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 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.

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

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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 radar 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 scatterometer-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

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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 0156 5.0 92.2 7.8 7.97 1.63 ϫ 10Ϫ2 0.61 0.64 9 5 May 1994 0158 255.4 13.9 Ϫ0.29 10 25 May 1994 0155 6.0 112.4 9.2 4.36 1.20 ϫ 10Ϫ2 1.64 0.65 11 7 Aug 1994 0157 245.8 2.2 2.55 9.09 ϫ 10Ϫ3 0.56 12 13 Sep 1994 0157 2.6 6.6 0.53 4.18 ϫ 10Ϫ3 2.50 13 22 Jan 1995 0157 248.8 14.4 8.09 1.65 ϫ 10Ϫ2 1.51 14 27 Apr 1995 0156 7.8 137.4 3.4 6.21 1.45 ϫ 10Ϫ2 0.48 2.29 15 1 Jun 1995 0156 51.8 1.8 8.06 1.64 ϫ 10Ϫ2 0.32 16 6 Jul 1995 0156 236.0 5.9 5.53 1.35 ϫ 10Ϫ2 0.50 17 10 Aug 1995 0156 272.7 8.4 1.09 5.95 ϫ 10Ϫ3 3.26 18 19 Oct 1995 0156 28.1 5.1 4.80 1.27 ϫ 10Ϫ2 0.88 19 28 Dec 1995 0156 174.3 3.0 2.19 8.77 ϫ 10Ϫ3 0.88 20 11 Apr 1996 0156 344.2 7.7 1.83 21 16 May 1996 0156 53.5 2.9 6.30 1.45 ϫ 10Ϫ2 0.27 22 19 Sep 1997 0156 345.5 9.2 1.26 6.47 ϫ 10Ϫ3 3.03 23 13 Mar 1998 0156 296.8 9.8 3.87 1.16 ϫ 10Ϫ2 1.30 24 18 Dec 1998 0156 3.4 3.6 1.6 7.66 1.62 ϫ 10Ϫ2 0.32 2.13 25 17 Mar 2000 0156 345.4 7.5 Ϫ0.04 26 21 Apr 2000 0156 199.6 4.8 3.75 1.13 ϫ 10Ϫ2 1.28 27 8 Sep 2000 0156 6.2 112.3 6.1 4.96 1.27 ϫ 10Ϫ2 0.25 1.02 28 17 Nov 2000 0156 323.9 9.2 2.63 9.44 ϫ 10Ϫ3 1.20

4). Based on statistics of ship-reported ocean winds and then each digital number (DN) in the SAR images is waves, the coastal jet and associated wind-wave devel- converted to a normalized radar cross section (NRCS) opment are investigated with relation to the transition as, of large-scale atmospheric circulation in midsummer ϭ ͑ ͒ ϩ ͓ ͔ ͑ ͒ (section 4). Section 5 gives a summary. NRCS 20 log10 DN CF dB , 1

where CF is the conversion parameter provided by the 2. Observation data and atmospheric community Japan Aerospace Exploration Agency (JAXA). To ob- model tain a wind speed from an NRCS value, the C-band model function (CMOD4) proposed by Stoffelen and a. Observation data Anderson (1997) is applied. An incidence angle, a beam We used 28 SAR images obtained by the Active Mi- view angle, a wind direction and an NRCS value are crowave Instruments (AMI) aboard the European re- required for a wind-speed estimate. The first two pa- mote sensing (ERS) satellites. The AMI operated at a rameters are taken from the satellite orbital informa- C-band (5.3GHz) with vertical polarization and a con- tion. In this study, the wind direction is estimated from stant incidence angle of 23° at the midswath. It obtains the surface winds of the National Centers for Environ- strips of high-resolution (about 30 m) images of 100 km mental Prediction–National Center for Atmospheric width, with a repetition time of 35 days. Acquisition Research (NCEP–NCAR) reanalysis, at 6-h intervals, times of the 28 SAR images are listed in Table 1. Im- provided by the National Oceanic and Atmospheric aging processes from raw signal data are performed by Administration (NOAA)–Cooperative Institute for the SIGMA (⌺) SAR Processor (Shimada 1999) and Research in Environmental Sciences (CIRES) Climate

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Diagnostics Center, Boulder, Colorado (more informa- b. Atmospheric community numerical models tion online at http://www.cdc.noaa.gov/). The assump- A high-resolution atmospheric numerical model is tion that a wind direction is uniform over the image used to examine a formative mechanism of the coastal apparently gives rise to some errors for wind speed wind fields observed by the SAR images. We used the retrieval [e.g., 45° errors can give maximum errors of fifth-generation Pennsylvania State University–NCAR about 3.5 (1.0) m sϪ1 under 10 (5) m sϪ1 wind fields]. Mesoscale Model (MM5), which was developed from Nevertheless, since spatial wind distribution is a main the mesoscale numerical model by Anthes and Warner target to be discussed, a wind speed itself will not affect (1978). MM5, a well-known atmospheric community essential conclusions in this study. Precise detection of numerical model, accommodates multinested domains, wind directions near coasts is beyond the scope of the nonhydrostatic dynamics, four-dimensional data as- present study but will be required for a quantitative similations, and various physical parameterizations. A discussion in the future. detailed description is given by Dudhia (1993). The To get land/open-ocean wind fields at around the model is configured with two domains with horizontal SAR observation time, Automated Meteorological grid resolutions of 15 and 5 km. The outermost nest Data Acquisition System (AMeDAS) data and the covers the range of Fig. 1 and has a relaxation zone NCEP–NCAR winds are used. AMeDAS, a land-based around the boundaries, while the innermost one covers regional meteorological observation system operated the square in Fig. 1. Its boundary is updated every time by the Japan Meteorology Agency (JMA), automati- step in the coarse-mesh calculation and has no relax- cally obtains precipitation amount, wind direction, wind ation zone. It is performed with 24 vertical levels, where speed, temperature, and sunshine duration every hour. the top level is set to 100 hPa. As described later, the About 840 AMeDAS stations are distributed across Ja- numerical simulations are performed without moisture pan. Aerological observations of daily rawinsondes and prediction and surface flux computation to focus on ascents provided by JMA are used to investigate atmo- dynamic effects. spheric conditions. The rawinsonde soundings at Fukuoka (see Fig. 1) record atmospheric pressure, air temperature, humidity, wind directions and wind 3. Coastal wind jets and conditions for their speeds at altitudes up to about 30 km. From these data, formation potential temperature gradients and the Brunt–Väisälä a. Coastal wind jets observed by SAR images frequencies at about 500-m height are calculated for the respective SAR observations (Table 1). Coastal wind jets flowing out from SN into the For investigating the influence of the coastal wind Tsushima Strait appear in the wind fields estimated jets on wind-wave development, we use the wind speed from the SAR image. Seven of the 28 cases show the and SWH observed on 25 July 1999 by the ERS-2 al- coastal jet pattern as indicated in the third column timeter. The wind speed (SWH) is estimated from the (SAR EXspd) in Table 1. The SAR-derived wind stripe slope (power) of the altimeter return pulse (IFREMER/ around Kyushu is shown together with the other wind CERSAT 2001). For climatological data related to data to capture synoptic features of the wind field (Fig. wind waves and surface winds around Japan, the ship- 2). The two examples of coastal jets were observed at reported Beaufort wind and wave observations, and In- 0157 UTC 1 September 1992 and at 0157 UTC 30 ternational Comprehensive Ocean–Atmosphere Data March 1993 (See Table 1). The arrows depicted over Set (ICOADS) 1° data are used. The former, provided the land and ocean are the AMeDAS winds at 0200 by Japan Oceanographic Data Center (JODC), was in- UTC and the NCEP–NCAR reanalysis at 0000 UTC, formation about winds and ocean waves compiled from closest to the acquisition time of the SAR images. Am- about 850 000 ship reports around Japan over a period bient southeasterly (SE) air flows across Kyushu are of 20 yr (1978–97) (http://www.jodc.go.jp/harotokei/ found over the ocean from the NCEP–NCAR winds. index.html). These monthly climatological values along The wind speed on land is relatively strong in the south- with the observation numbers are summarized on a ern part of Kyushu, where land elevation is low, com- 1° ϫ 1° grid. The ICOADS data provided by the pared with that in the middle of Kyushu where the NOAA–CIRES Climate Diagnostics Center, Boulder, Kyushu Mountains are located. The SAR-derived Colorado (from their Web site at http://www.cdc.noaa. coastal wind fields west of Kyushu show that the wind gov/) compiles maritime meteorological observations feature described above connects downstream to the from ship reports for 1960–2002. Here we calculate East China Sea. Moderate tip jets and wakes are monthly climatological wind vector fields on a 1° ϫ 1° formed, corresponding to the upstream lower topogra- grid around Japan. phy and the mountainous areas, respectively. Another

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exit of the flows is the gap between Honshu and Ky- ushu, where a strong wind jet radiates from the Kan- mon Strait into the Tsushima Strait and there are wakes at both sides of the jet. Figure 3 shows the close-up images around the Kan- mon Strait acquired on 30 March 1993. The mapped wind field (Fig. 3b) clearly shows that the near gale jet and the surrounding light breeze wakes create strong horizontal shear fields, which are expected to be a strong driving force to the ocean (Yamaguchi and Kawamura 2005). In addition, the high-resolution image shows that the jet is divided into several subjets and wakes associated with the upstream land topography. Figure 4 shows vertical profiles of the potential tem- peratures and the wind vectors acquired at Fukuoka (see Fig. 1) at 0000 UTC 1 September 1992 and at 0000 UTC 30 March 1993, which correspond to Figs. 2a,b, respectively. Both indicate a stable stratification with a strong potential temperature gradient in the atmospheric boundary layer. The wind profile on 1 Septem- ber 1992 (Fig. 4a) shows the SE winds through 2 km above the ground. On the other hand, on 30 March 1993 (Fig. 4b), the SE winds are restricted in the lowest two levels and the southwesterly to westerly winds are dominant over the 1 km altitude, which suggests that the strong jet shown in Fig. 2b is confined to the lower levels. The AMeDAS land and the NCEP–NCAR ocean surface winds are averaged to create a composite map of the time when the coastal jets were being observed (Fig. 5). The synoptic wind field is thought to be a southeasterly moderate breeze of about 6 m sϪ1. The surface wind over the land is a light breeze weaker than 2msϪ1 at many stations, while an easterly gentle breeze is found in the southern part of Kyushu, which may be a passage connecting to the downstream jet as mentioned above. A gentle breeze is also seen at both of the coasts along the Seto Inland Sea (SIS) and SN (see Fig. 1). The wind directions there, which are almost parallel to the coasts, are headed to the Kanmon Strait. Therefore, SIS and SN can be an upstream path for the coastal jet blasting into the Tsushima Strait.

b. The formation mechanism of SAR-derived coastal jets To investigate the formation mechanism of the coastal jet, numerical simulations by the MM5 are per- FIG. 2. (a) The coastal wind fields derived from the SAR images formed. The MM5 is usually initialized with objectively acquired around Kyushu at 0156 UTC 1 Sep 1992 (shaded strips), along with wind vectors over the land and ocean, which are ob- analyzed data to reproduce and predict realistic atmo- tained from AMeDAS at 0200 UTC and the NCEP–NCAR re- spheric fields. In this study, we perform simulations ini- analysis at 0000 UTC, respectively. The AMeDAS winds stronger tialized with idealized atmospheric fields in order to Ϫ (weaker) than 4 m s 1 are shown with thin black (gray) arrows investigate effects of initial conditions on coastal jet and the NCEP–NCAR winds are shown with thick gray arrows. (b) Same as in (a) except for the SAR images acquired at 0156 UTC 30 Mar 1993.

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FIG. 3. (a) The ERS-1/SAR image around the Kanmon Strait acquired at 0156 UTC 30 Mar 1993; (b) the SAR-derived wind field and land topography.

FIG. 4. Vertical potential temperature profiles (gray lines) and wind vectors (arrows) over Fukuoka at (a) 0000 UTC 1 Sep 1992 and (b) 0000 UTC 30 Mar 1993.

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patterns (see Fig. 2). Some parts of the easterly flows facing Kyushu turn into the northerly flows and go around the Kyushu Mountains. Their velocities are increased in southern Kyushu, and then they flow into the East China Sea. Similarly, the flows blocked by the Chugoku Mountains (Fig. 1) go westward along the mountain range and finally form the coastal jet over the Tsushima Strait. This forms the flow path throughout SIS and SN. The flow path is almost the same as the composite wind field (see Fig. 5). In general, airflow patterns passing around or over mountains are characterized by the Froude number (Fr ϭ U/Nh, where U is the horizontal wind speed, h is the height of the mountain, and N is the Brunt–Väisälä frequency; e.g., Walter and Overland 1982; Overland 1984). For Fr K 1, the topographical blocking effect is dominant. For an isolated mountain, the flow with a low Fr tends to go around it or stagnate upstream. In addition, for mountain chains with a length of 100 km or more, the Coriolis force comes to have a certain effect on the flow pattern. In the Northern Hemisphere, the air mass that had stagnated upstream starts to turn FIG. 5. A composite wind field constructed from the AMeDAS winds (land) and the NCEP–NCAR reanalysis surface winds left and flows along the mountain chains on the right- (ocean) at the seven cases when coastal jets over the Tsushima hand side, and then it eventually goes around the left- Straits are observed: 1 Sep 1992, 30 Mar 1993, 12 Sep 1993, 25 May hand side of the mountain chains at their edge. In the 1994, 27 Apr 1995, 18 Dec 1998, and 8 Sep 2000 (see Table 1). The numerical experiments of the present study, when h is AMeDAS winds stronger (weaker) than 2 m sϪ1 are shown with thin black (gray) arrows and the NCEP–NCAR winds are shown defined as 500 m, Fr is small enough (0.77) to meet the with thick gray arrows. conditions of the blocking effect. Therefore, the flow goes around the mountain chains and then the wind speed is increased due to a confluence of the stream- formation through sensitivity experiments, which are lines at the edges of the mountain chains. Since the discussed in section 3c. As a basic experiment, we set up lengths of the Chugoku Mountains and Kyushu Moun- initial and boundary conditions according to the atmo- tains are longer than 150 km, the Rossby number, Ro ϭ spheric conditions at the lower levels on 30 March 1993 U/fL, where f is the Coriolis parameter and L is the (see Fig. 4b); that is, 1) SE geostrophic flows of 5 m sϪ1, mountain length, is much smaller than 1. In these con- and 2) a constant potential temperature gradient of 5°C ditions, the flows are apparently affected by the earth’s kmϪ1 throughout the troposphere (the Brunt–Väisälä rotation, and the ones that are blocked by the Chugoku frequency, N ϭ 1.3 ϫ 10Ϫ2 sϪ1). Furthermore, to evalu- Mountains and Kyushu Mountains go around the left ate the dynamic effect of topography on the coastal jet side of these mountains. formation, no surface heat and moisture fluxes are ap- A test simulation initialized with no winds and acti- plied in the simulations. The numerical integration is vated with heat and moisture flux calculations is per- performed for 24 h, and it reaches almost steady state formed to investigate topography-induced thermal ef- after 14 h. fects on the coastal jet formation. The result from the The model-predicted horizontal wind pattern at a second day of the 48-h integration represents the pre- 10-m height after the 14-h integration is depicted in Fig. dominant diurnal wind system accounted for by the 6. The coastal jet pattern is clearly reproduced. The land and sea breezes. In the inland seas surrounded by surface wind flowing out of the Kanmon Strait forms Honshu, Shikoku, and Kyushu [SN, Bungo Channel the strong jet over the Tsushima Strait and makes (BC), and the SIS], topographically induced coastal jets strong horizontal shears with the neighboring wakes. In along the coasts are formed by the large-scale sea the East China Sea, a wind-speed gap is formed be- breezes between the open ocean and the Japan main tween the tip jets flowing through southern Kyushu and islands (not shown). However they flow in opposite the northern wakes behind the Kyushu Mountains. This directions from the coastal jets seen by the SAR im- pattern is also similar to the SAR-derived surface wind ages. It is concluded that the principle mechanism of

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FIG. 6. A model-simulated horizontal wind field at 10 m above the surface after the 14-h integration. The model is initialized with southeasterly geostrophic flows of 5 m sϪ1, and a constant potential temperature gradient of 5°CkmϪ1 throughout the troposphere. The squares, EN and EX, denote upstream and downstream regions where wind speeds and directions are measured (see text). the coastal jet formation is not the topographically in- (EN shown in Fig. 6). Judgment of whether a jet is duced thermal effect. formed or not and the degree of the jet enhancement is determined from downstream wind directions at the c. Atmospheric conditions for the coastal jet exit from the Kanmon Strait (EX; see Fig. 6), and formation an increasing ratio of wind speeds from EN to EX Sensitivity simulations, in which the upstream wind (EX/EN). direction and Fr are variable parameters, are per- The values of EX/EN are plotted with shaded formed to examine atmospheric conditions required squares with respect to the upstream wind directions for the coastal jet and the degree of the jet enhance- and Fr (Fig. 7), where 0° and 90° in the x axis mean ment. Since the mountain height (h) is defined as a northerly and easterly upstream flows, respectively. constant of 500 m, Fr is dependent on the two atmo- Shading of the squares represents the EX/EN value. spheric parameters, horizontal wind speed (U) and the We define a criterion for the coastal jet formation by Brunt–Väisälä frequency (N). An upstream wind direc- downstream wind directions (averaged at EX) of 115°– tion is defined as model-predicted 10-m winds averaged 150° (135° Ϯ 20°). The cases that meet the criterion are in the area off Shikoku, 31.5°–32.0°N, 133.0°–133.5°E expressed with the large squares, which are mostly dis-

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with a negative potential temperature gradient or with Fr larger than 2.5 are plotted at Fr ϭ 2.5, the maximum range of the y axis in Fig. 7. For all 28 cases, Fr, the NCEP–NCAR winds off Shikoku, the potential temperature gradients, and the Brunt–Väisälä frequencies, are summarized in Table 1. Figure 7 shows that almost all the SAR-derived coastal jets (six of seven cases) are observed when the upstream wind direction off Shikoku is between 90° and 180° (from east to south), which is consistent with the results derived from the numerical simulations. The only exception is the case on 18 December 1998, in which the SAR-estimated wind field depicts the coastal jet despite the northerly (3.58°) upstream flows. The upstream flow off Shikoku at 0000 UTC 18 December 1998 is a light breeze (1.6 m sϪ1) and the synoptic flow FIG. 7. An upstream wind direction–Froude number diagram filed rotates around Kyushu. Thus, the exception is for low-level jet formation over the Tsushima Straits. Squares are probably due to the unreliability of the wind direction the results from the numerical simulations, where the large squares indicate low level jet formation over the Tsushima Straits, off Shikoku and the heterogeneity of the horizontal while the small squares indicate no jet formation. Circles and wind fields, as well as the acquisition time gaps. crosses are the results of the SAR observations, where the circles The 21 cases with no coastal jet (plotted by crosses) denote that coastal jets appear in SAR images, while the crosses are out of the upstream direction range from 90° to 180° denote that no jets are observed. The upstream wind direction and (Fig. 7), which again matches well with the results of the Froude number at the SAR acquisition are estimated from the 6-hourly NCEP–NCAR surface winds off Shikoku and rawin- numerical simulation. Among the 28 cases of SAR ob- sonde observations at Fukuoka. The cases where the potential servation listed in Table 1, 27 cases are well simulated temperature gradient is negative or the Froude number is larger in terms of the coastal jet formation (no formation of than 2.5 are plotted at 2.5. The shades of the squares and circles the coastal jet). This demonstrates the validity of the represent the increasing rates of wind speeds in the core of the jets conducted model simulation for the coastal jet of sur- compared to those off Shikoku. All values plotted are summarized in Table 1. face wind field and the above discussions for its formation mechanism. tributed between 90° and 180°. This means that easterly to southerly upstream winds are required for jet forma- 4. Coastal wind jet’s effects on wind-wave tion. The EX/EN value gets increased (lighter shad- development ings) inversely proportional to Fr when it is lower than 1.2, while it converges and keeps almost constant for Figure 8a shows a synthetic image of 1) altimeter- Fr Ͼ 1.2. These results show that the easterly to south- derived wind speeds along a ground track at 1326 UTC erly upstream flows off Shikoku have potential for 25 July 1999, 2) AMeDAS wind vectors at 1300 UTC coastal jet formation over the Tsushima Strait and the over the land, and 3) 6-hourly NCEP–NCAR wind vec- jet is more enhanced with the lower Fr. tors at 1200 UTC over the ocean. Figure 8b represents The upstream wind directions, Fr, and increasing ra- altimeter-derived squared wind speed profile, which is tios of the wind speeds (EX/EN) at the SAR acquisi- proportional to wind energy, and SWH profiles over tions are derived and superimposed onto those from the range of 33.5°–35.5°N. For a reference, time series the numerical experiments (Fig. 7). The upstream wind of wind speeds and directions at Shimonoseki (see Fig. direction is determined from the NCEP–NCAR 10-m 8), corresponding to the gap between Honshu and Ky- winds at 31.4281°N, 133.125°E (Fig. 5), and Fr is calcu- ushu, are depicted for 25–26 July 1999 in Figs. 9a,b, in lated from rawinsonde observations at Fukuoka (Table which the acquisition time of the altimeter observation 1). The values of EX/EN are defined as ratios of the is shown with a dashed line. A vertical potential tem- SAR-derived wind speed averaged in the jet core to the perature profile and wind vectors over Fukuoka (see NCEP–NCAR 10-m wind speed at 31.4281°N, Fig. 1) at 1200 UTC 25 July 1999 are also indicated in 133.125°E (Table 1). The seven cases with the coastal Fig. 9c. jets are plotted with shaded circles, while the remaining The altimeter-derived wind-speed distribution show- 21 cases are plotted with crosses in Fig. 7. The cases ing a moderate breeze of Ͼ7msϪ1 for 34°–34.8°N and

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FIG. 8. (a) Wind speed derived from the ERS altimeter at 1326 UTC 25 Jul 1999, along with wind vectors over the land and ocean, which are obtained from AMeDAS at 1300 UTC and the NCEP–NCAR reanalysis at 1200 UTC, respectively. The scale of the altimeter-derived wind speed is shown with shading. The AMeDAS winds stronger (weaker) than 4 m sϪ1 are shown with thin black (gray) arrows and the NCEP/NCAR winds are shown with thick gray arrows. Geographical position of Shimonoseki is shown with a circle. Black lines between the altimeter track and the coasts show fetches estimated from the model-simulated wind field. (b) Wind energy (a black line) and significant wave height (a gray line) profiles along the altimeter track, 33.5°–35.5°N.

gentle breezes on both sides indicates the moderate UTC 25 July and the altimeter-derived wind and SWH coastal wind jet and wakes. In fact, the SE winds off (1326 UTC) are observed at about 9.5 h after the wind- Shikoku meet the atmospheric condition for the coastal jet onset. jet formation discussed in the previous section. More- The SWH profile (Fig. 8(b)) corresponds to the over, the east-southeasterly (ESE) wind at Shi- squared wind-speed distribution for the range south of monoseki (Fig. 8) as well as the whole wind distribution 35°N, suggesting that the wind jet has developed the over land (Fig. 8) suggests that the conditions are simi- wind waves. We conduct rough verification for it by lar to that of the coastal jet. The potential temperature using empirical fetch graph formulas. In general, the gradient, the Brunt–Väisälä frequency, and Fr calcu- growth of wind waves under a condition of constant lated from Fig. 9c are 3.2 (°CkmϪ1), 1.02 ϫ 10Ϫ2 (sϪ1), winds from a shoreline is simplified as a fetch-limited, and 0.94, respectively. Although the atmospheric sta- one-dimensional, time-independent problem, and has bility for this case is not as strong compared to cases of been estimated by using empirical relations between the SAR-observed coastal jets, the values still meet the the nondimensional fetch (Fˆ ) and nondimensional conditions of the wind jet. The time series of the wind SWH (Hˆ ) (e.g., Wilson 1965; Mitsuyasu 1968; Hassel- speed and direction at Shimonoseki (Figs. 9a,b) indi- mann et al. 1973). The Fˆ and Hˆ are calculated as cate the ESE wind, implying that of the wind jet over ˆ ϭ ր 2 ˆ ϭ ր 2 ͑ ͒ the Tsushima Strait lasts almost one day from 0400 F gF U 10, H gH U 10, 2

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FIG. 9. The time series of (a) wind speed and (b) direction for 24–26 Jul 1999 at Shimonoseki (see Fig. 8a). (c) A vertical potential temperature profile (gray line) and wind vectors (arrows) over Fukuoka at 1200 UTC 25 Jul 1999.

where g is the acceleration of gravity and U10 is the wind speed at 10-m height. We estimate fetches (F) between the ground-track and the coasts from the model-derived wind fields, which were simulated with the closest atmospheric conditions on 25 July 1999 (Fig.

8(a)). The mean wind speeds along the fetch (U10) are then calculated based on the altimeter-derived wind speeds and the model-derived along-fetch wind speed profiles. Calculated Hˆ along the altimeter ground track are plotted with respect to Fˆ in Fig. 10, where those inside (outside) the jet core, 34°–35°N, are shown with gray dots (squares). The average and standard deviation of the gray dots are also shown by a square with bars. Most values north of 35°N are larger than 105 for Fˆ and 10° for Hˆ , due to the relatively high SWH and the weak wind speeds (Fig. 8b), which might suggest that they do not meet conditions of being fetch-limited, one- dimensional, and time-independent. ˆ ˆ Empirical relations between F and H have been FIG. 10. The relation between the nondimensional fetch (Fˆ ) and proposed by many researchers. Representative formu- the nondimensional significant wave height (Hˆ ) for points along las by Wilson (1965) and the Joint North Sea Wave the altimeter track shown in Fig. 8, where the values inside (out- Atmosphere Program (JONSWAP; Hasselmann et al. side) the jet core, 34°–35°N, are shown with gray dots (squares). The average and the standard deviation for the jet core (gray 1973) are superimposed in Fig. 10. Although the plots dots) are shown with a square with vertical bars. The solid line W are a little scattered, the relation inside the jet core and dashed line J show the empirical formulas of Wilson (1965) (shown with gray dots) almost corresponds to the Wil- and JONSWAP (Hasselmann et al. 1973), respectively.

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FIG. 11. Frequencies of appearance of wind waves in August based on the ship-reported Beaufort wind-wave observations at (a) 34°–35°N, 130°–131°E; (b) 34°–35°N, 131°–132°E; (c) 34°–35°N, 129°–130°E, and (d) 33°–34°N, 129°–130°E. Their values are expressed as the Beaufort wind-wave scales. son (1965) formula, which involves the saturation of the higher frequency compared with those in surrounding wave growth for long-fetch conditions as Fˆ Ͼ 104. areas. The tendency is particularly prominent in sum- The coastal jets and the associated wind waves over/ mertime. Figure 11 shows the frequency of appearance in the Tsushima Straits are examined further by using of the wind waves in August at (Fig. 11a) 34°–35°N, climatological wind and wave data. We use the mea- 130°–131°E; (Fig. 11b) 34°–35°N, 131°–132°E; (Fig. surements of the ship-reported Beaufort wind and 11c) 34°–35°N, 129°–130°E; and (Fig. 11d) 33°–34°N, waves recorded over 20 years (see section 2). Over the 129°–130°E. These areas are shown with dashed lines in Tsushima Strait, northeasterly (NE) and southwesterly Fig. 1. The wind-wave strengths are shown by the Beau- (SW) winds, which are parallel to the strait, as well as fort scales. At 34°–35°N, 130°–131°E (Fig. 11a), the SE the northwesterly (NW) one associated with the win- wind waves show a mode, having almost the same fre- tertime monsoon are generally dominant. In the wind quency as the NE and SW wind waves, where the ESE jet area over the Tsushima Strait, 34°–35°N, 130°– to south-southeasterly (SSE) wind waves reach up to 131°E, the southeasterly SE wind waves appear with 23.2%. On the other hand, the ESE–SSE wind waves

Unauthenticated | Downloaded 09/28/21 12:29 PM UTC 576 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 64 accounting for only 12% do not predominate in the surrounding grids (Figs. 11b–d). The prominent SE waves are expected to be brought about by the coastal jets, which implies that the coastal jets blowing out from the Kanmon Strait are consistent phenomena in Au- gust. At 34°–35°N, 130°–131°E, the appearance frequencies for the ESE–SSE wind waves in July, August, and September, are 18.3%, 23.3%, and 9.1%, respectively. After the frequency reaches its maximum (23.3%) in August, it decreases rapidly in September. Figure 12 shows the ICOADS climatological wind fields from July to September. The flow pattern south of Japan experiences a systematic change associated with the seasonal evolution of the Asia summer monsoon. In July, during the mature Baiu (rainy) season in Japan, SW winds blowing into the stationary Baiu front around the Japanese islands are dominant in the south. Accompanied with the northward shift of the Baiu frontal zone, the rainy season ends in late July. From then almost all of the Japanese islands are covered with the Pacific high when midsummer comes. The northward shift of the Pacific high entails a shift in the prevailing flow south of Japan, which goes around the western edge of the high, from SW to SE. After late August, the front, which escaped northward during midsummer, stays stationary again around the Japanese islands, which brings the NE or SW flows to the south of Japan. The SE flow in August corresponds to the condition of the wind direction off Shikoku for coastal FIG. 12. Monthly surface wind fields derived from the ICOADS jet formation. Thus the high-frequency appearance of surface winds from July to September. the coastal jets and the associated wind-wave development in midsummer can be explained by the seasonal transition of the large-scale atmospheric circulation 2) Using MM5 of the atmospheric community numeri- around Japan. cal model, the formation mechanism of coastal jets was examined. It is deduced that the coastal jet is formed due to the convergence of stratified flows at a gap between extremely steep topography. 5. Summary 3) Additional sensitivity simulations were performed to examine favorable conditions for the coastal jet, We investigated the coastal jets flowing into the changing the wind directions and Fr (the Froude Tsushima Strait and their influence on wind-wave de- number). The coastal jets were successfully simu- velopment by using the SAR-derived wind fields, altim- lated for the easterly to southerly upstream flows off eter-derived data and climatological wind and wind- Shikoku. The degree of the jet enhancement was wave data. The numerical simulations using the MM5 mostly dependent on Fr of the upstream flow, indi- were used for investigating the formation mechanism of cating that the blocking effect on stratified flows coastal jets. We obtained the following results. efficiently enhances the low-level jet. The SAR ob- 1) The coastal jets blowing from the topographical gap servations and the atmospheric conditions were con- between Honshu and Kyushu into the Tsushima sistent with the numerical simulations, which dem- Strait were observed in about 25% of the SAR- onstrates the validity of the conducted model- derived wind fields under the ambient southeasterly simulation for the coastal jet of surface wind field. conditions. The coastal jet and adjacent wakes 4) A case study using altimeter-derived wind speed and formed a strong horizontal shear field locally. SWH profiles crossing the coastal jet indicated the

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