NUMERICAL-SIMULATION OF A LOW-LEVEL WIND CREATED BY COMPLEX OROGRAPHY- A CIERZO CASE-STUDY

Three-dimensional simulations of the IOP 10 of the Pyrenees Experiment are presented. In this case, the northerly synoptic flow forces two r egional winds around the Pyrenees Mountain range: the cierzo in the Eb ro Valley and the tramontana over the Mediterranean Sea. First, experi mental data are used to validate the simulation. The local winds are w ell reproduced, and the computed flow splitting upwind of the Pyrenees compares favorably with the real flow. The computed turbulence kineti c energy and the turbulent fluxes are in fair agreement with the obser vations in the cierzo. Second, the good fit between the observations a nd the computation permits one to draw some conclusions from the simul ation, 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 up per part of the valley by the pressure gradient created by the Pyrenee s. 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 a nalyze the variation of the ground pressure drag from three simulation s where the mountain height is varied: the result is consistent with t he conclusions of Stein. The results also confirm the beneficial effec t of an enhanced orography for such simulations.