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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.

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.

Significant climatic changes over northern Eurasia during the twentieth century are revealed in numerous variables including those affecting and characterizing the state of the cryosphere. In addition to commonly used in situ observations of snow cover such as snow depth and snow courses, synoptic archives in the former Soviet Union contain regular daily and semidaily reports about the state of the ground in the area surrounding the station. Information about frozen, dry, wet, ponded, and snow-covered land, and in the case of snow-covered land, about the characteristics of snow cover, is available in these reports. A new Global Synoptic Data Network (GSDN) consisting of 2100 stations within the boundaries of the former Soviet Union created jointly by the National Climatic Data Center (NCDC) and Russian Institute for Hydrometeorological Information (RIHMI) was used to assess the climatology of snow cover, frozen and unfrozen ground reports, and their temporal variability for the period from 1936 to 2004. Comparison with satellite measurements of snow cover extent is also presented. During the second half of the twentieth century and over many regions in northern Eurasia, an increase in unfrozen ground conditions (5 days since 1956 over the Russian Federation) was observed. The most prominent changes occurred in the spring season in Siberia and the Far East north of 55°N during April and May by 3 to 5 days, which constitute a 15%–35% change in these regions compared to long-term mean values. Since the beginning of the dataset, surface temperature changes in high latitudes have not been monotonic. As a result, linear trend analyses applied to the entire period of observations can lead to paradoxical conclusions. Specifically, changes in snow cover extent during the 1936–2004 period cannot be linked with “warming” (particularly with the Arctic warming) because in this particular period the Arctic warming was absent.

Observed changes in intense precipitation (e.g., the frequency of very heavy precipitation or the upper 0.3% of daily precipitation events) have been analyzed for over half of the land area of the globe. These changes have been linked to changes in intense precipitation for three transient climate model simulations, all with greenhouse gas concentrations increasing during the twentieth and twenty-first centuries and doubling in the later part of the twenty-first century. It was found that both the empirical evidence from the period of instrumental observations and model projections of a greenhouse-enriched atmosphere indicate an increasing probability of intense precipitation events for many extratropical regions including the United States. Although there can be ambiguity as to the impact of more frequent heavy precipitation events, the thresholds of the definitions of these events were raised here, such that they are likely to be disruptive. Unfortunately, reliable assertions of very heavy and extreme precipitation changes are possible only for regions with dense networks due to the small radius of correlation for many intense precipitation events.

Over the contiguous United States, precipitation, temperature, streamflow, and heavy and very heavy precipitation have increased during the twentieth century. In the east, high streamflow has increased as well. Soil wetness (as described by the Keetch–Byram Drought index) has increased over the northern and eastern regions of the United States, but in the southwestern quadrant of the country soil dryness has increased, making the region more susceptible to forest fires. In addition to these changes during the past 50 yr, increases in evaporation, near-surface humidity, total cloud cover, and low stratiform and cumulonimbus clouds have been observed. Snow cover has diminished earlier in the year in the west, and a decrease in near-surface wind speed has also occurred in many areas. Much of the increase in heavy and very heavy precipitation has occurred during the past three decades.

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...

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...

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

A framework is presented to quantify observed changes in climate within the contiguous United States through the development and analysis of two indices of climate change, a Climate Extremes Index (CEI) and a U.S. Greenhouse Climate Response Index (GCRI). The CEI is based on an aggregate set of conventional climate extreme indicators, and the GCRI is composed of indicators that measure changes in the climate of the United States that have been projected to occur as a result of increased emissions of greenhouse gases. The CEI supports the notion that the climate of the United States has become more extreme in recent decades, yet the magnitude and persistence of the changes are not large enough at this point to conclude that the increase in extremes reflects a nonstationary climate. Nonetheless, if impacts due to extreme events rise exponentially with the index, then the increase may be quite significant in a practical sense. Similarly, the positive trend of the U.S. GCRI during the twentieth centu...

Contemporary large-scale changes in satellite-derived snow cover were examined over the Northern Hemisphere extratropical land (NEL) areas. These areas encompass 55% of the land in the Northern Hemisphere. Snow cover (S) transient regions, the 'centers of action' relative to interannual variations of snow cover, were identified for the years 1972-1992. During these years a global retreat in snow cover extent (SE) occurred in the second half of the hydrologic year (April-September). Mean annual SE has decreased by 10% (2.3 x 10(exp 6) sq km). Negative trends account for one-third to one-half of the interannual continental variance of SE. The historical influence of S on the planetary albedo and outgoing longwave radiation (OLR) is investigated. The mean annual response of the S feedback on the radiative balance (RB) is negative and suggests a largescale heat redistribution. During autumn and early winter (up to January), however, the feedback of S on the planetary RB may be positive. Only by February does the cooling effect of S (due to albedo increase) dominate the planetary warming due to reduced OLR over the S. Despite a wintertime maximum in SE, the feedback in spring has the greatest magnitude. The global retreat of spring SE should lead to a positive feedback on temperature. Based on observed records of S, changes in RB are calculated that parallel an observed increase of spring temperature during the past 20 years. The results provide a partial explanation of the significant increase in spring surface air temperature observed over the land areas of the Northern Hemisphere during the past century. The mean SE in years with an El Nino and La Nina were also evaluated. El Nino events are generally accompanied by increased SE over the NEL during the first half of the hydrological year. In the second half of the hydrologic year (spring and summer), the El Nino events are accompanied by a global retreat of SE.

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 smalle...

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 variab...

Monthly mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951–90 (0.84°C versus 0.28°C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied. The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diur...

Important surface observations such as the daily maximum and minimum temperature, daily precipitation, and cloud ceilings often have localized characteristics that are difficult to reproduce with the current resolution and the physical parameterizations in state-of-the-art General Circulation climate Models (GCMs). Many of the difficulties can be partially attributed to mismatches in scale, local topography. regional geography and boundary conditions between models and surface-based observations. Here, we present a method, called climatological projection by model statistics (CPMS), to relate GCM grid-point flee-atmosphere statistics, the predictors, to these important local surface observations. The method can be viewed as a generalization of the model output statistics (MOS) and perfect prog (PP) procedures used in numerical weather prediction (NWP) models. It consists of the application of three statistical methods: 1) principle component analysis (FICA), 2) canonical correlation, and 3) infla...

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.

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.