NUMERICAL-SIMULATION OF LEE WAVE EVENTS OVER THE PYRENEES

A two-dimensional, non hydrostatic, compressible model is used to simulate the airflow over the Pyrenees in two lee wave events (IOP-3 and IOP-9) during the PYREX program. For each case, the mean flow is initialized by a single upstream sounding. The bottom topography used in the model is taken from the real mountain profile after smoothing out the wavelength shorter than 10 km. In all cases, the model simulates well several characteristics of the lee waves: the wavelength, amplitude, and position. This is a strong indication that the lee waves observed in these events were excited by non linear wave-wave interactions of long mountain waves, because the topographic forcing in the model has negligible spectral amplitude at the wavelength of the lee waves (approximately 10 km). Sensitivity experiments show that the phases of the lee waves are very sensitive to a small change of the mean wind and that their amplitudes are increased by the addition of smaller-scale topography. In both cases, the simulated downward momentum fluxes agree well with the observed fluxes around 4 km height: -15 x 10(4) N/m in IOP-3 and +10 x 10(4) N/m in the third day of IOP-9. The simulated fluxes are, however, almost constant through the troposphere, while the observations show rapid decrease of absolute values above 4 km height. This overestimation of the simulated momentum fluxes in the upper half of the troposphere is caused by the overestimation of the amplitude of long mountain waves at these heights; these long mountain waves control the vertical profile of the momentum flux there. It is suggested that the time evolution of the mean wind and the lateral momentum flux divergence found in the real atmosphere give rise to this overestimation.