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G. Kukla, J. Gavin, and T. R. Karl

Abstract

Meteorological stations located in an urban environment in North America warmed between 1941 and 1980, compared to the countryside, at an average rate of about 0.12°C per decade. Secular trends of surface air temperature computed predominately from such station data are likely to have a serious warm bias.

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Thomas R. Karl and Frank T. Quinlan

Abstract

The global radiation climate associated with anomalously cold winter months and cold winters is analyzed for the contiguous United States. The radiation data consist of rehabilitated measured and modeled monthly values of global radiation on both horizontal and tilted surfaces from 116 SOLMET stations in the contiguous United States. The period of record for these stations is 1953-75, but it is extended to cover the years 1953-80 by using percent of possible sunshine (PoPS) data.

On the average the central portions of the United States receive significantly more global radiation than areas east of the Mississippi, but the radiation climate during anomalously cold winters is found to be substantially different from the mean radiation climate. Results indicate that in general, during anomalously cold winter months many eastern portions of the United States receive significantly more (up to 30% more on tilted surfaces) global radiation than normal, whereas in the central United States, particularly in the southern half, less than normal global radiation is received (up to 20% less on tilted surfaces) during the unusually cold winter months. Similar relationships are also reflected over longer averaging periods, i.e., entire winters and heating seasons. As a result solar heating systems have the potential to perform similarly in the eastern and central United States during anomalously cold winters in each area.

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T. R. Karl, G. Kukla, and J. Gavin

Abstract

An appreciable number of nonurban stations in the United States and Canada have been identified with statistically significant (at the 90% level) decreasing trends in the monthly mean diurnal temperature range between 1941–80. The percentage of stations in the network showing the decrease is higher than expected due to chance throughout the year, with a maximum reached during late summer and early autumn and a minimum in December. Monte Carlo tests indicate that during five months the field significance of the decreasing range is above the 99% level, and in 12 months above the 95% level. There is a negligible probability that such a result is due to chance. In contrast, trends of increasing or decreasing monthly mean maximum or minimum temperatures have at most only two months with field significance at or above the 90% level. This is related to the tendency toward increasing temperature in the western portions of North America and decreasing temperature in the east.

The physical mechanism responsible for the observed decrease in the diurnal range is not known. Possible explanations include greenhouse effects such as changes in cloudiness, aerosol loading, atmospheric water vapor content, or carbon dioxide. Change in circulation is also a possibility, but it will be difficult to isolate since the patterns of the decreased diurnal temperature range have high field significance throughout much of the year, relatively low spatial coherence, and occur at many stations where individual trends in the maximum and minimum temperature are not statistically significant. Our data show that the trends in the maximum and minimum temperatures may differ considerably from trends in the mean.

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T. R. Karl, G. Kukla, and J. Gavin

Abstract

A network of 14 nonurban stations was used as a first step toward understanding the character of change of daily temperature between 1948 and 1983 across the United States. Overall, in the eastern two-thirds of the country more stations tend to reflect statistically significant cooler weather than would be expected due to chance alone. In the western one-third of the country the opposite is found. Despite the cooling in the east, air masses with comparable characteristics (dewpoints, cloud cover) affecting these stations have warmed. This finding is in qualitative agreement with the expected impact of rising concentrations of greenhouse gases.

Considering all stations regardless of location, a marked increase of daily minimum temperature was found in comparable air masses under overcast skies, which, when combined with the tendency for decreasing maxima, results in a highly significant (at 0.1% level) decrease of an implied diurnal temperature range. Both maxima and minima increased in clear weather after stratification by air mass characteristics. It is highly unlikely that the reported changes are related to boundary layer humidity or direction of air flow since the data were stratified for air mass identification by both surface dew-point temperature and wind direction.

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P. Ya. Groisman, V. V. Koknaeva, T. A. Belokrylova, and T. R. Karl

Documenting the instrumentally observed precipitation climate record presents many challenges because scientists must rely on data from stations that undergo many changes in the course of their operation. Detecting changes from such networks is essential for adequate understanding of climate and global change. As an illustrative example, we review the history of the instrumentally observed precipitation in the USSR. In the USSR, similar to other countries, numerous problems must be addressed before reliable estimates of precipitation can be made. The types of problems range from inadequate and changing exposures of raingages to varying sampling periods used to measure precipitation. Using information about measurement procedures, instrument intercomparisons, and field studies, various methods have been devised to overcome biases in the measurements.

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Patrick D. Miller, Lawrence J. Pratt, Karl R. Helfrich, and Christopher K. R. T. Jones

Abstract

The method of lobe analysis is used and extended to analyze a time-dependent, boundary-trapped recirculation. The recirculation gyre occurs in a numerical model of wind-driven flow around an island, but the underlying geometry of the gyre is similar to persistent eddies such as the Alboran Gyre and the Great Whirl. Even in the steady (weak forcing) limit, the gyre leaks fluid due to the fact that the surface Ekman pumping above it is directed downward. The authors show that this leakage is rapidly superseded by chaotic transport into and out of the gyre when time dependence sets in. Variations of the traditional “turnstile” approach to transport are used to study the dynamics of the gyre. A Lagrangian recirculation boundary, consisting of pieces of stable and unstable manifolds joined by a gate, allows straightforward calculation and visualization of potential vorticity flux.

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S. S. P. Shen, H. Yin, K. Cannon, A. Howard, S. Chetner, and T. R. Karl

Abstract

This paper analyzes the long-term (1901–2002) temporal trends in the agroclimate of Alberta, Canada, and explores the spatial variations of the agroclimatic resources and the potential crop-growing area in Alberta. Nine agroclimatic parameters are investigated: May–August precipitation (PCPN), the start of growing season (SGS), the end of the growing season (EGS), the length of the growing season (LGS), the date of the last spring frost (LSF), the date of the first fall frost (FFF), the length of the frost-free period (FFP), growing degree-days (GDDs), and corn heat units (CHUs). The temporal trends in the agroclimatic parameters are analyzed by using linear regression. The significance tests of the trends are made by using Kendall’s tau method. The results support the following conclusions. 1) The Alberta PCPN has increased 14% from 1901 to 2002, and the increment is the largest in the north and the northwest of Alberta, then diminishes (or even becomes negative over two small areas) in central and southern Alberta, and finally becomes large again in the southeast corner of the province. 2) No significant long-term trends are found for the SGS, EGS, and LGS. 3) An earlier LSF, a later FFF, and a longer FFP are obvious all over the province. 4) The area with sufficient CHU for corn production, calculated according to the 1973–2002 normal, has extended to the north by about 200–300 km, when compared with the 1913–32 normal, and by about 50–100 km, when compared with the 1943–72 normal; this expansion implies that the potential exists to grow crops and raise livestock in more regions of Alberta than was possible in the past. The annual total precipitation follows a similar increasing trend to that of the May–August precipitation, and the percentile analysis of precipitation attributes the increase to low-intensity events. The changes of the agroclimatic parameters imply that Alberta agriculture has benefited from the last century’s climate change.

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Kenneth E. Kunkel, Thomas R. Karl, Michael F. Squires, Xungang Yin, Steve T. Stegall, and David R. Easterling

Abstract

Trends of extreme precipitation (EP) using various combinations of average return intervals (ARIs) of 1, 2, 5, 10, and 20 years with durations of 1, 2, 5, 10, 20, and 30 days were calculated regionally across the contiguous United States. Changes in the sign of the trend of EP vary by region as well as by ARI and duration, despite the statistically significant upward trends for all combinations of EP thresholds when area averaged across the contiguous United States. Spatially, there is a pronounced east-to-west gradient in the trends of the EP with strong upward trends east of the Rocky Mountains. In general, upward trends are larger and more significant for longer ARIs, but the contribution to the trend in total seasonal and annual precipitation is significantly larger for shorter ARIs because they occur more frequently. Across much of the contiguous United States, upward trends of warm-season EP are substantially larger than those for the cold season and have a substantially greater effect on the annual trend in total precipitation. This result occurs even in areas where the total precipitation is nearly evenly divided between the cold and warm seasons. When compared with short-duration events, long-duration events—for example, 30 days—contribute the most to annual trends. Coincident statistically significant upward trends of EP and precipitable water (PW) occur in many regions, especially during the warm season. Increases in PW are likely to be one of several factors responsible for the increase in EP (and average total precipitation) observed in many areas across the contiguous United States.

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D. R. Easterling, J. L. Evans, P. Ya. Groisman, T. R. Karl, K. E. Kunkel, and P. Ambenje

Variations and trends in extreme climate events have only recently received much attention. Exponentially increasing economic losses, coupled with an increase in deaths due to these events, have focused attention on the possibility that these events are increasing in frequency. One of the major problems in examining the climate record for changes in extremes is a lack of high-quality, long-term data. In some areas of the world increases in extreme events are apparent, while in others there appears to be a decline. Based on this information increased ability to monitor and detect multidecadal variations and trends is critical to begin to detect any observed changes and understand their origins.

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K. P. Gallo, A. L. McNab, T. R. Karl, J. F. Brown, J. J. Hood, and J. D. Tarpley

Abstract

A vegetation index and a radiative surface temperature were derived from satellite data acquired at approximately 1330 LST for each of 37 cities and for their respective nearby rural regions from 28 June through 8 August 1991. Urban–rural differences for the vegetation index and the surface temperatures were computed and then compared to observed urban–rural differences in minimum air temperatures. The purpose of these comparisons was to evaluate the use of satellite data to assess the influence of the urban environment on observed minimum air temperatures (the urban heat island effect). The temporal consistency of the data, from daily data to weekly, biweekly, and monthly intervals, was also evaluated. The satellite-derived normalized difference (ND) vegetation-index data, sampled over urban and rural regions composed of a variety of land surface environments, were linearly related to the difference in observed urban and rural minimum temperatures. The relationship between the ND index and observed differences in minimum temperature was improved when analyses were restricted by elevation differences between the sample locations and when biweekly or monthly intervals were utilized. The difference in the ND index between urban and rural regions appears to be an indicator of the difference in surface properties (evaporation and heat storage capacity) between the two environments that are responsible for differences in urban and rural minimum temperatures. The urban and rural differences in the ND index explain a greater amount of the variation observed in minimum temperature differences than past analyses that utilized urban population data. The use of satellite data may contribute to a globally consistent method for analysis of urban heat island bias.

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