OVERCOLLECTION OF SOLID PRECIPITATION
BY A STANDARD PRECIPITATION GAGE,
NIWOT RIDGE, COLORADO

Tim Bardsley and Mark W. Williams

Institute of Arctic and Alpine Research and
Department of Geography, CB 450,
University of Colorado, Boulder, CO

OVERVIEW

Standard precipitation collectors generally undersample solid precipitation. Here we report the first evidence of overcollection of solid precipitation by a standard rain gage, a shielded Fergusson-type (Figure 1) weighing rain gage located above treeline at 3,500 meters on the saddle of Niwot Ridge, located in the eastern Front Range of the Colorado Rocky Mountains Figure 2 . Precipitation amount is measured continuously at the saddle on Niwot Ridge and also at a companion site called D1 located two kilometers to the west of the saddle at an elevation of 3,700 meters Figure 2 . The saddle site has an uninterrupted fetch of about two kilometers and the D1 site is located close to the continental divide with little fetch. A comparison of annual precipitation totals from 1987 through 1996 show that the precipitation amount of 1854 mm measured at the saddle site was 54% greater than the 1200 mm measured at D1 Figure 3 . Annual rainfall amounts (June-September) over the ten years were similar at the two sites: 222 mm at the saddle compared to 217 mm at D1 Figure 4 . However, annual solid precipitation (October-May) of 1514 mm measured at the saddle was 61% more than the 938 mm measured at D1 and accounted for most of the difference in annual precipitation amount at the two sites Figure 5 . Event measurements ( Figure 6 ) show that in general snowfall amount was undersampled at the saddle site by the Belfort recording gage as a function of wind speed Figure 7 . However, snow at the saddle site continued to be collected under blowing snow conditions after storm events, accounting for the oversampling of snow on an annual basis Figure 8 . The oversampling of precipitation amount at the saddle causes numerous problems in calculating important biogeochemical parameters at the alpine research site, such as chemical loading from atmospheric deposition, available water for biotic processes, and potential and actual evapotranspiration.

DISCUSSION

Measurement of true winter precipitation amount at the Saddle on Niwot Ridge is difficult because of its exposed position and logistical problems. Mean wind speed during the winter is 10-13 m/s. After precipitation events wind speeds are often greater than 20 m/s. These high wind speeds, combined with low surface snow hardness after storms, results in large amounts of blowing snow.

There appears to be three patterns of snow catch by the Belfort precipitation gage at the Saddle on Niwot Ridge:

• Infrequent storms with low wind speeds and large amounts of precipitation. Probably accounts for about two-thirds of the snowfall catch by the Belfort gage. Little under-or-over catch.
• Frequent storms with high wind speeds and light snowfall. Significant undercatch of falling snow is compensated for by collection of blowing snow. This blowing and snowing event requires further investigation to determine the whether there is overcollection of snow.
• Blowng snow events. Occurs primarily as storms clear. Probably accounts for about one-third of the snow collected by the Belfort.

SUMMARY

• Event measurements show that in general snowfall amount was undersampled as a function of wind speed.
• Blowing snow events caused the Belfort gage to oversample snow by about 33%.

ACKNOWLEDGEMENTS

Thanks to Mark Losleben for all the years he's put into maintaining such a great meteorological data set on Niwot Ridge. Thanks to Lee Turner and Tom Davinroy for help with graphics; Don Cline supplied figure 2. This research was funded by NSF grant DEB 92-11776 to the NWT LTER project, NSF EAR-9526875 (Hydrology), and Army Research Office grant DAAH04-96-1-0033, and the Mountain Research Station of INSTAAR and CU-Boulder.