Impact of Rooftop Siting and other Observing Practices on Precipitation Trends at Select Midwest Urban Locations

Data are available to examine trends in rainfall at select long-term stations throughout the US, incorporating data from the 19th century. Minneapolis, Milwaukee, Chicago, Peoria, and St Louis have been examined in detail: St. Louis and Minneapolis-St. Paul from 1837 through 2014, Peoria and Milwaukee, from 1857 to 2014, and Chicago from 1867 through 2014. Daily data from the newly quality controlled CDMP-Forts project (http://mrcc.isws.illinois.edu/research/cdmp/cdmp.html ) and the NOAA TheadEX data (http://www.erh.noaa.gov/climate/workshop2007/ClimateMeetings/ThreadExFactsheet.pdf) were used to construct these time series. A general increase in precipitation was found during the 20th century, preceded by a period of heavier rains in the last third of the 19th century. This study examines differences caused by changing observation practices prior to the 1940's. This includes the practice of locating raingauges in urban areas on rooftops during the early years of the Weather Bureau, and the non-recording of small rainfall amounts (< 0.10”) that was common amongst observers in the 1800s. This study will examine differences in concurrent station data from nearby sites located at different elevations above ground level (AGL), and how these differences may have impacted long term precipitation trends. Preliminary results indicate that the rooftop locations also resulted in an underestimation of precipitation, likely from increased wind losses aloft.