DYNAMIC CHARACTERISTICS OF REGIONAL FLOWS AROUND THE PYRENEES IN VIEWOF THE PYREX EXPERIMENT - PART-I - ANALYSIS OF THE PRESSURE AND WIND FIELDS AND EXPERIMENTAL ASSESSMENT OF THE APPLICABILITY OF THE LINEAR-THEORY

The Pyrenees experiment (PYREX), launched by the French and Spanish me teorological services, had provided an extensive database that was use d in the present work to describe the airflow around the Pyrenees and to evaluate the applicability of the linear theory to stable flows tha t are blocked by a relatively large mountain range. The direction of i ncidence of the incoming synoptic wind was used to classify the flow i n northerly and southerly categories. In both categories, the directio ns of incidence bear westward from normal incidence to the main axis o f the mountain range. Froude (Fr) and Rossby (Ro) numbers were used to represent the thermodynamic characteristics of the incoming air masse s and the scale of interaction in terms of the mountain shape, Here, F r and Ro were found linearly correlated, and their combination corresp onding to blocked flow in all the cases. Average cross correlations be tween the pressure drag across the mountain and the vertical profiles of the wind components at selected locations demonstrate that the hori zontal flow is clearly separated in an upper undisturbed regime and a lower blocked regime in which regional winds are induced. The separati ng layer corresponding to the mean Froude number encountered during PY REX was found at an altitude of about two-thirds of the mountaintop. D ue to the perturbation induced by the deflected wind, the total wind d uring northerly flows is stronger on the east side than on the west si de, while during southerly incoming flows, the total wind distribution is almost symmetric. Good correlation was found among the pressure fi eld perturbation, the pressure drag, and Froude number of the incoming flow. Clear correlation rules can be deduced from the analysis of the experimental data at different altitudes for both categories of incom ing flows. Therefore, since the pressure drag can be easily determined using field measurements, it becomes a powerful tool for the predicti on of the regional winds around the Pyrenees. The experimental data ar e in agreement with the linear theory predictions of the linear model in Koffi et al. regarding (a) the perturbation of the surface pressure field, which resembles the predicted bipolar distribution; (b) the de pendence of the drag on Fr-1, which enables the assessment of the line ar theory and the definition of the conditions of applicability of two models [(i) a two-dimensional model, for which it was possible to def ine quantitatively the effective blocked area, and (ii) a three-dimens ional model, for which a scaling function that combines the direction of incidence, the mountain shape, and the Coriolis effect was found al most constant, with an average value of 0.2 for all the cases under st udy]; (c) the extension of the area affected by the blocking effect, e stimated to be 4.5-5 times the width of the barrier and the drift of t he strong deceleration point due to the Coriolis effect; (d) the depen dence of the wind velocities on Fr-1 at the edges of the barrier; and (e) the asymmetric flow deviation induced by the Coriolis effect and b iased by the departure of the flow from normal incidence.