U.S. Climate Extremes Index

Introduction

How has the climate changed over the past 50 or more years? In what ways and by how much? Many people, including climatologists, have been struggling with these questions for some time now, not only for scientific interest, but also to aid in policy decisions (IPCC 2001) and to inform the general public. In order to answer these questions, it is important to obtain comprehensive and intuitive information which allows interested parties to understand the scientific basis for confidence, or lack thereof, in the present understanding of the climate system. One tool, first developed as a framework for quantifying observed changes in climate within the contiguous Unites States, is the U.S. Climate Extremes Index (CEI).

The CEI was first introduced in early 1996 (Karl et al. 1996) with the goal of summarizing and presenting a complex set of multivariate and multidimensional climate changes in the United States so that the results could be easily understood and used in policy decisions made by nonspecialists in the field. The contiguous U.S. was selected as the focus for this study in part since climate change is of great interest to U.S. citizens and policy makers and since climate changes within the U.S. have not been given extensive coverage in intergovernmental or national reports which focus on climate change assessments (IPCC 2001; NRC 1992; NRC 2001).

In 2003, two notable modifications were made to the CEI. Indicators in the original CEI summarized trends in temperature, precipitation and drought data on an annual basis. The revised CEI now includes an experimental tropical system component and is calculated for multiple seasons. The newest indicator documents trends in tropical system activity based on the wind velocity of landfalling tropical storm and hurricanes. As of October 2004, CEI calculations begin in 1910 for all periods and are updated within a few weeks after the end of a particular season and include final quality controlled data as well as near-real-time data. In September 2005, the two components for each of four indicators (steps 1, 2, 3, and 5) are plotted separately to help in the identification of trends and variability of each component. All graphs are now plotted as bar graphs rather than dot plots. In December 2005, a year-to-date season was made available along with the other eight standard seasons. Additions and modifications made to the original CEI are explained in an article entitled "A Revised U.S. Climate Extremes Index", which was published in mid-2008 (Gleason et al. 2008).

In July 2011, a regional CEI (RCEI) was introduced, which computes the CEI acrss the 9 U.S. Standard Regions (Karl and Koss, 1984). Year-to-year varations in the regional index have higher amplitude swings and larger/smaller percentages of each region affected by extremes compared with the CEI. There is a good deal of spatial consistency among the RCEI indicators and similar extremes may span across or be absent from a region in any given season.

Data Used

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:

1. monthly maximum and minimum temperature
2. daily precipitation
3. monthly Palmer Drought Severity Index (PDSI)
4. 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 tbe 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.

Definition

The U.S. CEI is the arithmetic average of the following five or six^# indicators of the percentage of the conterminous U.S. area:

1. The sum of (a) percentage of the United States with maximum temperatures much below normal and (b) percentage of the United States with maximum temperatures much above normal.
2. The sum of (a) percentage of the United States with minimum temperatures much below normal and (b) percentage of the United States with minimum temperatures much above normal.
3. The sum of (a) percentage of the United States in severe drought (equivalent ot the lowest tenth percentile) based on the PDSI and (b) percentage of the United States with severe moisture surplus (equivalent to the highest tenth percentile) based on the PDSI.
4. Twice the value of the percentage of the United States with a much greater than normal proportion of precipitation derived from extreme (equivalent to the highest tenth percentile) 1-day precipitation events.
5. The sum of (a) percentage of the United States with a much greater than normal number of days with precipitation and (b) percentage of the United States with a much greater than normal number of days without precipitation.
6. ^*The sum of squares of U.S. landfalling tropical storm and hurricane wind velocities scaled to the mean of the first five indicators.

^# The sixth indicator is experimental and is included in the experimental version of the CEI.

^* The sixth indicator is only utilized when the period of interest includes months with significant tropical activity. For practical purposes, the CEI does not include the sixth indicator for the cold season (Oct-Mar), winter (Dec-Feb) or spring (Mar-May). It also cannot be calculated independent of the first five indicators.

In each case, we define much above (below) normal or extreme conditions as those falling in the upper (lower) tenth percentile of the local, period of record. In any given year, each of the five indicators has an expected value of 20%, in that 10% of all observed values should fall, in the long-term average, in each tenth percentile, and there are two such sets in each indicator. The fourth indicator, related to extreme precipitation events, has an opposite phase that cannot be considered extreme: the fraction of the country with a much below normal percentage of annual precipitation derived from extreme (i.e. zero) 1-day precipitation amounts. Hence, the fourth indicator is multiplied by twice its value to give it an expected value of 20%, comparable to the first four indicators. In the case of tropical systems, any landfalling system is considered extreme. Since precipitation from such a system is already accounted for in the precipitation steps and can also affect the PDSI, wind velocity at the time of landfall is the focus for this indicator. The square of the wind velocity of each tropical storm and hurricane at the time of landfall is used since a linear increase in wind velocity corresponds more closely to an exponential increase in wind impact and damage. Because this step only accounts for the strength and frequency of tropical systems at landfall (and could not theoretically affect 100% of the nation), it was necessary to scale the step 6 time series to make it comparable to the other five steps. This is done by setting the mean of the time series to that of the other five steps. A CEI both with and without the tropical cyclone indicator is made available in the plots below.

A value of 0% for the CEI, the lower limit, indicates that no portion of the period of record was subject to any of the extremes of temperature or precipitation considered in the index. In contrast, a value of 100% would mean that the entire country had extreme conditions throughout the year for each of the five/six indicators, a virtually impossible scenario. The long-term variation or change of this index represents the tendency for extremes of climate to either decrease, increase, or remain the same.

Choose a period/season and an indicator from the options below and click "Plot" to create a time series graph. Note that the sixth indicator (Step 6) is not calculated for the spring (March-May), winter (Decemeber-February) and cold (October-March) seasons since most months during these periods are outside of the traditional hurricane season.

*As of September 2005, the fourth and fifth indicators are computed with data from one station per each grid box rather than multiple stations. This was done to eliminate many of the artificial extremes which resulted from a changing number of available stations over time. Therefore, the percentages in the step 4 and 5 graphs above will likely be significantly different from the percentages calculated prior to the changes implemented in September 2005.

**As of December 2005, landfalling wind speeds in the sixth indicator are determined using the following logic:

1. For landfalls prior to 1915: Use wind speed observation prior to landfall as landfalling windspeed.
2. For landfalls from 1915 to 1930: Use wind speed observation prior to landfall if observation was at 12Z. Otherwise use midpoint windspeed value from assigned landfalling Saffir-Simpson Scale.
3. For landfalls from 1931 to 1979: Use wind speed observation prior to landfall if observation was at either 0Z or 12Z. Otherwise use midpoint windspeed value from assigned landfalling Saffir-Simpson Scale.
4. For landfalls from 1980 to the present: Use estimated landfalling windspeed as reproted in the Tropical Cyclone Reports issued by the National Hurricane Center.

View CEI Data Files

Limitations: Statistics for the most recent period/season are computed from a fraction of the US which has data available at that time. Extreme percentage values can and will likely change as final quality controlled data become available and fill in the data gaps.

Choose a period/season from the options below to display an analysis of the most recent year.

References

• Gleason, K.L., J.H. Lawrimore, D.H. Levinson, T.R. Karl, and D.J. Karoly 2008: A Revised U.S. Climate Extremes Index. J. Climate, 21, 2124-2137.
• IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovermental Panel on Climate Change, World Meteorological Organization/United Nations Environment Programme, J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson, Eds. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.
• Karl, T.R., 1986: The sensitivity of the Palmer drought severity index and Palmer's Z-index to their calibration coefficients including potential evapotranspiration. J. Climate Appl. Meteor., 25, 77-86.
• ----, and W.J. Koss, 1984: "Regional and National Monthly, Seasonal, and Annual Temperature Weighted by Area, 1895-1983." Historical Climatology Series 4-3, National Climatic Data Center, Asheville, NC, 38 pp.
• ----, and C.N. Williams, 1987: An approach to adjusting climatological time series for discontinous inhomogeneities. J. Climate Appl. Meteor., 26, 1744-1763.
• ----, C.N. Williams Jr., and P.J. Young, 1986: A model to estimate the time of observation bias associated with monthly mean maximum, minimum, and mean temperatures for the United States J. Climate Appl. Meteor., 25, 145-160.
• ----, H. Diaz, and G. Kukla, 1988: Urbanization: Its detection in the U.S. climate record. J. Climate Appl. Meteor., 1, 1099-1123.
• ----, C.N. Williams Jr., F.T. Quinlan, and T.A. Boden, 1990: United States historical climatology network (HCN) serial temperature and precipitation. Dept. of Energy, Oak Ridge National Lab. ORNL/CDIAC-30, NDP-019/R1, 83 pp. plus appendixes.
• ----, R.W. Knight, D.R. Easterling, R.G. Quayle, 1996: Indices of Climate Change for the United States. Bull. Amer. Meteor. Soc., 77, 279-292.
• NRC, 1992: Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base. National Academy Press, 918 pp.
• ----, 2001: Climate Change Science: An Analysis of Some Key Questions. National Academy Press, 42 pp.
• Quayle, R.G., D.R. Easterling, T.R. Karl, and P.Y. Hughes, 1991: Effects of recent thermometer changes in the cooperative station network. Bull. Amer. Meteor. Soc., 72, 1718-1723.

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