Cooperative Institute for Research in Environmental Sciences

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Influence of Sample Volume on Disdrometer and Radar Reflectivity Estimates

Christopher R. Williams

CIRES and NOAA ESRL Physical Sciences Division

The accuracy of any instrument measuring rainfall is dependent on the size of the instrument’s sample volume. Increasing the sample volume increases the instrument’s measurement accuracy because it can observe more raindrops. But, increasing the sample volume decreases the instrument’s ability to represent the fine detailed spatial distribution of the raindrops, often called the representativeness error. For an example of the spatial distribution of raindrops, visualize the patterns of raindrops falling on a lake.

If we want to resolve the features of discrete raindrops (for example, clustering of raindrops or turbulence generated raindrops), then the sample volume needs to be large enough to reduce the measurement uncertainties, but small enough to resolve interesting discrete raindrop phenomena.

This study uses simulations to address the relationship of sample volume on the measurement accuracy of surface disdrometers that count the number and size of raindrops reaching the surface and of vertically pointing profiling radars that observe a volume of raindrops above the ground.