PO025

Comparison of offshore winds from ASCAT and WRF model in near coastal waters

Yuko Takeyama 1, Teruo Ohsawa2, Susumu Shimada3, Katsutoshi Kozai2, Tetsuya Kogaki3
1Tokyou University of Marine Science and Technology, Tokyo, Japan, 2Kobe University, Kobe, Japan, 3AIST, Koriyama, Japan

Abstract

Required spatial resolutions of an offshore wind resource map are different for between coastal waters and open waters. For coastal waters, meteorological and oceanographic phenomena have mesoscale and microscale structures and the required spatial resolution is higher than that in open waters. To make the map effectively, a microwave scatterometer, ASCAT wind with a 12.5 km spatial resolution and a numerical mesoscale model, WRF with a spatial resolution of a few hundred meters are appropriate for open waters and coastal waters, respectively.
This work provides the consistency between the ASCAT wind and the WRF wind around a boundary between open waters and coastal waters in two domains, D2 and D3. For the low and middle wind speed ranges (less than 14 m/s), the ASCAT wind speed is higher than the WRF wind speed. In contrast, the ASCAT wind speed is lower than the WRF wind speed at the range of high wind speeds (more than 14 m/s). At around 25 km from the coast, the mean differences are approximately a few percent. Beyond 25 km from the coast, the difference is almost constant regardless of the distance in D2. Meanwhile, in D3, the differences decrease gradually and become approximately zero at 35 km from the coast. A continuous examination of a reason of the difference would allow for a more accurate and reliable offshore wind resource map.

Method

Sea surface winds from the microwave scatterometer, ASCAT with a 12.5 km coastal product and the numerical mesoscale model, WRF with a 500 m spatial resolution are used. The ASCAT wind speed and direction observed from 1 January 2013 to 31 December (one year) are resampled to 0.1 degrees latitudinally and 0.125 degrees longitudinally with the inverse distance weighting method. Meanwhile the WRF simulation with a spatial resolution of 2.5 km 2.5 km (outer domain, D1) and 500 m 500 m (nested domain, D2 and D3) is also conducted during a same period of the ASCAT wind. The ASCAT and WRF winds are compared each other.

Results

For the low and middle wind speed ranges, the ASCAT wind speed is higher than the WRF wind speed. In contrast, the ASCAT wind speed is lower than the WRF wind speed at the range of high wind speeds. It is also found that the wind speed difference between the ASCAT and WRF winds in D2 and D3 is large within 25 km from the coast. At around 25 km, the mean differences, which are calculated for each 2 km of the distance, are a few percent in both domains. Beyond 25 km from the coast, the difference is almost constant regardless of the distance in D2. Meanwhile, in D3, the differences decrease gradually and become approximately zero at 35 km from the coast.

Conclusions

The ASCAT wind and the WRF are appropriate methods to make the wind resource map for open waters and coastal waters, respectively. This study found that the two wind speeds have differences of a few percent at maximum. Additionally the difference decreases according as the distance from the coast is longer in D3 though the difference is constant regardless of the distance. The two types map finally should be connected around a boundary between open waters and coastal waters. Larger validation areas would be needed to clarify the relation between the difference and the distance from the coast.

Objectives

Dedicators, who are interested in the assessment, may learn state of the art methods to make an offshore wind resource assessment using a satellite remote sensing and a numerical mesoscale model.