Adjustment of daily precipitation data at 10 climate stations in Alaska: Application of World Meteorological Organization intercomparison results

A methodology for adjusting the daily precipitation measured by the U.S. National Weather Service (NWS) 8-inch standard gauge for wind-induced undercatch, wetting loss, and trace amount of precipitation is provided. The application of the proposed adjustment procedures was made at 10 NWS climate stations in Alaska for 1982 and 1983. The results show the following: (1) Daily adjustment for wind-induced undercatch, wetting loss, and trace amount of precipitation increases the gauge-measured annual precipitation by 65-800 mm for the 2 years (about 10-140% of the gauge-measured yearly total) at the 10 stations in Alaska; (2) compared to wetting loss and trace amount of precipitation, wind-induced undercatch is the source of greatest error, although wetting loss and trace amount of precipitation are also significant systematic errors in the northern Alaska regions of low precipitation; (3) in the similar climate condition, the NWS 8-inch standard gauges with an Alter wind shield have a much lower adjustment for wind-induced undercatch than the unshielded gauges at nearby stations, and the unshielded gauges placed on the roof of the weather office building have a higher adjustment for wind-induced errors than those gauges mounted on the ground; (4) monthly adjustment factors (adjusted/measured precipitation) differ by station, and at an individual station by type of precipitation; (5) considerable intra-annual variation of the magnitude of the adjustments has been found in Alaska owing to the fluctuation of wind speed, air temperature, and frequency of snowfall. Using the constant correction factors (derived at a single intercomparison site) to the archived monthly precipitation records produces significant undercorrection of the wind-induced errors at high wind sites and overcorrecting of the errors at low wind sites. To avoid the undercorrection or overcorrection of the wind-induced errors, a constant correction factor should not be applied to gauge-measured snow data, Daily adjustments for systematic errors need to be applied to the archived precipitation data. It is expected that the adjustments will have an impact on climate monitoring.