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R. C. Schnell


Ice nuclei were measured in seawater, fog water and the free atmosphere from 28 July to 11 August during the 1975 Hayes Fog Cruise off the east coast of Nova Scotia, Canada. Some seawater samples were found to contain ice nuclei active at −4 to −5°C, although the majority of seawater samples contained no nuclei active at temperatures warmer than −14°C. Half of the fog water samples contained ice nuclei active at temperatures warmer than −10°C; some nuclei were active at −2°C. Atmospheric ice nucleus concentrations varied from 1.1 to 580 nuclei m−3 active at −15°C. Some bacteria isolated from fog water were observed to initiate ice at −1.5°C. High concentrations of active ice nuclei in seawaters and fog waters were associated with high concentrations of biological materials in the same samples.

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R. C. Schnell and Gabor Vali


Decayed plant leaf litters from North America, Europe and Asia have been found to contain copious numbers of ice nuclei, some active at −4°C. The abundance of nuclei in a litter was noted to vary according to the climate of the plant's origin; litters from tropical, A-type climates (according to the Köppen classification) contain fewer ice nuclei (103 g−1 active at −10°C) than litters from mid-latitude, C-type climates (105 g−1 active at −10°C) which in turn contain fewer nuclei than litters from high-latitude, D-type climates (109 g−1 at −10°C). The rate of release of freezing nuclei to the atmosphere from in situ litters from D-type climates was determined experimentally: the flux of nuclei active at −12°C was found to be 101−103 cm−2 day−1 during daylight hours.

Active ice nuclei also have been found in seawaters rich in phytoplankton; seawaters devoid of plankton are poor sources of ice nuclei. Some of these nuclei are active at temperatures around −4°C and concentrations reach up to 107−103 nuclei at −10°C per gram of plankton.

Using numerous measurements from around the globe, atmospheric ice nucleus concentrations, and also freezing nucleus concentrations in rainfall, were shown to exhibit a climatic dependence similar to that of biogenic nuclei sources at the surface. This correlation suggests that large proportions of atmospheric ice nuclei are possibly of biogenic origin.

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G. Vali, M. Christensen, R. W. Fresh, E. L. Galyan, L. R. Maki, and R. C. Schnell


Transient appearance of ice nuclei active at temperatures of −2 to −5°C has been noted to accompany the natural decay of plant leaf materials. It was shown that the development of these nuclei results from the presence of a bacterium which was identified as Pseudomonas syringae. These bacteria produce highly active nuclei in a variety of growth media. Evidence points to the fact that the bacterial cells themselves are the nuclei, but that nucleating capacity is a rare and changeable property of the cells. The findings raise the possibility that bacteria may play a role in atmospheric precipitation processes.

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Jonathan D. W. Kahl, Nina A. Zaitseva, V. Khattatov, R. C. Schnell, Dina M. Bacon, Jason Bacon, V. Radionov, and M. C. Serreze

An historical archive of over 25 000 radiosonde observations from the former Soviet “North Pole” series of drifting ice stations has been compiled and made available to interested researchers. This archive is the only long-term set of meteorological sounding data over the Arctic Ocean. The digital archive is a result of the multiyear, collaborative efforts of a group of United States and Russian scientists and keypunch operators working under the auspices of Working Group VIII, an area of study within the United States–Russian Federation Agreement for Protection of the Environment and Natural Resources. The archive contains soundings from 21 drifting stations over the period 1954–90 and is being distributed by the National Snow and Ice Data Center in Boulder, Colorado.

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Gerald D. Bell, Michael S. Halpert, Russell C. Schnell, R. Wayne Higgins, Jay Lawrimore, Vernon E. Kousky, Richard Tinker, Wasila Thiaw, Muthuvel Chelliah, and Anthony Artusa

The global climate during 1999 was impacted by Pacific cold episode (La Niña) conditions throughout the year, which resulted in regional precipitation and atmospheric circulation patterns across the Pacific Ocean and the Americas that are generally consistent with those observed during past cold episodes. The primary La Niña-related precipitation anomalies included 1) increased rainfall across Indonesia, and a nearly complete disappearance of rainfall across the east-central and eastern equatorial Pacific; 2) above-normal rains across northwestern and northern Australia; 3) increased monsoon rains across the Sahel region of western Africa; 4) above-average rains over southeastern Africa, 5) above-average rains over the Caribbean Sea and portions of Central America, and 6) below-average rains in southeastern South America.

The La Niña also contributed to persistent cyclonic circulation anomalies in the subtropics of both hemispheres, which flanked the area of suppressed convective activity over the eastern half of the equatorial Pacific. In the Northern Hemisphere this anomaly feature contributed to a pronounced westward retraction of the wintertime East Asian jet stream, which subsequently impacted precipitation and storm patterns across the eastern North Pacific and western North America. The La Niña-related pattern of tropical rainfall also contributed to a very persistent pattern of anticyclonic circulation anomalies in the middle latitude of both hemispheres, extending from the eastern Pacific across the Atlantic and Africa eastward to Australasia. This anomaly pattern was associated with an active Atlantic hurricane season, an inactive eastern North Pacific hurricane season, above-average rains in the African Sahel, and an overall amplification of the entire southeast Asian summer monsoon complex.

The active 1999 North Atlantic hurricane season featured 12 named storms, 8 of which became hurricanes, and 5 of which became intense hurricanes. The peak of activity during mid-August–October was accompanied by low vertical wind shear across the central and western Atlantic, along with both a favorable structure and location of the African easterly jet. In contrast, only 9 tropical storms formed over the eastern North Pacific during the year, making it one of the most inactive years for that region in the historical record. This relative inactivity was linked to a persistent pattern of high vertical wind shear that covered much of the main development region of the eastern North Pacific.

Other regional aspects of the short-term climate included: 1) above-average wintertime precipitation and increased storminess in the Pacific Northwest, United States; 2) above-average monsoonal rainfall across the southwestern United States; 3) drought over the northeastern quadrant of the United States during April–mid-August; 4) hurricane-related flooding in the Carolinas during September; 5) drought over the south-central United States during July–November; 6) below-average rainfall in the Hawaiian Islands throughout the year, with long-term dryness affecting some parts of the islands since October 1997; 7) a continuation of long-term drought conditions in southeastern Australia, with most of Victoria experiencing below-average rainfall since late 1996; and 8) above-average rainfall in central China during April–August.

Global annual mean surface temperatures during 1999 for land and marine areas were 0.41°C above the 1880–1998 long-term mean, making it the fifth warmest year in the record. However, significant cooling was evident in the Tropics during 1999 in association with a continuation of La Niña conditions. In contrast, temperatures in both the Northern Hemisphere and Southern Hemisphere extratropics were the second warmest in the historical record during 1999, and only slightly below the record 1998 anomalies.

The areal extent of the Antarctic ozone hole remained near record levels during 1999. The ozone hole also lasted longer than has been observed in past years.

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Jay H. Lawrimore, Michael S. Halpert, Gerald D. Bell, Matthew J. Menne, Bradfield Lyon, Russell C. Schnell, Karin L. Gleason, David R. Easterling, Wasila Thiaw, William J. Wrightand, Richard R. Heim Jr., David A. Robinson, and Lisa Alexander

The global climate in 2000 was again influenced by the long-running Pacific cold episode (La Niña) that began in mid-1998. Consistent with past cold episodes, enhanced convection occurred across the climatologically convective regions of Indonesia and the western equatorial Pacific, while convection was suppressed in the central Pacific. The La Niña was also associated with a well-defined African easterly jet located north of its climatological mean position and low vertical wind shear in the tropical Atlantic and Caribbean, both of which contributed to an active North Atlantic hurricane season. Precipitation patterns influenced by typical La Niña conditions included 1) above-average rainfall in southeastern Africa, 2) unusually heavy rainfall in northern and central regions of Australia, 3) enhanced precipitation in the tropical Indian Ocean and western tropical Pacific, 4) little rainfall in the central tropical Pacific, 5) below-normal precipitation over equatorial east Africa, and 6) drier-than-normal conditions along the Gulf coast of the United States.

Although no hurricanes made landfall in the United States in 2000, another active North Atlantic hurricane season featured 14 named storms, 8 of which became hurricanes, with 3 growing to major hurricane strength. All of the named storms over the North Atlantic formed during the August–October period with the first hurricane of the season, Hurricane Alberto, notable as the third-longest-lived tropical system since reliable records began in 1945. The primary human loss during the 2000 season occurred in Central America, where Hurricane Gordon killed 19 in Guatemala, and Hurricane Keith killed 19 in Belize and caused $200 million dollars of damage.

Other regional events included 1) record warm January–October temperatures followed by record cold November–December temperatures in the United States, 2) extreme drought and widespread wildfires in the southern and western Unites States, 3) continued long-term drought in the Hawaiian Islands throughout the year with record 24-h rainfall totals in November, 4) deadly storms and flooding in western Europe in October, 5) a summer heat wave and drought in southern Europe, 6) monsoon flooding in parts of Southeast Asia and India, 7) extreme winter conditions in Mongolia, 8) extreme long-term drought in the Middle East and Southwest Asia, and 9) severe flooding in southern Africa.

Global mean temperatures remained much above average in 2000. The average land and ocean temperature was 0.39°C above the 1880–1999 long-term mean, continuing a trend to warmer-than-average temperatures that made the 1990s the warmest decade on record. While the persistence of La Niña conditions in 2000 was associated with somewhat cooler temperatures in the Tropics, temperatures in the extratropics remained near record levels. Land surface temperatures in the high latitudes of the Northern Hemisphere were notably warmer than normal, with annually averaged anomalies greater than 2°C in parts of Alaska, Canada, Asia, and northern Europe.

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