An Intercomparison of Isentropic Trajectories over the South Atlantic

Abstract

The authors have compared isentropic trajectories computed from meteorological fields from different analysis centers. The analysis was performed for the South Atlantic, where a springtime maximum in tropospheric ozone has sparked considerable interest in the transport meteorology. Using the model of Schoeberl et al., isentropic forward trajectories were computed from an array of points over southern Africa and backward trajectories from an array of points over the South Atlantic. The model was run for an 8-day period in October 1989 with input taken from the twice-daily global gridded data fields from the National Meteorological Center (NMC) and from the European Centre for Medium-Range Weather Forecasts (BCMWF). There were large differences between the trajectories based on the two fields in terms of travel distance, horizontal separation, and vertical separation. Best comparisons for individual trajectories were found in the low-latitude easterlies, and the poorest comparisons were found in the westerlies and in the vicinity of the center of the South Atlantic subtropical anticyclone. Significant differences in wind speeds between the two analyses also led to large trajectory differences. Trajectories were also computed using once-daily NMC fields. The effect of this degradation of the data was small. Trajectories computed from balanced winds computed from the NMC geopotential height and temperature fields showed the largest differences when compared with the ECMWF trajectories. The balanced wind fields should not be used in trajectory construction in the tropical lower troposphere. It is difficult to make a definitive recommendation concerning which set of fields should be used in future transport analysts in this region due to the very large trajectory differences found in this analysis and the lack of any independent verification data. Any extensive analysis of transport in this region should be done only in conjunction with considerable additional data collection.