Air mass trajectories are calculated using the FLEXTRA trajectory model (Stohl et. al., 1995; Stohl and Seibert, 1998) and using meteorological data provided from ECMWF (European Centre for Medium Range Weather Forcasts). The meteorological data used are analyses with a spatial resolution of 1.25 degree and a temporal resolution of 6 hours. 3-dimensional trajectories are calculated using the vertical wind provided by ECMWF. This trajectory type is believed to be the most accurate in the troposphere where diabatic processes (e.g. water vapour condensation) are important. The accuracy of the trajectories is typically of the order of 20% of the travel distance (Stohl, 1998), but uncertainties in individual cases can also be much larger. An important error source for back trajectories from surface stations is the turbulence in the boundary layer which cannot be accounted for properly by single trajectories. Therefore, for every site 3 trajectories arriving at different altitudes are calculated here. Depending on the meteorological situation, their spread can either over- or underestimate the area from which the air is coming from, but it gives at least a rough estimate on the pathway of the air mass.

Publications using the FLEXTRA results presented on this webpage are requested to include a statement similar to: NILU is acknowledged for providing the FLEXTRA trajectories (www.nilu.no/trajectories) used in this study.

Stohl, A., G. Wotawa, P. Seibert, and H. Kromp-Kolb (1995):
Interpolation errors in wind fields as a function of spatial and temporal resolution and their impact on different types of kinematic trajectories.
J. Appl. Meteor. 34, 2149-2165.

Stohl, A., and P. Seibert (1998):
Accuracy of trajectories as determined from the conservation of meteorological tracers.
Q. J. Roy. Met. Soc. 124, 1465-1484.

Stohl, A. (1998):
Computation, accuracy and applications of trajectories - a review and bibliography.
Atmos. Environ. 32, 947-966.