Out-of-Level Instruments: Errors in Hydrometeor Spectra and Precipitation Measurements

Abstract

Meteorological instruments which are designed to measure the size spectra of hydrometeors such as hailstones or raindrops or instruments designed to measure cumulative precipitation such as raingages can produce errors if they are not installed and maintained with their sensing surfaces level. Errors from this source have generally been assumed to be negligible or ignored completely in most studies of rainfall and hailfall apparently because levelness is often taken for granted. This paper examines the effects on spectra and integrated precipitation measurements when the wind is blowing and the measuring instrument is out of level. Examples of out-of-level instruments are given based on National Hail Research Experiment hailpad data, data from the Illinois State Water Survey raindrop camera, and government-operated and volunteer-observer raingages. Raingages, for example, that are out of level by 2° in the presence of winds of 10 m s?1 can produce errors on the order of 9%.