Katsutoshi Kozai

Comparison of Geophysical Model Functions for SAR Wind Speed Retrieval in Japanese Coastal Waters

This work discusses the accuracies of geophysical model functions (GMFs) for retrieval of sea surface wind speed from satellite-borne Synthetic Aperture Radar (SAR) images in Japanese coastal waters characterized by short fetches and variable atmospheric stability conditions. In situ observations from two validation sites, Hiratsuka and Shirahama, are used for comparison of the retrieved sea surface wind speeds using CMOD (C-band model)4, CMOD_IFR2, CMOD5 and CMOD5.N. Of all the geophysical model functions (GMFs), the latest C-band GMF, CMOD5.N, has the smallest bias and root mean square error at both sites. All of the GMFs exhibit a negative bias in the retrieved wind speed. In order to understand the reason for this bias, all SAR-retrieved wind speeds are separated into two categories: onshore wind (blowing from sea to land) and offshore wind (blowing from land to sea). Only offshore winds were found to exhibit the large negative bias, and short fetches from the coastline may be a possible reason for this. Moreover, it is clarified that in both the unstable and stable conditions, CMOD5.N has atmospheric stability effectiveness, and can keep the same accuracy with CMOD5 in the neutral condition. In short, at the moment, CMOD5.N is thought to be the most promising GMF for the SAR wind speed retrieval with the atmospheric stability correction in Japanese coastal waters, although there is ample room for future improvement for the effect from short fetch.

Estimation of Offshore Wind Resources in Coastal Waters off Shirahama Using ENVISAT ASAR Images

Offshore wind resource maps for the coastal waters off Shirahama, Japan were made based on 104 images of the Advanced Synthetic Aperture Radar (ASAR) onboard the ENVISAT satellite. Wind speed fields were derived from the SAR images with the geophysical model function CMOD5.N. Mean wind speed and energy density were estimated using the Weibull distribution function. These accuracies were examined in comparison with in situ measurements from the Shirahama offshore platform and the Southwest Wakayama buoy (SW-buoy). Firstly, it was found that the SAR-derived 10 m-height wind speed had a bias of 0.52 m/s and a RMSE of 2.33 m/s at Shirahama. Secondly, it was found that the mean wind speeds estimated from SAR images and the Weibull distribution function were overestimated at both sites. The ratio between SAR-derived and in situ measured mean wind speeds at Shirahama is 1.07, and this value was used for a long-term bias correction in the SAR-derived wind speed. Finally, mean wind speed and wind energy density maps at 80 m height were made based on the corrected SAR-derived 10 m-height wind speeds and the ratio U80/U10 calculated from the mesoscale meteorological model WRF.

Wind-induced upwelling in the western equatorial Pacific Ocean observed by multi-satellite sensors

Abstract R/V MIRAI (MR01-K05 Leg3) of Japan Marine Science and Technology Center (JAMSTEC) was stationed at the point of 2° North and 138° East in the western equatorial Pacific Ocean from November 9 to December 9, 2001. During this air–sea interaction research cruise SeaWiFS and NOAA/AVHRR local area coverage (LAC) scenes were received by the station onboard R/V MIRAI and the products derived from these satellites are verified against oceanographic observations including the parameter of sea surface temperature, salinity, chlorophyll-a and current velocity profile to the depth of 300 m. Level 3 wind vector products derived from QuickSCAT onboard SeaWinds are also collected and validated against in situ wind vectors. The diurnal change of amplitude of sea surface temperature decreases from 1.5° to 0° after the week-long prevailing northwest monsoon wind with the maximum gust more than 20 m/s while the observed surface salinity and chlorophyll-a concentration increase from 34.1 to 34.37 and from 0.05 to 0.14 mg/m3, respectively. The increase of chlorophyll-a and the decrease of sea surface temperature in this region are shown in the multi-date SeaWiFS chlorophyll-a concentration products and multi-channel sea surface temperature (MCSST) products from NOAA/AVHRR. Wind vector patterns before and after the gust more than 20 m/s are also observed by QuickSCAT. During the period of the week-long northwest monsoon wind the current velocity of upper 70 m reaches about 70 cm/s in the southeastward direction while the current velocity at the depth from 80 to 120 m indicates 50 cm/s in the northwestward direction. The current of upper 70 m corresponds to the northwest monsoon current (NMC) and the intrusion of NMC enhanced by the strong northwest monsoon winds (westerly wind bursts) causes a reversal in the sub-surface current (New Guinea coastal undercurrent, NGCUC) which creates a temporal upwelling in this region.

Satellite-derived sea surface height and sea surface wind data fusion for spilled oil tracking

Abstract An attempt is made to estimate the trajectory of the spilled oil from the sunken tanker Nakhodka occurred on January 2, 1997 in the Japan Sea by fusing two microwave sensor data, namely ERS-2 altimeter and ADEOS/NSCAT scatterometer data. In this study ‘fusion’ is defined as the method of more reliable prediction for the trajectory of spilled oil than before. Geostrophic current vectors are derived from ERS-2 altimeter and wind-induced drift vectors are derived from ADEOS/NSCAT scatterometer data These two different satellite-derived vectors are ‘fused’ together in the surface current model to estimate and evaluate the trajectory of spilled oil from the sunken tanker Nakhodka. The distribution of component of spill vector is mostly accounted for by the distribution of geostrophic velocity component during the study period with some discrepancies during March, 1997.

Assessment of Offshore Wind Resources Within Japan's EEZ Using QuikSCAT Data

In this paper, offshore wind resources within the Japans EEZ (Exclusive Economic Zone) are assessed using wind speed data from the microwave scatterometer SeaWinds onboard QuikSCAT. At first, from the 10m-height wind speed from QuikSCAT, 60 m-height wind speed is estimated by using an empirical equation for height correction. Based on the 60 m-height wind speeds, annual energy Production is calculated under an assumption of installing 2 MW wind turbines every . The annual energy production is then accumulated for the entire Japans territorial waters and EEZ (). As a result, it is shown that the total energy Production is estimated to be TWh/yr. This offshore wind energy Potential within the EEZ is approximately 50 times higher than the actual annual electricity production in Japan.

Airborne polarimetric SAR monitoring for ships and coastal seas

Airborne polarimetric and interferometric synthetic aperture radar (PiSAR) images are analyzed for monitoring ships and coastal seas especially for estimating sea surface wind speed and direction. Though in situ ships data through AIS (Automatic Identification System) were not available by the time of analysis, PiSARs X band images showed the potential to map coastal wind vectors with high spatial resolution of 2.5 m

Aerosol properties from MIRAI research cruise in the tropical pacific ocean

Abstract Aerosol parameters are observed with a photopolarimeter (PSR-1000 device) and by SeaWiFS sensor during two research cruises conducted by R/V MIRAI in the equatorial Pacific regions. Observation of aerosol particles has shown the background characteristics of aerosol particles. The aerosol optical thickness calculated from PSR-1000 data coincide well with those from SeaWiFS data at 0.865 μm and shows difference at 0.443 μm. It is also shown that the data with low aerosol optical thickness has an ambiguity in the calculation of the Angstrom exponent for various sets of wavelengths. The refractive index of aerosol particles is retrieved by combining data from direct solar measurements and those from polarization measurements of scattered sun-light.

Assessment of the offshore wind resource in Japan with the ASCAT microwave scatterometer

ABSTRACT We analysed wind speed and direction off the coast of Japan using data from the satellite-borne Advanced Scatterometer (ASCAT) and the Weather Research and Forecasting model (WRF), validated these data using in situ wind measurements from 20 buoys, and evaluated the effect of the long time intervals from ASCAT observations on wind resource assessment. More than 25 km from the coast, and at heights of 10 m, the ASCAT wind speed has negative biases of up to 3.4% and root mean square errors of up to 18.5%; its wind direction has 11° to 27° of mean absolute error compared to buoy measurements at a height of 10 m. These accuracies are better than either the expected accuracies reported in the technical manual or those simulated with WRF with its spatial resolution of 10 km. We also evaluated long-term average ASCAT wind speeds in comparison to 4- and 5-year averages of in situ buoy wind speeds measured at three buoys, with resulting differences of –0.3%, –6.3%, and – 1.6%. Furthermore, wind roses show that appearance frequencies of the ASCAT wind direction for the long term are in a good agreement with those of the measurements at the three buoys. Our results show that the ASCAT-derived wind speed and direction are appropriate more than 25 km from the coast, and that the long time interval between ASCAT observations has an insignificant effect on wind resource assessment, if at least 4 or 5 years of averaged ASCAT data are used.

Methodologies for offshore wind resource assessment

Needs for offshore wind resource assessment have been rapidly growing all over the world with increase of interests in renewable energy and of knowledge that offshore winds can be a promising energy source. However, it is generally difficult to know wind climate over coastal waters, where offshore wind farms are usually constructed, because wind observation is mostly few and moreover the winds vary complicatedly in both time and space compared with those over open oceans. In order to cope with these difficulties statistical methods using Synthetic Aperture Radar (SAR) and numerical simulation with mesoscale model such as MM5 have been proposed for offshore wind resource assessment. The first half of the tutorial will cover how offshore wind resources are evaluated by using MM5 and SAR in the case of Japanese coastal waters. The second half of the tutorial will provide not only assessment of offshore wind resources but also integrated tools for wind farm design and management considering the life cycle of the wind farm starting from site selection to decommissioning in the case of European coastal waters. This tutorial will cover the following areas. 1. Resource assessment with MM5 (Ohsawa) 2. Resource assessment with SAR (Kozai) 3. EO-wind farm and WEMSAR projects and S-WASP (Hasager and Christiansen) 4. Wake analysis (Christiansen) This tutorial is intended for not only end-users such as electric power companies but also wind energy market players, marine construction and consulting companies, and satellite information service providers.

Investigation on ballast water exchangeable area in the Bay of Bengal using MODIS/Aqua

The ballast water exchange at seas has been recognized as one of the operational countermeasures to cope with the invasion of non-indigenous species through the ballast water. The Bay of Bengal is traditionally considered to have low chlorophyll-a concentration thus low phytoplankton counts, which is the reason why the Bay of Bengal (BoB) has been selected as a suitable ballast water exchangeable sea. However an anomalously high K(490) area was found off the coast of Sri Lanka during the northeast monsoon in 2005, which corresponds to higher plankton cell densities than the criterion set by the regulation of International Maritime Organization (IMO). The regression equation between K(490) and corresponding in situ plankton cell densities in the Bay of Bengal is developed to identify suitable ballast water exchangeable area based on the regulations of IMO. According to the results the central and eastern portions in the Bay of Bengal during the northeast monsoon season are found to be suitable, while the unsuitable area broadens during the southwest monsoon season at the western Bay of Bengal. Seasonal and annual variability of K(490) and corresponding cell density is discussed to establish an early routing system for avoiding the high cell density area in advance.