Estimating present day extreme water level exceedance probabilities around the coastline of Australia: tides, extra-tropical storm surges and mean sea level

The occurrence of extreme water levels along low-lying, highly populated and/or developed coastlines can lead to considerable loss of life and billions of dollars of damage to coastal infrastructure. Therefore it is vitally important that the exceedance probabilities of extreme water levels are accurately evaluated to inform risk-based flood management, engineering and future land-use planning. This ensures the risk of catastrophic structural failures due to under-design or expensive wastes due to over-design are minimised. This paper estimates for the first time present day extreme water level exceedence probabilities around the whole coastline of Australia. A high-resolution depth averaged hydrodynamic model has been configured for the Australian continental shelf region and has been forced with tidal levels from a global tidal model and meteorological fields from a global reanalysis to generate a 61-year hindcast of water levels. Output from this model has been successfully validated against measurements from 30 tide gauge sites. At each numeric coastal grid point, extreme value distributions have been fitted to the derived time series of annual maxima and the several largest water levels each year to estimate exceedence probabilities. This provides a reliable estimate of water level probabilities around southern Australia; a region mainly impacted by extra-tropical cyclones. However, as the meteorological forcing used only weakly includes the effects of tropical cyclones, extreme water level probabilities are underestimated around the western, northern and north-eastern Australian coastline. In a companion paper we build on the work presented here and more accurately include tropical cyclone-induced surges in the estimation of extreme water level. The multi-decadal hindcast generated here has been used primarily to estimate extreme water level exceedance probabilities but could be used more widely in the future for a variety of other research and practical applications.     

Changes in the occurrence of storm surges around the United Kingdom under a future climate scenario using a dynamic storm surge model driven by the Hadley Centre climate models

 A potential consequence of climate change is an alteration of the frequency of extreme coastal storm surge events. It is these extreme events which, from an impacts point of view, will be of more concern than the slow inundation of coastal areas by century scale changes in mean sea level. In this study, a 35 km resolution storm surge model of the North west European continental shelf region has been driven by winds and pressures from the Hadley Centre nested regional climate model. Simulations of both present day and future climate (the end of the twentyfirst century) have been performed. The results suggest that, in addition to the effect of rising mean sea level, at many locations around the United Kingdom coastline future changes in local meteorology will lead to further significant changes in the return periods of extreme storm surge events. At most sites, this meteorologically forced change represents a reduction in return period. Received: 18 September 2000 / Accepted: 8 February 2001     

Evaluation of the combined risk of sea level rise, land subsidence, and storm surges on the coastal areas of Shanghai, China

Shanghai is a low-lying city (3–4?m elevation) surrounded on three sides by the East China Sea, the Yangtze River Estuary, and Hangzhou Bay. With a history of rapid changes in sea level and land subsidence, Shanghai is often plagued by extreme typhoon storm surges. The interaction of sea level rise, land subsidence, and storm surges may lead to more complex, variable, and abrupt disasters. In this paper, we used MIKE 21 models to simulate the combined effect of this disaster chain in Shanghai. Projections indicate that the sea level will rise 86.6?mm, 185.6?mm, and 433.1?mm by 2030, 2050, and 2100, respectively. Anthropogenic subsidence is a serious problem. The maximum annual subsidence rate is 24.12?mm/year. By 2100, half of Shanghai is projected to be flooded, and 46?% of the seawalls and levees are projected to be overtopped. The risk of flooding is closely related to the impact of land subsidence on the height of existing seawalls and levees. Land subsidence increases the need for flood control measures in Shanghai.     

拉格朗日方法诊断2007年7月中国东部系列极端降水的水汽输送路径及其可能蒸发源区

以2007年7月中、下旬中国中东部地区系列极端降水为研究对象,分别以NCEP/NCAR分析资料和中尺度气象模式(WRF)模拟输出驱动拉格朗日三维粒子输送模式(FLEXPART),通过对极端降水有贡献的大气粒子群(气块)的后向轨迹,诊断了极端降水事件的水汽输送路径及其可能蒸发源区,并定量估算了不同水汽源区对降水事件的相对贡献大小。结果表明,极端降水事件的水汽输送可以向上游追溯到阿拉伯海和西亚地区,青藏高原的地形和副热带高压对水汽输送路径具有重要影响。源于热带和副热带低纬度地区的气块在到达降水区域之前,经历了多次的降水和蒸发过程,其中,阿拉伯海、印度半岛、孟加拉湾、中南半岛的缅甸以及中国西南部的川、滇等地区都是水汽的蒸发源区,上述所有源区为极端降水事件贡献了约80%的水汽。但是,不同水汽源区的相对贡献计算结果发现,陆地蒸发对水汽贡献相对重要,尤其是中南半岛的缅甸、中国的川、滇等地区的地表前期蒸发对这次极端降水的贡献超过了40%,这表明上游地区前期的土壤湿度异常可能对极端降水的发生具有重要的指示意义。     

热带风暴形成过程中的涡旋自组织及其复杂性：一个典型个例的红外云图分析

应用高分辨率的红外卫星云图资料,对台风8807从热带低压发展成热带风暴的过程进行了分析,给出了热带风暴形成过程伴随着复杂涡旋自组织现象的观测证据。结果发现,涡旋自组织可在从数十到十万km2的各种尺度扰动云团之间发生,发生合并的云团除了理想数值试验中的两类前景之外,还存在第三类前景,即较大尺度的云团会发生分裂,部分与其他云团合并。合并形成的新云团可能发展也有可能减弱,这应取决于新云团所处的环境以及是否有其他云团补充并入。用二维Fourier分析可以清楚地看到8807发展成热带风暴的过程可分为两个阶段,一是多涡共存,不同方向、不同尺度系统能量转换频繁阶段,另一是单涡发展阶段,谱密度等值线形态稳定少变。分维计算结果表明,热带风暴形成过程中的相关云团较已有研究分析的云团略显规则,分维略小于已有结果。此外,临界温度取值不同,分维是有差异的,边界温度越低,分维越小。     

Numerical investigation of an extreme storm with the Canadian Regional Climate Model: the case study of windstorm VIVIAN, Switzerland, February 27, 1990

 The windstorm VIVIAN that severely affected Switzerland in February 1990 has been investigated using the Canadian Regional Climate Model (CRCM). This winter storm was characterised by a deep cyclone in the North Atlantic and by strong geopotential and baroclinic north-south gradients in the troposphere over Western Europe resulting in high windspeeds in Switzerland. Our principal emphasis is to demonstrate the ability of the CRCM to simulate the windfield intensity and patterns. In order to simulate winds at very high resolution we operate an optimal multiple self-nesting with the CRCM in order to increase the horizontal and vertical resolution. The simulation starts with downscaling NCEP-NCAR reanalyses at 60 km with 20 vertical levels, followed by an intermediate 5-km simulation with 30 vertical levels nested in the former. The 5-km output is in a final phase used for initial and lateral conditions for a 1-km resolution simulation with 46 vertical levels. The multiple self-nesting in the horizontal is necessary to reach sufficient resolution to better capture the orographic forcing that modulates the atmospheric circulation at fine scales, whereas the vertical resolution enhancement helps to better simulate the boundary layer that modulates the windspeed along the surface and better represents the atmospheric circulation with a complex vertical structure (low-level jets, gravity waves and frontal features). It has also been found that the simulated temporal variability of the windfield and of most variables at the finer scales increases with the increasing nesting frequency. This indicates that as we progress towards finer scales in the horizontal, the vertical and the nesting frequency enhancement helps to simulate windspeed variability. However, the variability within the larger domain is limited by the archival frequency of reanalysis data that cannot resolve disturbances with time scale shorter than 12 h. Results show that while the model simulates well the synoptic-scale flow at 60-km resolution, cascade self-nesting is necessary to capture fine-scale features of the topography that modulate the flow that generate localised wind enhancement over Switzerland. Received: 6 July 2000 / Accepted: 13 February 2001     

Relationship between cross-equatorial flows over the Bay of Bengal and Australia in boreal summer:Role of tropical diabatic heating

本文通过对1979-2017年夏季925 hPa经向风异常进行经验正交函数(EOF)分解,研究了亚澳季风区内越赤道气流的年际变化特征.结果表明,越赤道气流的第一模态表现为亚澳季风区内不同通道间的同相变化,即一致加强或减弱;第二模态表现为孟加拉湾和澳大利亚越赤道气流的反相变化,其中新几内亚和孟加拉湾越赤道气流的反相变化最...     

TraCE-21ka的模拟评估及误差分析

TraCE-21ka是全球首个利用全耦合模式针对末次盛冰期(LGM)至今气候演变的瞬变模拟。利用现代再分析资料和历史特征时期重建的连续冻土边界对TraCE-21ka模拟做了评估。结果表明TraCE-21ka能够较好地模拟现代半球尺度环流和降水的空间形态,对东亚地区的模拟冬季较好而夏季欠佳。TraCE-21ka模拟的现代时期与再分析资料相比偏冷,北半球年平均表面温度比再分析资料低3～4 ℃,基于现代温度误差的分析表明TraCE-21ka对东亚地区气候演变的模拟欠佳。对于历史特征时期,重建的连续冻土边界线指示TraCE-21ka模拟的亚欧大陆在LGM偏暖,全新世中期偏冷,即低估了LGM以来的变温幅度。利用连续冻土边界线的年均表面温度约为-7 ℃这一特性,进一步定量评估出TraCE-21ka模拟的亚欧大陆中纬地区从LGM至今的升温幅度约为真实气候的40％。通过分析近百年全球升温速率证实TraCE-21ka的气候敏感性显著偏低,由此产生的误差在瞬变模拟中会不断累积。     

Zonally resolved water vapour coupling with tropical tropopause temperature: Seasonal and interannual variability,and influence of the Walker circulation

Climate Dynamics - Global stratospheric water vapour is strongly coupled to the tropical cold-point tropopause temperatures. We quantified the coupling between temperature and water vapour in the...     

Climate change impact on water and salt balances: an assessment of the impact of climate change on catchment salt and water balances in the Murray-Darling Basin, Australia

Climate change has potentially significant implications for hydrology and the quantity and quality of water resources. This study investigated the impacts of climate change and revegetation on water and salt balance, and stream salt concentration for catchments within the Murray-Darling Basin, Australia. The Biophysical Capacity to Change model was used with climate change scenarios obtained using the CSIRO DARLAM 125 (125 km resolution) and Cubic Conformal (50 km resolution) regional climate models. These models predicted up to 25% reduction in mean annual rainfall and a similar magnitude of increase in potential evapotranspiration by 2070. Relatively modest changes in rainfall and temperature can lead to significant reductions in mean annual runoff and salt yield and increases in stream salt concentrations within the Basin. The modelled reductions in mean annual runoff were up to 45% in the wetter/cooler southern catchments and up to 64% in the drier/hotter western and northern catchments. The maximum reductions in salt yield were estimated to be up to 34% in the southern catchments and up to 49% in the northern and western catchments. These changes are associated with average catchment rainfall decreases of 13 to 21%. The results suggest that percentage changes in rainfall will be amplified in runoff. This study demonstrates that climate change poses significant challenges to natural resource management in Australia.     

Simulated variations of eolian dust from inner Asian deserts at the mid-Pliocene, last glacial maximum, and present day: contributions from the regional tectonic uplift and global climate change

Northern Tibetan Plateau uplift and global climate change are regarded as two important factors responsible for a remarkable increase in dust concentration originating from inner Asian deserts during the Pliocene?CPleistocene period. Dust cycles during the mid-Pliocene, last glacial maximum (LGM), and present day are simulated with a global climate model, based on reconstructed dust source scenarios, to evaluate the relative contributions of the two factors to the increment of dust sedimentation fluxes. In the focused downwind regions of the Chinese Loess Plateau/North Pacific, the model generally produces a light eolian dust mass accumulation rate (MAR) of 7.1/0.28?g/cm²/kyr during the mid-Pliocene, a heavier MAR of 11.6/0.87?g/cm²/kyr at present, and the heaviest MAR of 24.5/1.15?g/cm²/kyr during the LGM. Our results are in good agreement with marine and terrestrial observations. These MAR increases can be attributed to both regional tectonic uplift and global climate change. Comparatively, the climatic factors, including the ice sheet and sea surface temperature changes, have modulated the regional surface wind field and controlled the intensity of sedimentation flux over the Loess Plateau. The impact of the Tibetan Plateau uplift, which increased the areas of inland deserts, is more important over the North Pacific. The dust MAR has been widely used in previous studies as an indicator of inland Asian aridity; however, based on the present results, the interpretation needs to be considered with greater caution that the MAR is actually not only controlled by the source areas but the surface wind velocity.     

Paleo-evolution of the Black Sea watershed: sea level and water transport through the Bosporus Straits as an indicator of the Lateglacial–Holocene transition

The paleo-evolution of the Black Sea level during the Lateglacial–Holocene transition is still unclear, which motivates us to provide new estimates for that period based on the analysis of water budget. Hydrological conditions in the Black Sea catchment area are reconstructed here using water balance equation, available data, and constraints based on simple theory relating the runoff ratio with climatic characteristics. In order to estimate the impact of the aridity of climate we consider two alternative scenarios: (1) climate change under constant in time gradient in precipitation and evaporation over land and sea, and (2) climate change accounting for changes in the horizontal distribution of precipitation and evaporation. Hydrological data are compiled from available present-day data and paleo-observations. A number of sensitivity experiments is carried out revealing that the hydrological conditions in the Black Sea watershed should have evolved towards a very arid climate (similar to the present-day climate in the Caspian Sea area) in order to initiate a drop of sea level of ∼100 m below the sill depth of the Bosporus Straits, as speculated in some recent research. Estimates of sea level changes reveal a qualitative agreement with the coast-line evolution inferred from paleo-observations. The Younger Dryas draw down of the Black Sea starts at about 13.3 to 13 kyr BP, with a maximum low-stand of 104 m at 11.5 kyr BP. In an arid climate scenario the sea level reaches the outlet at about 8.8 to 7.4 kyr BP. Approximately at that time, Mediterranean sea level was about 10 m higher, making flooding events of the Black Sea possible. However, the nature and exact timing of the Holocene reconnection depends also on other (not well known) factors; one of them is the Bosporus sill depth, motivating further research in this field. Estimates of the water transport through the Bosporus Straits are also provided for the time of Lateglacial–Holocene transition.