A Characterization of Wind Flow in and around an Alter Shielded Snowgauge

Single Alter shields are currently the standard wind shield used for many snowgauges around the world. While it is known that a correction factor in measuring the true liquid precipitation amount needs to be applied to correct for various wind speeds, it is not entirely understood how the wind flow over the single alter shield changes the collection efficiency of the gauge. In an attempt to better understand the wind flow in and around a single alter shielded snowgauge, a study was undertaken using sonic anemometers to measure the horizontal and vertical flow of wind over the snowgauge orifice. A sonic anemometer was setup inside an alter shield with a precipitation gauge in the middle and the sonic anemometer placed at the gauge orifice level. A second sonic was placed inside an alter shield with no precipitation gauge in the middle. A reference sonic was placed in the free-stream wind. The sonic anemometers were all initially setup in the free-stream wind so that a correction for any tilt of the sonic anemometer could be calculated and removed from the data and the true wind directions (in both the vertical and horizontal) could be measured. Once the tilt corrections were calculated, the shields were built around two of the sonics and the geonor snowgauge casing placed around the one sonic. The sonics were then left to run for the 2002-2003 winter season to collect data during precipitation and non-precipitation events. The characteristics of the wind flow and its likely impact on precipitation efficiency will be presented.