著者

Tsubasa Kohyama Hiroaki Miura Shoichiro Kido

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.17, pp.246-250, 2021 (Released:2021-12-23)

参考文献数

14

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A modified version of the principal component analysis (PCA) is introduced by reconsidering statistical degrees of freedom in spatial dimensions based on spatial auto-correlations. In the conventional PCA, data points that represent equal areas are assumed to have equal amount of information. In our new method, the intensive variability extraction (IVE), data points correlated with less other data points are weighted more before performing PCA. Hence, variability with independent information is emphasized, even if the variability is confined to small areas.Sea surface temperature (SST) data at each grid in the tropics are shown to have fewer spatial statistical degrees of freedom than that in the extratropics. Tropical SSTs exhibit covariability with large areas, because oceanic equatorial waves and atmospheric gravity waves share temperature information with surrounding areas. As to the extratropics, grids along the western boundaries of oceanic basins are more independent than those in the east, following dynamical requirement of the Earth's rotation.Using IVE, climate modes that involve interscale covariability are extracted. IVE performed for the Pan-Pacific SSTs extracts the Pacific Decadal Oscillation assuming the aforementioned a priori dynamical expectation. Using extratropical SSTs, it is demonstrated that IVE detects synchronicity of small-scale variability between distant narrow regions.

著者

Shuhei Matsugishi Hiroaki Miura Tomoe Nasuno Masaki Satoh

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.16A, no.Special_Edition, pp.12-18, 2020 (Released:2020-06-27)

参考文献数

38

被引用文献数

6

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We show that a modification to the latent heat flux (LHF) formulation in Non-hydrostatic Icosahedral Atmospheric Model (NICAM) impacts the representation of a Madden–Julian oscillation (MJO) event during the Pre-Years of the Maritime Continent (Pre-YMC) field campaign in 2015. First, we compare the LHFs computed by the standard NICAM setting with those estimated from the ship observation during Pre-YMC. In this comparison, the NICAM LHF is smaller than observation in the low wind speed region and larger in the high wind speed region. Consequently, the MJO signal weakens when it passes over the Maritime Continent (MC). Next, sensitivity experiments are conducted with a modification to the threshold minimum wind speed in the bulk formula, to enhance the LHFs in the low wind speed region. With this modification, propagation of the MJO is better simulated over the MC, although a bias still remains without corrections in the high wind speed regions. This result indicates that increasing the LHF in the low wind speed region likely contributes to a more effective accumulation of moisture over the eastern MC region and consequently allows the MJO to pass over the MC in the model.

著者

Chia Rui Ong Hiroaki Miura

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.14, pp.170-173, 2018 (Released:2018-11-13)

参考文献数

17

被引用文献数

2

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Empirical formulae of the terminal velocity and shape of a water droplet in microphysics parametrizations are derived from experiments or theoretical works and they are only verified under room temperature and standard atmospheric pressure. A two-phase direct numerical simulation model can be a strong tool to improve those empirical formulae under general conditions. Recently, the global B-spline fitting has been applied and its smooth one dimensional (1D) surface reconstruction of water droplets has enabled stable simulations of falling two dimensional (2D) droplets by the immersed boundary method (IBM). However, an extension of the global reconstruction from 1D to 2D is highly complex and is almost impossible to use in a model. To overcome this limitation, an iterative algorithm is proposed for a local smooth surface reconstruction in this work. One significant advantage is its straightforward extension to 2D surfaces. To test the new method, simulations of an axisymmetric free-oscillating water droplet are compared between the global and local surface reconstructions. A further simulation of a rising air bubble is performed to examine the robustness of the new algorithm for the highly distorted interface. This new method opens a pathway to three dimensional (3D) water droplet simulations by the IBM.

著者

Tsubasa Kohyama Hiroaki Miura Shoichiro Kido

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

pp.2021-043, (Released:2021-11-09)

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A modified version of the principal component analysis (PCA) is introduced by reconsidering statistical degrees of freedom in spatial dimensions based on spatial auto-correlations. In the conventional PCA, data points that represent equal areas are assumed to have equal amount of information. In our new method, the intensive variability extraction (IVE), data points correlated with less other data points are weighted more before performing PCA. Hence, variability with independent information is emphasized, even if the variability is confined to small areas.Sea surface temperature (SST) data at each grid in the tropics are shown to have fewer spatial statistical degrees of freedom than that in the extratropics. Tropical SSTs exhibit covariability with large areas, because oceanic equatorial waves and atmospheric gravity waves share temperature information with surrounding areas. As to the extratropics, grids along the western boundaries of oceanic basins are more independent than those in the east, following dynamical requirement of the Earth's rotation.Using IVE, climate modes that involve interscale covariability are extracted. IVE performed for the Pan-Pacific SSTs extracts the Pacific Decadal Oscillation assuming the aforementioned a priori dynamical expectation. Using extratropical SSTs, it is demonstrated that IVE detects synchronicity of small-scale variability between distant narrow regions.

著者

Shuhei Matsugishi Hiroaki Miura Tomoe Nasuno Masaki Satoh

出版者

Meteorological Society of Japan

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

pp.16A-003, (Released:2020-05-21)

被引用文献数

6

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We show that a modification to the latent heat flux (LHF) formulation in Non-hydrostatic Icosahedral Atmospheric Model (NICAM) impacts the representation of a Madden–Julian oscillation (MJO) event during the Pre-Years of the Maritime Continent (Pre-YMC) field campaign in 2015. First, we compare the LHFs computed by the standard NICAM setting with those estimated from the ship observation during Pre-YMC. In this comparison, the NICAM LHF is smaller than observation in the low wind speed region and larger in the high wind speed region. Consequently, the MJO signal weakens when it passes over the Maritime Continent (MC). Next, sensitivity experiments are conducted with a modification to the threshold minimum wind speed in the bulk formula, to enhance the LHFs in the low wind speed region. With this modification, propagation of the MJO is better simulated over the MC, although a bias still remains without corrections in the high wind speed regions. This result indicates that increasing the LHF in the low wind speed region likely contributes to a more effective accumulation of moisture over the eastern MC region and consequently allows the MJO to pass over the MC in the model.

著者

Hiroaki Miura Takafumi Maeda Masahide Kimoto

出版者

(公社)日本気象学会

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.8, pp.165-169, 2012 (Released:2012-12-28)

参考文献数

27

被引用文献数

1 2

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A pair of 20-year simulations by two different versions of MIROC (Model for Interdisciplinary Research on Climate) was examined by using a standardized set of Madden-Julian Oscillation (MJO) diagnostics. One of the major differences between version 4 (MIROC4) and version 5 (MIROC5) of MIROC is the cumulus parameterization scheme. MIROC4 uses a prognostic Arakawa-Schubert scheme, whereas MIROC5 uses the Chikira scheme. MIROC5 reproduced the MJO better than MIROC4: a stronger signal in the wavenumber-frequency diagram, a slower and more noticeable eastward movement in the lag-correlation plot, and a better phase relationship between outgoing longwave radiation and zonal winds.To investigate the impact of mid-tropospheric humidity on cumulus development in MIROC5, the atmosphere-only version of MIROC5 was used for a series of sensitivity runs, each with different entrainment parameter values. The entrainment parameter settings significantly influenced the simulated MJO. Large-scale cloud systems tended to move westward with smaller entrainment parameter values, accompanied by a pair of rotations with quasi-symmetry about the equator in the lower troposphere, whereas eastward movements were faster, with larger entrainment parameter values.

著者

Hiroaki Miura

出版者

(公社)日本気象学会

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.13, pp.69-73, 2017 (Released:2017-04-25)

参考文献数

18

被引用文献数

3

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A shallow water model is developed on the regular hexagonal mesh by combining the hexagonal B1-grid and B2-grid schemes. The new scheme called as the hexagonal synchronized B-grid (SB-grid) scheme in this work allows avoiding a computational mode problem of the ZM-grid scheme. It is known that the problem is caused by the mismatch of degrees of freedoms of the prognostic variables. The SB-grid uses the same variable arrangement as the ZM-grid, placing fluid depths and fluid velocities at the centers and corners of hexagonal cells, respectively, but the nonlinear terms of the momentum equation are discretized using wider spatial stencils than those of the ZM-grid. This change results in the inhibition of extra interactions in the velocity fields that enhances a computational mode in the ZM-grid. Geostrophic adjustment tests on a regular hexagonal mesh confirm that the SB-grid shallow water model behaves almost equivalently to the Z-grid model, and the computational mode problem is certainly settled down.

著者

Daisuke Takasuka Tomoki Miyakawa Masaki Satoh Hiroaki Miura

出版者

(公社)日本気象学会

雑誌

SOLA (ISSN:13496476)

巻号頁・発行日

vol.11, pp.170-176, 2015 (Released:2015-12-21)

参考文献数

42

被引用文献数

3 16

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The roles of topography on the propagation of the Madden-Julian Oscillation (MJO) are discussed using an aqua-planet of the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) with a 220-km horizontal mesh. Four topographical configurations with different land-sea masks and elevations are tested using a zonally non-uniform fixed-SST distribution. Explicit cloud microphysics is used to obtain MJO-like signals. Broad land cover generally weakens convection because of reduced surface latent heat flux (LHF). Forced lifting because of topography enhances local convection on the upwind side of high topography. It is suggested that the zonal contrasts of LHF are one reason for the delayed eastward propagation of the MJO-like disturbances. When only the eastern portion of the convective envelope is over land where the LHF is small, the LHF becomes rear-heavy, resulting in delayed eastward propagation. As the entire convective envelope proceeds over land, its contrast decreases or even reverses, resulting in faster eastward propagation.