Standard Rain Gauge for the NOAA Cooperative Observer Network - image NOAA CRH

Normally I focus on the temperature component, but the reason I’m posting this will become evident soon.  – Anthony

Our New Analysis of United States Precipitation Trends

By John Nielsen-Gammon, Texas state climatologist

I’m going to be talking a lot about various aspects of the Cooperative Observer (COOP) Network this week and next. The COOP network is our primary source for climate data for the United States since the 1890s. Observations include daily maximum and minimum temperatures, precipitation, and snowfall. A unique aspect of the COOP network is that almost all of the observations are taken by volunteers, making it an impressive example of coordinated, dedicated effort having an enduring impact.

There are two primary ways that COOP data get combined into coherent, long-term data sets. One is by using a subset of stations that have particularly long-term records: the United States Historical Climatology Network (USHCN). The other is by combining the COOP observations into regional averages, known as climate divisions.

The climate division data is the most common tool for monitoring month-to-month variations in the climate. When the National Oceanic and Atmospheric Administration (NOAA), which by the way administers the COOP network, says that Texas had the driest seven consecutive months on record this past October through April, they’re using the climate division data to make that statement.

There’s one big problem with using the climate division data to make that sort of statement, or to determine in general how climate has changed over time: COOP stations come and go. Volunteers move, or stop taking measurements. New volunteers might live in a wetter or warmer part of the climate division, making the new climate division averages wetter or warmer than before. Plus the climate division data before 1931 was reconstructed based on statewide averages because many parts of the country didn’t have enough stations within each climate division.

Barry Keim (the Louisiana State Climatologist) and others have looked at a few parts of the country and found that these sorts of changes can seriously affect the long-term record. I found the same thing with the Edwards Plateau climate division in Texas. Individual long-term stations showed that annual precipitation was increasing in central Texas, but the climate division average was going down. It turns out that in the early part of the century most stations were in the eastern Edwards Plateau, which gets most of the rain. Later on there were more dry stations, and the average for the division went down.

We decided to fix this problem. And if we were going to fix it for Texas, we may as well fix it for the rest of the country at the same time. This became Brent McRoberts’ master’s thesis, and with a little bit of refinement it because a paper that is going to be published in the Journal of Applied Meteorology and Climatology in the next couple of months.

Read the rest of the post here at the Houston Choronicle blog