U.S. Climate Extremes Index (CEI)
The U.S. CEI is based on an aggregate set of conventional climate extreme indicators which, at the present time, include the following types of data:
- monthly maximum and minimum temperature
- daily precipitation
- monthly Palmer Drought Severity Index (PDSI)
- landfalling tropical storm and hurricane wind velocity
* experimental (not used with the Regional CEI)
Each indicator has been selected based on its reliability, length of record, availability, and its relevance to changes in climate extremes.
Mean maximum and minimum temperature stations were selected from the U.S. Historical Climatology Network (USHCN) (Karl et al. 1990). Stations chosen for use in the CEI must have a low percentage of missing data within each year as well as for the entire period of record. Data used were adjusted for inhomogeneities: a priori adjustments included observing time biases (Karl et al. 1986), urban heat island effects (Karl et al. 1988), and the bias introduced by the introduction of the maximum-minimum thermistor and its instrument shelter (Quayle et al. 1991); a posteriori adjustments included station and instrumentation changes (Karl and Williams 1987). In April 2008, maximum and minimum temperature data from the USHCN were replaced by the revised USHCN version 2 dataset. In October 2012, a refined USHCN version 2.5 was released and replaced version 2 data for maximum and minimum temperature indicators.
Beginning in March 2012, daily precipitation data were extracted from the GHCN-Daily dataset. Prior to this, daily precipitation stations were extracted from the USHCN daily database and supplemented by Summary of the Day (TD3200) and pre-1948 (TD3206) daily precipitation stations. A mimimal number of missing days within each year and period of record were also a requirement for precipitation stations to be included in the CEI calculations.
The National Climatic Data Center (NCDC) climate division precipitation and temperature databases are used to calculate the PDSI (Karl 1986). The PDSI categorizes moisture conditions in increasing order of intensity as near normal, mild to moderate, severe, or extreme for droughts and wet periods, which fit nicely into the CEI framework.
Tropical storm and hurricane wind data, extracted from the National Hurricane Center's North Atlantic Hurricane Database (HURDAT), are the newest addition to the CEI and are included in the index when such a storm crossed over contiguous U.S. land. Multiple landfalls from tropical systems are considered valid and are used as many times as they hit land.