Numerical simulation of a low-level wind created by complex orography: A cierzo case study

Abstract

Three-dimensional simulations of the IOP 10 of the Pyrénées Experiment are presented. In this case, the northerly synoptic flow forces two regional winds around the Pyrénées Mountain range: the cierzo in the Ebro Valley and the tramontana over the Mediterranean Sea. First, experimental data are used to validate the simulation. The local winds are well reproduced, and the computed flow splitting upwind of the Pyrénées compares favorably with the real flow The computed turbulence kinetic energy and the turbulent fluxes are in fair agreement with the observations in the cierzo. Second, the good fit between the observations and the computation permits one to draw some conclusions from the simulation, using the model as numerical laboratory to amplify the utility of the dataset. In particular, the acceleration of the wind along the Ebro Valley is examined, and it is found that it is governed in the upper part of the valley by the pressure gradient created by the Pyrénées. Next, the balance of forces in the planetary boundary layer in the two wind systems is considered. Surprisingly, the cierzo has an Ekman-type balance of forces, but not the tramontana. Finally, the authors analyze the variation of the ground pressure drag from three simulations where the mountain height is varied: the result is consistent with the conclusions of Stein. The results also confirm the beneficial effect of an enhanced orography for such simulations.