Frequency distributions of daily precipitation from a high density volunteer network

The Community Collaborative Rain, Hail and Snow network (CoCoRaHS) now has thousands of volunteers measuring and reporting daily precipitation amounts. CoCoRaHS Observers use the same type of rain gauge and receive the very similar training resulting in a consistent quality of data across the country. Daily measurements of rain and the melted water content of snow and ice are reported to the nearest 0.01". (www.cocorahs.org)

This study examines large samples of daily precipitation data from different parts of the country and different seasons of the year and compares the statistical distributions of daily precipitation amounts. The basic shape of this distribution is shown to be similar both geographically and seasonally. The majority of daily precipitation amounts are small -- less than 0.05" -- in all seasons and all regions with the frequencies rapidly decreasing with increasing amounts. However, seasonal and geographic differences are apparent. For example, there are a noticeably higher proportion of larger daily precipitation amounts in summer than in winter, and in the humid Southeastern States compared to northern and western climatic regions. In the Pacific Northwest, there is a disproportionately large number of moderate daily amounts (0.20 to 0.50") compared to other regions of the country.

For areas of the country where there are high concentrations of CoCoRaHS volunteers (the Colorado Front Range, the Chicago area, New Jersey and Florida) a comparison will be made with long-term daily precipitation data from selected National Weather Service cooperative stations. This analysis shows how well precipitation frequency distributions from a high concentration of supplemental volunteer stations with only a short record length compare to data from a few official stations with many years of data. The results of this analysis provide insight on extreme value statistical assumptions such as those needed to estimate design rainfall and flooding for engineering and design applications.

Analysis in assymetries in daily rainfall reports will also be discussed. With this large dataset, it can be shown that manual observers continue to prefer to report certain digits more so than others. The heavier the rainfall amount, the greater the tendency for human observers to report amounts to whole numbers or fractions as has been shown in other studies.