Correcting the Automated Surface Observing System (ASOS) All-Weather Precipitation Accumulation Gauge (AWPAG) for Wind-Induced Undercatch during Snowfall Events

It is well documented that precipitation gauges located above ground collect lesser amounts of precipitation, particularly snow, than gauges at ground level due primarily to wind effects. Various windbreaks and shields have been designed to surround gauges to slow the wind and allow the hydrometeors to fall more vertically, thus increasing the collection efficiency of the gauge. The ASOS AWPAG gauge has operated with two different types of shields at roughly 350 sites around the country. Initially, a Russian-designed Tretyakov shield was used. While this increased the collection efficiency, it was still undercatching compared to results given from other types of shield designs. A “double-Alter” shield design was later incorporated. This design added an 8-foot-diameter Alter shield around the existing Tretyakov shield and is the design that is currently used. While this double-Alter design again increased the collection efficiency of the gauge, it still under-collected precipitation compared to the generally accepted measurement of a gauge in a Double-Fenced Intercomparison Reference (DFIR) shield. The National Center for Atmospheric Research (NCAR) installed identical versions of the ASOS AWPAG gauge using both types of shield designs at the Marshall Field Site, located just southwest of Boulder, Colorado. These configurations have been operated at the site for several years alongside a co-located GEONOR gauge in a DFIR shield. Wind speed at both gauge height and at 10-meter height has also been collected so that a transfer function can be developed to correct the gauge undercatch for wind speed. This paper will present the derived transfer functions for correcting precipitation accumulations for wind speed from the AWPAG gauge for both shield configurations and will present selected case studies to highlight the effect of the existing undercatch associated with both shield designs.

This research is in response to requirements and funding by the Federal Aviation Administration (FAA). The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.