ACCURACY OF TRAJECTORIES AS DETERMINED FROM THE CONSERVATION OF METEOROLOGICAL TRACERS

Dynamical structures as well as transport processes are often investig ated using trajectories. Several alternative techniques are currently in use to calculate trajectories, which may produce significantly diff erent results. In this study, three-dimensional, isentropic, isobaric and isoeta (terrain-following coordinate surfaces) trajectories are ca lculated. In the case of the isentropic trajectories, both the kinemat ic and the dynamic calculation method are applied. Using a tracer that is conserved along the real three-dimensional trajectories, it is pos sible to determine which of these trajectories are the most accurate. Here, conservative meteorological quantities, namely potential vortici ty, specific humidity and potential temperature, are used. As these tr acers are conserved only in the absence of diabetic processes, regions of the atmosphere where these processes are important, i.e. the bound ary layer and moist regions, are excluded. Tracer conservation along t he different trajectories is studied to find the mast accurate traject ory type. It is found that three-dimensional trajectories are the most accurate ones, and kinematic isentropic trajectories, affected more b y dynamical inconsistencies between meteorological fields, are the sec ond best in the troposphere. In the stratosphere, three-dimensional an d isentropic trajectories may be of similar accuracy. Isobaric and iso eta trajectories are much less accurate, both in the troposphere and i n the stratosphere. Dynamic trajectories tend to perform unrealistic i nertial oscillations and thus give clearly worse results than any of t he kinematic trajectories. Since there exists no direct relationship b etween the average errors in tracer conservation and average spatial p osition errors of the trajectories, a transformation of the tracer con servation errors into spatial position errors is difficult.