Shozo Yamane

Local Ensemble Transform Kalman Filtering with an AGCM at a T159/L48 Resolution

A local ensemble transform Kalman filter (LETKF) is developed and assessed with the AGCM for the Earth Simulator at a T159 horizontal and 48-level vertical resolution (T159/L48), corresponding to a grid of 480 240 48. Following the description of the LETKF implementation, perfect model Observing Systems Simulation Experiments (OSSEs) with two kinds of observing networks and an experiment with real observations are performed. First, a regular observing network with approximately 1% observational coverage of the system dimension is applied to investigate computational efficiency and sensitivities with the ensemble size (up to 1000) and localization scale. A 10-member ensemble is large enough to prevent filter divergence. Using 20 or more members significantly stabilizes the filter, with the analysis errors less than half as large as the observation errors. There is nonnegligible dependence on the localization scale; tuning is suggested for a chosen ensemble size. The sensitivities of analysis accuracies and timing on the localization parameters are investigated systematically. A computational parallelizing ratio as large as 99.99% is achieved. Timing per analysis is less than 4 min on the Earth Simulator, peak performance of 64 GFlops per computational node, provided that the same number of nodes as the ensemble size is used, and the ensemble size is less than 80. In the other set of OSSEs, the ensemble size is fixed to 40, and the real observational errors and locations are adapted from the Japan Meteorological Agency’s (JMA’s) operational numerical weather prediction system. The analysis errors are as small as 0.5 hPa, 2.0 m s 1 , and 1.0 K in major areas for sea level pressure, zonal and meridional winds, and temperature, respectively. Larger errors are observed in data-poor regions. The ensemble spreads capture the actual error structures, generally representing the observing network. However, the spreads are larger than the actual errors in the Southern Hemisphere; the opposite is true in the Tropics, which suggests the spatial dependence of the optimal covariance inflation. Finally, real observations are assimilated. The analysis fields look almost identical to the JMA operational analysis; 48-h forecast experiments initiated from the LETKF analysis, JMA operational analysis, and NCEP–NCAR reanalysis are performed, and the forecasts are compared with their own analyses. The 48-h forecast verifications suggest a similar level of accuracy when comparing LETKF to the operational systems. Overall, LETKF shows encouraging results in this study.

Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part I: North Atlantic Basin

Abstract Variability in the monthly mean flow and storm track in the North Atlantic basin is examined with a focus on the near-surface baroclinicity, B = Bxi + Byj. Dominant patterns of anomalous B found from empirical orthogonal function (EOF) analyses generally show patterns of shift and changes in the strength of B. Composited anomalies in the monthly mean wind at various pressure levels based on the signals in the EOFs display robust accompanying anomalies in the mean flow up to 50 hPa in the winter and up to 100 hPa in other seasons. Anomalous eddy fields accompanying the anomalous Bx patterns exhibit, broadly speaking, structures anticipated from linear theories of baroclinic instabilities and suggest a tendency for anomalous wave fluxes to accelerate/decelerate the surface westerly accordingly. Atmospheric anomalies accompanying By anomalies have patterns different from those that accompany Bx anomalies but are as large as those found for Bx. Anomalies in the sea surface temperature (SST) found for...

Influence of the Northern Hemisphere annular mode on ENSO by modulating westerly wind bursts

and high latitudes. Conversely, Thompson and Lorenz [2004] attempted to detect the effects of both hemispheric annular modes (AMs) on the tropics. They showed coupling between the AMs and tropical atmosphere and that a wintertime positive (negative) AM generates the upper tropospheric zonal mean westerly (easterly) anomalies in the tropics after a 2-week lag. These results suggest a link between the NAM and tropical variability. However, they did not note any direct influences of the NAM on the ENSO. [3] This study examined the influences of NAM on the ENSO. Based on the results of previous studies, we examined the lead and lag relationships between the NAM and ENSO in all seasons using a reanalysis data set. The observational evidence revealed a statistically significant relationship between the NAM signature and the ENSO. To strengthen this finding, we also used an atmospheric general circulation model (AGCM) to simulate climatological monthly mean sea surface temperature (SST) distributions.

Impacts of the Aleutian–Icelandic Low Seesaw on Surface Climate during the Twentieth Century

Abstract An interannual seesaw between the intensities of the Icelandic and Aleutian lows and its impact on surface climate observed during the twentieth century are investigated. In a recent period from the late 1960s to the early 1990s, their seesaw relationship was particularly apparent in late winter. The associated anomalies in surface air temperature were significant in many regions over the extratropical Northern Hemisphere except in central portions of the continents. The seesaw also modified the ocean–atmosphere exchange of heat and moisture extensively over the North Atlantic and North Pacific by changing evaporation and precipitation. Since the seesaw formation was triggered by eastward propagation of stationary Rossby wave trains from the North Pacific into the North Atlantic, anomalous circulation over the North Pacific in January was identified as a good precursor for February surface air temperatures in the Euro–Atlantic sector during that period. The seesaw relationship between the two low...

Observing-System Research and Ensemble Data Assimilation at JAMSTEC

Recent activities on ensemble data assimilation and its application to observing-system research at the Japan Agency for Marine-Earth Science and Technology are reviewed. A revised version of an ensemble-based data assimilation system for global atmospheric data has been developed on the second-generation Earth Simulator. This system assimilates conventional atmospheric observations and satellite-based wind data into an atmospheric general circulation model using the local ensemble transform Kalman filter (LETKF), a deterministic ensemble Kalman filter algorithm that is extremely efficient with parallel computer architecture. The updated system incorporates improvements to the previous system in the forecast model, data assimilation algorithm and input data. Using the LETKF system, observations taken during field campaigns are evaluated by data assimilation experiments involving adding or removing observations. The results of these observing-system experiments successfully demonstrate the value of the observations and are highly useful for exploring the predictability of atmospheric disturbances.

Formation, Mechanisms, and Predictability of the Aleutian–Icelandic Low Seesaw in Ensemble AGCM Simulations

Abstract The potential predictability associated with the remote influence of midlatitude tropospheric anomalies over the North Pacific or the North Atlantic, via a seesawlike interannual oscillation between the surface Aleutian and Icelandic lows (AL and IL, respectively) is investigated. Data from a 24-member ensemble of 50-yr atmospheric general circulation model simulation forced with observed sea surface temperature (SST) conditions are analyzed by separating the total simulated fluctuations into the external component forced by the prescribed SST and the internal component generated by atmospheric internal dynamics. The AL–IL seesaw can be identified in both the external and internal components of the variability. In the external variability, determined through the ensemble mean, the seesaw is gradually formed from December to March through the development of a Pacific–North American (PNA) pattern–like wave train, remotely forced by the El Nino–Southern Oscillation. The amplitudes of the externally ...

Quasigeostrophic Transient Wave Activity Flux: Updated Climatology and Its Role in Polar Vortex Anomalies

Abstract The climatology of transient wave activity flux defined by Plumb has been calculated for each calendar month, for high-frequency (HF) and low-frequency (LF) waves, using the NCAR–NCEP reanalyses for both hemispheres. Wave activity flux of both HF and LF waves shows upward propagation of waves from the lower troposphere into the upper troposphere, then into the lower stratosphere during the summer and at least up to the midstratosphere during other seasons. While the upward flux emanating from the lower troposphere is particularly large in the two storm tracks in the Northern Hemisphere (NH), it is large in most of the extratropics in the Southern Hemisphere (SH). The HF waves radiate equatorward most noticeably in the upper troposphere, whereas the LF waves do not show visible signs of equatorward radiation. The total horizontal flux is generally dominated by the advective flux that represents the eddy enstrophy advection by the mean flow and appears predominantly pseudoeastward. Divergence of th...