Hengchun Ye

The Changes in Russian Winter Snow Accumulation during 1936–83 and Its Spatial Patterns

Abstract Winter snow depth observations from 119 Russian stations during the years 1936–83 are selected. These irregularly spaced station data are then interpolated into 220 regular grids of 2° lat × 5.24° long that cover a region of 50°–70°N, 30°–140°E. The spatial variation patterns of the annual Russian winter snow accumulation during the period of 1936–83 are identified by using principal components analyses. Statistically significant trends in major snow depth variation patterns are detected. A method is constructed to estimate the spatial distributions of the total amount of snow depth change based on the significant trends of component scores during the period of 1936–83. The study found that snow depth has increased over most of northern Russia and decreased over most of southern Russia during the study period. Exceptions are found in northern European Russia, where a slight decrease in snow depth has occurred and in southern west Siberia where the snow depth has increased. The total amount of sno...

Seasonal and Synoptic Variations in Near-Surface Air Temperature Lapse Rates in a Mountainous Basin

To accurately estimate near-surface (2 m) air temperatures in a mountainous region for hydrologic prediction models and other investigations of environmental processes, the authors evaluated daily and seasonal variations (with the consideration of different weather types) of surface air temperature lapse rates at a spatial scale of 10 000 km 2 in south-central Idaho. Near-surface air temperature data (Tmax, Tmin, and Tavg) from 14 meteorological stations were used to compute daily lapse rates from January 1989 to December 2004 for a medium-elevation study area in south-central Idaho. Daily lapse rates were grouped by month, synoptic weather type, and a combination of both (seasonal–synoptic). Daily air temperature lapse rates show high variability at both daily and seasonal time scales. Daily Tmax lapse rates show a distinct seasonal trend, with steeper lapse rates (greater decrease in temperature with height) occurring in summer and shallower rates (lesser decrease in temperature with height) occurring in winter. Daily Tmin and Tavg lapse rates are more variable and tend to be steepest in spring and shallowest in midsummer. Different synoptic weather types also influence lapse rates, although differences are tenuous. In general, warmer air masses tend to be associated with steeper lapse rates for maximum temperature, and drier air masses have shallower lapse rates for minimum temperature. The largest diurnal range is produced by dry tropical conditions (clear skies, high solar input). Cross-validation results indicate that the commonly used environmental lapse rate [typically assumed to be 0.65°C (100 m) 1 ] is solely applicable to maximum temperature and often grossly overestimates Tmin and Tavg lapse rates. Regional lapse rates perform better than the environmental lapse rate for Tmin and Tavg, although for some months rates can be predicted more accurately by using monthly lapse rates. Lapse rates computed for different months, synoptic types, and seasonal–synoptic categories all perform similarly. Therefore, the use of monthly lapse rates is recommended as a practical combination of effective performance and ease of implementation.

The impacts of weather and pollution on human mortality in Birmingham, Alabama and Philadelphia, Pennsylvania.

Past studies have examined how both extreme weather and atmospheric pollutants influence human mortality. However, the differential and/or synergistic impacts of weather and pollution on mortality are poorly understood. This relationship is particularly notable in summer, when both extreme weather and high pollution episodes are frequent. The goal of this study is to describe the relationship between atmospheric conditions (as characterized by weather and air pollution) and mortality in the summer season for Birmingham, Alabama and Philadelphia, Pennsylvania. To assess the health impacts of both weather and pollution (ozone and total suspended particulates, or TSP), we used a synoptic, or air mass-based, approach to take into account the entire weather situation, rather than individual weather elements. This method was used to identify ‘offensive’ air masses, which are associated with elevated mortality, and then to investigate which individual or combination of atmospheric conditions poses the greatest threat to human health in terms of acute (daily) mortality. In both cities, offensive weather events have a greater impact on acute mortality than high concentrations of TSP or ozone. The highest mortality levels occur when the hottest, but not the most polluted, air mass is present in each city. In Philadelphia, neither TSP nor ozone appear to contribute an ‘add on’ effect to weather-related mortality. Under non-offensive weather situations, pollution concentrations are associated with increased mortality in Philadelphia. Yet, regardless of pollution concentration, mortality levels are much lower for these air masses than for offensive events. Pollution appears to be more important for mortality in Birmingham. Although Birminghams high-mortality (offensive) air mass is not the most polluted, offensive air mass days with high pollution concentrations nonetheless exhibit higher mean mortality than offensive air mass days with low pollution concentrations. Also different from Philadelphia is a lack of a relationship in Birmingham between pollution levels and mortality on non-offensive air mass days. The relationship between summer weather and mortality is strong in both cities, while the role of pollution is less clear. This research underscores the imperative need for the development of a weather/health watch-warning system to alert the public that an offensive synoptic situation is imminent. Copyright

Observational evidence of the influence of Pacific SSTs on winter precipitation and spring stream discharge in Idaho

Abstract Forty years of winter precipitation (23 stations) and spring stream flow discharge records (five stations) from across Idaho are analyzed to reveal regional patterns of association with sea surface temperatures (SSTs) in the Pacific Ocean. Results indicate that winter precipitation in the northern Idaho mountains, between 45° and 48°N, is negatively correlated with fall SSTs in the eastern tropical Pacific Ocean (El Nino and La Nina). Winter precipitation north of 45°N, is negatively correlated with winter SSTs in the northern Pacific (Pacific Decadal Oscillation, PDO). Spring stream discharge in Idaho is also negatively correlated with SSTs in the eastern tropical and northern regions of the Pacific Ocean. The association is asymmetric with stronger responses during negative SSTs for both regions in the Pacific Ocean. Wet and dry conditions are most likely associated with the combination of La Nina–negative PDO and El Nino–positive PDO, respectively. The greatest anomalies occur during the optimal combination of La Nina with negative PDO conditions. The revealed connections are valuable for climatic predictions based on the previous seasons SST conditions in the eastern tropical Pacific and slowly evolving SSTs in the northern Pacific Ocean.

Quasi-Biennial and Quasi-Decadal Variations in Snow Accumulation over Northern Eurasia and Their Connections to the Atlantic and Pacific Oceans

Abstract Spatial and temporal characteristics of winter snow depth variation over northern Eurasia and their connections to sea surface temperatures (SSTs) and associated atmospheric circulation anomalies, surface air temperatures, and precipitation are examined by using 60 yr (1936–95) of station data records. This study found that snow depth variation over the region east of the Caspian Sea and west of China, explaining 10.1% of total snow depth variance, has a quasi-biennial variability of about 2.5 yr. The snow depth variation over central European Russia and western-central Siberia, explaining 8.1% of the total snow depth variance, has a quasi-decadal variability of about 11.8 yr. The snow depth variation over the northern Ural Mountains, explaining 7.5% of the total snow depth variance has, variability of about 8 and 14 yr. The quasi-biennial snow depth variation is associated with SSTs over the northern North Pacific and tropical western Atlantic extending into the Gulf of Mexico. The associated at...

Divergence in seasonal hydrology across northern Eurasia: Emerging trends and water cycle linkages

[1] Discharge from large Eurasia rivers increased during the 20th century, yet much remains unknown regarding details of this increasing freshwater flux. Here, for the three largest Eurasian basins (the Ob, Yenisei, and Lena) we examine the nature of annual and seasonal discharge trends by investigating the flow changes along with those for precipitation, snow depth, and snow water equivalent. On the basis of a multiperiod trend analysis and examination of station data, we propose two characteristic regimes to explain the long-term discharge increase from these large Eurasian rivers. Over the early decades from approximately 1936 to 1965, annual precipitation correlates well with annual discharge, and positive discharge trends are concurrent with summer/fall discharge increases. The latter decades were marked by a divergence between winter/ spring flows, which increased, amid summer/fall discharge declines. A comparison of cold season precipitation (CSP) and spring discharge trends across subbasins of the Ob, Yenisei, and Lena shows limited agreement with one precipitation data set but good agreement (R 2 > 0.90) when a second is used. While natural variability in the Arctic system tends to mask these emerging trends, spatial and temporal changes can generally be characterized by increased solid precipitation, primarily to the north, along with a drier hydrography during the warm season.

Decadal variability of Russian winter snow accumulation and its associations with Atlantic sea surface temperature anomalies.

Russian winter snow depth data over a 48-year period (1936–1983) are analysed to reveal variation characteristics and associations to Atlantic sea surface temperature (SST) anomalies using methods of rotated principal component analysis (RPCA), singular spectrum analysis (SSA), and singular value decomposition (SVD) analysis. The study demonstrates that four time scales (4 years interannual, 11.8 years quasidecadal, 20 years bi-decadal and trend) characterize Russian winter snow depth variations. The decadal and longer time scale variations are found to be significantly associated with Atlantic SST anomalies. The trend, which occurred over much of the study region, is associated with SST trends over the northern north and tropical south Atlantic. Bi-decadal snow depth variation over central Siberia is associated with western tropical north Atlantic SSTs. A quasi-decadal variation over western European Russia is connected to a major Atlantic SST variation pattern of opposite signs over alternative latitudinal belts. This study suggests that the connections between the Atlantic Ocean and regional climate may be better reflected at decadal time scales than interannual and seasonal ones, as the dominant variability over the ocean is at slow modes. Copyright

POLAR SNOW COVER CHANGES AND GLOBAL WARMING

Many general circulation models suggest that current precipitation amounts in polar latitudes will increase under double CO 2 scenarios. Even though temperatures in such high-latitude regions should also increase under a doubling of CO2, as long as those temperatures remain below freezing, the increased precipitation should accumulate as snow. A study of both current and double CO2 temperature and precipitation data for all land areas poleward of 60 latitude using three different general circulation models suggests possible changes in snow accumulation due to increasing CO2. Increased snow accumulation will occur in the Antarctic whereas a small decrease in snow depth is to be expected in the Northern Hemisphere. Total snow accumulation for all land areas poleward of latitude 60 is found to increase under a double CO2 scenario. Many of the general circulation models (GCMs) that have been used to estimate climatic responses to increasing concentrations of CO2 and other trace gases in the atmosphere suggest a marked warming in polar latitudes. Although the GCMs do not provide direct information on snow cover or even snowfall, available polar warming scenarios have been cited as evidence to suggest increased melting of polar ice masses. The present study investigates temperature and precipitation conditions in the polar regions under conditions of a doubling of CO 2 using three readily available GCMs to provide some understanding of the possible range of future estimates of snow accumulation. Evaluating snowfall amounts under current and GCM simulated conditions permits an estimate of snow depth change under possible global warming scenarios. In 1934, Sir George Simpson postulated a unique theory for the cause of ice ages in the Pleistocene that required two fairly uniform increases in solar radiation with a period of decreased radiation between the radiation maxima. He theorized that a gradual rise in solar radiation would be accompanied by a gradual rise in air temperature. This, in turn, would result in an increase in precipitation, which would fall as snow in the polar regions. The increased precipitation would result from increased evaporation (due to the warmer water temperatures) and the increased water vapour capacity of the warmer air. He reasoned that as long as air temperatures in polar regions remained below freezing the increased precipitation would fall as snow and accumulate as permanent snow fields on the ground. This would lead to advancing ice sheets and a glacial period. As temperatures finally increased too much, more precipitation would fall as rain rather than snow and, at the same time, there would be increased melting of continental ice sheets, leading to their retreat and ultimate disappearance. Although Simpsons theory linking ice-age occurrence with an increase in solar radiation and air temperature has not survived the test of time, his recognition that an increase in air temperature might lead to increased snowfall in polar regions deserves further evaluation. Thomas (1986) in identifying three major processes relating global warming to average sea-level change, suggested that although two of the processes would lead to sea-level rise (increased ice melting and thermal expansion of sea water), one process might lead to a sea-level fall. That process was increased snowfall on polar

Characteristics of Winter Precipitation Variation over Northern Central Eurasia and Their Connections to Sea Surface Temperatures over the Atlantic and Pacific Oceans

Abstract This study reveals spatial and temporal characteristics of precipitation variability and their teleconnections to sea surface temperatures (SSTs) over the Atlantic and Pacific Oceans by analyzing 68 yr of recent precipitation records over the former Soviet Union. In addition to a general increasing trend of about 0.4 mm yr−1 or 6% decade−1 over much of the study region, three major modes of precipitation variation are identified. A quasi-biennial variation of 2–3 yr is found over the region surrounding the Caspian and Aral Seas. An El Nino timescale precipitation variation of 4–5 yr is present over southern central Siberia and is associated with eastern tropical Pacific SSTs. A quasi-decadal timescale variation of about 11–12 yr is evident over central European Russia. This quasi-decadal precipitation variation is closely linked to a major SST anomaly pattern of alternating latitudinal belts over the Atlantic and SST variations over the equatorial Pacific Ocean. These associations to SSTs are str...

Increases in snow season length due to earlier first snow and later last snow dates over north central and northwest Asia during 1937–94

Trends of the first and last snow date and the length of snow season, defined by the time period between first and last snow date, during 1937–94 over northern Asia are examined. The length of snow season over North Central and Northwest Asia has increased by about 4 days per decade during the study period. The increased length of the snow season is due to earlier snowfalls at the beginning of the season and to a lesser extent to later last snowfalls at the end of the season.