of the travel distance in heterogeneous flow and 5-15% in h omogeneous flow. The vertical motions, however, do not show a strong agreem ent. We then use numerical simulations to assess the sensitivity of the tra jectories to the meteorology, Vertical wind shear was observed for both Lag rangians and had a major impact on the tetroon trajectories when coupled wi th the overnight decrease in altitude, We also find that the trajectories a re more sensitive to the initial position in the first Lagrangian than in t he second. This sensitivity is probably due to the presence of a nearby col d front. Finally, we assess the impact of the liquid-water loading on the t etroon trajectories through the use of composite numerical trajectories in which the altitude is prescribed hourly. The error over the entire time per iod was reduced to 15-23% if the trajectories were lowered to 100 m altitud e overnight, and 5-15% if they were lowered to 10 m. This suggests that the tetroons interacted with the surface layer throughout the night. Budget an d evolution studies of the ACE-I Lagrangians must take this into account.