Improving the calculation of particle trajectories in the extra-tropical troposphere using standard NCEP fields

The calculation of particle trajectories in the extra-tropical troposphere is improved by a hybrid model that employs the temperature and geopotential fields to supplement the velocity field. The hybrid model uses the temperature and geopotential fields to construct the Montgomery Stream function, which, together with a Rayleigh friction force and the Coriolis force determine the evolution of a “correctional velocity” based on Newtonʹs 2nd law of motion. This velocity, however, is decoupled from the continuity equation so its horizontal divergence does not affect the pressure. The improvement of the trajectory calculation is obtained by integrating a linear combination of National Centers for Environmental Predictions’ (NCEP) velocity field and the “correctional velocity” computed from NCEPʹs temperature and geopotential fields. The improvement of the model-generated trajectories over those obtained from a straightforward advection by the velocity field is verified by comparing the calculated trajectories to the observed trajectories of 379 constant-level balloons launched in 1971 as part of the EOLE experiment. For flight times between 2 and 10 weeks the new algorithm generates trajectories that are statistically closer to the observed EOLE trajectories than those obtained from advection by the velocity field only. There are, however, several balloon flights where for certain, isolated, values of its parameters the hybrid model generates trajectories that are actually less accurate than those of straightforward advection. For balloon flights between 2 and 9 weeks the worst hybrid model trajectories are only about 4% less accurate than those of straightforward advection. The modelʹs improvement over advection by the velocity field reaches a maximum value of over 15% for 4–7 week long trajectories while for 1 week long, or longer than 10 week long trajectories the model offers no improvement over straightforward advection by the velocity field.