Double Structure of the Wind Jet through the Tsushima Strait

Abstract

We have investigated the northeasterly/southwesterly wind jets blowing through the Tsushima Strait from two case studies. Using high-resolution winds derived from SAR, we have presented the detailed structure of wind in the strait. The general ∞ow is generally close to geostrophic wind in the strait, inferring from the seal level pressure flelds. Wind jets are induced by two channels and by the terrestrial gap at the center of the Tsushima Island. The wind jets are clearly separated from the coastlines. The Tsushima Strait is a generic term indicating the sea area between the Korean Peninsula and the Japanese archipelago, with the Japanese islands of Honshu to the east, Kyushu to the south and the Goto Islands to the southeast (Figure 1). In the center of the strait, a slender island, the Tsushima Island is located along the strait. This island principally consists of two mountainous islands. The Tsushima Island divide the strait into eastern and western channels; the width and maximum depth are 140km and 110m in the eastern channel, and 40km and 200m in the western channel, respectively. Both channels connect the East China Sea and the Japan Sea. From these geographical conditions, the Tsushima Strait is one of the most important sea areas where the continuous and intense monitoring researches are required from oceanographic and meteorological point of views as well as marine security point of view. Extensive previous studies have investigated the Tsushima Strait hydrography by considering wind efiects on it. However, higher-resolution and wider-area wind observations are required for better understanding of the oceanographic and meteorological environment because of the complex coastal topography of the region. This is because few studies pay attention to the formation of wind jets in the strait, wind jet structures, and the role of the Tsushima Island on the formation of the wind jets. These are open issues at the present stage. The purpose of this paper is to investigate the wind blowing through the Tsushima Strait by using high-resolution satellite wind measurements. We look into the evolution and distribution of the wind jets during representative time periods and illustrate the contribution of the Tsushima Island to the wind jet formation. Speciflc questions we would like to address are: First question is typical structures of the wind jets found in the Tsushima Strait. When the wind blows through the Tsushima Strait in southwest-northeast direction, strong winds are formed. Second, what are the roles of the Tsushima Island on the formation of the wind jets? 2. DATA We analyze wind flelds derived from two SARs. They are the European Space Agency (ESA) Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (ASAR) and the Cana- dian RADARSAT-1. Both SARs operate at C-band (5.3GHz). While RADARSAT operates in horizontal (HH) polarization, ENVISAT/ASAR operates in HH or vertical (VV) polarization. In this study, we use wide-swath or ScanSAR mode products. That is, the ENVISAT/ASAR Wide- swath mode images covers an area of approximately 500 £ 500km, with a pixel size of 75m. The RADARSAT ScanSAR Narrow mode image covers an area of approximately 300 £ 300km, with a pixel size of 25m. Wind speed maps are derived from the ENVISAT/ASAR and RADARSAT images by applying SAR wind retrieval by using CMOD IFR2 scatterometer model function (1) and wind direction from objective analysis data (GPV: Grid Point Data). In cases of HH polarization images, a polarization ratio conversion factor (2) is applied. While the C-band geophysical model function is originally developed for VV polarized radar cross section, the polarization ratio conversion factor enables to convert it to HH polarized radar cross section. It has been shown by numerous studies that wind speeds can be retrieved from ENVISAT/ASAR (3) and RADARSAT-1 (4) with a root mean square error of less than 2m/s and with negligible bias.