Computing 3-D atmospheric trajectories for complex orography: application to a case study of strong convection in the western Mediterranean

A three-dimensional Lagrangian model (TRACOMX) has been developed to simula te large-scale atmospheric transport over complex terrain. Trajectories are integrated in time steps, assuming constant velocity during the time inter val, non-interaction with the environment and non-diffusive processes. The vertical velocity is computed, at high levels, by three-dimensional relaxat ion from the e-vector formulation of the omega equation. At low levels, the vertical velocity is computed by means of a 'specific treatment' of the or ography consisting of the application of the kinematic boundary condition t o the cells intersected by the orography, and taking into account the kinet ic energy of the flow, the atmospheric stability and the presence of horizo ntal thermal gradients. These effects are parameterized by means of a multi plicative factor obtained from the Froude number value. The trajectory comp utation is achieved by successive approximations, using an iterative method , to the final position for each trajectory vector. The model has been appl ied to a case study of strong convection over NE Spain and the results have been compared with those obtained from an isentropic method. (C) 2001 Else vier Science Ltd. All rights reserved.