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


Siems, S.T., Hess, G.D., Suhre, K., Businger, S., Draxler, R.R.


Abstract - As part of the first Aerosol Characterization Experiment (ACE-1), two Lagrangian experiments were conducted with the intent of observing the evolution of boundary layer air over the course of 24 hours. Smart tetroons, which adjust their internal pressure to maintain a fixed elevation, were used to mark an ideal air mass. During both Lagangians, however, the smart tetroons were observed to artificially lose elevation overnight; they dropped to near 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 prediction models. Comparable to other studies, the computed trajectory errors range from 12-34% of the travel distance in heterogeneous flow and 5-15% in homogeneous flow. The vertical motions, however, do not show a strong agreement. We then use numerical simulations to assess the sensitivity of the trajectories to the meteorology. Vertical wind shear was observed for both Lagrangians and has major impact on the tetroon trajectories when coupled with the overnight decrease in altitude. We also find that the trajectories are more sensitive to the initial position in the first Lagrangian than in the second. This sensitivity is probably due to the presence of a nearby cold front. Finally, we assess the impact of the liquid-water loading on the tetroon trajectories through the use of composite numerical trajectories in which the altitude is prescribe hourly. The error over the entire time period was reduced to 15-23% if they were lowered to 10m. This suggests that the tetroons interacted with the surface layer throughout the night. Budget and evolution studies of the ACE-12 Lagrangians must take this into account.




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