Magnitude and variation in the contribution of bank erosion to the suspended sediment load of the River Severn,UK

The wide range of studies describing the role of bank erosion in fluvial sediment supply have mostly lumped amounts of bank erosion into coarse temporal units, such as years. This paper investigates sediment yields from individual bank erosion events within the upper River Severn, UK (basin area 380 km²). Manual erosion pins and photo-electronic erosion pins were used to estimate bank erosion, and turbidity meters were used to determine suspended sediment transport. At the annual time-scale, the silt-clay fraction of bank-derived sediment accounted for an equivalent of 17 per cent of the suspended load, increasing to an average of 38 per cent at the monthly timescale, and then to an average of 64 per cent at the event timescale. This research highlighted that for an upland catchment, bank erosion was an important supply of suspended sediment, and that for some flood events bank erosion can supply more sediment than is transported. © 1997 John Wiley & Sons, Ltd.     

Relative scales of time and effectiveness of climate in watershed geomorphology

Peak rainfalls and peak runoff rates per unit area are comparable over a worldwide spectrum of climates. However, while the magnitude of the external contribution of energy or force in diverse regions is similar, the impact on the landscape varies markedly between regions. Absolute magnitudes of climatic events and absolute time intervals between such events do not provide satisfactory measures of the geomorphic effectiveness of events of different magnitudes and recurrence intervals. Although geomorphic processes are driven by complex sets of interrelated climatic, topographic, lithologic, and biologic factors, the work done by individual extreme events can be scaled as a ratio to mean annual erosion and the effectiveness of such events in forming landscape features can be related to the rate of recovery of channel form or mass wasting scars following alteration by the extreme event. Thus, a time scale for effectiveness may relate the recurrence interval of an event to the time required for a landform to recover the form existing prior to the event. River channels in temperate regions widened by floods of recurrence intervals from 50 to more than 200 years may regain their original width in matters of months or years. In semi-arid regions, recovery of channel form depends not only upon flows but upon climatic determinants of the growth of bottomland vegetation resulting in variable rates of recovery, on the order of decades, depending upon coincidence of average flows and strengthened vegetation. In truly arid regions the absence of vegetation and flow precludes recovery and the width of channels increases in drainage areas up to 100 km² but remains relatively constant at larger drainage areas. Area as well as time controls the effectiveness of specific events inasmuch as the likelihood of simultaneous peak discharges or rainfalls and large areas is less, particularly in arid regions where events spanning areas of more than several thousand km² are extremely rare if experienced at all. To some extent a decrease in area in a humid region is comparable with a regional change from humid toward more arid climate reflected in the increase in importance of episodic as contrasted with more continuous processes. Exceedingly rare floods of extreme magnitudes, estimated recurrence intervals of 500 years or longer, may exceed thresholds of competence otherwise unattainable in the ‘normal’ record resulting in ‘irreparable’ transformations of valley landforms. Denudation of hillslopes by mass wasting during relatively rare events can also be related to mean rates of denudation and to recovery of hillslope surfaces after scarring by different kinds of landslides. Measured recovery times described in the literature vary from less than a decade for some tropical regions to decades or more in temperate regions. Recurrence intervals of high magnitude storms which trigger mass wasting range from 1 to 2 years in some tropical areas, to 3 or 4 per hundred years in some areas of seasonal rainfall and to 100 or more years in some temperate regions. The effectiveness of climatic events on both hillslopes and rivers is not separable from gradient, lithology or other variables which control both thresholds of activity and recovery rate.     

Role of major terrestrial cratering events in dispersing life in the solar system

The larger and most energetic cratering events from comet and asteroid collisions with the Earth are probably associated with ejection of solid material faster than escape speeds every 100 Myr or so. Metre-sized boulders, we estimate, may have been ejected directly into Venus-crossing and perhaps Mars-crossing orbits from comet impacts at higher speeds and of larger mass, at least on 10 occasions in the last 3.5 Ga. Subsequent close encounters with Earth can also enable slower boulders to reach Mars-crossing orbits. Orbit perturbations from Mars and Jupiter would then have sent a fraction of the boulders to the outer planets and their icy satellite systems. In the so-called late bombardment epoch at 3.9 Ga, when primitive life was developing, ejection-causing impacts were much more frequent, at 30 per 0.1 Ga, yielding an increased probability of distributing seeds of terrestrial biology to the outer regions of the solar system.     

Volcanoes, the stratosphere, and climate

A list of volcanic eruption plumes observed to ascend into or near the stratosphere since 1883 shows that the volcanoes divide readily into two groups, one at low and one at higher latitudes. A model for the rise of a buoyant volcanic plume rise as applied to volcanic eruptions is corrected for realistic temperature profiles and for the finite vertical extent of the resultant debris clouds. The utility of the model can be questioned, however, owing to the highly uncertain and variable nature of the efficiency of use of heat energy of buoyant rise. The observed correlation of stratospheric plumes with climatic effects indicates that those plumes nearer the equator have the largest impact on surface temperatures. Analysis of the observations also suggests that injection of debris into the stratosphere is more important in determining the effect on climate than either the total volcanic explosivity of the eruption or the actual height reached within the stratosphere.     

Simulations of stratospheric sudden warmings in the Berlin troposphere-stratosphere-mesosphere GCM

Stratospheric sudden warming events in the Northern Hemisphere of the Berlin TSM GCM are investigated. In about 50% of the simulated years (13 out of 28), major midwinter warmings occur. This agrees well with observations but, whereas real events tend to occur approximately every second season, those in the model are clustered, most of them occur in the period between years 15/16 and years 24/25. In most other years, minor warming events take place. The warming events are found earlier in the winter than in reality. Many of the observed characteristics of warming events are well captured by the model: pulses of wave activity propagate out of the troposphere; these transient events force the zonal-mean zonal wind in the stratosphere and coincide with increases of the temperature at the North Pole and cooling at low levels in the tropics; temperature changes of opposite sign are modelled at higher levels. Synoptically, the modelled stratosphere evolves quite realistically before the warmings: the cyclonic vortex is displaced from the Pole by an amplifying anticyclone. After minor warmings, the stratosphere remains too disturbed as the cyclonic centre does not return to the North Pole as quickly as in reality. In the aftermath of major warmings the cyclonic vortex is not fully eroded and the anticyclonic circulation does not develop properly over the Pole; furthermore, the wintertime circulation is not properly restored after the event.     

Balloon observations of organic and inorganic chlorine in the stratosphere: the role of HClO4 production on sulfate aerosols

Simultaneous observations of stratospheric organic and inorganic chlorine were made in September 1993 out of Fort Sumner, New Mexico, using JPL balloon-borne MkIV interferometer. Between 15 and 20 km, a significant fraction (20-60%) of the inorganic chlorine could not be accounted for by the sum of measured HCl, ClONO2, and HOCl. Laboratory measurements of the reaction of ClO radicals on sulfuric acid solutions have indicated that, along with HCl, small amounts of perchloric acid, HClO4, were formed. Very little is known about the fate of HClO4 in the stratosphere and we use a photochemical box model to determine the impact of this new species on the partitioning of inorganic chlorine in the stratosphere. Assuming that HClO4 is photochemically stable, it is shown that in the enhanced aerosol loading conditions resulting from Mt. Pinatubo's eruption, HClO4 could represent a significant reservoir of chlorine in the lower stratosphere, sequestering up to 0.2 ppbv (or 50%) of the total inorganic chlorine at 16 km. The occurrence of this new species could bring to closure the inorganic chlorine budget deficiency made apparent by recent ER-2 aircraft in situ measurements of HCl.     

The boreal spring stratospheric final warming and its interannual and interdecadal variability

Based on the daily NCEP/DOE reanalysis II data,dates of the boreal spring Stratospheric Final Warming(SFW) events during 1979–2010 are defined as the time when the zonal-mean zonal wind at the central latitudes(65°–75°N) of the westerly polar jet drops below zero and never recovers until the subsequent autumn.It is found that the SFW events occur successively from the mid to the lower stratosphere and averagely from the mid to late April with a temporal lag of about 13 days from 10 to 50 hPa.Over the past 32 years,the earliest SFW occurs in mid March whereas the latest SFW happens in late May,showing a clear interannual variability of the time of SFW.Accompanying the SFW onset,the stratospheric circulation transits from a winter dynamical regime to a summertime state,and the maximum negative tendency of zonal wind and the strongest convergence of planetary-wave are observed.Composite results show that the early/late SFW events in boreal spring correspond to a quicker/slower transition of the stratospheric circulation,with the zonal-mean zonal wind reducing about 20/5 m s-1 at 30 hPa within 10 days around the onset date.Meanwhile,the planetary wave activities are relatively strong/weak associating with an out-of-/in-phase circumpolar circulation anomaly before and after the SFW events in the stratosphere.All these results indicate that,the earlier breakdown of the stratospheric polar vortex(SPV),as for the winter stratospheric sudden warming(SSW) events is driven mainly by wave forcing;and in contrast,the later breakdown of the SPV exhibits more characteristics of its seasonal evolution.Nevertheless,after the breakdown of SPV,the polar temperature anomalies always exhibit an out-of-phase relationship between the stratosphere and the troposphere for both the early and late SFW events,which implies an intimate stratosphere–troposphere dynamical coupling in spring.In addition,there exists a remarkable interdecadal change of the onset time of SFW in the mid 1990s.On average,the SFW onset time before the mid 1990s is 11 days earlier than that afterwards,corresponding to the increased/decreased planetary wave activities in late winter-early spring before/after the 1990s.     

Calculating the global mass exchange between stratosphere and troposphere

Large-scale cross-tropopause mass fluxes are diagnosed globally from 1979 to 1989 for Northern Hemisphere winter conditions (December, January, and February). Results of different methods of approaches with regard to the definition of the tropopause and the way to calculate the mass fluxes are compared and discussed. The general pattern of the mass exchange from the tropopause into the stratosphere and vice versa agrees fairly well when using different methods, but the absolute values can differ up to 100%.An inspection of the temporal development of the mass fluxes for solstice conditions indicates a complex picture. Whereas a permanent significant downward flux from the stratosphere into the troposphere is detected for latitude regions nearly between 25^°N and 40^°N and between 30^°S and 50^°S (initiated by the poleward branches of the Hadley cells), a non-uniform behaviour is observed at higher latitude bands. Periods of strong mass exchange from the troposphere into the stratosphere are disrupted by periods of an opposite mass exchange. A comparison of the stratoshere-troposphere (ST) exchange with the exchange at higher altitudes through surfaces, quasi-parallel to the tropopause, excludes a general connection. Only a few strong upward directed ST mass exchange events have counterparts at higher altitudes. The composition of the stratosphere may be influenced directly by the ST exchange only in a thin layer above the tropopause.     

Sulphur emissions to the stratosphere from explosive volcanic eruptions

 Two methods were used to quantify the flux of volcanic sulphur (as the equivalent mass of SO₂) to the stratosphere over different timescales during the Holocene. A combination of satellite-based measurements of sulphur yields from recent explosive volcanic eruptions with an appropriate rate of explosive volcanism for the past 200 years constrains the medium-term (∼10²years) flux of volcanic sulphur to the stratosphere to be ∼1 Mt a^–1, with lower and upper bounds of 0.3 and 3 Mt a^–1. The short-term (∼10- to 20-year) flux due to small magnitude (10¹⁰–10¹²kg) eruptions is of the order of 0.4 Mt a^–1. At any time the instantaneous levels of sulphur in the stratosphere are dominated by the most recent (0–3 years) volcanic events. The flux calculations do not attempt to address this very short timescale variability. Although there are significant errors associated with the raw sulphur emission data on which this analysis is based, the approach presented is general and may be readily modified as the quantity and quality of the data improve. Data from a Greenland ice core support these conclusions. Integration of the sulphate signals from presumed volcanic sources recorded in the GISP2 core provides a minimum estimate of the 10³–year volcanic SO₂ flux to the stratosphere of 0.5–1 Mt a^–1 over the past 9000 years. The short-term flux calculations do not account for the impact of rare, large events. The ice-core record does not fully account for the contribution from small, frequent events. Received: 27 September 1995 / Accepted: 13 December 1995     

火山活动对北半球平流层气候异常变化的影响

文中利用逐次滤波法滤除北半球平流层70 hPa约15~22 km高空大气温度异常变化中太阳活动的影响之后,进一步分析了火山活动的气候效应,分析结果表明,火山活动能引起平流层较大幅度增温,对于北半球70hPa高空气候异常变化的影响超过了总方差的30%;火山活动影响最显著的高度是平流层70 hPa约15~22 km高空,由此高度向上或向下,火山活动的影响都逐渐减小;火山活动引起平流层大气升温的同时还将引起对流层大气降温,其分界线大致位于对流层顶300 hPa附近;强火山爆发如皮纳图博火山爆发、阿贡火山爆发和堪察加北楮缅奴等火山爆发是引起未来两年左右平流层中下层温度异常变化最重要的因素,其方差贡献率超过百分之五十三！;火山喷发高度越高,引起平流层增温效应的层次也越高;北半球大气温度异常变化对南半球火山活动响应的滞后时间比北半球火山活动长. 平流层高空气候异常变化还具有显著的22年变化周期,分析认为是大气温度场对太阳磁场磁性周期22年异常变化的响应,其方差贡献率超过9%.     

Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6-15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8-4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.     

UHF radar observation of strato-tropospheric transfers on the anticyclonic side of a jet streak

An observation by UHF ST radar of a subsidence pattern on the right side of the exit region of a jet streak is reported. The onset of the subsidence pattern occurred at 23:30 UTC on the 29 November 1991, when a downward motion was initiated above 14 km. The injections of stratospheric air in this region seem to have an intermittent nature; they occur during at least three intervals during the lifetime of the subsidence pattern. Comparison of these results with an ECMWF analysis suggests that it is an unfolding case. However, observation of turbulent intensities w’ greater than 60 cm s⁻¹ at the tropopause level also suggests the existence of a turbulent flux between the stratosphere and the troposphere. From the turbulence characteristics measured by the radar and the potential temperature profile obtained by radiosonde data, the eddy diffusivity at the tropopause level has been calculated. An eddy diffusion coefficient ranging between 5 and 7 m² s⁻¹ is found. From these values, and with the assumption of a climatological gradient of the volume mixing ratio of ozone in the lower stratosphere, it is possible to deduce a rough estimate of the amount of ozone injected from the stratosphere into the troposphere during this event. A rate of transfer of 1.5×10²⁰ molecules of ozone per day and per square meter is found.     

夏季亚洲季风区对流层-平流层不可逆质量交换特征分析

基于2005年NCEP/GFS分析资料和拉格朗日粒子扩散模式的“Domain Filling”技术,以气块穿越对流层顶后的滞留时间为标准,诊断分析了夏季亚洲季风区对流层-平流层质量交换,重点讨论了对平流层大气成分收支具有实际意义的不可逆双向质量交换过程,并利用前向（后向）轨迹追踪方法,分析了其4天的“源（汇）”特征.研究结果表明：(1)对流层-平流层质量交换(Troposphere-Stratosphere mass Exchange,STE)的计算对滞留时间阈值的选择具有较强敏感性,大多数的气块在1～2天内可频繁地往返对流层顶.这些瞬时交换事件的考虑与否对穿越对流层顶的质量交换计算的准确性具有重要影响,尤其在中纬度的风暴轴区域.(2)从亚洲季风区对流层-平流层质量净交换纬向平均上看,45°N以南的区域为对流层向平流层的质量输送(Troposphere to Stratosphere mass Transport,TST),副热带地区为最强的上升支,而在45°N~55°N的中纬度地区是平流层向对流层质量输送（Stratosphere to Troposphere mass Transport,STT）.地理分布上,STT主要分布在青藏高原以北的东亚地区,与亚洲季风区夏季大尺度的槽区相对应.夏季整个亚洲季风区都是TST发生的区域,最大值位于青藏高原东南侧及其附近区域,该区域占亚洲季风区不可逆TST夏季平均总量的46%.(3)对流层-平流层质量交换的“源汇”特征分析表明,STT主要源于100°E以西、50°N以北的高纬地区,向下可以输送到中国东北部及朝鲜半岛北部等中纬度区域.而TST主要来源于中纬度和副热带地区的大气输送,向上穿越对流层顶高度以后,可分别向高纬的极地和热带地区输送,这意味着亚洲季风区夏季的TST水汽输送可能进入“热带管”中,进而可能对全球平流层水汽平衡产生重要影响.     

The ineffectiveness of Joule heating in the stratosphere

Several papers have recently invoked Joule heating in the stratosphere, generated from electric currents induced by solar wind interactions with Earth, as possibly playing a significant role in warming the polar stratosphere. This commentary assesses the accuracy of that contention and demonstrates that in situ Joule heating can take no significant part in warming the stratosphere, and thus cannot be used to suggest a link between stratospheric temperatures and solar activity.     

Study of the recent geodynamic processes in the Kopet-Dag region

The time series of the uniquely long geodetic observations of recent geodynamic processes in the Kopet-Dag region are analyzed. The regional observations of contemporary vertical movements cover a 75-year period; the zonal and local systems of leveling measurements, which provide an increased degree of spatiotemporal detail (the distance between the benchmarks is less than 1 km and the measurements are repeated with a frequency of once per month to two times per annum), have been functioning for 50 years. It is shown that during the last 40–50 years, the regional stress field in the forefront of the Main Kopet-Dag thrust and collision zone of the Turanian and Iranian plates is quasi static. The annual average trend rate of strains estimated from a set of the time series of uniquely long geodetic observations is (3–5) × 10^?8 yr^?1, which is only one to two amplitudes of tidal deformations of the solid Earth. The local deformations in the fault zone reach the rates that are by 1.5–2 orders of magnitude higher than in the block part of the region. It is found that the segments of the Earth’s surface within the axial part of the depression experience persistent uplifting, which indicates that they do not follow the scheme of inherited evolution characteristic of the fault-block structures of the region. It is demonstrated that these anomalous uplifts can be caused by the variations in the weak seismicity in the zone of the North Ashgabat Fault.     

Influence of wave activity on the composition of the polar stratosphere

The planetary wave impact on the polar vortex stability, polar stratosphere temperature, and content of ozone and other gases was simulated with the global chemical–climatic model of the lower and middle atmosphere. It was found that the planetary waves propagating from the troposphere into the stratosphere differently affect the gas content of the Arctic and Antarctic stratosphere. In the Arctic region, the degree of wave activity critically affects the polar vortex formation, the appearance of polar stratospheric clouds, the halogen activation on their surface, and ozone anomaly formation. Ozone anomalies in the Arctic region as a rule are not formed at high wave activity and can be registered at low activity. In the Antarctic Regions, wave activity affects the stability of polar vortex and the depth of ozone holes, which are formed at almost any wave activity, and the minimal ozone values depend on the strong or weak wave activity that is registered in specific years.     

Rise of volcanic plumes to the stratosphere aided by penetrative convection above large lava flows

Turbulent volcanic plumes disperse fine ash particles and toxic gases in the atmosphere and can lead to significant temperature drops in the atmosphere. In the geological past, the emplacement of large continental flood basalts (CFB) has been associated with large changes in the global environment and extinctions of biological species. The variable intensity of environmental changes induced by otherwise similar CFB events, however, begs for a reevaluation of physical controls on the environmental impact of volcanic eruptions. The climatic impact of an eruption depends on its ability to inject gases in the stratosphere and on the eruption rate. Using integral models of turbulent plumes above line and point sources, we find that mass rate estimates for CFBs are in general not large enough for volcanic plumes to reach the stratosphere on their own. Basaltic eruptions, however, are also associated with widespread lava flows which lose large amounts of heat and generate convection in the atmosphere. This form of convection, known as penetrative convection, acts to erode the stably stratified lower atmosphere and generates a thick well-mixed heated atmospheric layer in a few hours. The added buoyancy provided by such a layer almost always ensures that volcanic gases get transported to the stratosphere. The environmental consequences of CFBs are therefore controlled not by the inputs to the atmosphere from individual volcanic plumes, but by the dynamic response of the climate system to a succession of short eruptive pulses within a longer-lasting eruption sequence.     

Exploring new methods to analyse spatial impact distributions on debris-flow fans using data from south-western British Columbia

Predicting the spatial impact of debris flows on fans is challenging due to complex runout behaviour. Debris flow mobility is highly variable and flows can sporadically avulse the channel. For hazard and risk assessments, practitioners typically base the probability of spatial impact or avulsion on their experience and expert judgement. To support decision-making with empirical observations, we studied spatial impact distributions on 30 active debris-flow fans in south-western British Columbia, Canada. We mapped 146 debris-flow impact areas over an average observation period of 74 years using orthorectified airphotos, satellite imagery, topographic base maps, LiDAR data, orthophotos, and field observations. We devised a graphical method to convert our geospatial mapping into spatial impact heat maps normalized by fan boundaries, enabling comparison of runout distributions across different fans. About 90% of the mapped debris flows reached beyond the mid-points of fans, while less than 10% avulsed more than half-way across the fan relative to the previous flow path. Most avulsions initiated at distances of 20% to 40% of the maximum fan length from the fan apex and upstream of the fan intersection point. Large volume events tend to be more mobile in the down-fan direction, but the relation between volume and cross-fan runout (e.g., avulsions) is more complex. Differences in spatial impact distributions can be explained, in part, by the degree of fan incision and whether a fan is truncated at its toe by a river or lake. There were no significant differences in spatial impact distributions based on the geology of the source area, sediment supply condition, or hydrogeomorphic process classification.     

The impact of moderate-scale explosive eruptions on stratospheric gas injections

Volcanic gases such as SO ₂, H ₂S, HCl and COS emitted during explosive eruptions significantly affect atmospheric chemistry and therefore the Earth's climate. We have evaluated the dependence of volcanic gas emission into the atmosphere on altitude, latitude, and tectonic setting of volcanoes and on the season in which eruptions occurred. These parameters markedly influence final stratospheric gas loading. The latitudes and altitudes of 360 active volcanoes were compared to the height of the tropopause to calculate the potential quantity of volcanic gases injected into the stratosphere. We calculated a possible stratospheric gas loading based on different volcanic plume heights (6, 10, and 15 km) generated by moderate-scale explosive eruptions to show the importance of the actual plume height and volcano location. At a plume height of 15 km for moderate-scale explosive eruptions, a volcano at sea level can cause stratospheric gas loading because the maximum distance to the tropopause is 15–16 km in the equatorial region (0–30°). Eruptions in the tropics have to be more powerful to inject gas into the stratosphere than eruptions at high latitudes because the tropopause rises from ca. 9–11 km at the poles to 15–16 km in the equatorial region (0–30°N and S). The equatorial region is important for stratospheric gas injection because it is the area with the highest frequency of eruptions. Gas injected into the stratosphere in equatorial areas may spread globally into both hemispheres.