An intercomparison of results from three trajectory models

Three three-dimensional trajectory models (LAGRANTO, TRAJKS and FLEXTRA), a ll driven with analysis wind fields from the European Centre for Medium-Ran ge Weather Forecasts, are intercompared. The comparison has three parts: fi rst, a case study of strong ascent in a warm conveyor belt is performed; se cond, a large set of back trajectories from the tropopause region over Euro pe and the mid-latitude Atlantic Ocean is investigated third:, a set of low -level trajectories is compared The intercomparison shows that all three mo dels have been implemented correctly. The degree of model accordance depend s on the interpolation methods used Deviations between the results from a s ingle model using different interpolation schemes are of the same magnitude as the deviations of different models. If all models use linear spatial in terpolation, their respective trajectories closely agree with each other, w ith deviations of 2% or less for the average distance between the starting and the ending positions in the free atmosphere after 48 h. Close to the su rface, where the differences in the model formulations are largest, average horizontal position deviations may be up to 10%. Compared with other sourc es of errors, such as inaccuracies in the wind fields or insufficient tempo ral and spatial resolution of the data Set, these differences are much smal ler Non-linear spatial interpolation lead to stronger vertical motions than linear interpolation and in the case study, enhanced the quality of the re sults.