A 3-DIMENSIONAL ATMOSPHERIC TRAJECTORY MODEL - APPLICATION TO A CASE-STUDY OF ALPINE LEE CYCLOGENESIS

In this paper the problem of computing large-scale trajectories of atmospheric parcels is examined in the framework of a numerical limited-area model used for weather prediction. The parcel of air is considered to be a completely passive tracer, having no interaction with the environment, even when going through an area of deep atmospheric convection. The usual method of computing the trajectory after the integration, using model-produced wind fields every six hours, is compared with the method of computing the trajectory in an almost continuous fashion, during the integration of the model itself, providing in this way an estimate of the time-truncation errors associated to the first, more usual, method. This second method is found to be quite superior and can, in selected instances, lead to largely different results, as documented. Examples of beams of trajectories going through a strongly diffluent flow-field, as well as through an area of strong convection are shown and found to be very realistic, within the limitations imposed by model resolution. In addition, the sensitivity of trajectories to the definition of the vertical coordinate is explored and found to be large.