Excessive Snow Research Articles

Structural failures due to snow in Japan

About half the area of Japan is subject to annual continuous heavy snow. Approximately 15% of the population lives in the area. In this paper, snow-induced failure and structural design of building roofs in Japan are reviewed. First, an overview of the characteristics of snowfall and structural design for snow load in Japan is given. Then, the snowstorm of 2014 is reported, which hit a region without snow zone designation in the design standard and caused many structural failures along with 26 deaths. The snow load for flat roofs for a general area where heavy snow is not expected was increased following this event. Investigation of the roof failure of a power plant due to compacted snow in 2015 is also reported. Compaction of snow due to melting in earlier winter caused excessive snow density compared to what had been expected, while judgement of snow-removal operation was based on a visual evaluation of snow depth. After these events, real-time estimation of snow load has been developed and implemented for a wide area using meteorological observation. Also, a simple snow-load-estimation method based on snow depth measurement is developed to assist judgement of a specific plant.

Scale‐Aware and Definition‐Aware Evaluation of Modeled Near‐Surface Precipitation Frequency Using CloudSat Observations

AbstractCloudSat's 94‐GHz Cloud Profiling Radar provides unique near‐global observations of precipitation frequency and intensity. Here CloudSat‐based diagnostics for near‐surface precipitation frequency are implemented in publicly available software that is widely used for climate model evaluation. The new diagnostics are “definition aware” and “scale aware.” As a result, the diagnostics enable robust assessment of modeled near‐surface precipitation frequency at a range of intensity classes. The new diagnostics are used to evaluate precipitation frequency in a state‐of‐the‐art climate model, the Community Earth System Model version 1 (CESM1). CESM1 rains and snows too frequently, a bias that is especially pronounced for light rain. Conversely, while rare in both observations and CESM1, the heaviest rainfall events occur too infrequently in CESM1. Though the spatial distribution of snowfall events matches observations well, CESM1 also exhibits excessive snow frequency biases. Despite these biases, projected CESM1 changes in reflectivity‐based diagnostics provide interesting insights into what a future 94‐GHz radar could detect in a warmer world. With 3 °C of global warming, a future CloudSat‐class mission would detect substantial conversion of snow to rain at midlatitudes, a narrowing of the Tropical Pacific rain belt, increased light rain in subtropics, and increased snow frequency in polar regions. The future CESM1 simulations also provide evidence that present‐day spatial and magnitude biases imprint themselves on precipitation frequency changes. In summary, new precipitation frequency diagnostics for a range of precipitation intensities robustly expose climate model biases and inform expectations for observable future precipitation changes in a warming world.

How autumn Eurasian snow anomalies affect east asian winter monsoon: a numerical study

Previous studies have found that snow Eurasian anomalies in autumn can affect East Asian winter monsoon (EAWM), but the mechanisms remain controversial and not well understood. The possible mechanisms by which Eurasian autumn snow anomalies affect EAWM are investigated by numerical experiments with a coupled general circulation model and its atmospheric general circulation model component. The leading empirical orthogonal function mode of the October–November mean Eurasian snow cover is characterized by a uniform anomaly over a broad region of central Eurasia (40°N–65°N, 60°E–140°E). However, the results from a 150-ensemble mean simulation with snow depth anomaly specified in October and November reveal that the Mongolian Plateau and Vicinity (MPV, 40°–55°N, 80°–120°E) is the key region for autumn snow anomalies to affect EAWM. The excessive snow forcing can significantly enhance EAWM and the snowfall over the northwestern China and along the EAWM front zone stretching from the southeast China to Japan. The physical process involves a snow-monsoon feedback mechanism. The excessive autumn snow anomalies over the MPV region can persist into the following winter, and significantly enhance winter snow anomalies, which increase surface albedo, reduce incoming solar radiation and cool the boundary layer air, leading to an enhanced Mongolian High and a deepened East Asian trough. The latter, in turn, strengthen surface northwesterly winds, cooling East Asia and increasing snow accumulation over the MPV region and the southeastern China. The increased snow covers feedback to EAWM system through changing albedo, extending its influence southeastward. It is also found that the atmosphere–ocean coupling process can amplify the delayed influence of Eurasian snow mass anomaly on EAWM. The autumn surface albedo anomalies, however, do not have a lasting “memory” effect. Only if the albedo anomalies are artificially extended into December and January, will the EAWM be affected in a similar way as the impacts of autumn snow mass anomalies.

High-Resolution Interannual Mass Anomalies of the Antarctic Ice Sheet by Combining GRACE Gravimetry and ENVISAT Altimetry

Knowledge of interannual mass variations of the Antarctic ice sheet (AIS) associated with its surface mass change is important for correctly interpreting the long-term mass trend and evaluating the fidelity of surface mass balance from regional climate models. Here, we revisit the interannual anomalies of mass change from Gravity Recovery and Climate Experiment and elevation change from ENVISAT over the AIS during 2003–2009, with the objective of obtaining higher resolution interannual mass anomalies based on ENVISAT data. High positive correlations (>0.6) between the two interannual anomalies are primarily found over the west AIS and coastal regions in the east AIS, occupying more than 40% of the AIS. By combining the two interannual anomalies, we are able to estimate the density of snow/ice changing interannually over regions in the AIS. Especially over the Amundsen Sea sector with significant interannual signals, the temporal variability of the density of snow/ice associated with interannual anomalies is shown for the first time, which agrees with the events of excess snow accumulation and the accelerated ice discharge occurring there. Furthermore, we demonstrate the feasibility of obtaining higher resolution interannual mass anomalies over the west AIS, based on the density-corrected ENVISAT data. Negative correlations, which were also found in a previous study, are likely related to errors in the relatively weak interannual signals.

Understanding the surface temperature cold bias in CMIP5 AGCMs over the Tibetan Plateau

The temperature biases of 28 CMIP5 AGCMs are evaluated over the Tibetan Plateau (TP) for the period 1979–2005. The results demonstrate that the majority of CMIP5 models underestimate annual and seasonal mean surface 2-m air temperatures (Tas) over the TP. In addition, the ensemble of the 28 AGCMs and half of the individual models underestimate annual mean skin temperatures (Ts) over the TP. The cold biases are larger in Tas than in Ts, and are larger over the western TP. By decomposing the Ts bias using the surface energy budget equation, we investigate the contributions to the cold surface temperature bias on the TP from various factors, including the surface albedo-induced bias, surface cloud radiative forcing, clear-sky shortwave radiation, clear-sky downward longwave radiation, surface sensible heat flux, latent heat flux, and heat storage. The results show a suite of physically interlinked processes contributing to the cold surface temperature bias. Strong negative surface albedo-induced bias associated with excessive snow cover and the surface heat fluxes are highly anticorrelated, and the cancelling out of these two terms leads to a relatively weak contribution to the cold bias. Smaller surface turbulent fluxes lead to colder lower-tropospheric temperature and lower water vapor content, which in turn cause negative clear-sky downward longwave radiation and cold bias. The results suggest that improvements in the parameterization of the area of snow cover, as well as the boundary layer, and hence surface turbulent fluxes, may help to reduce the cold bias over the TP in the models.

高寒草甸地下根系生长动态对积雪变化的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 高寒草甸地下根系生长动态对积雪变化的响应 DOI: 10.5846/stxb201605240995 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点基础研究发展计划资助项目（2013CBA01807）；国家自然科学基金项目（31370542）；中央高校基本科研业务专项资金项目（2014NZYTD01）；西南民族大学创新型科研项目（CX2016SZ073） Response of belowground root growth dynamics to snow cover change in alpine meadow Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:2013年11月至2014年8月在青藏高原东缘红原县高寒草甸通过人工堆积的方法，进行了积雪量野外控制试验。以自然降雪的积雪量为对照（CK），设置了S1、S2和S3（积雪量分别为自然对照的2倍、3倍和4倍）3个处理，运用微根窗法追踪研究了积雪量改变后高寒草甸植被根系生长动态，并测定了积雪变化对土壤温度的影响。结果表明：高寒草甸植被根系生长存在明显的季节性变化，随着时间的推移，根系表面积、根尖数量及现存量逐渐增加并在8-9月达到最大值；当冬季积雪量达到143.4 mm（S1），对根系生长最为有利（根系表面积、根尖数量、现存量及生产量最大），根系生长旺盛期（净生产速率较高）有所提前和延长，但随着积雪量进一步增加，积雪对根系生长的正效应逐渐降低，根系生长旺盛期逐渐推迟甚至消失；研究还发现，随着积雪量增加，0-10 cm土层土壤温度逐渐降低，相似的变化规律也出现在10-20 cm土层，但在时间上有所延迟；相关性分析表明，在不同土层中，根系生长与土壤温度均呈正相关。因此，积雪变化通过改变土壤温度影响高寒草甸植物根系的生长发育，最终可能会影响高寒草甸生态系统的碳分配与碳循环过程。 Abstract:Permanent or seasonal snow cover is widespread on the Tibetan Plateau. Seasonal snow cover, which is affected by global climate change, is dominant in the northwestern part of Sichuan Province, on the eastern edge of the Tibetan Plateau. The root system is sensitive to environmental change. The effects of seasonal snow accumulation and thawing on soil physicochemical properties and microorganisms change could influence the root system in alpine meadow. Understanding the environmental effect on the root system of alpine meadow is essential to better understand the response mechanisms of terrestrial ecosystems to global climate change. From November 2013 to August 2014, we conducted the field study on controlled snow cover gradients in an alpine meadow of northwestern Sichuan. Four snow cover gradients were established: natural snowpack (control), and snowpack manipulated to be 2-, 3-, and 4-folds that of control. The root growth dynamics of plant communities and their responses to snow cover gradients were continuously monitored using the Minirhizotron method. The effects of the snow cover gradients on soil temperature were also measured. The results showed that soil temperature decreased with increasing snowpack volume. The optimum root system growth was observed with the natural snowpack. The root growth period was lower in the doubled natural snowpack compared with other treatments at the beginning, but the final root growth of was faster than that of the control. Belowground root system growth was restricted in the tripled snowpack. Correlation analysis showed that the root growth was positively correlated with soil temperature. Our results indicate that the winter snowpack change directly impacts subsurface ice storage, which in turn affects hydrothermal regimes in the alpine meadow soil and thus root system growth. Belowground root systems may suffer damage when the melt water refreezes underground after infiltrating into the soil and undergoes subsequent freeze-thaw cycles. Overall, moderately increased snowfall is conducive to the growth of the belowground root system, but excessive snow inhibits the belowground root system. Therefore, change in winter snowpack could alter carbon distribution and the carbon cycle in alpine meadow ecosystem. 参考文献 相似文献 引证文献

Summer monsoon rainfall variability over North East regions of India and its association with Eurasian snow, Atlantic Sea Surface temperature and Arctic Oscillation

This observational study during the 29-year period from 1979 to 2007 evaluates the potential role of Eurasian snow in modulating the North East-Indian Summer Monsoon Rainfall with a lead time of almost 6 months. This link is manifested by the changes in high-latitude atmospheric winter snow variability over Eurasia associated with Arctic Oscillation (AO). Excessive wintertime Eurasian snow leads to an anomalous cooling of the overlying atmosphere and is associated with the negative mode of AO, inducing a meridional wave-train descending over the tropical north Atlantic and is associated with cooling of this region. Once the cold anomalies are established over the tropical Atlantic, it persists up to the following summer leading to an anomalous zonal wave-train further inducing a descending branch over NE-India resulting in weak summer monsoon rainfall.

Experimental Assimilation of the GPM Core Observatory DPR Reflectivity Profiles for Typhoon Halong (2014)

Abstract Space-based precipitation radar data have been underused in data assimilation studies and operations despite their valuable information on vertically resolved hydrometeor profiles around the globe. The authors developed direct assimilation of reflectivities (Ze) from the Dual-Frequency Precipitation Radar (DPR) on board the Global Precipitation Measurement (GPM) Core Observatory to improve mesoscale predictions. Based on comparisons with Ze observations, this cloud resolving model mostly reproduced Ze but produced overestimations of Ze induced by excessive snow with large diameter particles. With an ensemble-based variational scheme and preprocessing steps to properly treat reflectivity observations including conservative quality control and superobbing procedures, the authors assimilated DPR Ze and/or rain-affected radiances of GPM Microwave Imager (GMI) for the case of Typhoon Halong in July 2014. With the vertically resolving capability of DPR, the authors effectively selected Ze observations most suited to data assimilation, for example, by removing Ze above the melting layer to avoid contamination due to model bias. While the GMI radiance had large impacts on various control variables, the DPR made a fine delicate analysis of the rain mixing ratio and updraft. This difference arose from the observation characteristics (coverage width and spatial resolution), sensitivities represented in the observation operators, and structures of the background error covariance. Because the DPR assimilation corrected excessive increases in rain and clouds due to the radiance assimilation, the combined use of DPR and GMI generated more accurate analysis and forecast than separate use of them with respect to the agreement of observations and tropical cyclone position errors.

Interannual Variability of Snow Water Equivalent (SWE) over Western Himalayas

Considering the importance of snow and glaciers in the Himalayas for understanding the water cycle and for water resource management of the rivers originating from the Himalayan, interannual variability of snow accumulation process over Himalayas and surrounding region has been studied using snow water equivalent (SWE) data. Remote sensing data from National Snow and Ice Data Centre have been used. These data have been compared against ground (in situ) observations of SWE measured at several gauge stations in the Indian part of the Satluj River basin. Accumulated SWE from remote sensing data and ground observations in the Satluj River basin have good and significant correlation. These data have also been compared against the Climate Forecast System Reanalysis and the European Centre for Medium Range Weather Forecast reanalysis-Interim (ERA-I). Upper air and surface data from the reanalyses have also been used to examine the atmospheric conditions when snowfall occurs and snow accumulates for the season. In this study, it is found that there is large interannual variation in SWE over western Himalayas and Satluj River basin (domain of interest). During excess years of snowfall, strong westerly winds are observed at 500 hPa over India. In wind anomaly, a cyclonic circulation is seen over northern parts of India with a deep trough along Pakistan, Rajasthan and Gujarat region. As a consequence of this trough, a moisture convergence zone is established in the region leading to more amount of snowfall. At the same time, during excess snow accumulation years, the air temperature from the surface to 500 hPa is colder than other years enabling the fallen snow to accumulate through the season.

Evaluation of Snow Load Using a Wind Tunnel on the Arched House

A damage occurrence of single-span type greenhouse by heavy snowfall in Korea, M shape collapse where excessive snow load of the roof occurs frequently regardless of the area damaging all the roof rafters. Therefore, in this paper, the snowdrift pattern of the arched house by shape using snow wind tunnel simulation is necessary. Experimental results indicated that the snowfalls on the roof of the arched house occur variously depending on the shape and location. As a result of estimating the load standard by roof slope angle and the values of snow loads using wind tunnel experiment, roof slopes showed 30% in 0°-15°and 80% in 45°-70° respectively, which were significant percentages in the snowdrift experiment.

Consistent patterns of Antarctic ice sheet interannual variations from ENVISAT radar altimetry and GRACE satellite gravimetry

SUMMARY Interannual variations of the Antarctic ice sheet due to surface mass balance (SMB) fluctuations are important for mass balance estimates and interpretations. To date, these variations are primarily assessed by global or regional atmospheric modelling. Satellite altimetry and satellite gravimetry over the ice sheet provide complementary observations of the related volume and mass effects, respectively. Yet, so far the interannual signal contents of these observations have not been extensively studied. We compare and jointly interpret ENVISAT radar altimetry (RA) and GRACE satellite gravimetry results, relying on RA products from the along-track repeat satellite RA approach and on the GRACE 10-d solutions by CNES/GRGS. RA results and GRACE results are expressed in terms of variations of ice sheet thickness, Δz(t), and ice-equivalent thickness, Δzice(t), respectively. In view of the different errors and limitations of both techniques and of differences between Δz(t) and Δzice(t) expected due to firn-related processes, our principal approach is a comparison of qualitative patterns in space and time. To adjust the spatial resolution of both data sets, we describe the spatial filtering inherent to the regularization of the CNES/GRGS GRACE solutions and apply this filtering to the ENVISAT RA height changes in a consistent fashion. After correction for glacial isostatic adjustment, the spatial patterns of linear trends seen by ENVISAT RA and GRACE over the period 2002 October to 2009 August agree well, not only for the extreme ice losses in the West Antarctic Amundsen Sea Sector but also for an alternating sequence of gains and losses along the East Antarctic coast. Our main focus is on interannual signals, which we represent by the low-pass filtered non-linear, non-seasonal components of the Δz(t) and Δzice(t) time-series. These components should reflect interannual SMB variations, apart from effects of changes in ice flow. We find an agreement between the interannual variation patterns from ENVISAT RA and GRACE with temporal correlation coefficients typically in the order of 0.8. This qualitative agreement, prevailing even in the theoretical absence of a simple proportionality between Δz(t) and Δzice(t), indicates that both observational signals primarily originate in common geophysical variations. Combining both geodetic data sets aids their interpretation and promises to be valuable for the reduction of SMB uncertainties. In a case study based on the analysis of both GRACE and ENVISAT RA, we identify a prominent interannual feature in West Antarctica in 2005 September/October as an event of excess snow accumulation with a mass effect of 82 ± 31 Gt.

A Bayesian approach to forecast intermittent demand for seasonal products

This paper investigates the forecasting of a large fluctuating seasonal demand prior to peak sale season using a practical time series, collected from the US Census Bureau. Due to the extreme natural events (e.g. excessive snow fall and calamities), sales may not occur, inventory may not replenish and demand may set off unrecorded during the peak sale season. This characterises a seasonal time series to an intermittent category. A seasonal autoregressive integrated moving average (SARIMA), a multiplicative exponential smoothing (M-ES) and an effective modelling approach using Bayesian computational process are analysed in the context of seasonal and intermittent forecast. Several forecast error indicators and a cost factor are used to compare the models. In cost factor analysis, cost is measured optimally using dynamic programming model under periodic review policy. Experimental results demonstrate that Bayesian model performance is much superior to SARIMA and M-ES models, and efficient to forecast seasonal and intermittent demand.

Inter‐decadal variation of the summer precipitation in China and its association with decreasing Asian summer monsoon Part II: Possible causes

AbstractThe present article is the second part of a study on the inter‐decadal variability of the summer precipitation in East China, which mainly addresses the possible cause of this change. Firstly, an updated analysis of the long‐term variations of snow cover, snow days and snow depth in the preceding winter and spring over the Tibetan Plateau (TP) was done by using station and satellite data. The abrupt increase in the winter and spring snow over the TP since around 1977 has been well documented. At that time, the inter‐decadal variation of the atmospheric heating over the TP in spring and summer had been estimated. It has been revealed that the atmospheric heating fields in subsequent spring and summer over the TP assumed a significant weakening after the late 1970s. This weakening is closely related to the significantly reduced surface sensible heat flux into the atmosphere and subsequent cooling over the TP and its surrounding atmosphere. The latter was produced by the increase of surface albedo and soil hydrological effect of melting snow under the condition of abrupt increase in the preceding winter and spring snow over the TP. On the other hand, three phases of significant inter‐decadal warming of the sea surface temperature (SST) in the tropical central and eastern Pacific, which occurred in the mid‐1960s, the late 1970s and the early 1990s, respectively, have been found. The above inter‐decadal variability of heating fields over the land area in the Asian region and neighbouring oceanic region of the West Pacific has consistently reduced the land–sea thermal contrast in summer in the Asian monsoon region based on the estimate of atmospheric heating fields. This cause is likely to lead to weakening of the Asian summer monsoon. In such case, the northward moisture transport in East Asia is greatly weakened and cannot reach North China, thus causing the condition of less precipitation or droughts. In contrast, the Yangtze River basin and South China receive a large amount of moisture supply and have strong upward motion, creating favourable conditions for frequent occurrence of heavy rainfall. In the process of the southward shift of the high‐precipitation zone, two abrupt or rapid regime shifts observed in the late 1970s and the early 1990s were possibly in response to the increase in the winter and spring snow over the TP, and two major rapid warming events of the SST in the tropical central and eastern Pacific in the late 1970s and the early 1990s.Correlative analysis has further confirmed that high TP snow and oceanic forcing factors have a positive correlation with the subsequent summer precipitation in the Yangtze River basin and most of South China, and a negative correlation with the summer precipitation in North China. This correlative relationship implies that if the TP has excessive (deficient) snow in the preceding winter and spring and the tropical central and eastern Pacific anomalously warms up (cools down), North China will have decreasing (increasing) summer precipitation, whereas the Yangtze River basin and South China will have increasing (decreasing) summer precipitation. Copyright © 2009 Royal Meteorological Society

Verification of Hydrometeor Properties Simulated by a Cloud-Resolving Model Using a Passive Microwave Satellite and Ground-Based Radar Observations for a Rainfall System Associated with the Baiu Front

This paper compares satellite microwave radiometer and ground-based radar observations with the Japan Meteorological Agency nonhydrostatic model (JMA-NHM) simulations, using a bulk microphysical parameterization, for a typical rainfall system associated with the Baiu front around the Okinawa Islands, Japan, on 8 June 2004. The JMA-NHM correctly replicated the shape, location and intensity of the precipitation associated with the observed rainfall system. Radar reflectivities and microwave brightness temperatures (TBs) were simulated using output from the JMA-NHM simulations. They were then compared with concurrent corresponding observations by the National Institute of Information and Communications Technology (NICT), CRL Okinawa Bistatic Polarimetric Radar (COBRA), and Advanced Microwave Scanning Radiometer for EOS (AMSR-E). Fairly good agreement was obtained between the simulated and observed reflectivities under the melting layer and TBs at low frequency (18.7 GHz), indicating that the JMA-NHM adequately simulated the amount of liquid hydrometeors. However, the intensity of scattering in the simulations was stronger than that in the COBRA observations above the melting layer and the AMSR-E observations at high frequencies (36.5 and 89.0 GHz). This was due to the fact that the JMA-NHM overestimated the amount and size of snow particles as a result of large depositional growth. The excessive snow contents were reduced by adjusting some of the microphysical processes in the JMANHM: the snowfall speeds were increased and a riming threshold for snow to graupel conversion was changed. These adjustments helped to reduce the amount and size of snow, resulting in further agreement with the COBRA observations. These adjustments also further improved the simulated TBs at high frequencies, especially at 36.5 GHz. However, differences still exist between the simulated and the observed TBs at high frequencies, suggesting that additional adjustment to and improvement of the snow microphysical processes are needed for the application of the model to microwave remote sensing of precipitation.

Improving Simulations of Convective Systems from TRMM LBA: Easterly and Westerly Regimes

Abstract The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of deep convection. Eliminating the dry growth of graupel in the bulk microphysics scheme effectively removed the unrealistic presence of high-density ice in the simulated anvil. However, comparisons with radar reflectivity data using contoured-frequency-with-altitude diagrams (CFADs) revealed that the resulting snow contents were too large. The excessive snow was reduced primarily by lowering the collection efficiency of cloud water by snow and resulted in further agreement with the radar observations. The transfer of cloud-sized particles to precipitation-sized ice appears to be too efficient in the original scheme. Overall, these changes to the microphysics lead to more realistic precipitation ice contents in the model. However, artifacts due to the inability of the one-moment scheme to allow for size sorting, such as excessive low-level rain evaporation, were also found but could not be resolved without moving to a two-moment or bin scheme. As a result, model rainfall histograms underestimated the occurrence of high rain rates compared to radar-based histograms. Nevertheless, the improved precipitation-sized ice signature in the model simulations should lead to better latent heating retrievals as a result of both better convective–stratiform separation within the model as well as more physically realistic hydrometeor structures for radiance calculations.

Northern Hemisphere snow cover and atmospheric blocking variability

The interseasonal relationship between Northern Hemisphere (NH) snow cover and regional blocking patterns is explored for a 31‐year data set. It is found that snow cover exerts an important influence on regional atmospheric blocking, which, in turn, modulates snow cover extent at subcontinental scales. Observational results provide strong evidence of two primary linkages in the seasonal snow cover‐blocking relationship that support an interannual persistence cycle: The first one links winter blocking over the Atlantic and the subsequent spring (summer) Eurasian (North American) snow cover anomalies; the second one implies that spring (summer) Eurasian (North American) snow cover precedes an anomalous winter Atlantic blocking activity. We describe the temporal stages of the snow cover‐blocking relationship in the framework of a six‐step conceptual model. According to that, an enhanced Atlantic blocking activity in winter favors a later spring snow disappearance through an enhanced cold advection toward western Eurasia. The resulting snow cover anomalies partially force an opposite‐sign blocking response over west and central Pacific which is sustained through spring and early summer, presumably because of the persistence of snow cover anomalies. This anomalous pattern seems to play a role in the propagation of snow cover anomalies from Eurasia in spring to the Hudson's Bay region of North America in summer. The excessive snow cover over this region induces an asymmetrical temperature distribution, which, in turn, favors blocking activity over Europe and the West Pacific. The connection between autumn and winter climates is not clear but it could be related with the ability of autumn high ATL blocking activity to determine an early snow cover appearance in October over western Eurasia. This linkage completes a snow cover‐blocking cycle of interactions which identifies snow cover as a candidate for the recently observed blocking trends and a contributor to the interannual persistence of winter climate.

Evaluation of surface albedo and snow cover in AR4 coupled climate models

Surface albedo (ALB), snow cover fraction (SCF) and snow water equivalent (SWE) of state‐of‐the‐art coupled climate models are compared and validated against ground‐based and remote‐sensed climatologies. Most IPCC AR4 climate models predict excessive snow mass in spring and suffer from a delayed spring snow melt while the onset of the snow accumulation is generally well captured. This positive SWE bias is mainly caused by too heavy snowfall during the winter and spring season. Seasonal cycles of snow cover area (SCA) at continental scales are captured reasonably well by most participating models. Two models clearly overestimate SCA over both Eurasia and North America. Year‐to‐year variations are reasonably well captured over both Eurasia and North America in winter and spring. The most pronounced underestimation in the interannual SCA variability is generally simulated during snow melt. The pronounced negative SCA trend that has been observed from 1979 to 2000 is only partly reproduced in the AR4 model simulations. Furthermore, the computed trends show a large spread among the models. Results from time slice simulations with the ECHAM5 climate model suggest that accurate sea surface temperatures are vital for correctly predicting SCA trends. Simulated global mean annual surface albedos are slightly above the remote‐sensed surface albedo estimates. The participating AR4 models generally reproduce the seasonal cycle of the surface albedo with sufficient accuracy while systematic albedo biases are predicted over both snow‐free and snow‐covered areas, with the latter being distinctly more pronounced. The study shows that the surface albedo over snow‐covered forests is probably too high in various state‐of‐the‐art global climate models. The analysis demonstrates that positive biases in SCA are not necessarily related to positive albedo biases. Furthermore, an overestimation of area‐averaged SWEs is not necessarily related to positive SCA anomalies since the relationship between SWE and SCF is highly nonlinear.

Forcing of the Asian monsoon on the Tibetan Plateau: Evidence from high‐resolution ice core and tropical coral records

Climatic influences on snow accumulation across the Tibetan Plateau are examined using records of net snow accumulation (An) and oxygen isotopic ratios (δ18O) since 1801 from two ice cores from opposite sides of the Plateau. From ∼1880 to the 1990s, summer monsoon precipitation has been a significant component of the annual accumulation on the Dasuopu glacier in the Himalayas, but during the latter part of the Little Ice Age (∼1810 to ∼1880) total An was 30% higher than the summer monsoon amounts in northern India. This was possibly the result of increased early winter snowfall as westerly low‐pressure systems linked to the North Atlantic pushed farther east along the Himalayas than they normally do today. The decades of high accumulation and the colder temperatures allowed excess snow and ice to persist late into each year, which may have weakened the subsequent Asian summer monsoon. Consequently, precipitation in the northeast Tibetan Plateau where the Dunde ice cap is located may have been affected primarily by Eurasian continental processes rather than tropical meteorology during this time. Since the onset of the recent warming over the last century, the south Asian summer monsoon intensified and influenced summer climate farther to the north and west, and expressions of tropical Pacific and Indian Oceanic/atmospheric processes are noticeable in the Dunde net accumulation record. The Dunde and Dasuopu glaciers, which are located on the northern and southern rims of the Tibetan Plateau, were situated in regions of environmental transition as the Northern Hemisphere climate shifted from a neoglacial to a warming climate mode, which is something to consider when interpreting the longer ice core climate records.

Decadal Change of the Spring Snow Depth over the Tibetan Plateau: The Associated Circulation and Influence on the East Asian Summer Monsoon*

The decadal change in the spring snow depth over the Tibetan Plateau and impact on the East Asian summer monsoon are investigated using station observations of snow depth data and the NCEP–NCAR reanalysis for 1962–93. During spring (March–April), both the domain-averaged snow depth index (SDI) and the first principal component of the empirical orthogonal function (EOF) analysis exhibit a sharp increase in snow depth after the late 1970s, which is accompanied by excessive precipitation and land surface cooling. The correlation between SDI and precipitation shows a coherent remote teleconnection from the Tibetan Plateau–northern India to western Asia. It is found that the increased snow depth over the plateau after the mid-1970s is concurrent with a deeper India–Burma trough, an intensified subtropical westerly jet as well as enhanced ascending motion over the Tibetan Plateau. Additional factors for the excessive snowfall include more moisture supply associated with the intensification of the southerly flow over the Bay of Bengal and an increase of humidity over the Indian Ocean. While the extensive changes of the circulation in Eurasia and the Indian Ocean are associated with a climate shift in the Northern Hemisphere after the mid-1970s, some regional factors such as the enhanced coupling between the sea surface temperature (SST) warming in the northern Indian Ocean/Maritime Continent and the tropical convective maximum (TCM), as well as local feedback of the land surface cooling due to excessive snow cover and the atmosphere may contribute to the regional circulation changes. The former enhances the western Pacific subtropical in the South China Sea–Philippine Sea through modulation of the local Hadley circulation and results in stronger pressure gradients and fronts in southeastern and eastern Asia. A close relationship exists between the interdecadal increase of snow depth over the Tibetan Plateau during March–April and a wetter summer rainfall over the Yangtze River valley and a dryer one in the southeast coast of China and the Indochina peninsula. It is proposed that the excessive snowmelt results in a surface cooling over the plateau and neighboring regions and high pressure anomalies that cause a more northwestward extension of the western Pacific subtropical high in the subsequent summer. Additionally, the increased surface moisture supply provides more energy for the development of the eastward-migrating low-level vortex over the eastern flank of the Tibetan Plateau. Both factors lead to a wetter summer in the vicinity of the Yangtze River valley.

Basic biomechanics of self-supporting plants: wind loads and gravitational loads on a Norway spruce tree

Mechanical constraints are major determinants of size and shape of self-supporting plants, for instance upright stems will become mechanically unstable due to their own weight if too slender (Euler buckling). Both gravitational forces and wind loads induce bending moments which the structure has to be able to withstand. Straightforward is the computation of gravitational loads on side branches and stems leaning to various degrees, provided that the geometrical parameters like their tapering mode and the number and size of primary and secondary branches is known. The limit of the structure is reached if at any point the bending moment induced is larger than the critical bending moment. This in turn depends on structural parameters (the geometry of the cross-section) and properties of the material (the modulus of elasticity and the critical stresses at which failure occurs). In many cases, depending on the root–soil interaction, the bending moment on the trunk may lead to root lodging. Most important also from an economical point of view is the assessment of wind loads. It requires an estimate of the effective sail-area (which due to the flexibility of the branches will depend on the wind speed) and the drag coefficient and particularly the knowledge of the wind profile, which may be very different for a solitary tree than for a tree within a dense stand. Flexibility combined with viscoelastic behaviour is the plant’s answer to dynamic wind loads. Although our comprehension may still seem fragmentary, one general conclusion emerges from a comparison of biomechanical calculations with empirical data: plants are often structured in such a way that they approach their biomechanical limits within controlled safety margins. This safety factor applies to ‘normal’ environmental loads, either gravitational loads or wind loads and may be overruled by excessive snow loads or by extreme winds like hurricanes. For a biologist it is a challenge to trace the ‘principle of constant safety factors’ to adaptive growth as response to mechanical stimuli. The phenomenon has been demonstrated qualitatively, but neither the mechanoreceptor nor the signal transduction chain has as yet been identified.