共查询到20条相似文献，搜索用时 15 毫秒
1.
J. UOTILA 《地球，A辑:动力气象学与海洋学》2001,53(1):112-128
The wind dependence of sea‐ice motion was studied on the basis of ice velocity and wind observations, and weather model output. The study area was a transition zone between open water and the ice‐covered ocean in the northern Baltic Sea. In the centre of the basin the sea‐ice motion was highly wind‐dependent and the linear relationship between the wind and the drift velocities explained 80% of the drift's variance. On the contrary, the wind‐drift dependence was low near the coast. The wind‐drift coherence was significant over a broader frequency range in the central part of the basin than for the coastal drift. The ice motion was simulated by a numerical model forced with five types of wind stress and with two types of current data, and the outcome was compared with the observed buoy drift. The wind and the wind‐induced surface current were the main factors driving the ice in the basin's centre, while internal ice stresses were of importance in the shear zone near the fast ice edge. The best wind forcing was achieved by applying a method dependent on atmospheric stability and ice conditions. The average air–ice drag coefficient was 1.4×10−3 with the standard deviation of 0.2×10−3 . The improvement brought about by using an accurate wind stress was comparable with that achieved by raising the model grid resolution from 18 km to 5 km. 相似文献
2.
This paper addresses some fundamental methodological issues concerning the sensitivity analysis of chaotic geophysical systems. We show, using the Lorenz system as an example, that a naïve approach to variational ("adjoint") sensitivity analysis is of limited utility. Applied to trajectories which are long relative to the predictability time scales of the system, cumulative error growth means that adjoint results diverge exponentially from the "macroscopic climate sensitivity"(that is, the sensitivity of time‐averaged properties of the system to finite‐amplitude perturbations). This problem occurs even for time‐averaged quantities and given infinite computing resources. Alternatively, applied to very short trajectories, the adjoint provides an incorrect estimate of the sensitivity, even if averaged over large numbers of initial conditions, because a finite time scale is required for the model climate to respond fully to certain perturbations. In the Lorenz (1963) system, an intermediate time scale is found on which an ensemble of adjoint gradients can give a reasonably accurate (O(10%)) estimate of the macroscopic climate sensitivity. While this ensemble‐adjoint approach is unlikely to be reliable for more complex systems, it may provide useful guidance in identifying important parameter‐combinations to be explored further through direct finite‐amplitude perturbations. 相似文献
3.
S. VANNITSEM 《地球，A辑:动力气象学与海洋学》2001,53(1):56-73
We compare the short‐ and medium‐range predictability of weather regimes of a quasi‐geostrophic model as defined by a hierarchical cluster algorithm and a Lyapunov‐based clustering method recently introduced in the literature. Both procedures lead to weather regimes displaying very different predictability properties on the short and medium range bases. While the former does not distinguish between stable and unstable weather regimes, the latter leads to clusters which do not display a good medium range predictability. We introduce a new clustering method taking advantages of the two previous techniques. Its application in the context of the quasi‐geostrophic model gives rise to regimes possessing at the same time a good medium range skill and well separated instability properties, indicating the possibility to build a systematic cartography of the short‐term predictability of weather fields in phase space. 相似文献
4.
A methodology for the correction of systematic errors in a simplified atmospheric general‐circulation model is proposed. First, a method for estimating initial tendency model errors is developed, based on a 4‐dimensional variational assimilation of a long‐analysed dataset of observations in a simple quasi‐geostrophic baroclinic model. Then, a time variable potential vorticity source term is added as a forcing to the same model, in order to parameterize subgrid‐scale processes and unrepresented physical phenomena. This forcing term consists in a (large‐scale) flow dependent parametrization of the initial tendency model error computed by the variational assimilation. The flow dependency is given by an analogues technique which relies on the analysis dataset. Such empirical driving causes a substantial improvement of the model climatology, reducing its systematic error and improving its high frequency variability. Low‐frequency variability is also more realistic and the model shows a better reproduction of Euro‐Atlantic weather regimes. A link between the large‐scale flow and the model error is found only in the Euro‐Atlantic sector, other mechanisms being probably the origin of model error in other areas of the globe. 相似文献
5.
The operability of marine operations, that is, the estimation of their weather downtime and duration, is traditionally determined either by means of risk analysis or Monte Carlo simulation techniques. The work presented herein establishes analytically the probability distribution and statistical parameters of the duration of individual activities of a marine project based on the theory of Markov chains. According to the proposed Markov model the distribution of the duration of an activity is associated with certain statistical properties of the return time, that is the period between two successive passages from the non-operable state. Information about individual activities is then combined according to the PNET methodology, as proposed by Ang, A. H. S., Abdelnour, J. & Chaker, A. A., Analysis of activity networks under uncertainty. J. Engng Mech. Div., ASCE, 101 (EM4) (1975) 373–387 which considers the sequence of the execution of the activities, as defined by the operations scenario of the project, in order to establish analytically the probability distribution of the duration of the project.
The statistical analysis distinguishes between activities which do not require a weather window for their execution and those which do. Policies which affect the execution of an activity are incorporated into the analysis by taking into consideration secondary tasks which are performed before or after its temporary suspension. Furthermore, performance efficiency factors are also introduced in order to reflect the influence of the prevailing sea state/vessel responses on the ability of the crew to carry out the activity.
The Markov model is applied for a range of uninterrupted durations to activities which do or do not require a weather window and the results are compared with those from a Monte Carlo simulation. Good agreement is obtained for the mean durations but significant deviation is evident for the second order moments. This behaviour is attributed to the length of the record and also to the distribution of the return times. Agreement between the results of the two models is generally better for activities of low uninterrupted duration which do not require a weather window.
Finally, the combined Markov/PNET methodology is illustrated with an example for a hypothetical project and results are compared with those from a Monte Carlo simulation. Similar conclusions to those mentioned above are drawn. 相似文献
6.
Tuomo M. Saloranta 《地球，A辑:动力气象学与海洋学》2000,52(1):93-108
A numerical 1‐dimensional fine grid sea ice thermodynamic model is constructed accounting specially for: (1) slush formation via flooding and percolation of rain‐ and snow meltwater, (2) the consequent snow ice formation via slush freezing, and (3) the effects of snow compaction on heat diffusion in snow cover. The model simulations from ice winter period 1979–90 are viewed against corresponding observations at the Kemi fast ice station (65 °39.8' N, 24° 31.4' E). The 11‐year averaged model results show good overall consistency with corresponding total ice thickness observations. The model slightly overestimates the snow ice thickness and underestimates the snow thickness in February and March, which is mainly addressed to the model assumption of isostatic balance (i.e., slush formation via flooding), which was probably not fully satisfied at the coastal Kemi fast ice station. Supposing that this assumption is nevertheless generally valid away from the very coastal fast ice zone, an estimate for sea ice sensitivity to changes in winter precipitation rate is produced. Increased precipitation leads to an increase only in snow ice thickness with little change in total ice thickness, while a reduction in precipitation of more than {213}50% causes a significant increase in total ice thickness. The difference in modeled total ice thickness for the case of artificially neglecting snow ice physics is about 25%, which indicates the importance of including snow ice physics in a sea ice model dealing with the seasonal sea ice zone. 相似文献
7.
海洋盐度在水循环、海洋环流、海洋生态系统、全球天气和气候变化等方面起着至关重要的作用。然而, 受观测的限制, 以往对海洋盐度的研究相对匮乏, 对其进行预报的工作更为少见。本文采用线性马尔可夫模型对印度洋海表面盐度(sea surface salinity, SSS)开展初步的预报工作。根据混合层盐度收支方程, 选择海表面高度(sea surface height, SSH)、海表面温度 (sea surface temperature, SST)、SSS等物理量的异常值作为模型的组成部分, 对印度洋SSS开展预报工作。结果表明, 马尔可夫模型可提前9个月对印度洋SSS进行较好的预报。此外, 南太平洋海表面温度异常(sea surface temperature anomaly, SSTA), 海表面高度异常(sea surface height anomaly, SSHA)和印度洋偶极子(Indian Ocean dipole, IOD)系数等遥相关因素的加入可将线性马尔可夫预报对印度洋SSS的预报效果(相关系数)平均提高10%。利用改进的模型对印度洋SSS进行提前1~11个月的“实时”预测, 得出预报的SSS时空变化特征与观测场相吻合。综上所述, 改进的线性马尔可夫模型对印度洋SSS具有一定的预测能力, 未来可进一步完善。 相似文献
8.
The use of linear estimation for the study of the information content of a given satellite radiance data set for temperature and humidity profile retrievals is first reviewed. A particular formulation of the retrieval approach is then used to obtain an intrinsic characterisation of the Infrared Atmospheric Sounding Interferometer (IASI) data set, in terms of accuracy versus vertical resolution of retrieved profiles. The performance of the IASI instrument alone is analysed and compared to that of the currently‐used HIRS‐TOVS. The problem is then regularized by addition of a priori independent information to the initial data set. The potential use of IASI data for some particular choices of the a priori information associated with practical problems such as profile inversion or data assimilation for weather forecasting is analysed. The approach is finally used to derive an "empirical" objective framework to define the vertical discretization adapted to these problems. 相似文献
9.
3‐dimensional variational algorithms are widely used for atmospheric data assimilation at the present time, particularly on the synoptic and global scales. However, mesoscale and convective scale phenomena are considerably more chaotic and intermittent and it is clear that true 4‐dimensional data assimilation algorithms will be required to properly analyze these phenomena. In its most general form, the data assimilation problem can be posed as the minimization of a 4‐dimensional cost function with the forecast model as a weak constraint. This is a much more difficult problem than the widely discussed 4DVAR algorithm where the model is a strong constraint. Bennett and collaborators have considered a method of solution to the weak constraint problem, based on representer theory. However, their method is not suitable for the numerical weather prediction problem, because it does not cycle in time. In this paper, the representer method is modified to permit cycling in time, in a manner which is entirely internally consistent. The method was applied to a simple 1‐dimensional constituent transport problem where the signal was sampled (perfectly and imperfectly) with various sparse observation network configurations. The cycling representer algorithm discussed here successfully extracted the signal from the noisy, sparse observations 相似文献
10.
A coupled air–sea general circulation model is used to simulate the global circulation. Different parameterizations of lateral mixing in the ocean by eddies, horizontal, isopycnal, and isopycnal plus eddy advective flux, are compared from the perspective of water mass transformation in the Southern Ocean. The different mixing physics imply different buoyancy equilibria in the surface mixed layer, different transformations, and therefore a variety of meridional overturning streamfunctions. The coupled‐model approach avoids strong artificial water mass transformation associated with relaxation to prescribed mixed layer conditions. Instead, transformation results from the more physical non‐local, nonlinear interdependence of sea‐surface temperature, air–sea fluxes, and circulation in the model's atmosphere and ocean. The development of a stronger mid‐depth circulation cell and associated upwelling when eddy fluxes are present, is examined. The strength of overturning is diagnosed in density coordinates using the transformation framework. 相似文献
11.
RICHARD BINTANJA 《地球，A辑:动力气象学与海洋学》2001,53(2):215-232
A one‐dimensional atmospheric surface layer model including turbulent diffusion and gravitational settling of suspended snow particles is used to simulate vertical profiles of snowdrift sublimation rates and the associated effects on the humidity and temperature profiles in the lowest 10 m. The simulations show that the thermodynamic feedback effects associated with snowdrift sublimation, i.e., strong increases in humidity and cooling, can significantly reduce the snowdrift sublimation rate, in particular in strong winds when large numbers of particles are being suspended. This negative feedback occurs because snowdrift sublimation depends on the undersaturation and temperature. Mechanisms that take away moisture from the surface layer, such as entrainment or horizontal advection of dry air, tend to weaken this feedback and enhance modelled snowdrift sublimation as the air generally remains undersaturated. Near the surface, however, the thermodynamic feedbacks dominate in strong winds, reducing the upward moisture flux from the surface. Then, snowdrift sublimation is the main contributor to the upward moisture flux at 10 m. Interestingly, in strong winds, the simulated total upward moisture flux in snowdrifting conditions is less than that in similar non‐drifting conditions. Hence, the model results indicate that occurrence of snowdrift sublimation may, counterintuitively, eventually lead to a reduction of the surface‐atmosphere moisture transport. 相似文献
12.
With the accelerated warming of the world, the safety and use of Arctic passages is receiving more attention.Predicting visibility in the Arctic has been a hot topic in recent years because of navigation risks and opening of ice-free northern passages. Numerical weather prediction and statistical prediction are two methods for predicting visibility. As microphysical parameterization schemes for visibility are so sophisticated, visibility prediction using numerical weather prediction models inclu... 相似文献
13.
A reliability based approach for analysis of offloading operations with two identical LNG carriers moored in a side-by-side configuration is suggested. The approach, although adopted for two tankers, is generally applicable to different types of LNG terminals. Different heading controls are conceivable for the discharging vessel in the case that the heading is not fixed: heading toward wind-sea, heading toward swell, heading toward wind and free weather vaning. The analyses show that heading toward the governing sea state is the most beneficial option for two identical LNG carriers moored in a side-by-side configuration. Strategies are discussed for estimation of limiting weather criteria that are consistent with prescribed target failure probabilities. 相似文献
14.
In this paper, we present a numerical procedure for solving a 2‐dimensional, compressible, and nonhydrostatic system of equations. A forward‐backward integration scheme is applied to treat high‐frequency and internal gravity waves explicitly. The numerical procedure is shown to be neutral in time as long as a Courant–Friedrichs–Lewy criterion is met. Compared to the leap‐frog‐scheme most models use, this method involves only two time steps, which requires less memory and is also free from unstable computational modes. Hence, a time‐filter is not needed. Advection and diffusion terms are calculated with a time step longer than sound‐wave related terms, so that extensive computer time can be saved. In addition, a new numerical procedure for the free‐slip bottom boundary condition is developed to avoid using inaccurate one‐sided finite difference of pressure in the surface horizontal momentum equation when the terrain effect is considered. We have demonstrated the accuracy and stability of this new model in both linear and nonlinear situations. In linear mountain wave simulations, the model results match the corresponding analytical solution very closely for all three cases presented in this paper. The analytical streamlines for uniform flow over a narrow mountain range were obtained through numerical integration of Queney's mathematical solution. It was found Queney's original diagram is not very accurate. The diagram had to be redrawn before it was used to verify our model results. For nonlinear tests, we simulated the famous 1972 Boulder windstorm and a bubble convection in an isentropic enviroment. Although there are no analytical solutions for the two nonlinear tests, the model results are shown to be very robust in terms of spatial resolution, lateral boundary conditions, and the use of the time-split scheme. 相似文献
15.
Salinity in a shallow estuary is affected by upland freshwater inputs (surface runoff, stream/canal flows, groundwater), atmospheric processes (precipitation, evaporation), marine connectivity, and wind patterns. In Everglades National Park (ENP) in South Florida, the unique Everglades ecosystem exists as an interconnected system of fresh, brackish, and salt water marshes, mangroves, and open water. For this effort a coastal aquifer conceptual model of the Everglades hydrologic system was used with traditional correlation and regression hydrologic techniques to create a series of multiple linear regression (MLR) salinity models from observed hydrologic, marine, and weather data. The 37 ENP MLR salinity models cover most of the estuarine areas of ENP and produce daily salinity simulations that are capable of estimating 65–80% of the daily variability in salinity depending upon the model. The Root Mean Squared Error is typically about 2–4 salinity units, and there is little bias in the predictions. However, the absolute error of a model prediction in the nearshore embayments and the mangrove zone of Florida Bay may be relatively large for a particular daily simulation during the seasonal transitions. Comparisons show that the models group regionally by similar independent variables and salinity regimes. The MLR salinity models have approximately the same expected range of simulation accuracy and error as higher spatial resolution salinity models. 相似文献
16.
This note provides a detailed theoretical derivation for the removal of non‐physical finite‐amplitude computational oscillations from the solution of the adjoint of a discretized model using the leapfrog finite‐difference scheme. Numerical results are shown using a 1‐dimensional shallow water equation model. 相似文献
17.
MICHAEL K. TIPPETT STEPHEN E. COHN RICARDO TODLING DAN MARCHESIN 《地球，A辑:动力气象学与海洋学》2000,52(5):533-553
Ensemble and reduced‐rank approaches to prediction and assimilation rely on low‐dimensional approximations of the estimation error covariances. Here stability properties of the forecast/analysis cycle for linear, time‐independent systems are used to identify factors that cause the steady‐state analysis error covariance to admit a low‐dimensional representation. A useful measure of forecast/analysis cycle stability is the bound matrix , a function of the dynamics, observation operator and assimilation method. Upper and lower estimates for the steady‐state analysis error covariance matrix eigenvalues are derived from the bound matrix. The estimates generalize to time‐dependent systems. If much of the steady‐state analysis error variance is due to a few dominant modes, the leading eigenvectors of the bound matrix approximate those of the steady‐state analysis error covariance matrix. The analytical results are illustrated in two numerical examples where the Kalman filter is carried to steady state. The first example uses the dynamics of a generalized advection equation exhibiting non‐modal transient growth. Failure to observe growing modes leads to increased steady‐state analysis error variances. Leading eigenvectors of the steady‐state analysis error covariance matrix are well approximated by leading eigenvectors of the bound matrix. The second example uses the dynamics of a damped baroclinic wave model. The leading eigenvectors of a lowest‐order approximation of the bound matrix are shown to approximate well the leading eigenvectors of the steady‐state analysis error covariance matrix. 相似文献
18.
The physical mechanism by which seasonally varying atmospheric wind stress exerted on the sea surface is communicated to the solid earth as oceanic pressure torque (continental torque) and bottom frictional torque is investigated with a linear shallow‐water numerical model of barotropic oceans. The model has a realistic land–ocean distribution and is driven by a seasonally varying climatic wind stress. A novel way to decompose the wind stress into rotational and non‐rotational components is devised. The rotational component drives ocean circulations as classical theories of wind‐driven circulations demonstrate. The non‐rotational component does not produce ocean circulations within the framework of a barotropic shallow‐water model, but balances with the pressure gradient force due to surface displacement in the steady state. Based on this decomposition, it is shown that most of the continental torque which plays a major role in producing the seasonal variation of length of day (LOD) is caused by the non‐rotational component of the wind stress. Both continental torque due to the wind‐driven circulation produced by the rotational component of the wind stress and the bottom frictional torque are of minor importance. 相似文献
19.
The present case study evaluates the downward longwave radiation at the surface (DLR) in several high‐resolution (≈1°) general circulation models (GCMs) using surface observations from a semiarid continental site in New South Wales, Australia (Uardry, 34.39°S, 142.30°E). This site is located on a large grassland plain uniform in both its land use and landcover type, and is therefore particularly well suited for a comparison with GCM grid mean values. Monthly averages of newly constructed clear‐sky and all‐sky DLR climatologies and the resulting cloud‐radiative forcing are compared. It is shown that the GCMs exceed the observed DLR under cloud‐free conditions by 10–20 W m−2 at this semiarid site on an annual basis, with a strong seasonal dependence. The calculated clear‐sky fluxes are overestimated during the warmer summer season, with large absolute values of DLR, while the biases are reduced in the colder and dryer winter season with smaller fluxes. This gives direct support for recent evidence that the DLR model biases depend systematically on the thermal and humidity structure of the cloudless atmosphere. Fluxes from strongly emitting atmospheres tend to be overestimated, but may be underestimated from atmospheres with smaller emission. This points to common problems inherent in the simulation of the emission from the cloudless atmosphere in current longwave radiation codes.
The comparisons of the all‐sky climatologies at Uardry show that the clear‐sky biases are partly masked in the models with an insufficient cloud‐radiative forcing, thereby counterbalancing the excessive DLR of the cloud‐free atmosphere. On the other hand, when the cloudradiative forcing is improved, the biases in the cloud‐free atmosphere become fully apparent in the all‐sky fluxes. 相似文献
The comparisons of the all‐sky climatologies at Uardry show that the clear‐sky biases are partly masked in the models with an insufficient cloud‐radiative forcing, thereby counterbalancing the excessive DLR of the cloud‐free atmosphere. On the other hand, when the cloudradiative forcing is improved, the biases in the cloud‐free atmosphere become fully apparent in the all‐sky fluxes. 相似文献
20.
This paper presents an analysis of the solutions for a steady state latent heat polynya generated by an applied wind stress acting over a semi‐enclosed channel using: (a) a dynamic–thermodynamic sea ice model, and (b) a steady state flux model. We examine what processes in the sea ice model are responsible for the maintenance of the polynya and how sensitive the results are to the choice of rheological parameters. We find that when the ice is driven onshore by an applied wind stress, a consolidated ice pack forms downwind of a zone of strong convergence in the ice velocities. The build‐up of internal stresses within the consolidated ice pack becomes a crucial factor in the formation of this zone and results in a distinct polynya edge. Furthermore, within the ice pack the across‐channel ice velocity varies with the across‐channel distance. It is demonstrated that provided this velocity is well represented, the steady state polynya flux model solutions are in close agreement with those of the sea ice model. Experiments with the sea ice model also show that the polynya shape and area are insensitive to (a) the sea ice rheology; (b) the imposition of either free‐ slip or no‐slip boundary conditions. These findings are used in the development of a simplified model of the consolidated ice pack dynamics, the output of which is then compared with the sea ice model results. Finally, we discuss the relevance of this study for the modelling of the North Water Polynya in northern Baffin Bay. 相似文献