The role of free-stream turbulence in attenuating the wind updraft above the collector of precipitation gauges

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, thus is responsible for a significant reduction of the collection performance. Previous approaches to this problem, using Computational Fluid Dynamic simulations and wind tunnel tests, mostly assumed steady and uniform free-stream conditions. Wind is rather turbulent in nature, though. The role of the natural free-stream turbulence on the collection performance is investigated in this work for the case study of a calix shape precipitation gauge and wind velocity between 10 and 18 ms−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 in order 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 about 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 employing the traditional uniform free-stream conditions.