The long‐term (1935–1999) monthly records of temperature, precipitation, stream flow, river ice thickness, and active layer depth have been analyzed in this study to examine Lena River hydrologic regime and recent change. Remarkable hydrologic changes have been identified in this study. During the cold season (October–April), significant increases (25–90%) in stream flow and decrease in river ice thickness have been found due to warming in Siberia. In the snowmelt period (May–June), strong warming in spring leads to an advance of snowmelt season into late May and results in a lower daily maximum discharge in June. During summer months (July–September) the changes in stream flow hydrology are less significant in comparison to those for winter and spring seasons. A slight stream flow increase is discovered for both July and August, mainly owing to precipitation increase in May and June. Discharge in September has a slight downward trend due to precipitation decrease and temperature increase in August. The magnitudes of changes in stream flow and river ice thickness identified in this study are large enough to alter the hydrologic regime. Investigation into the hydrologic response of the Lena River to climate change and variation reveals strong linkages of stream flow with temperature and precipitation. We therefore believe that Lena River hydrologic regime changes are mainly the consequence of recent climate warming over Siberia and also closely related to changes in permafrost condition.

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 (T max , T min , and T avg ) 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 T max 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 T min and T avg 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 Ϫ0.65°C (100 m) Ϫ1] is solely applicable to maximum temperature and often grossly overestimates T min and T avg lapse rates. Regional lapse rates perform better than the environmental lapse rate for T min and T avg , 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.

[1] Changes in active layer thickness (ALT) over northern high-latitude permafrost regions have important impacts on the surface energy balance, hydrologic cycle, carbon exchange between the atmosphere and the land surface, plant growth, and ecosystems as a whole. This study examines the 20th century variations of ALT for the Ob, Yenisey, and Lena River basins. ALT is estimated from historical soil temperature measurements from 17 stations , Lena basin only), an annual thawing index based on both surface air temperature data and numerical modeling . The latter two provide spatial fields. Based on the thawing index, the long-term average ALT is about 1.87 m in the Ob, 1.67 in the Yenisey, and 1.69 m in the Lena basin. Over the past several decades, ALT over the three basins shows positive trends, but with different magnitudes. Based on the 17 stations, ALT increased about 0.32 m between 1956 and 1990 in the Lena. To the extent that results based on the soil temperatures represent ground ''truth,'' ALT obtained from both the thawing index and numerical modeling is underestimated. It is widely believed that ALT will increase with global warming. However, this hypothesis needs further refinement since ALT responds primarily to summer air temperature while observed warming has occurred mainly in winter and spring. It is also shown that ALT exhibits complex and inconsistent responses to variations in snow cover.Citation: Zhang, T., et al. (2005), Spatial and temporal variability in active layer thickness over the Russian Arctic drainage basin,

We analyzed rain‐on‐snow (ROS) events in two reanalysis products. ROS events are a relatively rare phenomenon outside of a few regional maxima including western Eurasia, the higher elevations of western North America, the northeastern United States, and southeastern Canada. ROS events occur at the high latitudes, especially away from the continental interior, and no robust trends were found in the frequency of ROS events. We also explored the variability of ROS events with dominant large climate modes. The most robust relationship was found with the Arctic Oscillation or North Atlantic Oscillation (AO/NAO). The most notable variability associated with the AO/NAO was a northeast/southwest dipole feature across western Eurasia. More ROS events were found for northeastern Europe for the positive phase of the AO/NAO due to the increased frequency of rainfall. However, more ROS events were found for Central Europe for the negative phase of the AO/NAO due to the increased frequency of snow cover.

No abstract

Atmospheric rivers (ARs) are narrow, long, transient, water vapor-rich corridors of the atmosphere that are responsible for over 90% of the poleward water vapor transport in and across midlatitudes. However, the role of ARs in modulating extratropical and polar hydroclimate features (e.g., water vapor content and precipitation) has not been fully studied, even though moistening of the polar atmosphere is both a key result and amplifier of Arctic warming and sea ice melt, and precipitation is key to the surface mass balance of polar sea ice and ice sheets. This study uses the Modern-Era Retrospective analysis for Research and Applications, Version 2 reanalysis to characterize the roles of AR water vapor transport on the column-integrated atmospheric water vapor budget in the extratropical and polar regions of both hemispheres. Meridional water vapor transport by ARs across a given latitude (examined for 40°, 50°, 60°, and 70°) is strongly related to variations in area-averaged (i.e., over the cap poleward of the given latitude) total water vapor storage and precipitation poleward of that latitude. For the climatological annual cycle, both AR transport (i.e., nonlocal sources) and total evaporation (i.e., local sources) are most correlated with total precipitation, although with slightly different phases. However, for monthly anomalies, the water budget at higher latitudes is largely dominated by the relationship between AR transport and precipitation. For pentad and daily anomalies, AR transport is related to both precipitation and water vapor storage variations. These results demonstrate the important role of episodic, extreme water vapor transports by ARs in modulating extratropical and polar hydroclimate. Plain Language SummaryThe term atmospheric river (AR) was coined by scientists Zhu and Newell in the early 1990s with the main result highlighting the importance of relatively infrequent, long conduits of strong moisture transport being responsible for most of the poleward transport of moisture across the midlatitudes and into the polar regions. While it is generally understood that this moisture is critical to the water and energy budgets of high latitudes, there have been no studies that have ever quantified the relationship between AR poleward moisture transports and the hydroclimate features of high latitudes. After a long hiatus in the consideration of the role of ARs on global climate since those of Zhu and Newell, this study quantifies the connections between water vapor transport by ARs across specific latitudes (e.g., 40°) and the hydroclimate poleward of this latitude. The findings show there are strong, time scale-dependent (e.g., daily and monthly) connections between ARs and high-latitude hydroclimate features. For example, the findings show a strong relationship between AR water vapor transport at a given latitude and the areaaveraged total precipitation of the region poleward. This and other results in this study indicate the importance of ARs in shaping our global weather and climate.

Abstract:This study examines the characteristics of winter (Dec-Feb) rain-on-snow events and their relationship to surface air temperatures to reveal potential changes in rain-on-snow days under a warming climate over northern Eurasia. We found that rain-on-snow events mostly occur over European Russia during winter. Rain-on-snow days increase as air temperature increases and are primarily attributable to the increase in rainfall days. Air temperature is the primary cause for these changes, while the North Atlantic Oscillation has some influence on the rain on snow and rainfall over the northern part of European Russia. The magnitude of rain-on-snow increase ranges from 0Ð5 day to 2Ð5 days per degree Celsius increase in air temperature. Higher rates of increase in rain-on-snow days occur in the northern and eastern parts of European Russia where the air temperature is lower, in contrast to rainfall days which have higher rates at locations with higher air temperatures. This suggests that a decrease in snowfall days might be limiting the rate of increase in rain-on-snow events over warmer regions where the temperature is about 8°C or higher. This study also implies that rain-on-snow days will become more common over regions in which it is currently a rare event as air temperatures increase.

Daily synoptic observations were examined to determine the critical air temperatures and dewpoints that separate solid versus liquid precipitation for the fall and spring seasons at 547 stations over northern Eurasia. The authors found that critical air temperatures are highly geographically dependent, ranging from 21.08 to 2.58C, with the majority of stations over European Russia ranging from 0.58 to 1.08C and those over southcentral Siberia ranging from 1.58 to 2.58C. The fall season has a 0.58-1.08C lower value than the spring season at 42% stations. Relative humidity, elevation, the station's air pressure, and climate regime were found to have varying degrees of influences on the distribution of critical air temperature, although the relationships are very complex and cannot be formulated into a simple rule that can be applied universally. Although the critical dewpoint temperatures have a spread of 21.58 to 1.58C, 92% of stations have critical values of 0.58-1.08C. The critical dewpoint is less dependent on environmental factors and seasons. A combination of three critical dewpoints and three air temperatures is developed for each station for spring and fall separately that has improved snow event predictability when the dewpoint is in the range of 20.58-1.58C and has improved rainfall event predictability when the dewpoint is higher than or equal to 08C based on the statistics of all 537 stations. Results suggest that application of site-specific critical values of air temperature and dewpoint to discriminate between solid and liquid precipitation is needed to improve snow and hydrological modeling at local and regional scales.