12-16 February 2018









Concept of the Week: Developing a Quality Long-term Instrumental Climate Record

Systematic temperature and precipitation observations have been made at various locations across the nation for nearly two centuries. While only a handful of stations were available in the early 19th century, weather and climate observations currently are made from several hundred automatic weather sites operated by the National Weather Service and the Federal Aviation Administration as well as approximately 8000 stations in the Cooperative Observers Network administered by the National Weather Service. The weather data from these networks are also used to quantitatively assess changes of climate during the instrumental period of the past as well into the future. However, a variety of factors can affect the homogeneity of the record. For example, the locations of many of the stations have moved, from original downtown building roofs to current locations at airports. And the physical surroundings of the stations have changed, many becoming more urbanized.

In the late 1980s, the National Climatic Data Center (NCDC), now the National Centers for Environmental Information, in conjunction with the US Department of Energy's Oak Ridge National Laboratory created the United States Historical Climatology Network (USHCN) of 1218 stations across the 48 coterminous states having long-term records of both daily temperature and precipitation. This network was designed to provide an essential baseline data set for monitoring the nation's climate commencing in the late 19th century. These stations were created from a subset of the Cooperative Observers Network, chosen based upon long-term data quality that included length of record, percent of missing data, spatial distribution and number of station changes. Many of the selected USHCN stations were rural in an attempt to reduce the influence of urbanization. Using statistical analyses, data for these stations have been adjusted to account for movement of stations, or when a different thermometer type was installed. An urban warming correction was applied based upon population of the surrounding area.

More recently, NOAA began the US Climate Reference Network (USCRN), a project designed to collect and analyze climate data of the highest possible quality for the next 50 to 100 years. Each USCRN station would have electronic sensors that would make routine measurements of air temperature, precipitation, IR ground surface temperature, solar radiation and wind speed with a frequency of every five minutes and transmit these data to both NCDC and to National Weather Service offices via orbiting satellites on nearly a real-time basis. In addition to these measurements, additional sensors could be added to the USCRN stations that would measure soil temperature and soil moisture. Conscientious and detailed site selection was made for all stations so that they would not only be spatially representative, but that they would be in locations where the surrounding physical conditions would have a high likelihood of remaining the same over the next 50 to 100 years. Many of the sites were placed on federal or state owned lands, helping minimize the contamination of the climate record by urbanization or other changes in local ground cover.

These long-term, comparative, spatially representative values are vital to detect and verify the subtle changes in climatic conditions before they become overwhelmingly obvious.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2018, The American Meteorological Society.