Shoichiro Kojima

Kuroshio variations in the upstream region as seen by HF radar and satellite altimetry data

Variations of the position and speed of the Kuroshio are studied by a combination of high‐resolution high‐frequency (HF) ocean radar data and wide‐coverage altimetry data. The speed of the Kuroshio and its position are determined along 123.35° E northeast of Taiwan from daily maps of surface geostrophic velocity estimated from HF radar data from August 2001 to February 2005. These two are found to be well correlated for mesoscale variations with periods of a few months, as the Kuroshio tends to be faster (or slower) when its axis moves south (or north). This tendency is significant in summer when the Kuroshio is seasonally intensified and well‐defined in the HF radar data. By taking correlation with the sea surface dynamic height anomaly observed by satellite altimeters, these Kuroshio variations in the East China Sea are found to be induced by merging of mesoscale eddies from the east at 21° N and by coincident Kuroshio meanders east of Taiwan.

Enhancement of phytoplankton primary productivity in the southern East China Sea following episodic typhoon passage

The enhancement of primary productivity (PP enh ) in the southern East China Sea (ECS) following 16 typhoon passages was investigated using ocean color data and a primary productivity model. PP enh tended to be higher when typhoons traversed slowly with trajectories that allowed strong southerly winds to prevail over Yonaguni Island. Such long-lasting southerly winds were believed to push the Kuroshio current axis shelfward, enhancing the upwelling of nutrients, hence promoting new productivity (NP). The importance of long-lasting southerly winds as a proxy for physical perturbations underlying PP enh was expressed by an empirical equation by which 88% of PP enh variation could be explained. Applying this equation, we assessed that typhoon passages accounted for a minimum of 0.6― 11.8% of the ECS summer―fall NP, suggesting that typhoon passage over the southern ECS is an important phenomenon supporting NP in the ECS.

Verification of a Bayesian method for estimating directional spectra from HF radar surface backscatter

A Bayesian method for estimating directional wave spectra from the Doppler spectra obtained by HF radar is examined using data acquired during the SCAWVEX project. Applicability, validity and accuracy of the Bayesian method are demonstrated compared with the directional spectrum observed by a directional buoy. In addition the estimated spectra are compared with Wyatt (1990) and the Bayesian method is found to be more robust against noise. Necessary conditions of the Doppler spectral components to be used to estimate a reliable directional spectrum for the present method are also discussed.

Calibration experiments of advanced X-band airborne SAR system, Pi-SAR2

In the National Institute of Information and Communications Technology, Japan (NICT), we have developed an advanced X-band airborne synthetic aperture radar (SAR) system, Pi-SAR2, since 2006, to succeed the X-band polarimetirc and interferometric airborne SAR, Pi-SAR. The Pi-SAR2 has a higher spatial resolution of 0.3–0.6 m in the azimuth direction and 0.3–0.5 m in the range direction, as well as polarimetric and interferometric observation functions. The calibration experiments of the Pi-SAR2 were carried out as a part its test flights in December 2008 and February 2010. Calibration targets were deployed on a runway of the Taiki Aerospace Experiment Field in Hokkaido, Japan, to be observed.

Performance evaluation on cross-track interferometric SAR function of the airborne SAR system (PI-SAR2) OF NICT

The National Institute of Information and Communications Technology (NICT) developed an airborne X-band SAR system (Pi-SAR2: polarimetric and interferometric SAR 2) to monitor the earths environment and disasters [1]. Pi-SAR2 has polarimetric and interferometric functions with high spatial resolution of 0.3m [2]. In this paper we describe the single-pass interferometric function of Pi-SAR2 and show the performance evaluation on the interferometric function of Pi-SAR2 by using corner reflectors. Moreover we describe the volcanic monitoring by using interferometric SAR.

Development of the onboard processor for Pi-SAR2

The synthetic aperture radar (SAR) has been utilized for monitoring natural disaster areas because the SAR observation can be performed irrespective of the daylight and weather conditions. To reduce the time from observation to distributing SAR images to the headquarters for disaster control and disaster area, we have developed an onboard processor which is capable of doing a full-spec processing for the airborne SAR called as the polarimetric and interferometric SAR (Pi-SAR2) since 2009. We demonstrated that the developed onboard processor is effective to reduce the time from observation to distribution and the SAR image can be transferred to the ground within 15 min after observation via the communication satellite called as the engineering test satellite VIII (ETS-VIII) in the experiments on March 2011.

HF ocean radar observation of surface currents induced by a typhoon in the East China Sea

A long-range (200 km) high-frequency (HF) ocean radar system was developed by Japans Communications Research Laboratory (CRL) to monitor the Kuroshio upstream region of the East China Sea. Sea surface observation began in July 2001. In this paper, we present the validation results of surface currents obtained by the radar in comparison with currents measured using a moored current meter. We also present the results of observing temporal variations in the Kuroshio over a year and the surface currents induced by a strong typhoon.

Surface current patterns observed by HF radar: methodology and analysis of currents to the north of the Yaeyama Islands, East China Sea

A new method was developed to compare the classifications of different kind of data maps based on the self-organizing map (SOM) analysis. The surface current maps of the northern coast of Ishigaki and Iriomote Islands, East China Sea, observed by high-frequency (HF) radar were classified by SOM. Winds, sea surface temperatures (SST), and reanalysis data were also classified by SOM. The optimum area for classification was determined objectively by the new method, which relates these patterns to HF radar current patterns. We found two typical surface patterns: the first was that northeastward flows are dominant in the observation area and the second was that a clockwise eddy was also dominant in this area. The southwestward wind pattern was strongly related to the dominant clockwise eddy pattern of the HF radar current field.

Damage estimation of the Great East Japan earthquake with airborne SAR (PI-SAR2) data

A 9.0-magnitude earthquake occurred off the pacific coast of Tohoku area of Japan, in March 2011. Accompanied with the subsequent tsunami, it caused a lot of casualties and damages in the coast area. We conducted urgent observations of the damaged areas with an airborne X-band SAR system, Pi-SAR2, immediately after the earthquake, on March 12 and the following dates. The Pi-SAR2 is a fully polarimetric airborne radar, with high spatial resolution of 0.3 m. We found the high resolution radar images, with multi-temporal observations, are quite useful to estimate the area flooded by the tsunami and its changes. Also found is capability of the polarimetric radar images to estimate the amount of a pile of debris that is one of the greatest concerns for the residents.

Theoretical Characterization of X-Band Multiincidence Angle and Multipolarimetric SAR Data From Rice Paddies at Late Vegetative Stage

Various polarimetric decomposition techniques have been proposed and demonstrated for more than 20 years. However, they still have never been operational applications because of insufficient validations. The true composition ratio of scattering mechanisms within a radar backscatter has to be known. To achieve this, a novel comprehensive approach to accurately identify the contribution of each scattering mechanism by a multiincidence angle and multipolarimetric (MIMP) synthetic aperture radar (SAR) observation combined with a theoretical model simulation is introduced. Rice paddies in Niigata City in Japan having a simple vegetation structure without topography were observed by X-band polarimetric and interferometric SAR 2 (Pi-SAR2), by gradually varying the flight path in terms of incidence angle. In addition to the MIMP SAR observation, a dominant scattering mechanism is reliably isolated through the theoretical characterization of the data by a discrete scatterer model. In the case of the rice paddies at a late vegetative stage, a dominant scattering on HH gradually varies from the double-bounce scattering to the volume scattering of rice grains in terms of incidence angle, whereas VV is affected by various volume scatterings of grains, leaves, and stems at the small, medium, and large incidence angles, respectively. HV is simply affected by the volume scattering of grains at the small incidence angle, and then it gradually switches to the volume scattering of the stalks. This brand new approach by the MIMP SAR observation with the theoretical modeling for this specific stage could take forward the polarimetric decomposition studies.