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David R. Easterling

One of the signals expected with greenhouse warming is a change in what are now considered extreme temperatures. In this paper one type of extreme is examined for the 1948–99 period, that is, a change in the number of days when the minimum daily temperature dips below freezing or “frost days.” This is approached by looking at two questions: 1) have there been changes in the number of frost days per year, or per season, and 2) are there trends in the dates of the first autumn frost, last spring frost, and length of the frost-free season? Results show that the country as a whole has experienced a slight decrease in the number of days, with the biggest decreases in the winter and spring. Changes in frost dates for autumn show small changes to a later date, but the date of the last-spring freeze shows a distinct move to an earlier date. This results in an increase in the frost-free season. However, there is a distinct spatial pattern to the results that is consistent with the spatial pattern of annual temperature trends for the twentieth century. The geographical pattern shows the western United States with the largest decreases in frost days, and increases in frost-free season length. But the southeast United States, which is one of the few areas of the world showing cooling over the twentieth century, has no significant changes in the number of frost days or the frost-free season.

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David R. Easterling

Abstract

Observations of thunderstorm events by mouth for the period 1948–77 at 106 stations in the central United States are used to examine persistent spatial patterns in thunderstorm occurrence. A principal components analysis reveals that nearly 55% of the variance is explained by the first four components. An oblique rotation was performed using the first four components, with distinct synoptic-scale patterns revealed. The synoptic environments of four months representative of the four rotated components arc briefly described. A regionalization is presented that was developed by assigning each grid point to the rotated component on which it has the highest loading.

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Adam Terando, William E. Easterling, Klaus Keller, and David R. Easterling

Abstract

The authors examine recent changes in three agro-climate indices (frost days, thermal time, and heat stress index) in North America (centered around the continental United States) using observations from a historical climate network and an ensemble of 17 global climate models (GCMs) from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). Agro-climate indices provide the basis for analyzing agricultural time series that are unbiased by long-term technological intervention. Observations from the last 60 years (1951–2010) confirm conclusions of previous studies showing continuing declines in the number of frost days and increases in thermal time. Increases in heat stress are largely confined to the western half of the continent. The authors do not observe accelerating agro-climate warming trends in the most recent decade of observations. The spatial variability of the temporal trends in GCMs is lower compared to the observed patterns, which still show some regional cooling trends. GCM skill, defined as the ability to reproduce observed patterns (i.e., correlation and error) and variability, is highest for frost days and lowest for heat stress patterns. Individual GCM skill is incorporated into two model weighting schemes to gauge their ability to reduce predictive uncertainty for agro-climate indices. The two weighted GCM ensembles do not substantially improve results compared to the unweighted ensemble mean. The lack of agreement between simulated and observed heat stress is relatively robust with respect to how the heuristic is defined and appears to reflect a weakness in the ability of this last generation of GCMs to reproduce this impact-relevant aspect of the climate system. However, it remains a question for future work as to whether the discrepancies between observed and simulated trends primarily reflect fundamental errors in model physics or an incomplete treatment of relevant regional climate forcings.

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David R. Easterling and Peter J. Robinson

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Starting times of thunderstorms for 450 stations in the conterminuos United States for a 25-year period were analyzed using harmonic analysis techniques. Diurnal variations were expressed as both the time of maximum storm occurrence and the concentration of activity around this time. Distinct seasonal and spatial variations in diurnal activity occur. Analysis of these variations indicates that the country can be divided into nine thunderstorm regions. In the central states the majority of storms occur at night, but storms are frequent at any time. In both the east and the west there is a marked concentration of storms in the afternoon. In the west and northeast winter storms are rare, while along the Pacific Coast summer thunder is uncommon.

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Pavel Ya Groisman and David R. Easterling

Abstract

The biases and large-scale inhomogeneities in the time series of measured precipitation and snowfall over the United States and Canada are discussed and analyzed. The spatial statistical characteristics of monthly and annual snowfall and total precipitation are investigated and parameterized. After adjustments and selection of the “best” network, reliable “first guess” estimates of North American snowfall and precipitation are obtained. Century-long time series of unbiased annual precipitation over the regions to the south of 55°N and 40-year time series of unbiased area-averaged annual precipitation and snowfall for all of North America are developed. The analysis of their trends shows the following.

1) During the last 100 years, annual precipitation has increased in southern Canada (south of 55°N) by 13% and in the contiguous United States by 4%; however, the main domain of this century-scale precipitation increase is eastern Canada and adjacent to it northern regions of the United States.

2) Up to a 20% increase has occurred in annual snowfall and rainfall during the last four decades in Canada north of 55°N.

The relationships between century-long precipitation time series over North America with Northern Hemisphere surface air temperature and the South Oscillation index (SOI) are investigated. It is shown that ENSO (negative anomaly of SOI) is usually accompanied by an increase of precipitation whenever it affects the United States (especially in the southwestern region of the country).

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David R. Easterling, Thomas C. Peterson, and Thomas R. Karl

Abstract

At the National Climatic Data Center, two basic approaches to making homogeneity adjustments to climate data have been developed. The first is based on the use of metadata (station history files) and is used in the adjustments made to the U.S. Historical Climatology Network monthly dataset. The second approach is non-metadata based and was developed for use with the Global Historical Climatology Network dataset, since there are not extensive station history files for most stations in the dataset. In this paper the two methodologies are reviewed and the adjustments made using each are compared, then the results are discussed. Last, some brief guidelines on the limitations and uses of these data are provided.

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Kenneth E. Kunkel, Thomas R. Karl, and David R. Easterling

Abstract

A Monte Carlo analysis was used to assess the effects of missing data and limited station density on the uncertainties in the temporal variations of U.S. heavy precipitation event frequencies observed for 1895–2004 using data from the U.S. Cooperative Observer Network (COOP). Based on the actual availability of long-term station data, the effects of limited spatial density were found to be of greater importance than those of missing data. The Monte Carlo simulations indicate that there is a high degree of statistical confidence that the recent elevated frequencies in the United States are the highest in the COOP record since 1895, at least for event definitions using return periods of 5 yr or shorter. There is also high confidence that elevated frequencies seen early in the record are higher than those measured in the 1920s and 1930s, and are not simply an artifact of the limited spatial sampling. The statistically significant shift from high to low values in the early portion of the record, a reflection of natural variability, should not be ignored when interpreting the elevated levels of the most recent decades. Nevertheless, it does appear that the recent elevated levels exceed the variations seen in the earlier part of the record since 1895. The confidence in these statements decreases as the return period increases because of the diminishing number of events in the sample. When a linear trend is fit to the entire 1895–2004 period, the trends are positive and different from zero with a high level of statistical confidence for all return periods from 1 to 20 yr.

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Kevin P. Gallo, David R. Easterling, and Thomas C. Peterson

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The diurnal temperature range (DTR) at weather observation stations that make up the U.S. Historical Climatology Network was evaluated with respect to the predominant land use/land cover associated with the stations within three radii intervals (100, 1000, and 10 000 m) of the stations. Those stations that were associated with predominantly rural land use/land cover (LULC) usually displayed the greatest observed DTR, whereas those associated with urban related land use or land cover displayed the least observed DTR. The results of this study suggest that significant differences in the climatological DTR were observed and could be attributed to the predominant LULC associated with the observation stations. The results also suggest that changes in the predominant LULC conditions, within as great as a 10 000 m radius of an observation station, could significantly influence the climatological DTR. Future changes in the predominant LULC associated with observation sites should be monitored similar to the current practice of monitoring changes in instruments or time of observation at the observations sites.

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Kenneth E. Kunkel, Karen Andsager, and David R. Easterling

Abstract

This paper describes the results of an analysis of trends in short duration (1–7 days) extreme precipitation events that have a recurrence interval of 1 yr or longer for stations in the United States and Canada. This definition of extreme precipitation was chosen because such events are highly correlated with hydrologic flooding in some U.S. regions. The dominant temporal characteristic of a national event composite index is significant low-frequency variability. There were lengthy periods of a below-average number of events in the 1930s and 1950s and an above-average number of events in the early 1940s, early 1980s, and 1990s. Regional variations often differ substantially from the national composite. A simple linear analysis indicates that the overall trend covering the period 1931–96 has been upward at a highly statistically significant rate over the southwest United States and in a broad region from the central Great Plains across the middle Mississippi River and southern Great Lakes basins. The national trend for the United States is upward at a rate of 3% decade−1 for the period 1931–96. While the annual trend for Canada is upward for the period 1951–93, it is not statistically significant. Although the high statistical significance of the results is partially a consequence of the low frequency during the 1930s and 1950s located in the first half of the record, the latter half of the record exhibits an upward trend nearly identical to the entire record. However, an analysis of a 101-yr record of midwestern stations shows that heavy precipitation event frequencies around the turn of the twentieth century (1896–1906) were higher than for other periods of comparable length, except for 1986–96. Although data were not available in digital form to extend the analysis back to 1896 for the entire United States, the midwestern analysis shows that interpretation of the recent upward trends must account for the possibility of significant natural forcing of variability on century timescales.

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Kenneth E. Kunkel, David R. Easterling, David A. R. Kristovich, Byron Gleason, Leslie Stoecker, and Rebecca Smith

Abstract

Daily extreme precipitation events, exceeding a threshold for a 1-in-5-yr occurrence, were identified from a network of 935 Cooperative Observer stations for the period of 1908–2009. Each event was assigned a meteorological cause, categorized as extratropical cyclone near a front (FRT), extratropical cyclone near center of low (ETC), tropical cyclone (TC), mesoscale convective system (MCS), air mass (isolated) convection (AMC), North American monsoon (NAM), and upslope flow (USF). The percentage of events ascribed to each cause were 54% for FRT, 24% for ETC, 13% for TC, 5% for MCS, 3% for NAM, 1% for AMC, and 0.1% for USF. On a national scale, there are upward trends in events associated with fronts and tropical cyclones, but no trends for other meteorological causes. On a regional scale, statistically significant upward trends in the frontal category are found in five of the nine regions. For ETCs, there are statistically significant upward trends in the Northeast and east north central. For the NAM category, the trend in the West is upward. The central region has seen an upward trend in events caused by TCs.

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