Solid-precipitation (snowfall) measurement intercomparison, Bismarck, North Dakota

Abstract

Difficulties involved in obtaining accurate measurements of solid precipitation (snowfall) have been recognized for many years. Many studies have been conducted to evaluate the accuracy and performance of precipitation gages. These studies show that the type of collection method used can significantly affect the quantity of precipitation measured. Although the types of collection methods evaluated differed for each study and the magnitude of measurement error differed for each study, all of the studies concluded that wind is the major cause of measurement error. Measurement error caused by wind is the result of turbulence and wind speed in the vicinity of the gage orifice (opening). As air rises to pass over the gage, precipitation particles that would have entered the gage orifice are deflected and carried farther downwind. Measurement errors increase as wind speed increases. Studies have shown that measurement errors as great as 80 percent can be attributed to wind. Wind speed and turbulence can be reduced by shielding the gage from the wind. Several types of windshields have been built to decrease wind-related errors. The World Meteorological Organization recognized the need for an international comparison of current methods used for measuring solid precipitation and proposed the Solid Precipitation Measurement Intercomparison. The objectives of the Intercomparison are to: Determine the wind-related errors in methods of solid-precipitation measurements; Derive standard methods for correcting solid-precipitation measurements; Introduce a reference method of solid-precipitation measurement for general use to calibrate any type of precipitation gage; and Establish a complete solid-precipitation data set