Abstract
AbstractIn operational conditions, wind is the main environmental source of measurement biases for catching-type precipitation gauges. The gauge geometry induces a deformation of the surrounding airflow pattern, which is generally characterized by relevant updraft zones in front of the collector and above it. This effect deviates the trajectories of the lighter hydrometeors away from the collector and thus is responsible for a significant reduction of the collection performance. Previous approaches to this problem, using computational fluid dynamics simulations and wind-tunnel tests, mostly assumed steady and uniform free-stream conditions. Wind is turbulent in nature, though. The role of natural free-stream turbulence on collection performance is investigated in this work for the case study of a calyx-shaped precipitation gauge and wind velocity between 10 and 18 m s−1. The unsteady Reynolds-averaged Navier–Stokes model was adopted. Turbulent conditions were simulated by imposing constant free-stream velocity and introducing a fixed solid fence upstream of the gauge to generate the desired turbulence intensity. Wind-tunnel measurements allowed validating numerical results by comparing measured and simulated velocity profiles in representative portions of the investigated domain. Results revealed that in the case of turbulent free-stream conditions both the normalized magnitude of the flow velocity and the updraft above the collector are reduced by approximately 20% and 12%, respectively. The dissipative effect of the turbulent fluctuations in the free stream has a damping role on the acceleration of the flow and on the updraft. This would result in a reduced undercatch with respect to literature simulations that employed the traditional uniform free-stream conditions.
Highlights
National weather services commonly adopt catching type gauges for operational in situ precipitation measurements
In order to better single out the role of free-stream turbulence on the airflow features above the gauge collector we focused, in the present work, on the calyx-shape precipitation gauge
Results were visualized in terms of normalized maps and profiles on the vertical along-wind symmetry plane of the gauge collector (y 5 0)
Summary
National weather services commonly adopt catching type gauges for operational in situ precipitation measurements. These instruments are equipped with a collector (funnel) to convey precipitation into a container, where. Denotes content that is immediately available upon publication as open access Both instrumental and environmental factors act as sources of systematic errors in precipitation measurements, and can be adjusted by means of correction. Wind generally accelerates above the collector of the instrument, while vertical upward velocity components arise upwind of the collector (Warnick 1953) This aerodynamic effect induced by the gauge body deflects the hydrometeors (liquid/solid particles) away from the collector (Folland 1988; Nespor and Sevruk 1999). The main factors of influence are the gauge geometry, the wind speed and the characteristics of precipitation, including the particle size distribution and precipitation intensity (Thériault et al 2012; Colli et al 2015)
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