Correcting circulation biases in a lower-resolution global general circulation model with data assimilation

In this study, we aim at developing a new method of bias correction using data assimilation. This method is based on the stochastic forcing of a model to correct bias by directly adding an additional source term into the model equations. This method is presented and tested first with a twin experiment on a fully controlled Lorenz ’96 model. It is then applied to the lower-resolution global circulation NEMO-LIM2 model, with both a twin experiment and a real case experiment. Sea surface height observations are used to create a forcing to correct the poorly located and estimated currents. Validation is then performed throughout the use of other variables such as sea surface temperature and salinity. Results show that the method is able to consistently correct part of the model bias. The bias correction term is presented and is consistent with the limitations of the global circulation model causing bias on the oceanic currents.     

A non-breaking-wave-generated turbulence mixing scheme for a global ocean general circulation model

Ocean Dynamics - A novel vertical mixing scheme to describe the influence of the non-breaking surface waves in ocean general circulation models is proposed based on the second-order turbulence...     

Accelerated continental-scale snowmelt and ecohydrological impacts in the four largest Siberian river basins in response to spring warming

River runoff from the four largest Siberian river basins (the Ob, Yenisei, Lena, and Kolyma) considerably contributes to freshwater flux into the Arctic Ocean from the Eurasian continent. However, the effects of variation in snow cover fraction on the ecohydrological variations in these basins are not well understood. In this study, we analysed the spatiotemporal variability of the maximum snow cover fraction (SCF_max) in the four Siberian river basins. We compared the SCF_max from 2000 to 2016 with data in terms of monthly temperature and precipitation, night-time surface temperatures, the terrestrial water storage anomaly (TWSA), the normalised difference vegetation index (NDVI), and river runoff. Our results exhibit a decreasing trend in the April SCF_max values since 2000, largely in response to warming air temperatures in April. We identified snowmelt water as the dominant control on the observed increase in the runoff contribution in May across all four Siberian river basins. In addition, we detected that the interannual river runoff was predominantly controlled by interannual variations in the TWSA. The NDVI in June was strongly controlled by the timing of the snowmelt along with the surface air temperature and TWSA in June. The rate of increase in the freshwater flux from the four Siberian rivers decreased from 2000 to 2016, exhibiting large interannual variations corresponding to interannual variations in the TWSA. However, we identified a clear increase trend in the freshwater flux of ~4 km³/year when analysing the long-term 39-year historical record (1978–2016). Our results suggest that continued global warming will accelerate the transition towards the earlier timing of snowmelt and spring freshwater flux into the Arctic Ocean. Our findings also highlight the effects of earlier snowmelt on ecohydrological changes in the Northern Hemisphere.     

Streamflow dynamics in a Mediterranean temporary river

Abstract

Mediterranean rivers are characterized by the irregularity of flow, harsh hydrological fluctuations and a profound transformation as the result of human activity. In this study, we investigate the streamflow response of a Mediterranean temporary river in which different groundwater, agriculture and urban contributions play an important role. Streamflow was measured at three nested gauging stations installed along Na Borges River, a lowland agricultural catchment (319 km²) on the island of Mallorca. Based on two hydrological years (2004/05 and 2005/06), potential evapotranspiration and surface water–groundwater interaction generated a succession of four different hydrological periods playing an important role in baseflow dynamics. The runoff coefficients were very low (<5%). At the event scale, groundwater also controlled runoff response, being very different according to hydrogeology, antecedent conditions and human impacts. During dry seasons, wastewater and karstic spring discharges maintain an influent regime into some streams. As a result, intense rainstorms in late summer generated water volumes over the impervious urban surfaces involved, with the result that quickflow was significant because the hydrological pathways were active. Citation Estrany, J., Garcia, C. & Alberich, R. (2010 Estrany, J., Garcia, C. and Batalla, R. J. 2010. Hydrological response of a small Mediterranean agricultural catchment. J. Hydrol., 380(1-2): 180–190. doi:10.1016/j.jhydrol.2009.10.035[Crossref] , [Google Scholar]) Streamflow dynamics in a Mediterranean temporary river. Hydrol. Sci. J. 55(5), 717–736.      

Three-dimensional decomposition method of global atmospheric circulation

By adopting the idea of three-dimensional Walker, Hadley and Rossby stream functions, the global atmospheric circulation can be considered as the sum of three stream functions from a global per- spective. Therefore, a mathematical model of three-dimensional decomposition of global atmospheric circulation is proposed and the existence and uniqueness of the model are proved. Besides, the model includes a numerical method leading to no truncation error in the discrete three-dimensional grid points. Results also show that the three-dimensional stream functions exist and are unique for a given velocity field. The mathematical model shows the generalized form of three-dimensional stream func- tions equal to the velocity field in representing the features of atmospheric motion. Besides, the vertical velocity calculated through the model can represent the main characteristics of the vertical motion. In sum, the three-dimensional decomposition of atmospheric circulation is convenient for the further in- vestigation of the features of global atmospheric motions.     

用全球大气环流模式嵌套区域气候模式模拟东亚现代气候

将区域气候模式RegCM2与中国科学院大气物理研究所的9层全球格点大气环流模式IAP-AGCM单向嵌套,对东亚现代气候进行数值模拟研究,同时检验和分析该嵌套模式的性能.已完成的10年积分结果表明,单向嵌套RegCM2由于具有较高分辨率和较完善的物理过程,因此对地面气温和降水的空间分布形势和季节变化趋势都有较好的模拟能力,且较与之嵌套的IAP-AGCM的模拟效果有较大改善,如在中国区域,它模拟的年均地面气温与实况的空间相关系数由全球环流模式的0.92提高到0.94,模拟的年均降水由0.5提高到0.7. 这与嵌套RegCM2能模拟出IAP-AGCM所不能分辨的中尺度信号有很大关系.     

变网格大气模式对1998年东亚夏季风异常的模拟研究

本文利用法国国家科研中心(CNRS)动力气象实验室(LMD)发展的可变网格的格点大气环流模式LMDZ4,对1998年东亚夏季降水进行了模拟,考查了变网格模式对东亚夏季降水的模拟性能.结果表明,模式在一定程度上能模拟出东亚夏季降水的极大值中心、夏季风雨带以及降水由东南向西北递减的空间分布特征.模式基本再现了1998年夏季两次雨带的进退特征,包括降水强度、雨带范围等,从而合理再现了1998年夏季江淮地区的"二度梅"现象.与观测相比,模拟的不足在于:在陡峭地形区附近存在虚假降水;江淮和华北地区以及四川盆地存在水汽输送的气旋式辐合偏差,同时高层环流辐散偏强,使得下层暖湿空气辐合上升、降水偏多;在东南地区存在反气旋式的水汽输送偏差,30°N以南地区降水偏少.对于1998年的"二度梅"现象,模拟偏差主要表现为长江中下游地区两次(特别是第二次)较强降水持续时间偏短,强降水范围偏小,而黄淮和华南地区却降水偏多.分析表明,模式对两次梅雨期降水的模拟偏差直接受环流形势模拟偏差的影响.LMDZ4区域模式版本的特点一是区域加密,二是加密区内预报场每10天向再分析资料恢复一次.敏感试验结果表明,LMDZ4加密区向强迫场的10天尺度恢复总体上有利于提高模式对华北降水的模拟能力,而对长江流域和华南降水的模拟具有不利影响.较之均匀网格模拟试验,加密试验由于在东亚的分辨率大大提高,对东亚夏季降水模拟效果更好.     

Optimisation of a parallel ocean general circulation model

This paper presents the development of a general-purpose parallel ocean circulation model, for use on a wide range of computer platforms, from traditional scalar machines to workstation clusters and massively parallel processors. Parallelism is provided, as a modular option, via high-level message-passing routines, thus hiding the technical intricacies from the user. An initial implementation highlights that the parallel efficiency of the model is adversely affected by a number of factors, for which optimisations are discussed and implemented. The resulting ocean code is portable and, in particular, allows science to be achieved on local workstations that could otherwise only be undertaken on state-of-the-art supercomputers.     

Effects of alternative cloud radiation parameterizations in a general circulation model

Using the National Center for Atmospheric Research (NCAR) general circulation model (CCM2), a suite of alternative cloud radiation parameterizations has been tested. Our methodology relies on perpetual July integrations driven by ±2 K sea surface temperature forcing. The tested parameterizations include relative humidity based clouds and versions of schemes involving a prognostic cloud water budget. We are especially interested in testing the effect of cloud optical thickness feedbacks on global climate sensitivity. All schemes exhibit negative cloud radiation feedbacks, i.e., cloud moderates the global warming. However, these negative net cloud radiation feedbacks consist of quite different shortwave and longwave components between a scheme with interactive cloud radiative properties and several schemes with specified cloud water paths. An increase in cloud water content in the warmer climate leads to optically thicker middle- and low-level clouds and in turn negative shortwave feedbacks for the interactive radiative scheme, while a decrease in cloud amount leads to a positive shortwave feedback for the other schemes. For the longwave feedbacks, a decrease in high effective cloudiness for the schemes without interactive radiative properties leads to a negative feedback, while no distinct changes in effective high cloudiness and the resulting feedback are exhibited for the scheme with interactive radiative properties. The resulting magnitude of negative net cloud radiation feed-back is largest for the scheme with interactive radiative properties. Even though the simulated values of cloud radiative forcing for the present climate using this method differ most from the observational data, the approach shows great promise for the future.     

Passive tracers in a general circulation model of the Southern Ocean

Passive tracers are used in an off-line version of the United Kingdom Fine Resolution Antarctic Model (FRAM) to highlight features of the circulation and provide information on the inter-ocean exchange of water masses. The use of passive tracers allows a picture to be built up of the deep circulation which is not readily apparent from examination of the veloCity or density fields. Comparison of observations with FRAM results gives good agreement for many features of the Southern Ocean circulation. Tracer distributions are consistent with the concept of a global “conveyor belt” with a return path via the Agulhas retroflection region for the replenishment of North Atlantic Deep Water.     

On the ratio noise-signal of long-periodic oscillations in the general atmospheric circulation

Summary Characteristic features of long-periodic oscillations in the general atmospheric circulation may be deduced from the correlation existing between the monthly, the three-monthly, the six-monthly and the twelve-monthly figures of the barometric pressure at two distant stations and from the autocorrelation of the same figures at each station.It is shown that the monthly figures of the barometric pressure at Easter Island and those at Djakarta during the six years 1950–1955 have, when plotted as a function of time, one oscillation in common, whereas further these figures have both their own rhythms and their own noise. The period of the common oscillation is 28.6 months, while the phase difference between both stations amounts to 180°. The r.m.s. value of the noise is 0.76 times the value of the amplitude of the oscillation. The rhythms differ in wave length, amplitude and phase.It is also shown that the monthly figures of the barometric pressure at Ponta Delgada and those at Stykkisholm have, when plotted as a function of time, a whole spectrum of oscillations jointly, while all oscillations have the same phase difference (238°) between both stations. These features are deduced from observations during 40 years (1896–1915 and 1921–1940). If yearly figures are used instead of monthly ones, then the greater part of the spectrum of oscillations is smoothed away and only a few common oscillations are left. The first common oscillation has a wave length of 26.5 months and the second one has a wave length of 38.7 months.     

北京沙尘频次的年际变化及其全球环流背景分析

本文采用相关和合成的分析方法,研究北京沙尘频次的年际变化及其冬、春季的全球环流背景.结果表明北京沙尘频次有年际变化的特点,并与全球范围内的环流异常相联系,特别是南、北半球的中高纬的环流异常.南半球环流异常与沙尘的联系在冬、春两季有很好的持续性和显著性,北半球中高纬环流异常与沙尘的联系冬季较春季显著.春季对流层高层的东亚西风急流增强能使低空的蒙古气旋加强和锋生,从而引起地面大风,为沙尘天气频次的增加提供动力条件.     

Cascading ocean basins: numerical simulations of the circulation and interbasin exchange in the Azov-Black-Marmara-Mediterranean Seas system

In this paper, we use the unstructured grid model SCHISM to simulate the thermohydrodynamics in a chain of baroclinic, interconnected basins. The model shows a good skill in simulating the horizontal circulation and vertical profiles of temperature, salinity, and currents. The magnitude and phases of the seasonal changes of circulation are consistent with earlier observations. Among the mesoscale and subbasin-scale circulation features that are realistically simulated are the anticyclonic coastal eddies, the Sebastopol and Batumi eddies, the Marmara Sea outflow around the southern coast of the Limnos Island, and the pathway of the cold water originating from the shelf. The superiority of the simulations compared to earlier numerical studies is demonstrated with the example of model capabilities to resolve the strait dynamics, gravity currents originating from the straits, high-salinity bottom layer on the shallow shelf, as well as the multiple intrusions from the Bosporus Strait down to 700 m depth. The warm temperature intrusions from the strait produce the warm water mass in the intermediate layers of the Black Sea. One novel result is that the seasonal intensification of circulation affects the interbasin exchange, thus allowing us to formulate the concept of circulation-controlled interbasin exchange. To the best of our knowledge, the present numerical simulations, for the first time, suggest that the sea level in the interior part of the Black Sea can be lower than the sea level in the Marmara Sea and even in some parts of the Aegean Sea. The comparison with observations shows that the timings and magnitude of exchange flows are also realistically simulated, along with the blocking events. The short-term variability of the strait transports is largely controlled by the anomalies of wind. The simulations demonstrate the crucial role of the narrow and shallow strait of Bosporus in separating the two pairs of basins: Aegean-Marmara Seas from one side and Azov-Black Seas from the other side. The straits of Kerch and Dardanelles provide sufficient interbasin connectivity that prevents large phase lags of the sea levels in the neighboring basins. The two-layer flows in the three straits considered here show different dependencies upon the net transport, and the spatial variability of this dependence is also quite pronounced. We show that the blocking of the surface flow can occur at different net transports, thus casting doubt on a previous approach of using simple relationships to prescribe (steady) outflow and inflow. Specific attention is paid to the role of synoptic atmospheric forcing for the basin-wide circulation and redistribution of mass in the Black Sea. An important controlling process is the propagation of coastal waves. One major conclusion from this research is that modeling the individual basins separately could result in large inaccuracies because of the critical importance of the cascading character of these interconnected basins.     

Equilibration mechanisms in an adjoint ocean general circulation model

We examine the equilibrated and time-evolving adjoint solutions of an ocean general circulation model. Adjoint models calculate the sensitivity of a diagnostic, (here, the strength of the meridional overturning) to all forcing fields in a single integration. The time evolution of the sensitivity patterns demonstrates the validity of the adjoint modeling approach over climatological time scales in coarse-resolution ocean models. Our objective is to identify the principle adjustment mechanisms through which the meridional overturning strength adapts to perturbations in wind and buoyancy forcing. The adjoint approach is shown to be a valuable alternative to traditional perturbation methods in highlighting the processes and time scales important to ocean and climate modeling.     

近60年全球大气环流经向模态的气候变化

本文根据1948～2004年NCEP/NCAR 1000 hPa、500 hPa、100 hPa高度场逐月再分析资料,分析了近60年全球大气环流经向模态的气候变化. 结果表明：近60年来第一模态从低层到高层都表现出高纬与低纬地区之间明显的反向变化关系,且随时间有明显的增强趋势. 第一模态位相发生了相反的改变,低纬地区由负距平演变为正距平,高纬地区由正距平演变为负距平. 1000 hPa和500 hPa高度场上的南半球比北半球变化激烈,而100 hPa高度场上的北半球比南半球变化激烈. 第二模态在1000 hPa高度场上,主要表现为南极涛动（AAO）和北极涛动(AO),且两涛动在年际、年代际尺度上表现出明显的负相关关系;在100 hPa高度场上,主要表现为南北半球高纬度地区之间的反向变化;500 hPa高度场是1000 hPa和100 hPa的一个过渡层次,主要表现出明显的南极涛动(AAO). 第二模态可能是南北半球中高纬环流相互作用的桥梁.     

扰动位能与大气环流异常的耦合关系及机理研究

本文在前期局地扰动位能理论工作基础上,进一步着眼于扰动位能与大气环流异常的关系和机理问题,采用奇异值分解(SVD)方法分析了扰动位能与高、中、低层大气动能的耦合相关模态以及年际变率情况,同时考察它与作为大气质量分布的海平面气压之间的相关关系,并从物理上初步探讨了扰动位能与大气动能以及质量的相关机理问题.结果表明,扰动位能自身变率的第一模态同时也是与大气环流异常场耦合变化的主要模态,与之相对应的大气动能和质量场的主模态也是其自身变率的支配模态,说明它们之间存在相互制约的物理机制.而且,热带外地区大气环流主模态特征的形成,与扰动位能兼具全球尺度纬向对称结构和局地尺度纬向非对称性密不可分.另外,冬季南北半球环状模指数与扰动位能的前两阶矩存在非常好的相关关系,而在夏季这种关系明显削弱,仅存在于南半球.     

Effects of precipitation physics algorithms on a simulated climate in a general circulation model

The purpose of this study is to investigate the effects of precipitation physics in a general circulation model (GCM) on a simulated climate. Experiments are performed under the single column model (SCM) framework to examine basic features and under the general circulation model framework to investigate the impact on seasonal simulation. The SCM simulation shows that convection processes in the model have a considerable influence on the change in vertical thermodynamic structure, resulting in a change in precipitation, whereas in the GCM framework stratiform precipitation physics play a distinct role in changing the atmospheric structure. The GCM experiments also show that the overall reduction of precipitation in simulations with prognostic stratiform precipitation physics is highly related to changes in cloudiness and corresponding changes in radiative flux, which in turn leads to the reduction of convective activities.     

Modelling particle residence times in agricultural river basins using a sediment budget model and fallout radionuclide tracers

Contemporary patterns in river basin sediment dynamics have been widely investigated but the timescales associated with current sediment delivery processes have received much less attention. Furthermore, no studies have quantified the effect of recent land use change on the residence or travel times of sediment transported through river basins. Such information is crucial for understanding contemporary river basin function and responses to natural and anthropogenic disturbances or management interventions. To address this need, we adopt a process‐based modelling approach to quantify changes in spatial patterns and residence times of suspended sediment in response to recent agricultural land cover change. The sediment budget model SedNet was coupled with a mass balance model of particle residence times based on atmospheric and fluvial fluxes of three fallout radionuclide tracers (⁷Be, excess ²¹⁰Pb and ¹³⁷Cs). Mean annual fluxes of suspended sediment were simulated in seven river basins (38–920 km²) in south‐west England for three land cover surveys (1990, 2000 and 2007). Suspended sediment flux increased across the basins from 0.5–15 to 1.4–37 kt y^‐1 in response to increasing arable land area between consecutive surveys. The residence time model divided basins into slow (upper surface soil) and rapid (river channel and connected hillslope sediment source area) transport compartments. Estimated theoretical residence times in the slow compartment decreased from 13–48 to 5.6–14 ky with the increase in basin sediment exports. In contrast, the short residence times for the rapid compartment increased from 185–256 to 260–368 d as the modelled connected source area expanded with increasing sediment supply from more arable land. The increase in sediment residence time was considered to correspond to longer sediment travel distances linked to larger connected source areas. This novel coupled modelling approach provides unique insight into river basin responses to recent environmental change not otherwise available from conventional measurement techniques. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.