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Abstract
Numerical studies of the wind-induced bias of precipitation measurements assume that turbulence is generated by the interaction of the airflow with the gauge body, while steady and uniform free-stream conditions are imposed. However, wind is turbulent in nature due to the roughness of the site and the presence of obstacles, therefore precipitation gauges are immersed in a turbulent flow. Further to the turbulence generated by the flow-gauge interaction, we investigated the natural free-stream turbulence and its influence on precipitation measurement biases. Realistic turbulence intensity values at the gauge collector height were derived from 3D sonic anemometer measurements. Large Eddy Simulations of the turbulent flow around a chimney-shaped gauge were performed under uniform and turbulent free-stream conditions, using geometrical obstacles upstream of the gauge to provide the desired turbulence intensity. Catch ratios for dry snow particles were obtained using a Lagrangian particle tracking model, and the collection efficiency was calculated based on a suitable particle size distribution. The collection efficiency in turbulent conditions showed stronger undercatch at the investigated wind velocity and snowfall intensity below 10 mm h−1, demonstrating that adjustment curves based on the simplifying assumption of uniform free-stream conditions do not accurately portray the wind-induced bias of snow measurements.
Highlights
Precipitation measurements obtained from catching type gauges are subject to systematic biases ascribable to both instrumental [1] and environmental sources [2]
With the aim of obtaining suitable turbulence intensity values to characterize the wind near the ground surface, 3D sonic anemometer measurements from the Nafferton (UK) field test site were analyzed
On the right-hand axis, the sample size for each wind class is reported. This result reveals that turbulent fluctuations exhibited average values very close to zero, except for the wind classes characterized by a small sample size
Summary
Precipitation measurements obtained from catching type gauges are subject to systematic biases ascribable to both instrumental [1] and environmental sources [2]. The focus of this paper is on the catching bias induced by wind, the most important environmental error resulting in a lower amount of collected precipitation than what is expected in undisturbed conditions. Catching-type precipitation gauges themselves generate airflow deformations around their physical body, with significant acceleration and vertical velocity components arising especially above the gauge collector [4,5]. These airflow features have the potential to deviate the trajectories of the approaching hydrometeors, inducing a generally lower efficiency in the collection of precipitation than in the absence of wind. The wind effect on precipitation gauges is a well-known measurement issue, and is operationally mitigated either in the field, by employing wind shields [7], or in post-processing, by adjusting the measured data [8]
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