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Richard W. Knight

Abstract

A method is presented for quantifying the temporal variability found in hurricanes. The method is based upon several well-known nonparametric tests that use ranking techniques; among these are tests devised by Kendall, Kruskal-Wallis and Jonckheere. The tests area used in an attempt to answer the following questions:

1) How variable are selected meteorological elements in time?

2) Are there significant differences among profiles?

3) Are there differences in variability between the interior and exterior portions of profiles?

4) Do the selected elements exhibit a trend in time?

Data taken from Hurricane Debbie (1969) on two days are processed and a computational example is presented.

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Paul Tattelman and Richard W. Knight

Abstract

A method for extracting 1-min rain rates from original weighing raingage recordings is described. The method allows the retrieval of rates for long periods at approximately 300 United States weather stations. The process combines magnification of original chart records with modern digitizing and filtering techniques to obtain the 1-min data that are ordinarily unreadable. Analyses are presented of the frequency and duration of 1-min rates for these seven locations and an eighth location, for which data were collected using a high speed recorder.

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Richard W. Knight and Glenn W. Brier

Abstract

Plans are underway to attempt to reduce the destructive force of hurricanes by artificially modifying their structure by means of cloud seeding. Since the natural variability of meteorological elements observed in hurricanes is high, the success of the project depends upon establishing a cause and effect relationship between the seeding and the hurricane's response. The small sample of mature hurricanes coupled with rigorous selection criteria make a randomized experiment impractical. Therefore, an evaluation technique based on the concept of randomization in time is developed.

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Pavel Ya Groisman and Richard W. Knight

Abstract

A disproportionate increase in precipitation coming from intense rain events, in the situation of general warming (thus, an extension of the vegetation period with intensive transpiration), and an insignificant change in total precipitation could lead to an increase in the frequency of a potentially serious type of extreme events: prolonged periods without precipitation (even when the mean seasonal rainfall totals increase). This paper investigates whether this development is already occurring during the past several decades over the conterminous United States, for the same period when changes in frequency of intense precipitation events are being observed. Lengthy strings of “dry” days without sizeable (>1.0 mm) precipitation were assessed only during the warm season (defined as a period when mean daily temperature is above the 5°C threshold) when water is intensively used for transpiration and prolonged periods without sizable rainfall represent a hazard for terrestrial ecosystem’s health and agriculture. During the past four decades, the mean duration of prolonged dry episodes (1 month or longer in the eastern United States and 2 months or longer in the southwestern United States) has significantly increased. As a consequence the return period of 1-month-long dry episodes over the eastern United States has reduced more than twofold from 15 to 6–7 yr. The longer average duration of dry episodes has occurred during a relatively wet period across the country but is not observed over the northwestern United States.

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Thomas R. Karl and Richard W. Knight

The deadly heat wave of July 1995 that affected much of the U.S. midwest, most notably Chicago, Illinois, has been put into historical perspective. The heat wave has been found to be remarkably unusual, but only partially because of the extreme high apparent temperatures (an index of the combined effect of temperature and humidity on humans), where the authors calculate a return period of the peak apparent temperature of ≤ 23 yr. Of greater significance were the very high temperatures that persisted day and night over an extended 48-h period. Analysis presented here indicates that for Chicago such an extended period of continuously high day and night apparent temperature is unprecedented in modern times. The 2-day period where the minimum apparent temperature failed to go below 31.5°C (89°F) is calculated to be an extremely rare event (probability of occurrence <0.1%) based on a 10 000-yr-long simulation of a four-parameter (temperatures related to the mean, the intraseasonal daily variance, the interannual variance, and the day-to-day persistence of temperature) probabilistic model.

Such unusual heat waves evoke questions related to the future course of the climate and whether this recent event was merely an extreme anomaly or part of an ongoing trend toward more extreme heat waves. A Monte Carlo analysis of trends (1948–95) for various quantiles of the hourly apparent temperatures during the most severe heat waves each year from 26 midwestern stations reveals a modest, statistically insignificant increase of apparent temperatures for a wide range of quantiles without the inclusion of 1995 data. There is a statistically significant increase in apparent temperature with its inclusion, reflected most strongly for upper quantiles or daytime temperatures. It is argued, however, that because of the impact of changes in instrumentation at primary National Weather Service stations, the potential affects of urbanization, and little trend of summer mean temperatures, it is unlikely that the macroscale climate of heat waves in the Midwest or in Chicago is changing in any significant manner.

Trends notwithstanding, the authors demonstrate the difficulty associated with projecting changes in the frequency and severity of similar types of events, even if the mean apparent temperature could be accurately predicted for the next century, for example, global warming projections. This is demonstrated using Chicago temperatures. The authors show that accurate projections of the frequency, severity, and duration of heat waves in the Midwest require accurate projections not only of the mean, the interannual variance, the intraseasonal variance, and day-to-day persistence, but also the interrelationships among these quantities within different synoptic-climatic regimes.

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Thomas R. Karl and Richard W. Knight

Twentieth century trends of precipitation are examined by a variety of methods to more fully describe how precipitation has changed or varied. Since 1910, precipitation has increased by about 10% across the contiguous United States. The increase in precipitation is reflected primarily in the heavy and extreme daily precipitation events. For example, over half (53%) of the total increase of precipitation is due to positive trends in the upper 10 percentiles of the precipitation distribution. These trends are highly significant, both practically and statistically. The increase has arisen for two reasons. First, an increase in the frequency of days with precipitation [6 days (100 yr)−1] has occurred for all categories of precipitation amount. Second, for the extremely heavy precipitation events, an increase in the intensity of the events is also significantly contributing (about half) to the precipitation increase. As a result, there is a significant trend in much of the United States of the highest daily year–month precipitation amount, but with no systematic national trend of the median precipitation amount.

These data suggest that the precipitation regimes in the United States are changing disproportionately across the precipitation distribution. The proportion of total precipitation derived from extreme and heavy events is increasing relative to more moderate events. These changes have an impact on the area of the United States affected by a much above-normal (upper 10 percentile) proportion of precipitation derived from very heavy precipitation events, for example, daily precipitation events exceeding 50.8 mm (2 in.).

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Pavel Ya. Groisman, Richard W. Knight, and Thomas R. Karl

Abstract

In examining intense precipitation over the central United States, the authors consider only days with precipitation when the daily total is above 12.7 mm and focus only on these days and multiday events constructed from such consecutive precipitation days. Analyses show that over the central United States, a statistically significant redistribution in the spectra of intense precipitation days/events during the past decades has occurred. Moderately heavy precipitation events (within a 12.7–25.4 mm day−1 range) became less frequent compared to days and events with precipitation totals above 25.4 mm. During the past 31 yr (compared to the 1948–78 period), significant increases occurred in the frequency of “very heavy” (the daily rain events above 76.2 mm) and extreme precipitation events (defined as daily and multiday rain events with totals above 154.9 mm or 6 in.), with up to 40% increases in the frequency of days and multiday extreme rain events. Tropical cyclones associated with extreme precipitation do not significantly contribute to the changes reported in this study. With time, the internal precipitation structure (e.g., mean and maximum hourly precipitation rates within each preselected range of daily or multiday event totals) did not noticeably change. Several possible causes of observed changes in intense precipitation over the central United States are discussed and/or tested.

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Thomas R. Karl, Richard W. Knight, and John R. Christy

Abstract

Long-term (50 to 100 years) and short-term (10 to 30 years) global and hemispheric trends of temperature have an inherent unknown error due to incomplete and nonrandom spatial sampling. A number of experiments have been conducted to help quantify the potential magnitude of this error. The analysis includes the errors introduced into the climate record because of both incomplete global coverage and inadequate sampling within those areas presumed to have adequate observatory. In these experiments it is found that the uncertainty in calculating historical temperature trends is dependent upon the pattern of temperature change, the method of treating the effect of nonrandom spatial sampling, and the time and length over which the trend is calculated but is relatively insensitive to the random errors associated with estimating regional-scale (grid cell size) temperature anomalies.

Results imply that the errors associated with century-scale trends of temperature are probably an order of magnitude smaller than the observed global warming of nearly 0.5°C per 100 years since the late nineteenth century. The errors in estimates of decadal temperature trends are found to be large relative to century-scale trends and are unreliable during the nineteenth and early twentieth centuries. Even during the recent decade of the 1980s, the area-averaging techniques used in some analyses could be improved by addressing the over-sampling of Northern Hemisphere (especially over land) relative to the rest of the globe. Otherwise, significant positive biases are likely during the 1980s. These biases may have contributed to the reported differences between in situ surface and space-based temperatures during the 1980s.

The rather encouraging results with respect to the magnitude of the spatial sampling errors associated with the calculation of long-term trends beginning in the nineteenth century cast a positive light on efforts aimed at extending the proxy and observed global temperature record further back in time, despite limited geographic coverage.

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Pavel Ya. Groisman, Richard W. Knight, and Thomas R. Karl

Changes in several components of the hydrological cycle over the contiguous United States have been documented during the twentieth century: an increase of precipitation, especially heavy and very heavy precipitation, and a significant retreat in spring snow cover extent over western regions during the last few decades.

These changes have affected streamflow, including the probability of high flow.

In the eastern half of the United States a significant relationship is found between the frequency of heavy precipitation and high streamflow events both annually and during the months of maximum streamflow. Two factors contributed to finding such a relation: 1) the relatively small contribution of snowmelt to heavy runoff in the eastern United States (compared to the west), and 2) the presence of a sufficiently dense network of streamflow and precipitation gauges available for analysis. An increase of spring heavy precipitation events over the eastern United States indicates with high probability that during the twentieth century an increase of high streamflow conditions has also occurred. In the West, a statistically significant reduction of snow cover extent has complicated the relation between heavy precipitation and streamflow. Increases in peak stream flow have not been observed here, despite increases in heavy precipitation events, and less extensive snow cover is the likely cause.

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Thomas R. Karl, Pavel Ya Groisman, Richard W. Knight, and Richard R. Heim Jr.

Abstract

Contemporary large-scale changes in solid and total precipitation and satellite-derived snow cover were examined over the North American continent. Annual snow cover extent over the last 19 years decreased up to 6×105 km2 relative to a 0.93°C (0.33°C) increase in North American (Northern Hemisphere) temperature.

A strong correlation exists between snow cover and temperature where up to 78% of the variance in regional snow cover and snowfall is explained by the anomalies of monthly mean maximum temperature. Over the last two decades the decrease in snow cover during winter (December-March) has largely occurred through reduced frequency of snow cover in areas that typically have a high probability of snow on the ground with little change in the frequency of snow cover in other areas. Similar characteristics were observed during spring (April-May) in areas with high snow cover probability except for an expansion of the snow-free regions. Anomalies in these two seasons dominate the interannual variability (nearly three-fourths of the variance) of snow cover.

The apparent unprecedented global warmth of the 1980s was accompanied by a retreat of the mean annual North American snow cover, a 10% increase in annual Alaskan precipitation, a significant decrease (−7% ) in annual snowfall over southern Canada (while the total precipitation remained above normal), and a more than twofold increase in the variance of the ratio of frozen to total precipitation over the contiguous United States. An increase (4% –5% per decade) of both solid and total precipitation over northern Canada (zone 55°–70°N) occurred during the last four decades. A century-scale increase (1% per decade) of precipitation was found over southern Canada, but the proportion of the precipitation that falls in frozen form over this area decreased over the last four decades. Precipitation over the contiguous United States has significantly (2% –3% per decade) increased during the last four decades, but on a century time scale the increasing trend is not yet statistically significant.

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