Item: A streamlined collector for precipitation (ascop)
Title: A streamlined collector for precipitation (ascop)
Proceedings: Proceedings of the 1994 International Snow Science Workshop, Snowbird, Utah, USA
Authors: Thomas WIESINGER & Werner KRONEIS, Tauemkraftwerke, Department of Hydrology, A-5710 Kaprun. Austria, KRONEIS, manufacturer of meteorological equipment, Iglaseegasse 30-32, A-II91 Vienna
Abstract: Conventional precipitation gages with a cylindrical shape are a barrier in the wind field and cause the air flow to separate from the rim. to alter its direction and to accelerate as it passes over the opening (Sevruk et al.; 1989). The consequence of this aerodynamic blockage is a deficiency in catch. because the trajectories of hydrometeors are deflected away from the opening The problem increases in severity as windspeed increases and as the falling speed of the precipitation decreases. Thus. undercollection is greatest at high windspeeds. and for snow and tine rain as opposed to larger sized raindrops. The aerodynamic performance of exposed precipitation collectors depends on their relative depth. the ratio of depth to radius (aspect ratio), the size and shape of the rim (Sevruk et al., 1989) and upon other details of their shape. Reducing the depth of the collector reduces the displacement and acceleration of the flow over the opening, by reducing the aerodynamic blockage caused by the collector (Hall et al., 1992). If a precipitation collector is to make use of reduced depth in order to improve the flow characteristics over the opening, it must be quite shallow. The aspect ratio would need to be below 0.25 and preferably below 0.1 (meaning: for a 20 em diameter collector, the depth should be below 10 crn, preferably below 4 em). The aspect ratio of ASCOP is 0.66 (with a wind shelter that reduces cooling), compared to 3.79 ofthe Austrian standard gage (Paar AP 23). The precipitation collectors presented here are instruments for the measurement of all forms of precipitation with collector and container separated in order to reduce the aerodynamic blockage of the collector. By applying the principles of an airfoil or wing design, it is possible to redistribute the disturbances between the upper and lower surfaces of the collector, thus minimizing those over the opening. Acceleration takes place under the collector (there must be an acceleration associated with the barrier in the airflow) and the flow field remains almost unaffected above the opening. The goal is that the trajectories of snowflakes above the opening are not altered by the existence of the gage.
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Keywords: aerodynamic blockage, precipiation gages
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