Coastal Wind Jets Flowing into the Tsushima Strait and Their Effect on Wind-Wave Development

Osamu Isoguchi Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan

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Hiroshi Kawamura Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan

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Abstract

Coastal wind jets that flow into the Tsushima Strait, Japan, and their effects on wind waves are investigated using synthetic aperture radar (SAR) images and altimeter-derived wind and waves. The coastal wind jets appear in 7 of 28 SAR-derived wind fields under the conditions of ambient southeasterly winds. Atmospheric conditions corresponding to the appearance of the coastal wind jets are examined by a high-resolution atmospheric community model, which indicates that stratified flows are influenced by the land topography and then pass through the strait forming low-level jet flows. Sensitivity experiments reveal that easterly to southerly stratified flows are a necessary condition for a jet’s formation and the degree of its enhancement is mostly controlled by the Froude number of the upstream flows. Atmospheric conditions in which the SAR observed the coastal jets meet the model-derived necessary conditions, which corroborate the validity of the model simulation and the jet’s formation mechanism. Next, the authors present a case study concerning effects of the coastal jets on wind-wave development by using altimeter-derived wind speed and significant wave height (SWH). Both profiles show similar convex spatial distribution within the wind jet range. The nondimensional SWH and fetch indicate agreements with the empirical fetch graph formula, suggesting that the wind waves are locally developed by the coastal jet. Through investigation of ship-based climatological data, it is found that the coastal jets occur frequently in midsummer due to prevailing southeasterly flows, which is accounted for by the seasonal evolution of the Asian summer monsoon.

* Current affiliation: Earth Observation Research and Application Center, Japan Aerospace Exploration Agency, Tokyo, Japan

Corresponding author address: Dr. Hiroshi Kawamura, Aramaki-aza Aoba Aoba-ku, Sendai, 980-8579, Japan. Email: kamu@ocean.caos.tohoku.ac.jp

Abstract

Coastal wind jets that flow into the Tsushima Strait, Japan, and their effects on wind waves are investigated using synthetic aperture radar (SAR) images and altimeter-derived wind and waves. The coastal wind jets appear in 7 of 28 SAR-derived wind fields under the conditions of ambient southeasterly winds. Atmospheric conditions corresponding to the appearance of the coastal wind jets are examined by a high-resolution atmospheric community model, which indicates that stratified flows are influenced by the land topography and then pass through the strait forming low-level jet flows. Sensitivity experiments reveal that easterly to southerly stratified flows are a necessary condition for a jet’s formation and the degree of its enhancement is mostly controlled by the Froude number of the upstream flows. Atmospheric conditions in which the SAR observed the coastal jets meet the model-derived necessary conditions, which corroborate the validity of the model simulation and the jet’s formation mechanism. Next, the authors present a case study concerning effects of the coastal jets on wind-wave development by using altimeter-derived wind speed and significant wave height (SWH). Both profiles show similar convex spatial distribution within the wind jet range. The nondimensional SWH and fetch indicate agreements with the empirical fetch graph formula, suggesting that the wind waves are locally developed by the coastal jet. Through investigation of ship-based climatological data, it is found that the coastal jets occur frequently in midsummer due to prevailing southeasterly flows, which is accounted for by the seasonal evolution of the Asian summer monsoon.

* Current affiliation: Earth Observation Research and Application Center, Japan Aerospace Exploration Agency, Tokyo, Japan

Corresponding author address: Dr. Hiroshi Kawamura, Aramaki-aza Aoba Aoba-ku, Sendai, 980-8579, Japan. Email: kamu@ocean.caos.tohoku.ac.jp

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