A study of trajectory quality using methane measurements from Hateruma Island

Initial conditions of trajectory calculation in studies of atmospheric transport are often arbitrary, leading to questions on applying trajectories to atmospheric observations of green house gases. We used observed methane on Hateruma Island in 1996-2000 as a tracer to investigate the quality of variety length of trajectories obtained from a three-dimensional (3D) and an isentropic (IS) model with different starting altitudes. The trajectory altitude is also considered in trajectory quality analysis. Our approach is to analyze the correlation between methane variation and the displacement of trajectory pairs. The study shows that the 3D model gives more accurate trajectories than the IS model, especially when long trajectories with high-altitude endpoints are to be used; and that a low starting altitude should be used for trajectory calculation by the IS model; however, for the 3D model a starting altitude close to the surface may affect trajectory quality due to the effect of zero vertical wind on the surface. The appropriate trajectory length for Hateruma Island is 78 h when the 3D model is used and 54 h when the IS model is used. (C) 2003 Elsevier Science Ltd. All rights reserved.