AIRS资料质量控制对飓风路径模拟的影响试验

对WRFDA模式中AIRS亮温资料质量控制方案进行了检验,并以美国Earl飓风为例进行数值试验,研究了质量控制方案对飓风路径模拟的影响。试验结果表明:WRFDA模式中11条质量控制原则对红外高光谱AIRS亮温资料的同化效果影响很大,不论是加入逐条质量控制原则,还是缺席某条质量控制原则,飓风路径的模拟情况都比不上控制试验;而在所有质量控制原则都加入之后,在大部分模拟时段内同化试验中模拟的飓风路径偏差都要小于控制试验,而且同化试验中最大路径偏差也小于控制试验。不同的质量控制原则对观测资料的过滤能力也不一样,其中地表发射率Jacobian分量检测、临边检测、云检测和SST检测等4个质量控制原则剔除卫星资料数量相对较多。本文中AIRS亮温资料质量控制方案的对比试验,可以为中国发展红外高光谱卫星系统提供非常有益的借鉴和参考。     

Global comparison of the regional rainfall results of enhanced greenhouse coupled and mixed layer ocean experiments: Implications for climate change scenario development

The extent of agreement amongst current global climate models (GCMs) on the global pattern of rainfall change simulated under enhanced greenhouse conditions is assessed. We consider the results of five experiments which use a simple mixed layer ocean formulation and five which use a fully dynamic ocean model (coupled experiments). For many regions of the northern hemisphere there is strong agreement amongst both mixed layer and coupled experiments on the sign of simulated rainfall change. However, in the southern hemisphere there are large, and apparently systematic, differences between the coupled and mixed layer experiments. In particular, whereas the mixed layer experiments agree on simulated rainfall increase in summer in the tropics and subtropics of the Australian sector, the coupled experiments agree (although more weakly) on rainfall decreases. These differences appear to relate to the much reduced warming simulated by the coupled experiments in the high latitudes of the southern hemisphere. However, recent oceanographie evidence suggests that this suppressed warming may be considerably overestimated. We conclude therefore that despite the in-principle advantages of coupled models, it may be too soon to base some regionally specific climate change scenarios solely on the results of coupled experiments.     

Experiment-based policy change over time: Learning from experiences in the Dutch fen landscape

This study provides insight into the potential of experiment-based policy making to address complex environmental challenges. The investigation shows the usefulness of making an analytical distinction between incremental and synoptic policy experiments to understand successful change over time. Incremental refers here to gradual adjustment of policies in a specific context, synoptic to radical change of policies on a large scale. Success is understood in terms of drawing lessons and realising policy goals, enabling diffusion and improving environmental status. This unique study, including both the initial investigation and a follow-up 10 years after, analyses the dynamics that emerged from seven empirical cases to identify factors affecting the design and success of these two types of experiments. We studied three incremental experiments and four synoptic experiments in the Dutch fen landscape to learn from their practical experiences. This densely populated and commercially used landscape is vulnerable to environmental pressure such as precipitation peaks, sea level rise and droughts. Initiators of the incremental experiments were able to change land-use management practices, albeit staying relatively close to the status quo. In contrast, while initiators of the synoptic experiments expressed higher ambitions in terms of change, these experiments tended to experience stagnation and difficulties in implementation and diffusion. Observations reveal that experiment-based policy change may crucially depend on capacity to deal with stakeholder dynamics and to embed experiments within multi-level institutional settings.     

Experiments of the WRF three-/four-dimensional variational (3/4DVAR) data assimilation in the forecasting of Antarctic cyclones

The three-/four-dimensional variational data assimilation systems (3/4DVAR) of the Weather Research and Forecasting (WRF) model were explored in the forecasting of two Antarctic synoptic cyclones, which had large influence on the Ross Sea/Ross Ice Shelf region in October 2007. A suite of variational data assimilation experiments, including regular 3DVAR, high-resolution 3DVAR, and 4DVAR experiments, were designed to evaluate their performances in weather analysis and forecasting in Antarctica. In general, both 4DVAR and high-resolution 3DVAR experiments showed better forecasting skill than regular 3DVAR experiments. High-resolution 3DVAR experiments were the most efficient in reducing the analysis errors of surface winds and temperature, and had the best performance during the first 24 h of forecasting. However, during the following forecast period, 4DVAR experiments showed either better or about comparable performance to high-resolution 3DVAR experiments. These results indicate that increasing the spatial resolution during 3DVAR is an economical approach to improving the weather analysis and forecasting over Antarctica. At the same time, the 4DVAR approach had a longer impact on forecasting than the high-resolution 3DVAR approach. Understandably, both of the variational assimilation approaches are promising techniques toward improving the regional analysis and forecasting over Antarctica.     

Atmosphere and ocean dynamics: contributors to the European Little Ice Age?

The role of a reduction in the Atlantic meridional overturning and that of a persistently negative North Atlantic Oscillation in explaining the coldness of the European Little Ice Age (LIA) has been assessed in two sets of numerical experiments. These experiments are performed using an intermediate complexity climate model and a full complexity GCM. The reduction in the Meridional Overturning Circulation (MOC) of ca. 25% is triggered by a conventional fresh-water hosing set-up. A persistently negative NAO winter circulation, at NAO-index value ?0.5, is imposed using recently developed data-assimilation techniques applicable on paleoclimatic timescales. The hosing experiments lead to a reduction in oceanic meridional heat transport and cooler sea-surface temperatures. Next to a direct cooling effect on European climate, the change in ocean surface temperatures feedback on the atmospheric circulation modifying European climate significantly. The data-assimilation experiments showed a reduction of winter temperatures over parts of Europe, but there is little persistence into the summer season. The output of all model experiments are compared to reconstructions of winter and summer temperature based on the available temperature data for the LIA period. This demonstrates that the hypothesis of a persistently negative NAO as an explanation for the European LIA does not hold. The hosing experiments do not clearly support the hypothesis that a reduction in the MOC is the primary driver of LIA climate change. However, a reduction in the Atlantic overturning might have been a cause of the European LIA climate, depending on whether there is a strong enough feedback on the atmospheric circulation.     

The long-time decay of rotating homogeneous flows over variable topography

The long-time decay of rotating, homogeneous flows over variable topography is analyzed by means of laboratory experiments and numerical simulations. The influence of the topography on the flow evolution is associated with stretching and squeezing effects on fluid columns as they experience changes in depth, and with viscous effects produced by the boundary condition at the solid bottom of the tank. In particular, experiments with a sine-shaped topography in one of the horizontal directions are analyzed by using two different basic flows. First, the evolution of dipolar vortices drifting across the topography while slowly decaying, is examined. The second set of experiments considers the evolution and decay of an initially circular vortex transforming into a tripolar structure. The experiments are well represented by numerical simulations based on a quasi-two-dimensional formulation with variable topography.     

Mesoscale vortex shedding from large islands: A comparison with laboratory experiments of rotating stratified flows

Summary Mesoscale vortex shedding in the wake of large islands has been analysed with respect to the influence of stratification and background rotation. Comparison with results from laboratory experiments on rotating-stratified flows past isolated obstacles based on Rossby and Froude numbers have shown that forFr<0.4 vortex shedding can be expected for moderate rotation rates. Rossby numbers for cases of atmospheric vortex streets were found to be much larger than in laboratory experiments. Depending on the Froude number different flow pattern in the wake of three-dimensional obstacles have been identified in both, laboratory experiments and atmospheric observations.With 5 Figures     

Assimilation of Sea Surface Temperature in a Global Hybrid Coordinate Ocean Model

The Hybrid Coordinate Ocean Model(HYCOM) uses different vertical coordinate choices in different regions. In HYCOM, the prognostic variables include not only the seawater temperature, salinity and current fields, but also the layer thickness. All prognostic variables are usually adjusted in the assimilation when multivariate data assimilation methods are used to assimilate sea surface temperature(SST). This paper investigates the effects of SST assimilation in a global HYCOM model using the Ensemble Optimal Interpolation multivariate assimilation method. Three assimilation experiments are conducted from 2006–08. In the first experiment, all model variables are adjusted during the assimilation process. In the other two experiments, the temperature alone is adjusted in the entire water column and in the mixed layer. For comparison, a control experiment without assimilation is also conducted. The three assimilation experiments yield notable SST improvements over the results of the control experiment. Additionally, the experiments in which all variables are adjusted and the temperature alone in all model layers is adjusted, produce significant negative effects on the subsurface temperature. Also, they yield negative effects on the subsurface salinity because it is associated with temperature and layer thickness. The experiment adjusting the temperature alone in the mixed layer yields positive effects and outperforms the other experiments. The heat content in the upper 300 m and 300–700 m layers further suggests that it yields the best performance among the experiments.     

固体降水自动化观测试验

介绍了2006年1～9月在吉林长春站、黑龙江通河站、新疆大西沟站进行的固体降水自动化观测仪器对比试验,共有6个厂家3种类型21套仪器参加了为期9个月的对比试验,本次对比采用WMO（1985）推荐的双栅式对比用标准器（DFIR）作为试验对比标准器。通过对比试验考察了融雪型雨雪量计、称重式降水计、激光式降水计测量不同形态降水的性能特点,为固体降水自动化观测仪器选型提供依据,推进固体降水业务化工作。     

不同初始场及侧边界对台风路径数值预报的影响

利用T213L31及T106L19所提供的初始场及侧边界就不同初始场及侧边界对台风路径数值预报的影响进行了敏感性试验。试验结果表明，从总体平均距离误差来看，初始场及侧边界的改进可以明显提高台风路径数值预报能力。通过不同的初始场及侧边界的相互组合(T106L19的初始场及侧边界；T106L19的初始场、T213L31的侧边界；T213L31的初始场、T106L19的侧边界；T213L31的初始场及侧边界)，对不同类型的台风路径预报、对初始场及侧边界的敏感性进行了初步试验。结果表明：非转向阶段的台风对初始场较为敏感，而转向阶段的台风对侧边界及初始场都很敏感。