CIRCULATION PATTERNS AND TEMPERATURE FIELDS ASSOCIATED WITH EXTENSIVE SNOW COVER ON THE NORTH AMERICAN CONTINENT

Weekly snow cover areas, derived from the NOAA/NESS Northern Hemisphere Digitized Snow and Ice Cover Data Base, were correlated with weekly temperature anomalies across the United States and with weekly 700-mb geopotential heights over the North American sector. The correlations were computed for snow cover across the entire North American continent as well as the western and eastern United States for the winters 1966–67 through 1979–80. Extensive snow cover is associated with negative temperature anomalies across most of the continental United States. The strongest relationship occurs along the eastern flank of the Rocky Mountains from the Canadian border to the central Great Plains and reflects the southward movement of cold arctic air masses toward the Gulf of Mexico. An anomalous trough over the western part of North America is responsible for extensive snow cover in the winter. The surface storm track is displaced southward during winters with heavy snow cover, with cyclones occurring more frequently in the southern Plains and southeastern United States. Moist Gulf of Mexico air advected northward by the southerly flow aloft is an important prerequisite condition for the occurrence of extensive snow cover in the eastern United States during the first half of winter.     

REGIONAL-SCALE TROUGHING OVER THE SOUTHWESTERN UNITED STATES: TEMPORAL CLIMATOLOGY,TELECONNECTIONS, AND CLIMATIC IMPACT

Regional-scale middle- and upper-tropospheric troughing over the southwestern United States represents a departure from the modal circulation pattern for western North America. Once developed, southwestern troughs often are associated with positive vorticity advection aloft, surface cyclone formation, and moisture advection over areas of the western Great Plains and Intermountain West. These trough systems may play an important role in the precipitation climatology of the western and central United States. However, very little work has focused on the temporal climatology, developmental characteristics, or climatic impacts of southwestern troughs. This study provides a detailed climatology of southwestern troughing that focuses on: (1) the temporal frequency of these events; (2) the teleconnective circulation changes that are associated with their development; and (3) the importance of these systems in the precipitation climatology of the western and central United States.

The temporal climatology of southwestern troughs reveals that most of these systems occur during spring and autumn, with somewhat fewer events in winter and very few events in summer. An examination of 500-mb geopotential height and 24-hr height change composites during trough development shows that much of the wave-train activity that accompanies trough onset is limited to the North Pacific and North American regions. These changes are characterized by the amplification and eastward movement of a ridge/trough couplet over the eastern Pacific, which is preceded by synoptic-scale transient wave activity over the western and central Pacific. While southwestern troughs occur less than 30% of the time, southwestern trough-derived precipitation comprises over 60% of the monthly totals for some sites.     

SPATIAL VARIABILITY OF MIDTROPOSPHERIC CIRCULATION PATTERNS AND ASSOCIATED SURFACE CLIMATE IN THE UNITED STATES DURING ENSO WINTERS

Numerous studies have documented that the Pacific/North American (PNA) pattern is the dominant extratropical response to ENSO forcing affecting the circulation over North America. However, the PNA is not the sole pattern that occurs during ENSO events. This study identifies the dominant synoptic circulation patterns and associated temperature and precipitation departures that occur during ENSO winters. Using standardized departures of 500 mbar heights over North America and the North Pacific Ocean, a subjective classification of the anomaly maps for winter months identified as warm ENSO events identifies three basic categories of 500 mbar standardized anomaly patterns: Variations of the PNA pattern, the reverse PNA pattern, and patterns with no PNA signature. Composite standardized anomaly maps of the synoptic categories of 500 mbar heights as well as composites of standardized temperature and precipitation departures for the contiguous United States were constructed. Three variations of the PNA, accounting for nearly half of the ENSO winters, are presented, identifying various configurations of the 500 mbar anomaly field and their effect on precipitation and temperature distribution. Similar composites are presented for reverse PNA and non-PNA winters. [Key words: climatology, climate change, El Nińo/Southern Oscillation, troposphere.]     

Avalanche Climatology of the Western United States,with an Emphasis on Alta,Utah*

Climate and snowpack characteristics of avalanches vary spatially across the western United States, distinguishing three regions. The coastal mountain renges have warmer, denser snow; interior (continental) ranges have colder, less-dense snow; and intermountain ranges have intermediate characteristics. Avalanche character of Alta, Utah, is related to eastern Pacific 700-mb height anomalies for December, January, and March, but not for February. Avalanche conditions around Alta do not always relate to localized pressure gradient winds for December and February.     

Lateral variations of S velocity in the upper mantle from higher Rayleigh modes

Summary. The generalized inverse theory has been applied to interpret several sets of higher mode data, previously obtained for the United States and the Pacific Ocean. The depth-resolving power of these data allows us to find the distribution of S velocity down to about 600 km. The main lateral variations of S velocity are found in the uppermost 250 km, the south-western United States showing the lowest velocities and the central-north-eastern United States the highest velocities. Between 250 and 500 km an opposite situation seems to occur, western velocities being the greatest ones, but these lateral variations are 3 to 5 times less than above and they cannot be surely established under the variance estimated for the data. Finally no lateral variations are resolved between 500 and 700 km. Some remarks may be made about the corresponding absolute models: (1) the agreement is good with published models, built with the fundamental mode alone; (2) the slight lowvelocity zone which is not required when inverting the fundamental mode alone in the central and north-eastern United States, is required when highermode data are added; (3) a rather strong increase of the S -velocity gradient is found near 360 km depth, both for the average data across the United States and the Pacific Ocean.     

北冰洋陆架的划界现状

北冰洋面积约1 300×10⁴ km²,周边国家对200 M（海里）大陆架及专属经济区的划界现状及对200 M外大陆架的诉求,对于我们参与北极科考具有十分重要的参考意义。大陆架与专属经济区的划界,美国与苏联已签署协定,美国国会已批准,但前苏联和俄罗斯未批准,这直接导致两国的海域争议不断。俄罗斯与挪威也已签署协议。美国与加拿大之间未签署协议,争议区超过7 000 M²。丹麦与加拿大已签署协议,完成划界,但存在汉斯岛的主权争议。挪威与丹麦及挪威与冰岛已完成划界。根据《联合国海洋法公约》第76条规定,北冰洋周边国家拥有确定北冰洋200 M外大陆架的权利。俄罗斯和挪威已提交200 M外大陆架划界方案,挪威对南森海盆西部的划界方案已得到大陆架界限委员会原则同意,但俄罗斯方案仍有很大的争议。作为《斯瓦尔巴条约》的签署国,斯瓦尔巴协定区,我们有权进入;因美国没有签署《联合国海洋法公约》,阿拉斯加北部大陆架及200 M外的楚科奇海台,都是我们进行科考的选择。此外,也可以通过国际合作,参与不同海域的科考和研究。     

SYNOPTIC CONTROL OF REGIONAL TEMPERATURE TRENDS IN THE COTERMINOUS UNITED STATES BETWEEN 1949 AND 1981

Regional patterns of January and mean annual temperature change between 1949 and 1981 are compared with regional changes in January precipitation totals and related to upper-level atmospheric circulation. During cold periods, cooling is concentrated in the eastern United States and is associated with a southward shift in the zone of maximum precipitation in the eastern United States (reflecting the mean position of the polar front jet), and with a strengthening of meridional flow aloft. During this same period, enhanced meridionality yields an increase in temperatures in the Pacific Northwest, associated with amplification of the mean long wave ridge over the Rocky Mountains. Warm periods in the eastern United States, conversely, correlate with a stronger zonal flow aloft (resulting in cooler, wetter conditions in the Pacific Northwest). Accordingly, temporal variation in regional temperature trends may be more readily interpreted as responses to synoptic controls which yield departures of opposite sign in locales influenced by ridges vs. troughs, rather than as a reflection of uniform, progressive hemispheric or global temperature change.     

MOISTURE STRESS,MID-TROPOSPHERIC PRESSURE PATTERNS,AND THE FOREST/GRASSLAND TRANSITION IN THE SOUTH CENTRAL STATES

The climatic causes of the major forest/grassland ecotone in the south central United States (Kansas, Missouri, Oklahoma, Texas) are still poorly understood. Grassland and forest vegetation types differ markedly in their ability to withstand water stress induced by vapor pressure deficit (VPD), the difference between saturation vapor pressure and actual vapor pressure in the atmosphere. VPD is an airmass characteristic induced by ambient temperatures higher than dewpoint temperature. Mean summer airmass movement is from the Gulf of Mexico onto the continent in the central states area, but mean VPD displays a strong gradient approximately parallel to the ecotone. A subset of days having the strongest VPD gradients across the ecotone also had a 500 mb pressure height pattern identical to the long-term mean (modal) pattern. This 500 mb pattern, with a ridge over the Rocky Mountains and a trough over the Great Lakes, induces subsidence, stability, warming, and high VPD in Great Plains airmasses. Farther east, away from the zone of maximum subsidence, VPD is much lower. The grassland region coincides with the area of highest VPD. Because of the importance of the daily configuration of mid-tropospheric windflow patterns in controlling the distribution of VPD, moisture, and precipitation, more frequent occurrence of the modal 500 mb pattern is one mechanism for the occurrence of drought.     

A SYNOPTIC CLIMATOLOGY OF MOISTURE STRESS GRADIENTS IN THE WESTERN GREAT LAKES REGION

Surface pressure distributions and 500 millibar flow patterns are identified for summer season days with a moisture stress gradient across the western Great Lakes region that may be related to the location of the prairie-forest ecotone. Results indicate that days with moisture stress gradients are a characteristic feature of the summer climate in the area and that these days are associated with unique, nonrandom groupings of mid-tropospheric flow patterns. SW-NE and W-E stress gradients, those that appear to contribute most to defining and reinforcing the location of the ecotone, occur most frequently, and the modal 500-mb flow direction for these transect groups, an azimuth of 290°, is the same as the mode for the entire population of summer season flow directions. We conclude that the modal mid-tropospheric circulation of the summer season plays a significant role in determining the surface weather conditions that regulate the most common moisture stress patterns within the western Great Lakes region.     

Spatial and Temporal Characteristics of Tornado Path Direction*

Common perception is that tornadoes travel in paths from the southwest quadrant of directions toward the northeast. This study examines path directions for 6,194 tornadoes that occurred in the eastern two‐thirds of the United States during the twenty‐three‐year period 1980–2002. At the national scale, nearly 70 percent of tornadoes included in the study propagated from the west, west‐southwest, and southwest, with west‐southwest being the highest frequency origin direction. Nevertheless, distinct seasonal and regional variations were found. In central and northern areas of the country, a more westerly or northwesterly path origin prevails during late spring and summer. The midtropospheric flow, convective typology, and synoptic patterns of tornado outbreaks are thought to contribute to the distributions observed in the climatology.     

SYNOPTIC-SCALE VARIABILITY IN THE PROBABILITY OF PRECIPITATION FROM THUNDERSTORMS IN THE UNITED STATES

A synoptic-scale climatology of precipitation amounts from thunderstorms was developed by analysis of amounts from individual storms for 220 stations in the conterminous United States for the period 1948-1977. The probability of having a thunderstorm without rainfall was assessed for each station. For storms which did produce precipitation, the probability distribution of amounts was found to be well summarized by the incomplete gamma distribution. Sets of seasonal maps of the probability of receiving any measurable amount, less than 4 mm, and more than 20 mm of precipitation are presented. Consistent spatial patterns are found. Thunder without precipitation is most likely in the west. The greatest probability of heavy precipitation occurs along the Gulf Coast, extending in the summer throughout the mid-section of the nation. Topographic effects are apparent, with mountainous areas generally having less intense precipitation than surrounding regions. [Key World: thunderstorms, precipitation probabilities, gamma distribution.]     

Deep seismic reflection evidence for the role of extension in the evolution of continental crust

Summary. Deep seismic reflection profiling, as well as geologic studies, indicate that extensional basins are a common feature of continental crust. The wide-spread occurrence of extensional basins, combined with published models on the effects of extension on lower crustal rocks, suggests that extensional processes play an important role in the evolution of continental crust. Extension is now recognized to have been the last major tectonic event to affect approximately 50% of the United States. This observation and (he preservation of extensional features in areas which have experienced subsequent episodes of compression, suggests that extensional processes may lead to a strengthening of continental crust. In areas of active extension such as the western United States, seismic and petrologic data, as well as theoretical modeling of heat flow data, suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present Mohorovicic discontinuity formed during extension. Although the interpretation of the various data is somewhat speculative, we suggest that the volume of continental material in some areas has doubled as a result of extension. Thus, extension may result in the addition of a significant amount of new material to the continents.     

美国中东部温带稳定≥10℃积温

应用修订的康拉德公式计算美国中东部温带地区日均温稳定≥10℃积温,并同我国相应纬度地区作了对比。这一对比揭示出我国东部温带并非世界同纬度地带夏半年热量资源最丰富。     

天山山系的雪崩危险区分布

根据山地生态特征，地形切割深度及河谷横剖面形成和雪崩重视率等，将天山山系雪崩危险区分为：１）低山草原或半荒漠雪崩危险区；２）中山森林－草原雪崩危险区；３）中山草地雪崩危险区；４）高山冰缘带（石质）雪崩危险区；５）高山；极高山冰川带雪崩危险区。     

Coal Waste,Socioeconomic Change,and Environmental Inequality in Appalachia: Implications for a Just Transition in Coal Country

Resource-dependent communities are subject to both unstable economic patterns and environmental degradation associated with extractive industries. However, few studies have empirically analyzed these dynamics in tandem and over time. This paper explores whether Appalachian neighborhoods closer to coal waste impoundments experienced steeper poverty changes from 1990 to 2000. Impoundment failures have resulted in some of the largest environmental disasters in U.S. history and scientists have expressed concerns over their growing heights and risks. Spatial regression models show that neighborhoods closer to impoundments have higher poverty increases on average, even after controlling for mining employment and other variables. These findings highlight the need for a deeper consideration of the environmental inequalities associated with the continued downturn of coal production and employment in the United States. Further, discussions about just transitions must recognize the legacy hazards and externalities associated with extractive industries – and their impacts on communities.     

Evolving United States metropolitan land use patterns

We investigate spatial patterns of residential and nonresidential land use for 257 United States metropolitan areas in 1990 and 2000, measured with 14 empirical indices. We find that metropolitan areas became denser during the 1990s but developed in more sprawl-like patterns across all other dimensions, on average. By far, the largest changes in our land use metrics occurred in the realm of employment, which became more prevalent per unit of geographic area, but less spatially concentrated and farther from the historical urban core, on average. Our exploratory factor analyses reveal that four factors summarize land use patterns in both years, and remained relatively stable across the two years: intensity, compactness, mixing, and core-dominance. Mean factor scores vary by metropolitan population, water proximity, type, and Census region. Improved measurement of metropolitan land use patterns can facilitate policy and planning decisions intended to minimize the most egregious aspects of urban sprawl.     

CLIMATOLOGY OF STATEWIDE EXTREME TEMPERATURES DURING WINTER MONTHS IN THE COTERMINOUS UNITED STATES

A 20-year (1961-1980) record of statewide extreme maximum and minimum temperatures for the winter season is examined for the coterminous United States. Extreme maxima exhibit a zonal pattern, modified somewhat by terrain features in the west. Extreme minima are more dramatically skewed from a zonal orientation, because of the effects of elevation and proximity to oceans. Interannual variability in the record of extreme temperatures appears to be dictated by proximity to air-mass source regions (e.g., low variability of minima in the Upper Midwest, low variability of maxima in the Deep South) and sensitivity to the position and amplitude of mid-tropospheric circulation and snowpack dynamics (e.g., high variability of minima in the Ohio Valley). Strong, spatially coherent patterns of correlation between statewide extremes and amplitude of midtropospheric flow are evident (negative correlation with enhanced troughing in the east; positive correlation with enhanced ridging in the west). Lag correlations with ENSO indices suggest a relatively weak linkage between antecedent conditions in the Pacific and the degree of extremeness of maximum (but not minimum) temperatures in the eastern United States. [Key words: extreme maximum temperatures, extreme minimum temperatures, climatic variability, meridionality indices, El Nino-Southern Oscillation, snowpack.]     

Trends in Interdiurnal Temperature Variation for the Central United States, 1945–1985

Temporal variation in the absolute value of interdiurnal variability (AIDV) of January maximum temperatures is examined for the period 1945–1985 in the central United States (90°–105°W). AIDV indicates the magnitude of day-to-day changes in the maximum temperature. Five-year running-mean AIDV values decreased significantly throughout the period. This trend was apparent for the region overall and for the 10 of the 20 study sites analyzed individually. Temporal variation in AIDV values was significantly related to mid-tropospheric flow patterns over the central United States. Meridional circulation was associated with smaller AIDV values in all sites except those in the northwestern and southeastern corners of the study area, whereas zonal flow was linked to larger interdiurnal temperature change. The analysis of selected individual years indicated that mid-tropospheric circulation patterns were more persistent with meridional flow, and surface cyclones and their associated surface fronts tracked through the study area less frequently than in years with zonal flow. Analysis at a daily temporal resolution revealed an added dimension of trends in temperature variability beyond those apparent with a coarser monthly resolution. These results emphasize the importance of finer temporal resolution in the analysis of both past and projected climate change; regionally summarized monthly temperature values mask important variation in temperature trends apparent at shorter temporal and more local spatial scales.     

SNOW-AVALANCHE HAZARDS IN GLACIER NATIONAL PARK,MONTANA: METEOROLOGIC AND CLIMATOLOGIC ASPECTS

Hazardous snow avalanches in Glacier National Park, Montana, are associated with a variety of meteorologic conditions: heavy snow; heavy snows followed by a rise in air temperature to above freezing; a rise in air temperature to above freezing, without precipitation; and rain in association with above-freezing air temperatures. Years of major, widespread avalanching may be recognized by examination of historical information and tree-ring data. Avalanche types include slab avalanches, wet snow avalanches, and dry loose snow avalanches. February is the peak avalanche month. Intraannual seasonalities of avalanche trigger mechanisms and type of avalanche are related. The presence of sun crusts in some cases provides unstable stratigraphic planes in the snowpack over which freshly deposited snow may glide. Destructive windblasts also occur in association with some avalanches. Insufficient data from east of the Continental Divide precluded a comparison of avalanche type and trigger mechanisms from the western and eastern portions of Glacier National Park. The general avalanche climate is more similar to that reported from the southern Canadian cordillera than to continental locations such as the mountains of Colorado. [Key words: Snow avalanches; avalanche trigger mechanisms; avalanche types; avalanche seasonality; Glacier National Park, Montana; northern Rocky Mountains; hazard planning.]     

THE MORPHODYNAMICS OF THE MONT BLANC MASSIF IN A CHANGING CRYOSPHERE: A COMPREHENSIVE REVIEW

One of the most glacierized areas in the European Alps, the Mont Blanc massif, illustrates how fast changes affect the cryosphere and the related morphodynamics in high mountain environments, especially since the termination of the Little Ice Age. Contrasts between the north‐west side, gentle and heavily glaciated, and the south‐east side, steep and rocky, and between local faces with varying slope angle and aspect highlight the suitability of the study site for scientific investigations. Glacier shrinkage is pronounced at low elevation but weaker than in other Alpine massifs, and supraglacial debris covers have developed over most of the glaciers, often starting in the nineteenth century. Lowering of glacier surface also affects areas of the accumulation zone. While modern glaciology has been carried out in the massif for several decades, study of the permafrost has been under development for only a few years, especially in the rock walls. Many hazards are related to glacier dynamics. Outburst flood from englacial pockets, ice avalanche from warm‐based and cold‐based glaciers, and rock slope failure due to debuttressing are generally increasing with the current decrease or even the vanishing of glaciers. Permafrost degradation is likely involved in rockfall and rock avalanche, contributing to the chains of processes resulting from the high relief of the massif. The resulting hazards could increasingly endanger population and activities of the valleys surrounding the Mont Blanc massif.