The Role of Free-Stream Turbulence in Attenuating the Wind Updraft above the Collector of Precipitation Gauges

Abstract

In 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.

Calibration of non‐catching precipitation measurement instruments: A review

“…The effect of wind on precipitation measurement is also called the exposure effect in the literature (since Jevons, 1861). The gauge body, immersed in a wind field, behaves like a bluff‐body obstacle in the free flow, and produces strong velocity gradients, upwards or downwards components and turbulence close to the gauge (Cauteruccio et al, 2020). The hydrometeors trajectories are diverted by the velocity field around the instrument (Cauteruccio et al, 2021a; Cauteruccio et al, 2021b; Cauteruccio et al, 2021c; Colli et al, 2020; Folland, 1988; Jevons, 1861; Nešpor & Sevruk, 1999) and, depending on the gauge shape and wind speed, the number of hydrometeors that cross the sensing volume is affected, leading to an over‐ or under‐estimation of the precipitation intensity.…”

On Neglecting Free-Stream Turbulence in Numerical Simulation of the Wind-Induced Bias of Snow Gauges

“…Catch ratios obtained for each solid particle size d at Uref = 2.5 m s −1 using the LPT model and based on the large eddy simulation (LES) mean airflow fields for the Geonor precipitation gauge under uniform and turbulent free-stream conditions. In addition to the work presented in ref [15], where the free-stream turbulence effect was investigated using an unsteady Reynolds-averaged Navier-Stokes approach (URANS), and results were provided in terms of attenuation of the airflow updraft and acceleration above the collector of the gauge, in the present work the particle-fluid interaction (using a one-way coupled model) was also addressed, and the two free-stream turbulence conditions were compared in terms of catch ratios and collection efficiency.…”

“…This was due to the greater aptitude of the small size particles to follow the turbulent velocity fluctuations, while larger particles are more inertial. In particular, the free-stream turbulence had two main effects: it reduced the aerodynamic effect of the wind-gauge interaction, with lower velocity components near the gauge body (as demonstrated in ref [15]) and introduced velocity fluctuations in all directions. When the particle size is small, particles are more sensitive to the turbulent fluctuations, and therefore catch ratios in uniform free-stream conditions are larger than in turbulent conditions.…”

“…Further to the local generation of turbulence due to the flow-gauge interaction, in this work we investigated the natural free-stream turbulence intensity and its influence on precipitation measurement biases. To the best of the author's knowledge, only one attempt to assess the impact of free-stream turbulence on the wind-induced bias has been performed [15].…”

“…Results revealed that both the normalized vertical and horizontal components of the flow velocity above the gauge collector were less accentuated in the turbulent free-stream configuration than in uniform free-stream conditions, due to the energy dissipation induced by turbulent fluctuations. However, in a CFD approach based on the unsteady Reynolds-averaged Navier-Stokes (URANS) equations, the analysis was limited to the effect of turbulence on the airflow deformation around and above the gauge, while the impact on the hydrometeors trajectories was not quantified [15].…”