Precipitation measurement and derivation of precipitation inclination in a windy mountainous area in northern Costa Rica

Over small-scale topography in windy areas, precipitation tends to be redistributed by wind through the modification of precipitation inclination. The latter is often derived from wind speed and conventional rain gauge records by application of relations-derived mainly for convective rainfall conditions-between (1) precipitation intensity and drop diameter, and (2) drop diameter and terminal fall velocity. However, it remains to be seen whether such relationships give valid results for the typically low precipitation intensities prevailing in tropical montane cloud forests. On the basis of the assumption that the total amount of near-surface atmospheric liquid water, defined hereafter as the 'potential precipitation', is most likely to be identical over small distances, this paper introduces a device to measure amounts of potential precipitation. The gauge captures both the vertical and horizontal component of precipitation, and this allows derivation of precipitation inclination using simple trigonometry. Results on precipitation inclinations obtained with the 'potential precipitation gauge' on a wet and windy site in northern Costa Rica suggest the droplets to be smaller than those predicted by the commonly used relationships referred to above. The current gauge is also shown to be more effective in catching inclined precipitation than two different types of spherical gauges. In addition, 'effective' fall velocities were determined for each precipitation event using derivated precipitation inclination and wind speed. The assumption of spatially similar potential precipitation amounts and effective fall velocities throughout a small catchment allows the catchment-wide determination of precipitation inclination and therefore of hydrologically effective precipitation from a single-point measurement of potential precipitation and wind speed in combination with modelled spatial wind speed pattern. This approach is believed to yield appreciably better estimates of spatial precipitation inputs compared to reliance on conventional rain gauges and derivation of terminal fall velocities from precipitation intensity records. Copyright (C) 2010 John Wiley & Sons, Ltd.