Teruo Ohsawa

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.

In situ observational research of the gap wind “Hijikawa-Arashi” in Japan

The Hijikawa-arashi, a gap wind occurring in Ozu City, Ehime Prefecture, Japan, was investigated through in situ observations of horizontal and vertical directions. Analysis of surface air temperature data revealed that the inland Ozu Basin was radiatively cooled on the days on which the Hijikawa-arashi events occurred. This induced a greater difference in air temperature between the basin and the estuary of the Hijikawa River in comparison to days that no basin cooling occurred. In addition, the wind speeds of the Hijikawa-arashi observed at the estuary of the Hijikawa River were strongly proportional to the sea-level pressure difference between the inland Ozu Basin and the estuary. Theoretical calculations indicated that this pressure gradient force was sufficient for driving the strong wind of the Hijikawa-arashi. Moreover, calculation of the Froude number using vertical meteorological data revealed that the Hijikawa-arashi developed as a supercritical flow. That is, the flow was intensified at the exit of the gap, in accordance with the hydraulic theory. The vertical observations detected the inversion layer over the Hijikawa-arashi and suggested an application of the shallow water theory to this gap wind. The Hijikawa-arashi is an interesting gap flow with a strong wind, despite its small-scale geography relative to other gap winds worldwide. There is an important trigger getting higher basin pressure upstream due to the radiative cooling of the atmosphere and formation of a cold pool at the basin.

Current Situation and Difficulty of Wave Forecast From Viewpoint of Ship Management

Numerical forecasts of weather and oceanography are increasingly common in the field of ship operations due to advances in computer science. However, in some situations, the accuracy of forecasts is too unreliable to ensure safe operations. In the current study, a nationwide questionnaire is used to determine the effectiveness of wave forecasts in enhancing navigation safety. Data analysis is also shown for two cases of failed forecasts in low-pressure weather systems near Japan in the winter. Finally, recommendations are made for improving wave forecasts from the viewpoint of ship operations.Copyright

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.

Investigation of the Fetch Effect Using Onshore and Offshore Vertical LiDAR Devices

An offshore wind measurement campaign using vertical light detection and ranging (LiDAR) devices was conducted at the Hazaki Oceanographic Research Station (HORS) as part of an investigation into determining the optimal distance from the coast for a nearshore wind farm from a meteorological perspective. The research platform was a 427 m long pier located on a rectilinear coastline on the Pacific coast of the central Honshu Island in Japan. The relationship between the ratios of the increase of wind speed near the surface and fetch length within 5 km of the coast was analyzed via LiDAR observations taken at heights from 40 to 200 m. The results showed that the speed of the coastal wind blowing from land to sea gradually increased as the fetch length increased, by approximately 15–20% at 50 m above sea level around a fetch length of 2 km. Moreover, empirical equations were derived by applying the power law to the relationship between the increase of wind speed and fetch lengths at 1–5 km, as obtained from the LiDAR measurements. It was also found that the wind speed increase at a 2 km fetch length was equivalent to the effect of a 50–90 m vertical height increase on the coast in this region.

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.

Numerical Analysis of Failed Forecasts of Waves Under Low Pressures From Viewpoint of Ship Operation

Today, weather routing becomes more important under the current situation of international maritime transportation. However, the authors reveal that the current systems are not necessarily sufficiently reliable. It is also worthwhile to consider the background of the difficulty in numerically forecasting winds and waves. Here, two failed cases of wave forecasts are verified using WRF and SWAN, two numerical meso-meteorology models. In both failed cases, low pressures developed from the southern sea area of Japan with a northeast direction. First, wind direction can be reproduced correctly, even if the resolution of the simulation is changed. Second, if the resolution is insufficient, wind speed will be underestimated as smaller than the observed values. At the same time, wave height tends to be underestimated as compared to observed values, too. This is the feasibility study for improving the forecasting of wave growth pattern due to developing low pressures from the viewpoint of safe ship operation.Copyright

Offshore wind profile measurements using a Doppler LIDAR at the Hazaki Oceanographical Research Station

Vertical wind speed profiles near the coast were observed using a Doppler Light Detection and Ranging (LIDAR) system at the Hazaki Oceanographical Research Station (HORS) from September 17 to 26, 2013. The accuracies of the theoretical wind profile models of the log profile model and the Monin-Obukov similarity (MOS) theory were examined by comparing them to those of the observed wind profiles. As a result, MOS, which takes into account the stability effects during wind profile calculations, successfully estimated the wind profile more accurately than the log profile model when the wind was from a sea sector (from sea to land). Conversely, both models did not estimate the profile adequately when the wind was from a land sector (from land to sea). Moreover, the wind profile for the land sector was found to include an obvious diurnal cycle, which is relevant to the stability change over land. Consequently, it is found that the atmospheric stability plays an important roll to determine the offshore wind speed profiles near the coast for not only the sea sector but also the land sector.

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.