Four-dimensional variational data assimilation for high resolution nested models

Abstract. As with other western boundary currents globally, the East Australian Current (EAC) is inherently dynamic making it a challenge to model and predict. For the EAC region, we combine a high-resolution state-of-the-art numerical ocean model with a variety of traditional and newly available observations using an advanced variational data assimilation scheme. The numerical model is configured using the Regional Ocean Modelling System (ROMS 3.4) and takes boundary forcing from the BlueLink ReANalysis (BRAN3). For the data assimilation we use an Incremental Strong-Constraint 4-Dimensional Variational (IS4D-Var) scheme. This paper describes the data assimilative model configuration that achieves an optimised minimisation of the difference between the modelled solution and the observations to give a dynamically-consistent `best-estimate' of the ocean state over a 2-year period. The reanalysis is shown to represent both assimilated and non-assimilated observations well. It achieves mean spatially-averaged RMS residuals with the observations of 7 cm for SSH and 0.4 °C for SST over the assimilation period. The time-mean RMS residual for subsurface temperature measured by Argo floats is a maximum of 1 °C between water depths of 100–300 m and smaller throughout the rest of the water column. Velocities at several offshore and continental shelf moorings are well represented in the reanalysis with complex correlations between 0.8–1 for all observations in the upper 500 m. Surface radial velocities from a high-frequency radar array are assimilated and the reanalysis provides surface velocity estimates with complex correlations with observed velocities of 0.8–1 across the radar footprint. Comparison with independent (non-assimilated) shipboard CTD cast observations shows a marked improvement in the representation of the subsurface ocean in the reanalysis, with the RMS residual in potential density reduced to about half of the residual with the free-running model in the upper eddy-influenced part of the water column. This shows that information is successfully propagated from observed variables to unobserved regions as the assimilation system uses the model dynamics to determine covariance, such that the ocean state better fits and is in balance with the observations. This is the first study to generate a reanalysis of the region at such a high resolution, making use of an unprecedented observational data set and using an assimilation method that uses the time-evolving model physics to adjust the model in a dynamically consistent way. As such, the reanalysis potentially represents a marked improvement in our ability to capture important circulation dynamics in the EAC. The reanalysis is being used to study EAC dynamics, observation impact in state-estimation and as forcing for a variety of downscaling studies.

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Hybridizing sequential and variational data assimilation for robust high-resolution hydrologic forecasting

10.5194/hess-2016-454 ◽

2016 ◽

Author(s):

Felipe Hernández ◽

Xu Liang

Keyword(s):

High Resolution ◽

Data Assimilation ◽

Variational Data Assimilation ◽

Sequential Data ◽

State Variables ◽

Hydrologic Modelling ◽

Multi Objective ◽

High Resolution Models ◽

Computational Resources ◽

Traditional Approaches

Abstract. There are two main frameworks for the estimation of initial states in geophysical models for real-time and forecasting applications: sequential data assimilation and variational data assimilation. However, modern high-resolution models offer challenges, both in terms of indeterminacy and computational requirements, which render most traditional methods insufficient. In this article we introduce a hybrid algorithm called OPTIMISTS which combines advantageous features from both of these data assimilation perspectives. These features are integrated with a multi-objective approach for selecting ensemble members to create a probabilistic estimate of the state variables, which promotes the reduction of observational errors as well as the maintenance of the dynamic consistency of states. Additionally, we propose simplified computations as alternatives aimed at reducing memory and processor requirements. OPTIMISTS was tested on two models of real watersheds, one with over 1,000 variables and the second with over 30,000, on two distributed hydrologic modelling engines: VIC and the DHSVM. Our tests, consisting of assimilating streamflow observations, allowed determining which features of the traditional approaches lead to more accurate forecasts while at the same time making an efficient use of the available computational resources. The results also demonstrated the benefits of the coupled probabilistic/multi-objective approach, which proved instrumental in reducing the harmful effects of overfitting – especially on the model with higher dimensionality.

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Review of “Hybridizing Bayesian and variational data assimilation for robust high-resolution hydrologic forecasting” by Felipe Hernández and Xu Liang

10.5194/hess-2017-431-rc2 ◽

2017 ◽

Author(s):

Anonymous

Keyword(s):

High Resolution ◽

Data Assimilation ◽

Variational Data Assimilation

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Variational data assimilation system with nesting model for high resolution ocean circulation

Fluid Dynamics Research ◽

10.1088/0169-5983/47/5/051401 ◽

2015 ◽

Vol 47 (5) ◽

pp. 051401

Author(s):

Yoichi Ishikawa ◽

Teiji In ◽

Satoshi Nakada ◽

Kei Nishina ◽

Hiromichi Igarashi ◽

...

Keyword(s):

High Resolution ◽

Data Assimilation ◽

Ocean Circulation ◽

Variational Data Assimilation ◽

Data Assimilation System ◽

Assimilation System

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Hybridizing Bayesian and variational data assimilation for robust high-resolution hydrologic forecasting

10.5194/hess-2017-431 ◽

2017 ◽

Author(s):

Felipe Hernández ◽

Xu Liang

Keyword(s):

High Resolution ◽

Data Assimilation ◽

Predictive Accuracy ◽

Initial Conditions ◽

Time Estimation ◽

Variational Data Assimilation ◽

Streamflow Forecasting ◽

Infiltration Capacity ◽

Computational Overhead ◽

Capacity Model

Abstract. The success of real-time estimation and forecasting applications based on geophysical models has been possible thanks to the two main frameworks for the determination of the models’ initial conditions: Bayesian data assimilation and variational data assimilation. However, while there have been efforts to unify these two paradigms, existing attempts struggle to fully leverage the advantages of both in order to face the challenges posed by modern high-resolution models – mainly related to model indeterminacy and steep computational requirements. In this article we introduce a hybrid algorithm called OPTIMISTS (Optimized PareTo Inverse Modeling through Integrated STochastic Search) which is targeted at non-linear high-resolution problems and that brings together ideas from particle filters, 4-dimensional variational methods, evolutionary Pareto optimization, and kernel density estimation in a unique way. Streamflow forecasting experiments were conducted to test which specific parameterizations of OPTIMISTS led to higher predictive accuracy. The experiments analysed two watersheds, one with a low resolution using the VIC (Variable Infiltration Capacity) model and one with a high-resolution using the DHSVM (Distributed Hydrology Soil Vegetation Model). By selecting kernel-based non-parametric sampling, non-sequential evaluation of candidate particles, and through the multi-objective minimization of departures from the streamflow observations and from the background states, OPTIMISTS was shown to outperform a particle filter and a 4D variational method. Moreover, the experiments demonstrated that OPTIMISTS scales well in high-resolution cases without imposing a significant computational overhead and that it was successful in mitigating the harmful effects of overfitting. With these combined advantages, the algorithm shows the potential to increase the accuracy and efficiency of operational prediction systems for the improved management of natural resources.

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High-resolution synthetic monitoring by a 4-dimensional variational data assimilation system in the northwestern North Pacific

Journal of Marine Systems ◽

10.1016/j.jmarsys.2009.02.016 ◽

2009 ◽

Vol 78 (2) ◽

pp. 237-248 ◽

Cited By ~ 24

Author(s):

Yoichi Ishikawa ◽

Toshiyuki Awaji ◽

Takahiro Toyoda ◽

Teiji In ◽

Kei Nishina ◽

...

Keyword(s):

High Resolution ◽

Data Assimilation ◽

North Pacific ◽

Variational Data Assimilation ◽

Data Assimilation System ◽

Assimilation System

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Development of an Efficient Regional Four-Dimensional Variational Data Assimilation System for WRF

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-13-00076.1 ◽

2014 ◽

Vol 31 (12) ◽

pp. 2777-2794 ◽

Cited By ~ 16

Author(s):

Xin Zhang ◽

Xiang-Yu Huang ◽

Jianyu Liu ◽

Jonathan Poterjoy ◽

Yonghui Weng ◽

...

Keyword(s):

High Resolution ◽

Data Assimilation ◽

Computational Cost ◽

Wrf Model ◽

Forecast Accuracy ◽

Hurricane Sandy ◽

Variational Data Assimilation ◽

Full Resolution ◽

Computational Performance ◽

Multiple Data

Abstract This paper presents the development of a single executable four-dimensional variational data assimilation (4D-Var) system based on the Weather Research and Forecasting (WRF) Model through coupling the variational data assimilation algorithm (WRF-VAR) with the newly developed WRF tangent linear and adjoint model (WRFPLUS). Compared to the predecessor Multiple Program Multiple Data version, the new WRF 4D-Var system achieves major improvements in that all processing cores are able to participate in the computation and all information exchanges between WRF-VAR and WRFPLUS are moved directly from disk to memory. The single executable 4D-Var system demonstrates desirable acceleration and scalability in terms of the computational performance, as demonstrated through a series of benchmarking data assimilation experiments carried out over a continental U.S. domain. To take into account the nonlinear processes with the linearized minimization algorithm and to further decrease the computational cost of the 4D-Var minimization, a multi-incremental minimization that uses multiple horizontal resolutions for the inner loop has been developed. The method calculates the innovations with a high-resolution grid and minimizes the cost function with a lower-resolution grid. The details regarding the transition between the high-resolution outer loop and the low-resolution inner loop are introduced. Performance of the multi-incremental configuration is found to be comparable to that with the full-resolution 4D-Var in terms of 24-h forecast accuracy in the week-long analysis and forecast experiment over the continental U.S. domain. Moreover, the capability of the newly developed multi-incremental 4D-Var system is further demonstrated in the convection-permitting analysis and forecast experiment for Hurricane Sandy (2012), which was hardly computationally feasible with the predecessor WRF 4D-Var system.

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Comparison of the Impacts of Momentum Control Variables on High-Resolution Variational Data Assimilation and Precipitation Forecasting

Monthly Weather Review ◽

10.1175/mwr-d-14-00205.1 ◽

2015 ◽

Vol 144 (1) ◽

pp. 149-169 ◽

Cited By ~ 42

Author(s):

Juanzhen Sun ◽

Hongli Wang ◽

Wenxue Tong ◽

Ying Zhang ◽

Chung-Yi Lin ◽

...

Keyword(s):

High Resolution ◽

Data Assimilation ◽

Control Variable ◽

Wrf Model ◽

Variational Data Assimilation ◽

Error Modeling ◽

Limited Area ◽

Control Variables ◽

Background Error ◽

Precipitation Prediction

Abstract The momentum variables of streamfunction and velocity potential are used as control variables in a number of operational variational data assimilation systems. However, in this study it is shown that, for limited-area high-resolution data assimilation, the momentum control variables ψ and χ (ψχ) pose potential difficulties in background error modeling and, hence, may result in degraded analysis and forecast when compared with the direct use of x and y components of wind (UV). In this study, the characteristics of the modeled background error statistics, derived from an ensemble generated from Weather Research and Forecasting (WRF) Model real-time forecasts of two summer months, are first compared between the two control variable options. Assimilation and forecast experiments are then conducted with both options for seven convective events in a domain that encompasses the Rocky Mountain Front Range using the three-dimensional variational data assimilation (3DVar) system of the WRF Model. The impacts of the two control variable options are compared in terms of their skills in short-term qualitative precipitation forecasts. Further analysis is performed for one case to examine the impacts when radar observations are included in the 3DVar assimilation. The main findings are as follows: 1) the background error modeling used in WRF 3DVar with the control variables ψχ increases the length scale and decreases the variance for u and υ, which causes negative impact on the analysis of the velocity field and on precipitation prediction; 2) the UV-based 3DVar allows closer fits to radar wind observations; and 3) the use of UV control variables improves the 0–12-h precipitation prediction.

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