Evaluation of Meteorological Analyses, Simulations, and Long-Range Transport Calculations Using ANATEX Surface Tracer Data

Abstract

Perfluorocarbon tracer data collected during the Across North America Tracer Experiment (ANATEX) are used to evaluate different meteorological analysis, simulations, and long-range transport calculations. Three basic types of meteorological analysis and simulation are evaluated: objective analysis of observed data, prognostic simulation with observed lateral boundary conditions, and four-dimensional data assimilation (FDDA). The evaluation is based on 1) the root-mean-square separation between two-dimensional meteorological trajectories (constructed from different analyses or simulations) and surface tracer trajectories and 2) the relationship between the upward displacement of three-dimensional trajectories and the maximum value of the surface tracer concentration. The root-mean-square data indicate that the optimum value of the Newtonian nudging coefficient for the FDDA wind field in the lower troposphere is 6 × 10⁻⁴ s⁻¹, and the quality of the prognostic simulation is lower than that for FDDA or the objective analysis, particularly when surface fronts are present. These data also show that trajectory errors, with respect to transport distance, are larger in low-speed wind regimes than in medium- to high-speed regimes, and suggest that the rate of increase of trajectory error decreases with time, but the uncertainty of the rate of increase is quite large during the first 18 h of transit. The three-dimensional trajectories indicate that large-scale upward motion is a mechanism for removal of tracer from the boundary layer, and the strongest correlation between the upward displacement of the trajectory air parcels and the surface tracer concentration is obtained using the FDDA dataset. The overall results suggest that when both the vertical and horizontal components of the wind fields are considered, FDDA (using an appropriate value for the nudging coefficient) is better than the other methodologies.