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

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Coastal Wind Jets Flowing Into the Tsushima Strait and Their Effect on Wind-Wave Development 564 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 64 Coastal Wind Jets Flowing into the Tsushima Strait and Their Effect on Wind-Wave Development OSAMU ISOGUCHI* AND HIROSHI KAWAMURA Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Miyagi, Japan (Manuscript received 4 August 2005, in final form 14 June 2006) ABSTRACT Coastal wind jets that flow into the Tsushima Strait, Japan, and their effects on wind waves are investi- gated 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. 1. Introduction where sharp changes in heat, moisture, and momentum transfers, as well as in elevation, occur around the Local winds are driven by large/mesoscale atmo- coastline. The behavior of coastal wind fields has im- spheric circulations or induced by land topography. mediate effects on social activities such as marine traf- They consist of two types: those mechanically induced, fic, commercial fishing, and leisure. such as mountain waves, downslope winds, and wakes; A lack of observational data with temporal/spatial and those thermally induced, such as land and sea resolution enough to research the coastal wind fields breezes and slope and valley winds (e.g., Stull 1988). (e.g., Panel on Coastal Meteorology 1992) has been These local circulations over land have been studied resolved by satellite observations over the last decade. intensively because of human concerns over these phe- Satellite scatterometers provide us maps of the surface nomena and the easy setup of meteorological instru- vector wind with spatial resolution of 25 km (Liu 2002), ments, which were supported by strong social demands. and they are operationally assimilated into weather The local circulations are quite unique in coastal zones, prediction models. They have succeeded in showing orographically modified wind fields, such as surface * Current affiliation: Earth Observation Research and Appli- wind jets and wakes, and have allowed us to examine cation Center, Japan Aerospace Exploration Agency, Tokyo, Ja- detailed features, formation mechanisms, and oceanic pan. responses to them (e.g., Laing and Brenstrum 1996; Kawamura and Wu 1998; Luis and Kawamura 2000, Corresponding author address: Dr. Hiroshi Kawamura, Ara- 2002; Chelton et al. 2000a, 2000b, 2004; Xie et al. 2001; maki-aza Aoba Aoba-ku, Sendai, 980-8579, Japan. Pickart et al. 2003). However, the satellite scatterom- E-mail: [email protected] eter cannot observe ocean winds within about 50 km DOI: 10.1175/JAS3858.1 © 2007 American Meteorological Society Unauthenticated | Downloaded 09/28/21 12:29 PM UTC JAS3858 FEBRUARY 2007 I S OGUCHI AND KAWAMURA 565 from the coasts because the radar backscattering from land contaminates the ocean signals. So-called remote sensing gaps are a serious problem for coastal ocean- ography because wind distribution is a main driving force for coastal ocean circulation and many important related phenomena. Synthetic aperture radar (SAR) can provide high spatial resolution images of ocean surface roughness. It can be used to estimate wind speed even in the coastal seas, and in particular within the scatterometer remote sensing gap. Wind stress variations on the sea surface modulate the roughness and change backscattered ra- dar power, which is usually expressed as normalized radar cross sections (NRCS). Empirical relations, known as a model function, between NRCS and wind speeds have been established using scatterometer- measured NRCS of C-band and Ku-band and in situ or analyzed surface winds (e.g., Wentz et al. 1984; Stof- felen and Anderson 1997). Recently SAR-measured NRCS have been directly compared with scatterom- eter-estimated winds to establish the L-band model function (Shimada et al. 2003). SAR images can thus be FIG. 1. A map of the study area with topography and the nu- used to resolve high-resolution wind fields by applying merical model domains. The inner square indicates the 5-km grid the developed model functions. domain; (a)–(d) 1° ϫ 1° grids of the ship-reported Beaufort wind There are numerous works of research dealing with observations shown in Fig. 11. the surface wind fields over the coastal seas using SAR images (e.g., Alpers et al. 1998; Korsbakken et al. 1998; Pan and Smith 1999; Sandvik and Furevik 2002; Furevik development by using SAR images and altimeter- et al. 2002). Alpers et al. (1998) have investigated kata- derived data. Figure 1 shows the geographical map of batic wind fields over Mediterranean coastal waters by the study area. The Kanmon Strait separates Honshu using SAR-derived wind patterns and an atmospheric and Kyushu, over which a topographical gap, 30 km in model and demonstrated that SAR images were suit- width and 500 m in height, is formed by the mountains able for a study on local wind fields in coastal areas. higher than 500 m at the respective edges of Honshu Sandvik and Furevik (2002) have simulated a mesoscale and Kyushu. The Kanmon Strait, which connects Suo- coastal jet confirmed through in situ ship measurements Nada (SN) and the Tsushima Strait (see Fig. 1), is the and SAR images by using a high-resolution numerical choke point of sea traffic. Since more than 650 ships model. They have found that the coastal jets were a travel through this area everyday, sea accidents are a result of stratified flows around mountains. Through serious problem. Better understanding of the coastal the combined use of scatterometer, altimeter and SAR- jet/wake behavior is thus required for aspects of marine derived winds, Shimada and Kawamura (2004, 2006) safety. Ultrahigh resolution marine meteorology using have demonstrated that the SAR winds could fill the the SAR surface winds may contribute to management scatterometer remote sensing gaps, and that these oro- and control of coastal human activities. graphically modified winds affected wind-wave devel- In section 2, satellite and in situ data and methods opment in the downstream region. Yamaguchi and used are described. In section 3, SAR images reveal the Kawamura (2005) have examined the effects of spa- spatial structure of the coastal jets radiating from the tially dependent sea surface wind fields on ocean cur- Kanmon Strait into the Tsushima Strait. An atmo- rents in a semienclosed bay by using SAR-estimated spheric community numerical model with realistic to- wind fields and a numerical ocean model. Vorticity in- pography is used to investigate mechanisms of coastal put due to spatially dependent wind stress was essential jet formations. Through our case study, using altimeter- to the generation of the wind-driven ocean circulation derived wind and significant wave height (SWH) data, in the bay. it is demonstrated that the coastal wind jet affects the In the present study, we investigate a coastal jet flow- local wind-wave field, which has a SWH peak associ- ing into the Tsushima Strait and its effect on wind-wave ated with the high-resolution wind distribution (section Unauthenticated | Downloaded 09/28/21 12:29 PM UTC 566 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 64 TABLE 1. Summary of SAR observations and meteorological measurements. Displayed in each case are SAR observation time, the SAR-derived wind speed in the core of coastal jets (EXspd), NCEP–NCAR surface wind directions (Dir) and speeds (ENspd) off Shikoku, potential temperature gradients (d␪/dz), the Brunt–Väisälä frequency (N), and the Froude numbers (Fr), which are derived from rawinsonde soundings at Fukuoka, and increasing rates of the wind speeds from ENspd to EXspd (EX/EN) (see the text). SAR NCEP–NCAR off Shikoku Rawinsode at Fukuoka Date and time Case (in UTC) EXspd (m sϪ1) Dir (deg) ENspd (m sϪ1) d␪/dz (°CkmϪ1) N (sϪ1)FrEX/EN 1 1 Sep 1992 0156 10.3 105.8 7.0 4.10 1.15 ϫ 10Ϫ2 1.60 1.48 2 6 Oct 1992 0156 34.3 11.4 Ϫ0.46 3 10 Nov 1992 0156 277.2 11.2 1.51 7.26 ϫ 10Ϫ3 3.16 4 23 Feb 1993 0156 296.7 16.0 Ϫ0.16 5 30 Mar 1993 0156 15.0 124.3 5.0 5.28 1.35 ϫ 10Ϫ2 1.03 3.02 6 13 Jul 1993 1156 239.7 6.1 0.58 4.35 ϫ 10Ϫ3 4.13 7 17 Aug 1993 0156 227.4 10.2 6.25 1.43 ϫ 10Ϫ2 0.87 8 21 Sep 1993
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