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Kaoru Ichikawa Last modified date:2020.06.05

Associate Professor / Interdisciplinary Graduate School of Engineering Sciences, Department of Earth System Science and Technology, Environmental Metrology
Division of Earth Environment Dynamics
Research Institute for Applied Mechanics

Graduate School
Other Organization

 Reseacher Profiling Tool Kyushu University Pure
Academic Degree
PhD, MSc
Country of degree conferring institution (Overseas)
Field of Specialization
Physical Oceanography
ORCID(Open Researcher and Contributor ID)
Total Priod of education and research career in the foreign country
Outline Activities
Researches for providing synoptic views of ocean currents in coastal and marginal seas, where small and fast-moving phenomena are dominant, are conducted by mainly analyzing remote sensing (satellite and ocean radar) data; they include development of the coastal altimetry, sea surface height measurements by the GPS interferometry, cheap & robust surface drifting buoys, and monitoring system using reflected GNSS signals at the sea surface.
Such synoptic views will be used to study the movement/diffusion of materials/physical properties in the upstream Kuroshio area, which could be caused by variations of the Kuroshio due to interaction with westward-propagating mesoscale eddies in the Pacific Ocean originated from far eastern areas.

Except these research activities, I have committed educational activities in Interdisciplinary Graduate School of Engineering Science, and academic/social activities as a council member of Japan Oceanographic Society and Japan-French Oceanographic Society.
Research Interests
  • GNSS Reflectometry of Ocean Waves, Tides and Height
    keyword : GNSS Reflectometry, Sea surface height, Sea surace waves, full-time monitoring
  • Sea surface dynamic height measurements in coastal areas and degree of the geostrophy
    keyword : coastal area, GPS, satellite altimetry, geostrophic current, wind-driven current, tidal current
  • Optimal analysis of satellite altimetry data
    keyword : altimetry, optimal interpolation, objective mapping, satellite oceanography
    1988.04~2002.03A series of data processing methods are investigated to utilize satellite altimetry data in physical oceanography. They include improvement of marine geoid data by oceanographic observational data and removement of orbit error by an optimal interpolation method..
  • Interaction between mesoscale eddies and the Kuroshio
    keyword : Kuroshio Extension, mesoscale eddies, satellite altimetry, detached ring
    1993.04Using satellite altimetry data, interactions between mesoscale eddies and the Kuroshio and the Kuroshio Extension have been investigated. A series of phenomena, as pinching off of a cyclonic cold ring from the Kuroshio Extension, westward propagation of the ring, and coalescense of the ring into the Kuroshio, is clearly described. In addition, variations of the volume transport and position of the Kuroshio due to offshore mesoscale eddies are investigated..
  • Impacts of offshore variations on coastal sea level changes
    keyword : coastal sea level, mesoscale eddies, western boundary current, satellite altimetry
    2002.04Impacts of offsore variations are investigated on coastal sea level variations longer than a month. Offshore variations are determined from satellite altimetry data, and compared with coastal sea level variations measured by tide gauges. Mesoscale eddies dominates variations on islands in an open ocean. Meanwhile, effect of Kuroshio variations is found more important to coastal sea levels south of Japanese main islands..
Current and Past Project
  • We have achieved many descriptive physical oceanographic results on variations of flow fields in the western North Pacific, including the Kuroshio, Kuroshio Extension and Oyashio current sys-tems and associated variations in geodetic fields. Moreover, we have extended our description to the currents in marginal seas as the East China Sea, the Japan Sea and the Sea of Okhotsk. Fur-thermore, progress of the ocean data assimilation techniques has been achieved to the stage being able to accumulate these observations of large-scale current systems, meso-scale eddies and region-al currents in the marginal seas.

  • Various oceanic parameters such as sea surface height, winds and sea ice, are important for monitoring global environmental changes, and also they are necessary for weather forecast, maritime route determination, fishing, and ocean drilling. Satellites have provided wide and repeated observations that would be never acquired by research vessels alone, but due to limited number of expensive satellites, they still cannot capture fast-moving or rapidly-changing phenomena, such as the Tsunami propagation and the growth of typhoons. Meanwhile, new remote sensing technique using micro satellites that measure reflection of GNSS signals (GNSS-R) can provide all-time global observations, although its accuracy is less than the expensive special satellites. Such frequent observations would be most useful to monitor fast-moving or rapidly-changing significant signals, which are often related with disasters. In this project, fundamental relationships between GNSS-R and sea-truth data are obtained by in situ observations, and close cooperation between researchers of the ocean and space would be established.
  • Detection of Coastal Velocity Variations in the Tsushima Strait, CNES-ISRO SARAL/AltiKa Project;
    This proposal consists of two major targets. The first and fundamental one is to evaluate performances of the Ka-band altimeter in the coastal region and to establish proper data processing methods for estimation of coastal surface geostrophic velocity anomaly, by comparisons with the high-resolution surface velocity data obtained by the High-Frequency (HF) radar system located in the Tsushima Strait between Japan and Korea. The other and advanced target is to study causes and effects of the velocity variations in the Tsushima Strait by extending the spatially limited observation area of the HF radar system using multiple AltiKa sub-satellite tracks.
  • Variations of flow fields in the western North Pacific and surrounding marginal seas, CNES-EUMETSAT Ocean Surface Topography Science Team;
    The primary aim of this proposal is to understand variations of flow fields in the western North Pacific and surrounding marginal seas. Heat and volume transport of the Kuroshio and Oyashio current systems will be investigated by combining altimetry data with in situ observations and numerical models. At the same time, variations of the flow fields in the East China Sea and the Sea of Okhotsk will be also investigated. Those variations would be expected to have a possible relationship between the Kuroshio and Oyashio current systems, but they have not been described well due to their small-scale nature in marginal seas. Use of HF radar data with high spatio-temporal resolutions and improved accuracy of the altimetry data in marginal seas together with dense coverage by multi-satellite use would provide better insights of such variations.
    In this proposal, we shall continue in situ observations. They include hydrographic and mooring field observations, HF radars, and drifters. They are used for basic oceanographic studies, and may be eventually used as calibration/validation data for Jason-2/OSTM by any other OST-ST groups. Similarly, we shall maintain operational real-time Japan-GODAE data products produced by operating an ocean data assimilation system of JMA. Meanwhile, assimilated results from a more com-prehensive model will be used for basic studies.In addition, gravity changes due to huge oceanic motions such as a large Kuroshio meander will be examined by use of a dense network of accurate gravity field measurements in Japan. We intend to obtain new understandings of circulations in the western North Pacific through those aspiring studies.
  • "Study on the influence of global warming and rapid economic growth in the marine and atmospheric environments in the East Asia", RIAM, Kyushu University
Academic Activities
1. Interdis.Grad Sch Engg Sci, Kyushu Univ., Dept. Earth Sys Sci. Tech.,, Dept. Earth System Sci. and Tech., Kyushu Kyushu, Fukuoka, Japan, Fluid Dynamics for Global Environmental Studies, Springer Japan, 10.1007/978-4-431-56499-7, 2017.10, [URL], This book introduces the basic concepts of environmental fluid dynamics. It is intended for use by students, researchers, engineers, and specialists working not only in general fluid research but also in the atmospheric and oceanic research fields. The Earth is covered by atmosphere and oceans and is exposed to solar wind. Therefore, the knowledge of fluid dynamics is essential for tackling its environmental issues. Although many textbooks have treated fluid dynamics, practically no book has been published that clearly describes all essential ideas, from the fundamentals of fluid dynamics to advanced environmental sciences, with careful sequential explanations of the governing mathematics. This book has been developed to solve these educational problems and has actually been in use in lectures in the graduate school of Kyushu University for more than 15 years.

2. Fluid Dynamics to study Global Environment
Masayuki Oikawa, Masashi Kashiwagi, Keita Iga, Kaoru Ichikawa, Tetsuo Yanagi
Ed: Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University.
1. Xifeng Wang, Kaoru Ichikawa and Dongni Wei, Coastal Waveform Retracking in the Slick-Rich Sulawesi Sea of Indonesia, Based on Variable Footprint Size with Homogeneous Sea Surface Roughness, Remote Sensing, 10.3390/rs11111274, 11, 11, 1274, 2019.05, [URL], Waveforms of radar altimeters are often corrupted due to heterogeneous sea surface roughness within footprints, such as slicks. In past studies, subwaveform retrackers such as the adaptive leading edge subwaveform retracker (ALES) which use only a section of the waveform have been proposed. However, it is difficult to choose a reasonable estimation window from an individual waveform. In the present study, a post-processed subwaveform retracker is proposed which identifies the waveforms of surrounding along-track points. The size of the estimation window is variable and is determined to keep the sea surface roughness within the corresponding footprint homogeneous. The method was applied to seven years of 20 Hz Jason-2 altimeter data over the slick-rich Sulawesi Sea of Indonesia and compared with ALES and sensor geophysical data record (SGDR) products. The standard deviation of the sea surface dynamic heights was around 0.13 m, even without spatial smoothing or some geophysical corrections. This is only 75% and 25% of the ALES and SGDR results, respectively. Moreover, all retrievals of the range, SWH, and sigma0 include less outliers than the other products. These results indicate that the variable estimation windows determined in the present study can adapt well to the variation of sea surface roughness..
2. Kaoru Ichikawa, Takuji Ebinuma, Masanori Konda, Kei Yufu, Low-Cost GNSS-R Altimetry on a UAV for Water-Level Measurements at Arbitrary Times and Locations, Sensors (Basel, Switzerland), 10.3390/s19050998, 19, 5, 2019.02, Together with direct Global Navigation Satellite System (GNSS) signals, the signals reflected at the water surface can be received by an unmanned aerial vehicle (UAV). From the range difference between two GNSS signal paths, the height of the UAV above the water level can be geometrically estimated using the weighted least squares method, called GNSS reflectometry (GNSS-R) altimetry. Experimental low-cost GNSS-R altimetry flights with a UAV were conducted at the coast of Lake Biwa, Japan. Although the height estimated by the GNSS-R altimeter included large short-term noises up to 8 m amplitude, it agreed well with the UAV altitude measured by the post-processed kinematic positioning. By selecting better weight functions in the least square method and using sufficient temporal averaging, the GNSS-R altimetry achieved accuracy in the order of 0.01 m if a sufficient number of GNSS satellites with high elevation angles were available. The dependency of the results on the weight functions is also discussed..
3. Ichikawa K., Yoshikawa Y., Morimoto A., Fukudom K., Yoon JH., Complementary Remote Sensing Observations of the Tsushima Warm Current Patterns, Remote Sensing of the Asian Seas, 10.1007/978-3-319-94067-0_10, 191-204, Barale V., Gade M. (eds), Springer, Cham, ISBN 978-3-319-94067-0, 2018.09, [URL], When monitoring exchanges between adjacent marginal seas, measurements of the volume transport through connecting straits are essential. In such cases, acoustic Doppler current profilers (ADCPs) mounted at the bottom of ferries that regularly cross such straits can provide useful platforms for directly monitoring strait currents. However, since their observations would naturally be confined to each ferry's set route, it would be impossible for them to observe all relevant current patterns. In addition, a ship-mounted ADCP is very expensive to install, maintain, and (when necessary) remove. In this chapter, we will describe alternative indirect remote sensing techniques that can be used to observe surface velocities in the Korea Strait between Japan and Korea. One such technique involves a high-frequency (HF) ocean radar system that can provide synoptic views of the surface velocity field with high resolutions in both space and time, even though additional processes are required to separate the geostrophic ocean currents from the ageostrophic tidal and wind-driven currents. Alternatively, slopes of the sea surface dynamic height (SSDH) can provide a geostrophic component of the surface velocity. Unfortunately, the satellite altimeters that are most commonly used to measure the SSDH field in open oceans are unsuitable for use in narrow straits. Instead, the use of global navigation satellite systems (GNSS) is showing promise as a way to obtain the coastal SSDH, even though proper spatial smoothing processes are still required..
4. Ichikawa K., Remote Sensing of the Kuroshio Current System, Remote Sensing of the Asian Seas, 10.1007/978-3-319-94067-0_10, 205-220, Barale V., Gade M. (eds), Springer, Cham, ISBN 978-3-319-94067-0, 2018.09, [URL], Variations of the Kuroshio Current System, which is the western boundary current of the North Pacific, influence both large-scale climate changes and regional waters.
Satellite altimeters have been widely used to monitor various aspects of the Kuroshio, such as its position and volume transport.
Although both meanders of the Kuroshio axis and isolated mesoscale eddies result in similar mesoscale temporal anomaly, they can be distinguished when the mean sea surface dynamic height is retrieved, which cannot be obtained by altimetry data alone. This enables us to describe the shedding and merging of offshore mesoscale eddies and the development of the stable large meander south of Japan.
Spatial and temporal resolutions of satellite altimetry data are, however, not sufficient to describe fast-moving small-scale variations of the Kuroshio itself. The high-resolution surface velocity field around the Kuroshio can be directly obtained by high-frequency (HF) ocean radars, which can provide thorough descriptions of the Kuroshio in terms of variations of the maximum speed of the Kuroshio and the displacement of its axis position. In addition, short-term variations of the Kuroshio, such as responses to extraordinary winds of a typhoon, can be studied quantitatively..
5. XiFeng Wang, Kaoru Ichikawa, Coastal Waveform Retracking for Jason-2 Altimeter Data Based on Along-Track Echograms around the Tsushima Islands in Japan, Remote Sensing, 10.3390/rs9070762, 2017, 9, 7, 2017.07, Although the Brown mathematical model is the standard model for waveform retracking
over open oceans, due to heterogeneous surface reflections within altimeter footprints, coastal
waveforms usually deviate from open ocean waveform shapes and thus cannot be directly interpreted
by the Brown model. Generally, the two primary sources of heterogeneous surface reflections are land
surfaces and bright targets such as calm surface water. The former reduces echo power, while the
latter often produces particularly strong echoes. In previous studies, sub-waveform retrackers, which
use waveform samples collected from around leading edges in order to avoid trailing edge noise,
have been recommended for coastal waveform retracking. In the present study, the peaky-type noise
caused by fixed-point bright targets is explicitly detected and masked using the parabolic signature in
the sequential along-track waveforms (or, azimuth-range echograms). Moreover, the power deficit of
waveform trailing edges caused by weak land reflections is compensated for by estimating the ratio
of sea surface area within each annular footprint in order to produce pseudo-homogeneous reflected
waveforms suitable for the Brown model. Using this method, altimeter waveforms measured over
the Tsushima Islands in Japan by the Ocean Surface Topography Mission (OSTM)/Jason-2 satellite
are retracked. Our results show that both the correlation coefficient and root mean square difference
between the derived sea surface height anomalies and tide gauge records retain similar values
at the open ocean (0.9 and 20 cm) level, even in areas approaching 3 km from coastlines, which
is considerably improved from the 10 km correlation coefficient limit of the conventional MLE4
retracker and the 7 km sub-waveform ALES retracker limit. These values, however, depend on the
topography of the study areas because the approach distance limit increases (decreases) in areas with
complicated (straight) coastlines..
6. XiFeng Wang, Kaoru Ichikawa, Effect of High-Frequency sea waves on wave period retrieval from radar altimeter and buoy data, Remote Sensing, doi:10.3390/rs8090764, 8, 764, 2016.09, [URL], Wave periods estimated from satellite altimetry data behave differently from those calculated from buoy data, especially in low-wind conditions. In this paper, the geometric mean wave period Ta is calculated from buoy data, rather than the commonly used zero-crossing wave period Tz. The geometric mean wave period uses the fourth moment of the wave frequency spectrum and is related to the mean-square slope of the sea surface measured using altimeters. The values of Ta obtained from buoys and altimeters agree well (root mean square difference: 0.2 s) only when the contribution of high-frequency sea waves is estimated by a wavenumber spectral model to complement the buoy data, because a buoy cannot obtain data from waves having wavelengths that are shorter than the characteristic dimension of the buoy..
7. Akira Nagano, Kaoru Ichikawa, Hiroshio Ichikawa, Masanori Konda, Kiyoshi Murakami, Volume transports proceeding to the Kuroshio Extension region and recirculating in the Shikoku Basin, Journal of Oceanography, 10.1007/s10872-013-0173-9, 69, 3, 285-293, 2013.05, [URL], The volume transport of the Kuroshio, the western boundary current of the North Pacific subtropical gyre, varies vigorously due to merging of disturbances propagating from the entire North Pacific. Taking into account the recirculation in the Shikoku Basin by the zonal observation line at 30「ェN to the west of the Izu-Ogasawara Ridge, we estimated the volume transport in the top 1000 m layer toward the Kuroshio Extension region. The volume transport of the local recirculation gyre in the Shikoku Basin increases associated with the westward extension of the gyre, particularly in the period of the large meandering path of the Kuroshio south of Japan. Meanwhile, most of the transport variations toward the Kuroshio Extension region correspond to those of the Kuroshio transport on the continental slope south of Japan, which vary independently of those of the recirculation gyre..
8. Ichikawa Kaoru, Wen Chang Yang, Akihiko Morimoto, Yoshikawa Yutaka, Shigeo Sugitani, Wen-Shan Chiang, Jian-Wu Lai, En Yu Liang, Cho-Teng Liu, Chang-Wei Lee, Kei Yufu, Moeto Kyushima, Satoshi Fujii, Senjyu Tomoharu, Yoshihiko Ide, Preliminary results of the Japan-Taiwan joint survey on combining ocean radar data in the Kuroshio upstream region, Ocean Science Journal, 10.1007/s12601-013-0011-4, 48, 1, 141-148, 2013.03, [URL], Japanese and Taiwanese institutes have collaborated to obtain ocean radar data with significantly increased coverage in the upstream Kuroshio region. An international joint survey was conducted in June 2012, in which intensive in situ observations were performed within the radar coverage. Details of the joint survey are presented in this paper with brief descriptions of preliminary results on the surface and subsurface currents near and within the Kuroshio..
9. Akira Nagano, Kaoru Ichikawa, Hiroshio Ichikawa, H Tomita, H Tokinaga, Masanori Konda, Stable volume and heat transports of the North Pacific subtropical gyre revealed by identifying the Kuroshio in synoptic hydrography south of Japan, Journal of Geophysical Research, 10.1029/2009JC005747, 115, C09002, 14pp, 115, C09002, doi:10.1029/2009JC005747, 2010.09.
10. Kaoru Ichikawa, Synoptic flow structures in the confluence region of the Kuroshio and Ryukyu current, Journal of Geophysical Research, Vol. 114, C06007, doi:10.1029/2008JC005213, 2009.06.
11. Kaoru Ichikawa, Kuroshio variations in the upstream region as seen by HF radar and satellite altimetry data, International Journal of Remote Sensing, 29(21), 6317-6326, doi:10.1080/01431160802175454, 2008.11.
12. Tokeshi, R., K. Ichikawa, S. Fujii, K. Sato and S. Kojima, Estimating the geostrophic velocity obtained by HF radar observations in the upstream area of the Kuroshio, Journal of Oceanography, Vol 63(4), 711-720, 2007.06.
13. Kaoru Ichikawa, Flow of abyssal water into Wake Island Passage: Properties and transports from hydrographic surveys, Journal of Geophysical Research, Vol. 112, C04008, doi:10.1029/2006JC004000, 2007.04.
14. Kaoru Ichikawa, Influence of the Kuroshio fluctuations on sea level variations along the south coast of Japan, Journal of Oceanography, 10.1007/s10872-006-0014-1, 61, 5, 979-985, 61, 979-985, 2005.09, [URL].
15. K. Ichikawa, N. Gohda, M. Arai and A. Kaneko, Variations of the Subtropical Counter-Current as seen by repeated ADCP and satellite altimetry data, Proceedings of The Indonesia Ocean Forum 2005 and the 13th PAMS/JECSS Workshop, 161-164, 2005.07.
16. Kaoru Ichikawa, Estimating the Kuroshio axis south of Japan using combination of satellite altimetry and drifting buoys, Journal of Oceanography, 10.1023/B:JOCE.0000038343.31468.fe, 60, 2, 375-382, Vol. 60, 375-382, 2004.03, [URL].
Works, Software and Database
1. Ocean Flux data sets estimated with satellite remote sensing data. This includes sea surface fluxes such as shortwave radiation, longwave radiation, latent heat, sensible heat and momentum, togather with related quantities as sea-surface air temperature, sea surface height and surface velocity. The data set is available via internet (
1. , [URL].
  • Okada Prize, Japan Oceanographic Society, 2001
Educational Activities
Classes in Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Science, Kyushu University.
Co-author (Chapter 7) of the textbook "Fluid Dynamics to study global environment", which is now on sale in the Korean and English translations.
Other Educational Activities
  • 2017.11, Department's textbook "Fluid Dynamics for Global Environmental Studies" is on sale at Springer..
  • 2001.03.
Professional and Outreach Activities
Researcher of Japan Agency for Marine-Earth Science and Technology, 2000-2010
Visiting Scientist in Center for Marine Environmental Studies, Ehime University, 2001-
CNES/EUMETSAT Ocean Surface Topography Science Team Principal Investigator, 2006-
Member of Marine-Space Cooperation Committee, 2010-
NASA CYGNSS Science Team, Principal Investigator, 2015-
CNES TOSCA_SWOT2016 Science Team, Co investigator, 2015-.