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-induc ed undercatch, wetting loss, and trace amount of precipitation is prov ided. The application of the proposed adjustment procedures was made a t 10 NWS climate stations in Alaska for 1982 and 1983. The results sho w the following: (1) Daily adjustment for wind-induced undercatch, wet ting loss, and trace amount of precipitation increases the gauge-measu red annual precipitation by 65-800 mm for the 2 years (about 10-140% o f the gauge-measured yearly total) at the 10 stations in Alaska; (2) c ompared to wetting loss and trace amount of precipitation, wind-induce d undercatch is the source of greatest error, although wetting loss an d trace amount of precipitation are also significant systematic errors in the northern Alaska regions of low precipitation; (3) in the simil ar climate condition, the NWS 8-inch standard gauges with an Alter win d shield have a much lower adjustment for wind-induced undercatch than the unshielded gauges at nearby stations, and the unshielded gauges p laced on the roof of the weather office building have a higher adjustm ent 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 adjustme nts has been found in Alaska owing to the fluctuation of wind speed, a ir temperature, and frequency of snowfall. Using the constant correcti on 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 e rrors at low wind sites. To avoid the undercorrection or overcorrectio n of the wind-induced errors, a constant correction factor should not be applied to gauge-measured snow data, Daily adjustments for systemat ic errors need to be applied to the archived precipitation data. It is expected that the adjustments will have an impact on climate monitori ng.