On the Approximation of the Coriolis Terms in C-grid Models

A new representation of the Coriolis terms on the Arakawa C grid is proposed. The approximation dumps the grid-scale noise that arises because of spatial averaging of the Coriolis terms when the grid spacing is larger than the deformation radius. The proposed approximation can also be applied in C-grid schemes with semi-implicit treatment of the Coriolis terms. The new scheme is analyzed in the context of the linear inertial-gravity waves and its advantageous behavior is demonstrated with respect to the conventional technique

Covariance Localization with the Diffusion-Based Correlation Models

Improving the performance of ensemble filters applied to models with many state variables requires regularization of the covariance estimates by localizing the impact of observations on state variables. A covariance localization technique based on modeling of the sample covariance with polynomial functions of the diffusion operator (DL method) is presented. Performance of the technique is compared with the non-adaptive (NAL) and adaptive (AL) ensemble localization schemes in the framework of numerical experiments with synthetic covariance matrices in a realistically inhomogeneous setting. It is shown that the DL approach is comparable in accuracy with the AL method when the ensemble size is less than 100. With larger ensembles, the accuracy of the DL approach is limited by the local homogeneity assumption underlying the technique. Computationally, the DL method is comparable with the NAL technique if the ratio of the local decorrelation scale to the grid step is not too large

Simulations of Quasigeostrophic Currents Derived From Satellite Altimetry and Acoustic Tomography of an Open Ocean Region

Acoustic tomography (AT) and satellite altimetry (SA) measure properties of the ocean state with high temporal resolution. That makes these data suitable for long-term monitoring of mesoscale features in the open ocean regions, where the open boundaries are the major sources of model forecast uncertainties on timescales larger than 1 week. In this paper, a finite-difference quasigeostrophic model of an open ocean region is considered as a possible tool for interpolating AT-SA data in space and time. The assimilation algorithm is based upon the 4D variational data assimilation scheme controlled by the initial and boundary conditions of the model. The model configuration used in the simulations corresponds to the AT array deployed by the Japan Marine Science and Technology Center (JAMSTEC) in the region of the Kuroshio Extension in 1997. Twin data experiments show that mesoscale features in an area of 1000 km x 1000 km can be effectively monitored by five acoustic transceivers, measuring reciprocal travel times. The quality of assimilation is studied as a function of the position of the transceivers in the vertical and the effective number of monitored rays. It is shown that reciprocal travel time observations (differential tomography) in combination with SA provide a significant improvement of the quality of assimilation

Decadal Variability of Circulation in the Arctic Ocean Retrieved From Climatological Data by a Variational Method

[1] An inverse 3D finite-element ocean circulation model has been designed and used to study variability of the Arctic Ocean circulation in the last 4 decades. We obtained stationary model solutions with the temperature and salinity fields close to the ones given by the Environmental Working Group (EWG) [1998] atlas. Transports at the open boundaries, wind forcing and hydrographic fields are treated as unknowns, which are varied to minimize a quadratic cost function subject to model constraints. The inverse problem is solved for 10 gridded hydrographic data sets that were obtained as winter and summer averages of EWG data over each of the 4 decades (1950s to 1980s) and over the whole period of observations (1948-1993) documented in the atlas. The results show that Arctic circulation in the last 4 decades has undergone significant changes, which manifest themselves in (1) 10% reduction of the ventilation rate in the Atlantic sector of the Arctic Ocean; (2) substantial decrease of the advective heat and freshwater import at the lateral boundaries; (3) spinning down of the cyclonic gyre in the northern Greenland sea, which is partly driven by deep convection; and (4) 3400 km(3) increase of the net fresh water storage, with 75% taking place in the Atlantic sector of the Arctic Ocean. Most of these changes are similar to the ones observed on seasonal transition from winter to summer climatologies, and indicate that the Arctic Ocean is experiencing a shift toward a warmer state

A Hybrid Background Error Covariance Model for Assimilating Glider Data into a Coastal Ocean Model

A hybrid background error covariance (BEC) model for three-dimensional variational data assimilation of glider data into the Navy Coastal Ocean Model (NCOM) is introduced. Similar to existing atmospheric hybrid BEC models, the proposed model combines low-rank ensemble covariances B(m) with the heuristic Gaussian-shaped covariances B(0) to estimate forecast error statistics. The distinctive features of the proposed BEC model are the following: (i) formulation in terms of inverse error covariances, (ii) adaptive determination of the rank m of B(m) with information criterion based on the innovation error statistics, (iii) restriction of the heuristic covariance operator B(0) to the null space of B(m), and (iv) definition of the BEC magnitudes through separate analyses of the innovation error statistics in the state space and the null space of B(0). The BEC model is validated by assimilation experiments with simulated and real data obtained during a glider survey of the Monterey Bay in August 2003. It is shown that the proposed hybrid scheme substantially improves the forecast skill of the heuristic covariance model

A Method of Successive Corrections of the Control Subspace in the Reduced-Order Variational Data Assimilation

A version of the reduced control space four-dimensional variational method (R4DVAR) of data assimilation into numerical models is proposed. In contrast to the conventional 4DVAR schemes, the method does not require development of the tangent linear and adjoint codes for implementation. The proposed R4DVAR technique is based on minimization of the cost function in a sequence of low-dimensional subspaces of the control space. Performance of the method is demonstrated in a series of twin-data assimilation experiments into a nonlinear quasigeostrophic model utilized as a strong constraint. When the adjoint code is stable, R4DVAR\u27s convergence rate is comparable to that of the standard 4DVAR algorithm. In the presence of strong instabilities in the direct model, R4DVAR works better than 4DVAR whose performance is deteriorated because of the breakdown of the tangent linear approximation. Comparison of the 4DVAR and R4DVAR also shows that R4DVAR becomes advantageous when observations are sparse and noisy

Teaching History in Schools in Chita in Context of Implementation of Tasks of the Soviet State in Sphere of Historical Policy (late 1930-ies - early 1940-ies)

The article analyzes the specifics of teaching history in schools of Chita in the late 1930-ies - early 1940-ies. The sources were published party and government documents and correspondence from the collections of the State archive of the Transbaikal region, introduced into scientific circulation for the first time. It is shown that the pre-war years was a very important step for the development of school historical education in our country, which determined changes in the content of this discipline and methods of its learning. Special attention in research is paid to the peculiarities of organization of educational process on the lessons of history, the characteristics of personnel and material support of the Chita schools. The author dwells on the problems of realization of educational process on the subject, among which there are shortage of qualified personnel, lack of necessary educational and methodical literature, presence of students behind the school program, and others. It is noted that due to the remoteness of the Chita region from the center the solution of these questions objectively was slower. However, despite the shortcomings of the efforts of the education department, school administrators and all teachers of Chita gradually contributed to bringing the quality of the teaching of history in conformity with the essential requirements

Seasonal Variation of the North Atlantic Current

The seasonal circulation of the upper 1000 m of the North Atlantic between 40 degrees -55 degreesN and 20 degrees -40 degreesW is calculated using the traditional dynamic method and a circulation model with a density field that evolves with the flow. The model is of finite difference form and is based on dynamics that describe the nonlinear evolution of the ocean at low Rossby number. The model is controlled by initial and boundary conditions that include air-sea buoyancy and momentum fluxes. The model is run in two ways: with controls specified directly from observations and with controls inferred by the assimilation of all available data. These data include surface drifter trajectories, sea levels from the TOPEX/Poseidon altimeter, Bunker air-sea fluxes, and the Levitus climatological monthly means of temperature and salinity. We conclude that the North Atlantic Current transport is 40 +/- 18 Sv with seasonal variations of the order of 2 Sv. The mean vertical transport out of the region is 2 +/- 9 Sv and is subject to seasonal variations of 2 Sv. Overall, these estimates are in good agreement with integral North Atlantic Current features derived from independent long-term measurements made in the region over the past decade. The optimal ocean state has a volume transport across the western boundary of 51 +/- 3 Sv with a maximum transport of 61 +/- 5 Sv in April-May and a minimum of 42 +/- 3 Sv in October-November, This western inflow is compensated by mean outflows of 28 +/- 2 (east), 16 +/- 2 (north), 5 +/- 2 (south), and 1.8 +/- 0.4 Sv out of the domain at 1000 m. Sensitivity studies show that nonlinear mixing and seasonality are important in determining the overall circulation. Specifically, steady boundary forcing leads to annual mean transports that are 15-25% smaller than transports obtained with seasonal forcing. Winter convection is also shown to play a significant role in determining the overall circulation pattern