Morphodynamics of intertidal bars on a megatidal beach, Merlimont, Northern France

The morphology, bedforms and hydrodynamics of Merlimont beach, in northern France, characterised by intertidal bars and a spring tidal range of 8.3 m, were surveyed over a 10-day experiment with variable wave conditions that included a 2-day storm with significant wave heights of up to 2.8 m. The beach exhibited two pronounced bar-trough systems located between the mean sea level and low neap tide level. Waves showed a cross-shore depth modulation, attaining maximum heights at high tide. The mean current was characterised dominantly by strong tide-induced longshore flows significantly reinforced by wind forcing during the storm, and by weaker, dominantly offshore, wave-induced flows. Vertical tidal water-level variations (tidal excursion rates) showed a bimodal distribution with a peak towards the mid-tide position and low rates near low and high water. The two bar-trough systems in the mid-tide zone remained stable in position during the experiment but showed significant local change. The absence of bar migration in spite of the relatively energetic context of this beach reflects high macro-scale bar morphological lag due to a combination of the large vertical tidal excursion rates in the mid-tide zone, the cross-shore wave structure, and the pronounced dual bar-trough system. The profile exhibited a highly variable pattern of local morphological change that showed poor correlation with wave energy levels and tidal excursion rates. Profile change reflected marked local morphodynamic feedback effects due mainly to breaks in slope associated with the bar-trough topography and with trough activity. Change was as important during low wave-energy conditions as during the storm. Strong flows in the entrenched troughs hindered cross-shore bar mobility while inducing longshore migration of medium-sized bedforms that contributed in generating short-term profile change. The large size and location of the two pronounced bars in the mid-tide zone of the beach are tentatively attributed respectively to the relatively high wave-energy levels affecting Merlimont beach, and to the cross-shore increase in wave height hinged on tidal modulation of water depths. These two large quasi-permanent bars probably originated as essentially breakpoint bars and are different from a small bar formed by swash and surf processes in the course of the experiment at the mean high water neap tide level, which is characterised by a certain degree of tidal stationarity and larger high-tide waves.     

The impact of the friction of distance on the early settlement and development of Eyre Peninsula,South Australia

The cost of overcoming distance has played and continues to play an important role in the evolution of Australia's space‐economy. This paper examines the impact of the ‘friction of distance’, as well as the quality of the resource base, on the settlement and development of Eyre Peninsula in South Australia. A series of maps portraying the spread of pastoralism and then agriculture on Eyre Peninsula reveals clearly the importance of ‘accessibility’ in influencing the pattern and timing of settlement. It was not until well into this century that all the potentially agricultural land in the interior was occupied, for this required the construction of a railway and water pipeline up the centre of the peninsula.     

Graded-Density Reservoirs for Accessing High Stress Low Temperature Material States

In recently developed laser-driven shockless compression experiments an ablatively driven shock in a primary target is transformed into a ramp compression wave in a secondary target via unloading followed by stagnation across an intermediate vacuum gap. Current limitations on the achievable peak longitudinal stresses are limited by the ability of shaping the temporal profile of the ramp compression pulse. We report on new techniques using graded density reservoirs for shaping the loading profile and extending these techniques to high peak pressures.     

Microbial life in martian ice: A biotic origin of methane on Mars?

Despite the fact that microbial cells are unlikely to be found in the Martian soil in the near future, this paper is written on the assumption that some of the seasonally varying concentration of Martian methane is due to ongoing methanogenesis. It is first pointed out that life might have arisen on Mars first and been transported to Earth later. A case is made that an icy origin of life is more likely than a hot origin, especially if biomolecules take advantage of the high encounter rates and stability against hydrolysis, and that microorganisms feed on the ions that comprise eutectic solutions in ice. Although certain difficulties are avoided if RNA and DNA grow while adsorbed on clay grains, double strand-breaks of microbial DNA due to alpha radioactivity are a far greater threat to microbial survival on clay or other rock types than in ice. Developing a relation between the rate of microbial metabolism in ice and the experimentally determined rate of production of trapped gases of microbial origin, one can estimate the concentration of methanogens that could account for the methane production rate as a function of temperature of their habitat. The result, of order 1 cell cm⁻³ in the Martian subsurface, seems an attainable goal provided samples are taken from at least 1 or 2 m below the hostile surface of Mars. Instruments on NASA’s 2011 Mars Science Lab will measure stable isotopes for methane, water, and carbon dioxide, which on Earth served to distinguish abiotic, thermogenic, and microbial origins. Future measurements of chirality of biomolecules might also provide evidence for Martian life.     

空间运输联系的分布与交流规律研究

空间运输联系是指在自然、社会、经济诸要素综合作用下，区域间通过交通运输设施进行旅客和货物交流产生的相互联系与作用。采用定量与定性相结合的实证分析方法，本文研究了客货运输的地域分布特征、地域间交流联系生成的内在机制、基本类型和增长演变的趋势，划分出客运联系“单中心”、“多中心”，货运联系“互补性交流”、“竞争性交流”、“递接性交流等类型，并提出了非均衡系数、集中度、首位联系量等评价指标。     

26Al and 10Be dating of late pleistocene and holocene fill terraces: a record of fluvial deposition and incision,Colorado front range

Cosmogenic ²⁶Al, ¹⁰Be, and ¹⁴C dating of fluvial fill terraces in steep canyons of the Colorado Front Range provides a temporal framework for analysing episodic aggradation and incision. Results from Boulder Canyon show that terrace heights above the modern channel (grade) can be divided into: (1) Bull Lake (≳100 ka; 20–15 m above grade); (2) Pinedale (32–10 ka; 15–4 m above grade); and (3) Holocene age (<4 m above grade). No pre‐Bull Lake deposits are preserved along Boulder Canyon, and only three small remnants >15 m above grade record Bull Lake deposition. Well‐preserved terraces of Pinedale age suggest that the range of terrace height above grade reflects short‐term fluctuations in the river profile during periods of rapidly changing stream load and power. Net river incision apparently occurred during transitions to interglacial periods. Soil development and stratigraphic position, along with limited cosmogenic and ¹⁴C dating, suggest that ∼130 ka terraces in Boulder Canyon correlate with the Louviers Alluvium, and that 32 to 10 ka fills in the canyon correlate with the Broadway Alluvium on the adjacent High Plains. Late Pleistocene incision rates (∼0·15 m ka⁻¹) along Boulder Canyon exceed pre‐late Pleistocene incision rates, and are higher than middle to late Pleistocene incision rates (∼0·04 m ka⁻¹) on the High Plains. This study provides an example of how modern geochronologic techniques allow us to understand better rivers that drain glaciated catchments. Copyright © 2002 John Wiley & Sons, Ltd.     

A global satellite view of the seasonal distribution of mineral dust and its correlation with atmospheric circulation

Aerosols make a considerable contribution to the climate system through their radiative and cloud condensation nuclei effects, which underlines the need for understanding the origin of aerosols and their transport pathways. Seasonal distribution of mineral dust around the globe and its correlation with atmospheric circulation is investigated using satellite data, and meteorological data from ECMWF. The most important sources of dust are located in North Africa, the Middle East and Southwest Asia with an observed summer maximum, and East Asia with a spring peak. Maximum dust activity over North Africa and the Middle East in summer is attributed to dry convection associated with the summertime low-pressure system, while unstable weather and dry conditions are responsible for the spring peak in dust emission in East Asia. Intercontinental transport of mineral dust by atmospheric circulation has been observed, including trans-Atlantic transport of North African dust, trans-Pacific transport of Asian dust, and transport of dust from the Middle East across the Indian Ocean. The extent of African dust over the Atlantic Ocean and its latitudinal variation with season is related to the large-scale atmospheric circulation, including seasonal changes in the position of the intertropical convergence zone (ITCZ) and variation of wind patterns. North African aerosols extend over longer distances across the North Atlantic in summer because of greater dust emission, an intensified easterly low level jet (LLJ) and strengthening of the Azores-Bermuda anticyclonic circulation. Transport of East Asian aerosol is facilitated by the existence of a LLJ that extends from East Asia to the west coast of North America.     

2008年T213与德国降水数值预报产品对比检验

为了解各种数值预报的误差特点,更好地在预报过程中选择数值预报产品作为参考依据,将中国国家气象中心的T213降水预报与德国降水预报分别进行晴雨预报检验,对2008年5—8月东北地区降水资料进行对比分析。结果表明：两种模式24-120h预报正确率为60％-70％,随着预报时效的增加,正确率呈下降趋势,德国降水预报的正确率高于T213,两种预报漏报率均明显小于空报率,T213漏报率较低,为5％左右,德国降水预报空报率较低,为20％左右。对2008年4-6月出现东北冷涡过程的两种模式降水预报进行对比分析,发现德国降水预报正确率明显高于T213预报,对冷涡降水预报有一定的指示意义。     

Observations of Dynamic Turbulence in the Lower Stratosphere over Inner Mongolia Using a High-resolution Balloon Sensor Constant Temperature Anemometer

We present characterizations of the dynamic turbulence in the lower stratosphere measured by a new balloon-based system designed for detecting finer scale dynamic turbulence. The balloon-based system included a constant temperature anemometer(CTA) operating at a sampling rate of 2 k Hz at an ascent speed of 5 m s^-1(corresponding to a vertical resolution of 2.5 mm), an industrial personal computer, batteries, sensors for ambient temperature and humidity, an A/D converter, and others. The system was successfully launched to 24 km altitude over Bayannur City, Inner Mongolia Province. Results show the spatial intermittence of the turbulence layers, with clear boundaries between turbulent and nonturbulent regions. This is the first time that the dynamic turbulence spectrum down to the viscous sub-range has been obtained throughout the lower stratosphere over China. With that, the energy dissipation rates of dynamic turbulence could be calculated with high precision. The profile of the dissipation rates varied from 7.37 × 10^-7 to 4.23 W kg^-1 and increased with altitude in the stratosphere.     

Trans-Arctic shipping routes expanding faster than the model projections

Rapid declines in Arctic sea ice coverage over the past four decades have increased the commercial feasibility of trans-Arctic routes. However, the historical changes in navigability of trans-Arctic routes remain unclear, and projections by global circulation models (GCMs) contain large uncertainties since they cannot simulate long-term Arctic sea ice changes. In this study, we determined the changes in trans-Arctic routes from 1979 to 2019 by combining two harmonized high-quality daily sea ice products. We found that the trans-Arctic routes are becoming navigable much faster than projected by the GCMs. The navigation season for open water (OW) vessels along the Northeast Passage (NEP) has lengthened from occasionally navigable in the 1980 s to 92 ± 15 days in the 2010 s. In contrast, previous GCM projections have suggested that navigability would not be achieved until the mid-21st century. The 90-day safety shipping area for OW vessels expanded by 35% during 1979–2018, reaching 8.28 million km² in 2018, indicating an increasing rate of 0.08 ± 0.01 million km² per year. The shortest trans-Arctic routes were also shifted further north than the model projections. Regular ships have been able to safely travel north along the islands in the NEP and transit through the M’Clure Strait in the Canadian Arctic Archipelago during the 2010 s, while previous studies have projected that this would not be feasible until the mid-21st century. We also found that the improved navigability of trans-Arctic routes enables commercial ships to transport approximately 33–66% (at the same load factor) more goods from East Asia to Europe during the Arctic shipping season than by the traditional Suez Canal route. These findings highlight the need for aggressive actions to develop mandatory rules that promote navigation safety and strengthen environmental protection in the Arctic.