The distribution of wind-driven rainfall in a small valley: An empirical basis for numerical model verification

As raindrops are falling through the boundary layer over rugged terrain, their trajectories are being affected by topography-conditioned perturbations in the local wind field. Resulting fine-scale rainfall distributions have been observed in a small valley. Rather intricate, although distinctly persistent, patterns of local variations have been identified strikingly dependent on local wind conditions. A dense network of both standard and directional ('inclined') raingages and vectopluviometers has been used. From the latter one may calculate the 3-D direction, from which wind-driven rain is incident at respective measuring points: Results of this type have led to the reconstruction of some features of the locally perturbed wind field within the valley. A possible vortex became apparent in most cases, with its intensity varying in strict accordance with regional wind conditions and the angle of impact of winds relative to the valley axis. Results can serve to improve the design of rainfall monitoring networks, and more important, they may serve as a basis for adapting numerical models of local wind fields and raindrop trajectories, as a tool to estimate the fine-scale distribution of rainfall over irregular terrain.