Papers - KOMORI Nobumasa

Influence of the Gulf Stream on the troposphere

Shoshiro Minobe, Akira Kuwano-Yoshida, Nobumasa Komori, Shang-Ping Xie, Richard Justin Small

NATURE (NATURE PUBLISHING GROUP)  452 ( 7184 ) 206 - U51 2008.03

Accepted,  ISSN  0028-0836

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The Gulf Stream transports large amounts of heat from the tropics to middle and high latitudes, and thereby affects weather phenomena such as cyclogenesis(1,2) and low cloud formation(3). But its climatic influence, on monthly and longer timescales, remains poorly understood. In particular, it is unclear how the warm current affects the free atmosphere above the marine atmospheric boundary layer. Here we consider the Gulf Stream's influence on the troposphere, using a combination of operational weather analyses, satellite observations and an atmospheric general circulation model(4). Our results reveal that the Gulf Stream affects the entire troposphere. In the marine boundary layer, atmospheric pressure adjustments to sharp sea surface temperature gradients lead to surface wind convergence, which anchors a narrow band of precipitation along the Gulf Stream. In this rain band, upward motion and cloud formation extend into the upper troposphere, as corroborated by the frequent occurrence of very low cloud- top temperatures. These mechanisms provide a pathway by which the Gulf Stream can affect the atmosphere locally, and possibly also in remote regions by forcing planetary waves(5,6). The identification of this pathway may have implications for our understanding of the processes involved in climate change, because the Gulf Stream is the upper limb of the Atlantic meridional overturning circulation, which has varied in strength in the past(7) and is predicted to weaken in response to human- induced global warming in the future(8).

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Deep ocean inertia-gravity waves simulated in a high-resolution global coupled atmosphere-ocean GCM

Nobumasa Komori, Wataru Ohfuchi, Bunmei Taguchi, Hideharu Sasaki, Patrice Klein

GEOPHYSICAL RESEARCH LETTERS (AMER GEOPHYSICAL UNION)  35 ( 4 ) L04610-L04610-5 2008.02

Accepted,  ISSN  0094-8276

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In order to investigate the deep ocean inertia-gravity waves, a high-resolution global coupled atmosphere-ocean simulation is carried out with a coupling interval of 20 minutes. Large (similar to 10(-3) m s(-1)) root-mean-square variability of vertical velocity is found in middepths (2000-4000 m), which is not reported in previous studies using realistic ocean simulations. Horizontal distribution of the large variability roughly corresponds to the wintertime atmospheric storm tracks and is stretched equatorward due to beta-dispersion in open ocean with some "shadow regions'' behind the obstacles. Frequency spectrum of vertical velocity has strong peaks at around f and 2f (f is the local inertial period) in midlatitudes, and has additional peak at around (3/2) f or 3f at some points. These results suggest necessity of re-evaluation of wind-induced near-inertial energy with high-frequency atmospheric forcing.

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Description of AFES 2: Improvements for high-resolution and coupled simulations

Takeshi Enomoto, Akira Kuwano-Yoshida, Nobumasa Komori, Wataru Ohfuchi

HIGH RESOLUTION NUMERICAL MODELLING OF THE ATMOSPHERE AND OCEAN (SPRINGER)     77 - 97 2008

Accepted

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This chapter describes the updated version of Atmospheric General Circulation Model for the Earth Simulator (AFES 2). Modifications are intended (1) to increase the accuracy and efficiency of the Legendre transform at high resolutions and (2) to improve the physical performance. In particular, the Emanuel scheme replaces a simplified version of the Arakawa-Schubert scheme for the parametrization of cumulus convection. The Emanuel scheme parametrizes O(100m) drafts within subgrid-scale cumuli and does not have explicit dependency upon the grid size. Therefore the cloud model of the Emanuel scheme allows us to use it at high resolutions of O(10 km) where the validity of the ensemble cloud model of the Arakawa-Schubert scheme is questionable. Moreover, 10-year test runs indicate that the use of the Emanuel scheme improve the physical performance at a moderate resolution as well. Anomalies of the geopotential height and zonal winds in the middle to upper troposphere are reduced, although the improvements in terms of the distributions of precipitation and sea-level pressure are not significant. Improvements are attributable to a better vertical structure of temperature in the tropics due to more realistic estimation of mixing of the momentum, temperature, and moisture by the Emanuel scheme.

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Understanding and forecasting high-impact phenomena in the atmosphere and ocean

Hideharu Sasaki, Takeshi Enomoto, Nobumasa Komori, Akira Kuwano-Yoshida, Kazuhisa Tsuboki, Jun Inoue, Bunmei Taguchi, Wataru Ohfuchi

Annual Report of the Earth Simulator Center April 2008--September 2008    23 - 28 2008

High-resolution simulation of the global coupled atmosphere-ocean system: Description and preliminary outcomes of CFES (CGCM for the Earth Simulator)

Nobumasa Komori, Akira Kuwano-Yoshida, Takeshi Enomoto, Hideharu Sasaki, Wataru Hfuchi

HIGH RESOLUTION NUMERICAL MODELLING OF THE ATMOSPHERE AND OCEAN (SPRINGER)     241 - 260 2008

Accepted

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We have been developing a global, high-resolution, coupled atmosphere-ocean general circulation model, named CFES, which was designed to achieve efficient computational performance on the Earth Simulator. A brief description of CFES and some preliminary results obtained from 66-month integration are presented. Although some deficiencies are apparent in the results, realistically simulaled small-scale structures such as extratropical cyclones and sea surface temperature fronts in the mid-latitudes, and seasonal variation of tropical sea surface temperature and polar sea-ice extent encourage us to study mechanism and predictability of high-impact phenomena and their relation to the global-scale circulations using CFES.

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High-resolution simulation studies on the mechanisms and predictability of various atmospheric and oceanic phenomena

Wataru Ohfuchi, Takeshi Enomoto, Munehiko Yamaguchi, Nobumasa Komori, Hideharu Sasaki, Yoshikazu Sasai, Yoshio Masuda

Annual Report of the Earth Simulator Center April 2004--March 2005    51 - 56 2006

Description of sea-ice component of Coupled Ocean–Sea-Ice Model for the Earth Simulator (OIFES)

Komori, N, K. Takahashi, K. Komine, T. Motoi, X. Zhang, G. Sagawa

Journal of the Earth Simulator 4   31 - 45 2005

Accepted

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Application of sensitivity analysis using an adjoint model for short-range forecasts of the Kuroshio path south of Japan

Y Ishikawa, T Awaji, N Komori, T Toyoda

JOURNAL OF OCEANOGRAPHY (SPRINGER)  60 ( 2 ) 293 - 301 2004.04

Accepted,  ISSN  0916-8370

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A sensitivity experiment has been performed by assimilating altimetric data into a 1.5-layer primitive equation model as a first attempt to examine the impact of initialization on forecasts of the Kuroshio path variability south of Japan. By exploiting the advantage of an adjoint model, our approach clearly shows that a small meander off Shikoku Island has a large impact on the prediction of meander growth in the Kuroshio region. Further, the strengthening of the Kuroshio current and its recirculation clearly becomes an important factor in the development of the meander. These results demonstrate the effectiveness of our assimilation approach in identifying efficient initialization schemes on numerical forecasting of the Kuroshio south of Japan and should help in the construction of an effective observing system for improving the forecasting.

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A fifty-year eddy-resolving simulation of the world ocean: Preliminary outcomes of OFES (OGCM for the Earth Simulator)

Masumoto Yukio, Sasaki H, Kagimoto T, Komori N, Ishida A, Sasai Y, Miyama T, Motoi T, Mitsudera Humio, Takahashi K

Journal of the Earth Simulator 1   35 - 56 2004

Accepted

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Improved estimates of the dynamical state of the North Pacific Ocean from a 4 dimensional variational data assimilation

S Masuda, T Awaji, N Sugiura, Y Ishikawa, K Baba, K Horiuchi, N Komori

GEOPHYSICAL RESEARCH LETTERS (AMER GEOPHYSICAL UNION)  30 ( 16 ) 1868 2003.08

Accepted,  ISSN  0094-8276

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[1] A 4-dimensional variational data assimilation system has been used to better define the mean seasonal state of the North Pacific. The synthesis of available observational records and a sophisticated general circulation model produces a dynamically consistent time-varying dataset which exhibits realistic features of the global ocean circulation and requires no artificial sources or sinks for the temperature and salinity fields. The dataset enables us to clarify the water mass formation and movement processes. A sensitivity experiment using our system reveals that the origin of the North Pacific Intermediate Water can be traced back to the Okhotsk and Bering Seas in the subarctic region and to the subtropical Kuroshio region further south, consistent with recent observational findings. This result illustrates that the ocean state derived from our data assimilation has greater information and forecast potential than that obtained from earlier methods.

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Short-range forecast experiments of the Kuroshio path variabilities south of Japan using TOPEX/Poseidon altimetric data

N Komori, T Awaji, Y Ishikawa

JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS (AMER GEOPHYSICAL UNION)  108 ( C1 ) 3010 2003.01

Accepted,  ISSN  0148-0227

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[1] Capability for short-range forecasting of the Kuroshio path variabilities south of Japan is studied by assimilating the absolute sea surface dynamic height data, which are derived from in situ hydrography and TOPEX/Poseidon (T/P) altimetry, into a high-resolution 1-1/2 layer primitive equation model every 15 days for 5 years from 1993 to 1997. The initialization scheme used is the variational method, which contains the nonlinear dynamics of the model as a weak constraint and is suitable for initialization in the western boundary current regions where initialization by simple optimal interpolation is inappropriate. The time series of the analysis field well represents the Kuroshio path variations during the study period. For example, the root-mean-square (RMS) differences from the observations falls within 0.3degrees for the location in latitude of the current axis off Enshu-nada and 11.4 cm for the sea level at Hachijo-jima Island while their observed RMS variabilities are 0.5degrees and 30.7 cm, respectively. By using the resulting fields as the initial conditions, 116 cases of 90-day forecast experiments for the Kuroshio path variations are then performed to assess the statistical properties of our assimilation system. Though significant bias is found in the forecast field southeast of Kyushu, the accuracy of our forecasted field south of Japan is much higher than that of persistence experiments, and the typical evolution of the Kuroshio into a large-amplitude path off Enshu-nada is successfully reproduced after the initialization. In particular, the eastward progression speed and the horizontal scale of the meander are quite similar to those observed. These results and the comparison with the RMS variability of the analysis field enable us to infer that roughly 60-day forecasts of the Kuroshio path variabilities south of Japan are possible. This indicates that our assimilation system is suitable for the operational short-range forecasting of the upper Kuroshio.

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GODAE計画概要と観測船の役割

蒲池政文, 淡路敏之, 川村宏, 小守信正, 倉賀野連

月刊海洋 34 ( 9 ) 624 - 636 2002.01

Dynamical initialization for the numerical forecasting of ocean surface circulations using a variational assimilation system

Y Ishikawa, T Awaji, N Komori

JOURNAL OF PHYSICAL OCEANOGRAPHY (AMER METEOROLOGICAL SOC)  31 ( 1 ) 75 - 93 2001

Accepted,  ISSN  0022-3670

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A variational data assimilation system is presented for the initialization of an ocean surface circulation forecast system. The authors' variational data assimilation system is designed to satisfy both statistical and dynamical constraints. As is usual, the statistical part of the assimilation scheme corresponds to the optimal interpolation scheme while the dynamical part works as a weak constraint for the model equations except for the time differential terms. Thus this variational data assimilation scheme can be regarded as an extended optimal interpolation capable of obtaining the analysis field that satisfies the model dynamics. Comparison with the results of assimilating altimetric data into a 1.5-layer primitive equation model by the classical optimal interpolation clearly shows the advantage of this assimilation method. For example, the analysis field is significantly improved, in particular, in the western boundary current regions and their extensions, where some assumptions inherent in the optimal interpolation, such as the Gaussian function for the error covariance matrix and the geostrophic balance for the error fields, break down because of the nonlinear nature of the currents. To show the effectiveness of this variational assimilation system for the initialization process, several numerical forecasting experiments are carried out using the analysis fields from the variational assimilation scheme and from the optimal interpolation as initial conditions. The case initialized by the optimal interpolation scheme exhibits inertia-gravity waves generated by the geostrophic initialization leading to contamination of the forecasting result. In contrast, the variational assimilation case can effectively reduce the generation of the gravity waves, thus providing the dynamically consistent analysis field. It is also shown that the accuracy of the short-range forecast with this variational data assimilation is sensitive to the initial guess of the gradient descent method. The computational cost of this assimilation system is lower than that using other sophisticated schemes such as the adjoint method and the Kalman filter, suggesting that this system is suitable for the operational forecasting of western boundary currents.

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大洋域における熱・運動量収支 モデルによる西部熱帯太平洋暖水プールの熱収支解析

淡路敏之, 美山透, 石川洋一, 印貞治, 飯田昌虎, 根田昌典, 小守信正, 中村知裕

月刊海洋 31 ( 9 ) 574 - 581 1999.09

ISSN  0916-2011