Low-Level Easterly Winds Blowing Through the Tsugaru Strait, Japan

JUNE 2012 S H I M A D A E T A L . 1779

Low-Level Easterly Winds Blowing through the Tsugaru Strait, Japan. Part II: Numerical Simulation of the Event on 5–10 June 2003

TERUHISA SHIMADA Ocean Environment Group, Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Japan

MASAHIRO SAWADA AND WEIMING SHA Atmospheric Science Laboratory, Graduate School of Science, Tohoku University, Sendai, Japan

HIROSHI KAWAMURA Ocean Environment Group, Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Japan

(Manuscript received 5 February 2011, in ﬁnal form 3 January 2012)

ABSTRACT

This paper investigates the structures of and diurnal variations in low-level easterly winds blowing through the Tsugaru Strait and Mutsu Bay on 5–10 June 2003 using a numerical weather prediction model. Cool air that accompanies prevailing easterly winds owing to the persistence of the Okhotsk high intrudes into the strait and the bay below 500 m during the nighttime and retreats during the daytime. This cool-air intrusion and retreat induce diurnal variations in the winds in the east inlet of the strait, in Mutsu Bay, and in the west exit of the strait. In the east inlet, a daytime increase in air temperature within the strait produces a large air temperature difference with the inﬂowing cool air, and the resulting pressure gradient force accelerates the winds. The cool air ﬂowing into Mutsu Bay is heated over land before entering the bay during the daytime. The resulting changes in cool-air depth and in pressure gradient force strengthen the daytime winds. In the west exit, local pressure gradient force perturbations are induced by the air temperature difference between warm air over the Japan Sea and cool air within the strait, and by variations in the depth of low-level cool air. The accelerated winds in the west exit extend southwestward in close to geostrophic balance during the daytime and undergo a slight anticyclonic rotation to westerly during the nighttime owing to the dominance of the Coriolis effect.

1. Introduction months, particularly in June–July (Fig. 1 of Part I). These winds occur to the west of the east–west passage that Strong winds can develop in the exit region of a terres- connects the western North Pacific and the Japan Sea and trial gap when an along-gap pressure gradient is created consists of the Tsugaru Strait, Mutsu Bay, and circumjacent mostly in conjunction with cold-air surges (e.g., Overland low-level terrestrial gaps (Fig. 1); these winds are asso- and Walter 1981; Steenburgh et al. 1998; Chelton et al. ciated with cool maritime air accompanying the easterly 2000; Colle and Mass 2000; Sharp and Mass 2004). A wind over the Pacific. This easterly wind, commonly known companion paper to this study (Shimada et al. 2010, as Yamase in Japan (e.g., Takai et al. 2006), intermittently hereafter Part I) has focused on such strong winds that blows toward northern Japan from the Okhotsk high dur- frequently occur in northern Japan during the summer ing the summer months. Using observational and reanaly- sis data, Part I first presented the structures and evolutions of the easterly surface winds within and adjacent to the Corresponding author address: Teruhisa Shimada, Ocean Envi- TsugaruStraitandMutsuBay. ronment Group, Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Aramaki Aza The two main results derived from Part I are as fol- Aoba 6-3, Aoba-ku, Sendai, Miyagi 980-8578, Japan. lows: 1) the pressure gradient force along the Tsugaru E-mail: [email protected] Strait predominantly induces the easterly strong winds

DOI: 10.1175/MWR-D-11-00035.1

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cycles of the local wind accelerations. As possible causes, Part I has suggested diurnally varying cool-air intrusion into the strait and the bay from the east, and complex distribution of land and sea with differential heating and cooling on the basis of Kawai et al. (2006). To address these challenges, analyses of numerical simulation data are indispensable. Therefore, this study investigates the diurnal variations in the low-level easterly winds within and adjacent to the Tsugaru Strait and Mutsu Bay using a numerical weather prediction model. We focus on a sustained easterly wind event occurring on 5–10 June 2003, described in previous studies (Shimada and Kawamura 2007, 2009; Part I). In particular, we compare the locally strong winds in the east inlet of the strait, in Mutsu Bay, and in the west FIG. 1. Study area and geographical names referred to in this exit of the strait, and we examine the differences and paper. Gray contours show monthly mean SST in June 2003 at every 18C. The framed rectangle indicates the inner domain of the similarities in wind acceleration and diurnal variation. model simulation. The color scale indicates terrain elevation. The This case study offers a comprehensive vision of sur- solid circles and triangle denote weather observation stations face winds under conditions frequently occurring dur- [Hakodate (HK) and Fukaura (FK)] and a buoy in Mutsu Bay, ing the summer in this study area, which leads to better respectively. The square indicates the location for which wind understanding of regional weather and climate. More speeds from the simulated data and the SeaWinds observations are compared. importantly, this study is the first to investigate diurnally varying gap winds; previous studies have focused on a one-time event of gap winds. in the west of the strait. The maritime cool air accom- We give brief descriptions of our meteorological model panying the easterly winds is blocked by the central simulation in the following section. In section 3, we spine of the mountains of northern Japan and is dammed present horizontal and vertical structures of the wind and on the east side (Fig. 1 of Part I). The resulting east–west the cool air within and adjacent to the Tsugaru Strait and air temperature differences create an along-strait sea Mutsu Bay, and we explore causes of the diurnal varia- level pressure (SLP) gradient on a regional to synoptic tions in the low-level winds in section 4. Section 5 is de- scale (Fig. 10 of Part I). 2) The strong winds in the west voted to the summary and discussion. In this study, we use of the strait vary diurnally under sustained upstream Japan standard time (JST, UTC 1 9 h) for descriptions winds from the east. Stronger (weaker) and easterly hereafter because this study deals with diurnal variation. (east northeasterly) winds are observed during the nighttime (daytime), corresponding to the cool-air intrusion from the east (retreat from the west; Figs. 4 and 11 of 2. Model simulations Part I). Meanwhile, the easterly winds over the land and a. Model description in Mutsu Bay are stronger (weaker) during the daytime (nighttime). Thus, the large-scale pressure gradient force The model simulation was designed with the fifth- responsible for the easterly strong winds to the west of the generation Pennsylvania State University–National Cen- strait is modified by the diurnal cycle of thermal forcing. ter for Atmospheric Research (PSU–NCAR) Mesoscale At the same time, Part I has presented the following Model (MM5; Grell et al. 1995). According to studies that challenges for further understanding of the low-level simulate and validate winds over the ocean (e.g., Song winds blowing through the Tsugaru Strait and Mutsu et al. 2004), we chose the following physical options for Bay. 1) Mechanisms of local wind accelerations within model simulation: the cumulus parameterization of Grell the strait and the bay remain unresolved issues. Al- et al. (1995), the simple ice scheme of Dudhia (1993), a though there is evidence that the local wind accelera- cloud radiation scheme accounting for longwave and tions occur in the east inlet of the strait (around 41.68N, shortwave radiative transfers in cloudy and clear air 140.88E) and in Mutsu Bay (Yamaguchi and Kawamura (Dudhia 1989), a five-layer soil temperature model with 2005; Part I), detailed investigations into the causes of a fixed substrate (Dudhia 1996), and the National Cen- the wind accelerations need to be conducted. The mech- ters for Environmental Prediction (NCEP) Medium- anism of the wind acceleration in the west of the strait also Range Forecast scheme for planetary boundary layer merits further study. 2) We need to look into the diurnal processes (Hong and Pan 1996).

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wind event. The simulation underestimates the SLP difference by an average of 0.95 hPa. However, the variation in the simulated SLP difference is generally consistent with that of the observed, and diurnal variation in the SLP difference is represented in the simulation. We next compare the surface wind fields shown in Fig. 3 with the observed data (Figs. 4 and 5 of Part I). FIG. 2. Hourly SLP differences between the weather observation The simulation underestimates the maximum wind station data (gray) and the simulated data (black) for the locations speeds exiting from the west of the strait. The simulated of stations HK and FK. Because station FK is located outside the 21 inner domain, data from the outer domain are used here. wind speed at the location indicated in Fig. 1 is 1.38 m s less than the SeaWinds scatterometer observations during the entire simulation period on an average. This un- We defined two model domains with grid sizes of 3 derestimation results from the reduced SLP differences and 1 km. The outer domain, which is indicated by the along the strait in the simulation (Fig. 2). Within the strait square in Fig. 1 of Part I, nests the inner domain shown and the bay, the wind speed differences are relatively in Fig. 1 using a two-way interface. Thirty-three full- small. In particular, the mean biases during the simulation sigma levels were used in the vertical, with higher res- period are 0.38 m s21 at station HK and 20.98 m s21 at 1 olution in the lower layers. The initial and boundary the buoy in the Mutsu Bay. Meanwhile, the curvatures of conditions for the model simulation were obtained the strong wind region in the west of the strait from the from the following two datasets: 1) the objective analysis simulation are quite consistent with those from the ob- data known as gridpoint value (GPV), which are outputs servations (Fig. 4 of Part I). The simulation also re- from the Mesoscale Nonhydrostatic Model of the Japan produces locally strong winds in the east inlet and in Meteorological Agency with 10-km and 6-h spatiotem- Mutsu Bay as well as in the west exit; strong wind ex- poral resolution; and 2) sea surface temperature (SST) tensions from the terrestrial gaps of the Tsugaru and from the Advanced Very High Resolution Radiometer Matsumae Peninsulas; and wind blockings with wakes (AVHRR) Pathfinder data at 4-km resolution. This study in the lee of the Shimokita Mountains, Mt. Osorezan, and used monthly mean SST in June 2003 as a constant sea the Matsumae Peninsula. These structures are consistent surface boundary condition (Fig. 1) because frequent with those in the wind field derived from Synthetic Ap- cloud cover in the study area made construction of time- erture Radar (SAR; Fig. 5 of Part I). Thus, we can con- varying SST maps difficult. A simulation was initialized firm the validity of the simulation. The only discrepancy at 0900 JST 5 June 2003 and the two domains were in- in the wind patterns is that the wind confluence from tegrated to 0900 JST 10 June 2003 for 120 h. We mainly Mutsu Bay to the west exit of the strait is not inferred analyzed hourly output from the inner domain. We ex- from the SAR observations. cluded the first 3-h simulations from the analyses hereafter since these periods were considered the spinup 3. Horizontal and vertical structures period of the simulation. a. Horizontal structures b. Validation of the simulation We first show 10-m wind fields on 8–9 June 2003 (Fig. 3) On the basis of the observational results shown in as a typical example of locally strong winds under sus- Part I, we present several examples to validate the sim- tained upstream winds from the east. The times for which ulated results; detailed data descriptions can be found in these wind fields are plotted correspond to times of the Part I. We first compare observed and simulated SLP SeaWinds observations shown in Fig. 4 of Part I. The differences between weather observation stations Ha- easterly or southeasterly winds blow into the strait from kodate (HK) and Fukaura (FK; Fig. 2). Part I has shown the east inlet and into Mutsu Bay from the southeastern that the SLP difference can represent a SLP gradient low-level gap (40.98N, 141.38E) during both daytime and along the Tsugaru Strait and the evolution of an easterly nighttime. The wind speeds increase and reach maxima (8 m s21) in the east inlet of the strait and in Mutsu Bay during the daytime (Figs. 3b,c). The maximum speeds are located downwind of the narrowest section of the strait 1 The 33 full-sigma levels are located at s 5 1.00, 0.99, 0.98, 0.97, 0.96, 0.95, 0.94, 0.93, 0.92, 0.91, 0.90, 0.89, 0.87, 0.85, 0.83, 0.81, 0.79, (41.68N, 140.88E) and at the center of the bay (41.18N, 21 0.75, 0.70, 0.65, 0.60, 0.55, 0.50, 0.45, 0.40, 0.35, 0.30, 0.25, 0.20, 0.15, 140.88E). The strong winds (;8ms )blowfromthe 0.10, 0.05, and 0.00. west exit toward the southwest along the direction of the

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FIG. 4. Hodographs of hourly mean wind during 4 days from 0900 JST 6 Jun 2003 to 0800 JST 10 Jun 2003 in (a) the east inlet, (b) Mutsu Bay, and (c) the west exit. Gray lines extending from the origin indicate hourly mean wind vectors, and black lines with dots indicate the path of the tip of the wind vector. The numbers near the four large dots denote local times. Arcs indicate wind speed contours every 2 m s21.

strait (Figs. 3b,c). During the nighttime (Figs. 3a,d,e), the local wind accelerations in the east inlet and in Mutsu Bay weaken. The winds from the east inlet and Mutsu Bay merge and accelerate (.9ms21) in the west exit. The accelerated winds extend westward, together with the other strong winds extending from the terrestrial gaps of the Tsugaru Peninsula (Figs. 3a,d,e). From these results, diurnal variations in the locally strong winds are represented by those corresponding to the three key locations in the east inlet (41.578N, 140.808E), in Mutsu Bay (41.078N, 140.808E), and in the west exit (41.308N, 140.208E) as shown in Fig. 3a. The diurnal wind variations are conﬁrmed by hodographs of the hourly mean winds at the three locations FIG. 3. (a)–(e) Simulated 10-m wind vectors overlaid on the wind speed map (color shade) from 0500 JST 8 Jun 2003 to 0500 JST during 4 days from 0900 JST 6 June to 0800 JST 10 June 9 Jun 2003. The selected times correspond to the times of the (Fig. 4). In the east inlet (Fig. 4a) and in Mutsu Bay SeaWinds observations shown in Fig. 4 of Part I. Three black circles (Fig. 4b), rapid accelerations begin at 0900 JST. Wind in (a) denote the locations at which data are sampled for sub- speed increases to a maximum (9 m s21) at 1500 JST in sequent analyses. the east inlet and at 1800 JST in Mutsu Bay. During the

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FIG. 6. Hourly mean 2-m air temperatures from 0900 JST 6 Jun 2003 to 0800 JST 10 Jun 2003 in the east inlet, in Mutsu Bay, and in the west exit.

nighttime, the wind speeds decrease and return to the original state with changing direction. Meanwhile, the wind speeds are smaller during the daytime (0900– 1800 JST) than the nighttime in the west exit (Fig. 4c). The wind speed increases after 2100 JST with a shift toward the west and the strong winds persist for 11 h (2100–0800 JST) with maximum wind speeds at 0300 JST. These hodographs show regular diurnal variations in the local strong winds with different time lags. We examine 2-m air temperature fields corresponding to the wind fields (Fig. 5). East–west gradient in air temperature is seen in all the fields with low (high) air temperature in the east (west). In addition, we can trace the paths of the cool air inflowing from east to west, corresponding to the stronger winds (Fig. 3). During the daytime (Figs. 5b,c), the inflowing cool air from the east inlet contrasts significantly with the high air temperature in the lee of Mt. Osorezan. The leeward high air temperature is attributed to the blocking of cool air, enhanced solar in- solation by clearance of the low-level clouds due to the mountain, and the weak winds. Meanwhile, the air temperature in Mutsu Bay is high (158C) because of advec- tion of air over the heated land to the southeast (40.98N, 141.38E; Fig. 5b). Over the land east of Mutsu Bay, inflowing cool air is limited on the eastern half, and heating over land becomes prominent on the western half. During the nighttime (Figs. 5a,d,e), the cool-air flows from the east inlet and Mutsu Bay are still enhanced by persistent higher air temperature in the lee of Mt. Osorezan. Also, the cool-air outflows from the west exit and the circumjacent terrestrial gaps become clearer during the nighttime than during the daytime. To see the difference in diurnal variations in the air temperature at the three locations, we show hourly mean air temperatures during the 4 days from 0900 JST 6 June

FIG. 5. (a)–(e) Simulated ﬁelds of 2-m air temperature (shaded) to 0800 JST 10 June in Fig. 6. In the east inlet, the diurnal and SLP (contours with an interval of 0.2 hPa) at the same times as amplitude is the smallest (0.98C) of the three, and the air those shown in Fig. 3. temperature peaks at 1800 JST. These results feature the inﬂowing cool air. In Mutsu Bay, air temperature rapidly increases up to the peak by 1200 JST and the diurnal

Unauthenticated | Downloaded 09/30/21 12:36 PM UTC 1784 MONTHLY WEATHER REVIEW VOLUME 140 amplitude is the largest (1.68C). These results reflect the diurnal cycle of air temperature over the upstream land. However, the difference between minimum air temperatures in the east inlet and in Mutsu Bay at around 0600 JST is relatively small (0.48C). In the west exit, the peak air temperature is the highest of the three, likely due to the heating from the warm surface water, the entrainment from the aloft, and the confluence of warm air from Mutsu Bay. The distributions of SLP bear some analogy to those of the air temperature within the strait and the bay (Fig. 5). These analogies suggest a significant hydrostatic contribution of low-level air temperature to SLP, as is subsequently discussed. During the daytime (Figs. 5b,c), a large SLP gradient is created in the east inlet due to the inflowing cool air and the high air temperature within the strait. The warm air corresponds to low pressure in the lee of Mt. Osorezan, in the west of the Tsugaru Peninsula, and in a small cove on the southwest of Mutsu Bay. The SLP contours cover the west exit at which the cooler air within the strait meets the warm air over the Japan Sea. During the nighttime (Figs. 5a,d,e), the SLP gradient is weak in the east inlet and Mutsu Bay because the cool air completely passes through these locations. Meanwhile, the fan-shaped dense SLP contours in the FIG. 7. Simulated winds (vectors) and air temperature (shaded) at west exit indicate cool-air outflow. While diurnal varia- 150 m at (a) 0500 JST 8 Jun 2003 and (b) 1800 JST 8 Jun 2003. tion in SLP reflects the low-level air temperature, the height of the cool-air layer also varies diurnally, as shown Fig. 8. The trajectories reaching the west exit and the in section 3b. northernmost trajectories channeling through a terres- Typical features of the surface fields described above trial gap of the Matsumae Peninsula trace back along the are confirmed in the horizontal fields below 300– strait to the east inlet. The low-level trajectories across the 500 m. As we shall see later, these heights correspond to center of Mutsu Bay mainly pass over the Tsugaru Pen- the depth of the low-level layer of the cool air inflowing insula and partly go into the strait. On the basis of these into the strait and the bay. Moreover, the height of results, we define representative routes of the flow 500 m is nearly the maximum terrain elevation in the (Fig. 8) used hereafter in the analyses. The northern route study area. For comparison, Fig. 7 shows the distribu- follows the curve of the strait and extends westward from tions of wind and air temperature at a height of 150 m the east inlet. To the west of 140.28E, the two routes are at 0500 JST 8 June and 1800 JST 8 June. This height separately defined to follow the wind directions during corresponds to that of wind speed maxima in the strong the nighttime and daytime. The southern route passes wind regions, as shown in section 3b. As in the case of the over Mutsu Bay from the southeast to the west exit of the surface fields (Figs. 3 and 5), stronger (weaker) winds strait. Moreover, we define meridional sections at 140.28 correspond to cooler (warmer) air owing to the to- and 140.88E (Fig. 8), crossing the key locations for the pography of this area. locally strong winds in the east inlet of the strait, in Mutsu Bay, and in the west exit of the strait (Fig. 3a). b. Vertical structures We begin with meridional cross sections of zonal wind To estimate key locations for investigating vertical components and potential temperature (Fig. 9). Local- structures of the low-level strong winds, we derive gen- ized high speeds or high-wind core structures with little eral wind flows according to examples in previous vertical shear are evident. These structures correspond studies (e.g., Colle and Mass 2000). Using wind fields to the well-mixed layer with constant potential temper- interpolated to 10 min, the backward trajectories, which ature of the inflowing cool air and are situated below the start every 20 min at a height of 150 m from an array of inversion. At 0500 JST 8 June (Figs. 9a,b), surface wind points along 139.88E, are calculated. The trajectories maxima (.8ms21) shown in Fig. 3a reach up to 500– from 0500 JST 8 June to 0500 JST 9 June are plotted in 600 m in the west exit (41.38N in Fig. 9a), in the east inlet

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FIG. 8. Backward trajectories (thin curves) starting at an array of points along 139.88E (small black dots) every 20 min from 0500 JST 8 Jun 2003 to 0500 JST 9 Jun 2003, originating at a height of 150 m. The gray shaded color of the trajectory indicates height. The terrain with heights greater than 150 m is shaded. Along the trajectories, the northern and southern routes are defined to represent the general flows, and are denoted by a solid and dotted black curved line, respectively. To the west of 140.28E, two routes are defined to follow the wind directions during the nighttime and daytime. Three points (circles) on the two routes denote the key locations defined in Fig. 3a. Longitude lines at 140.28 and 140.88E indicate the locations of cross- sectional analysis.

(41.68N in Fig. 9b), and in Mutsu Bay (41.18NinFig.9b). potential temperature at the three locations (Fig. 10). The core potential temperatures are 285 K. At 1800 JST 8 The height and potential temperature of the well-mixed June (Figs. 9c,d), the heights of the core structures of the layers vary diurnally in a fairly regular manner with large easterly wind components decrease by 100–300 m. consistent temporal lags and diurnal amplitudes of sur- The potential temperatures within the core structures face air temperature at the three locations (Fig. 6). An increase (;288 K) and the inversions weaken. additional important point is that the mixed-layer height We next examine the diurnal variation in the cool-air increases (decreases) with a decrease (increase) in po- intrusion and retreat from time–height diagrams of tential temperature. The mixed-layer height reaches up

FIG. 9. Meridional cross sections of zonal wind component (color shade) and potential temperature (contours) along (a),(c) 140.28 and (b),(d) 140.88E at (a),(b) 0500 JST 8 Jun 2003 and (c),(d) 1800 JST 8 Jun 2003. The contour interval is 1.5 K.

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speeds at the surface. A similar deepening and shallowing trend is seen upwind (141.28E) and downwind (141.088E) of the east inlet along the 285-K contour, and horizontal wind acceleration occurs with the downward wind component. Along the southern route (Fig. 11b), minimum potential temperature is found over the land (141.38E) due to the nighttime cooling. Wind speed increases over the bay and reaches a maximum at 140.88E with downward vertical wind components. The low-level cool-air layer (,290 K) corresponding to high horizontal winds become shallower by 100– 200 m at 1800 JST 8 June (Figs. 11c,d) than that seen at 0500 JST 8 June. The cool air with less than 285 K is limited to the east of 141.08E. Along the northern route (Fig. 11c), the slight increases in the cool-air height and the subsequent subsidences are seen in the east inlet and in the west exit. For the southern route (Fig. 11d), minimum potential temperature (,284 K) is confined to the region east of the land (141.38E), and the mixed layer increases up to 500 m over the land (141.38E). These two structures indicate mixed layer development over the FIG. 10. Time–height diagrams of potential temperature in (a) heated land. Winds accelerate on the western half of the the east inlet, (b) Mutsu Bay, and (c) the west exit. The sampling land (140.28E). Moreover, the horizontal wind acceler- locations are shown in Fig. 8. ates over the bay (140.98–141.18E) with subsidence of the cool air, corresponding to the strong surface winds. to 300–500 m in the east inlet (Fig. 10a) and in the west We now examine the relation between diurnal changes exit (Fig. 10c), and more than 500 m in Mutsu Bay in SLP gradient and changes in the temperature and (Fig. 10b) during the nighttime. Thus, the height of the height of the cool-air layer. In Fig. 12, the horizontal mixed layer on the routes of the cool air is determined by gradient magnitude of SLP is compared with that of the degree of intrusion of the well-mixed cool layer from a hydrostatic pressure component from the surface to the east. 700 m, which is defined as To examine the intrusion of low-level cool air and the ð depth change of the cool-air layer, we show in Fig. 11 700 p 5 rgdz, (1) cross sections of potential temperature and wind com- 0 ponents along the defined routes shown in Fig. 8. At 0500 JST 8 June (Figs. 11a,b), low-level cool air where r is the air density, g is the gravitational acceler- (,290 K) with a height more than 500 m passes through ation, and z is the vertical direction. In the three loca- the Tsugaru Strait and Mutsu Bay from east to west with tions, variations in both pressure gradient magnitudes a slight increase in temperature due to the probable are consistent with each other, and the ratios of the reasons mentioned in section 3a. Strong easterly winds hydrostatic pressure gradient to the SLP gradient are confined to the cool-air layer. Along the northern amount to 90%. The contribution of low-level cool air to route (Fig. 11a), the low-level cool-air layer deepens on SLP is more significant in the east inlet and the west exit the upwind (east) sides of the west exit (140.48E), and than in Mutsu Bay. In the east inlet and west exit, the becomes shallow by 200–300 m with a downward verti- gradient magnitude of a pressure component below cal wind component. This deepening can be attributed 500 m similarly shows a consistent variation with the to the forced ascent of cool air due to terrain blocking on SLP gradient magnitude. Thus, the hydrostatic pressure both sides of the routes and the decreasing width of the contribution of the cool air layer with diurnally varying strait, resulting in higher pressure at this location. The air temperature and depth induces diurnal variation in shallowing is attributed to horizontal spreading of the air the SLP gradient. after exiting the west exit (140.28E). The horizontal winds On the other hand, variations in the horizontal pres- in the cool-air layer accelerate up to 12 m s21 in the west sure gradient magnitude are compared with variations in exit, largely because of the drops in SLP from east to west. height of 290-K potential temperature. In the east inlet This acceleration corresponds to the maximum wind and in Mutsu Bay (Figs. 12a,b), the pressure gradient

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FIG. 11. Cross sections of potential temperature (contours with an interval of 1 K) and wind components (vectors) along the (a),(c) northern and (b),(d) southern routes defined in Fig. 8 at (a),(b) 0500 JST 8 Jun 2003 and (c),(d) 1800 JST 8 Jun 2003. Color shade indicates horizontal wind speed. Thick contours show potential temperature of 290 and 285 K. Dashed lines denote the key locations shown in Fig. 8. magnitudes negatively correlate with the heights of including Mutsu Bay, to the east of 141.18E. Thus, the 290-K potential temperature. This indicates that a large diurnal cycle of the cool air inflowing into the strait and pressure gradient is created during the daytime when the the bay create the downslope of the isentropic surface in inflowing cool air encounters warm air inside the strait the cool layer. or the bay. The pressure gradient is reduced during the nighttime when the deep layer of the cool air passes completely through the east inlet and Mutsu Bay. In the west exit, the pressure gradient magnitude and the 290-K height are roughly positively correlated. This indicates that a large pressure gradient is created when the cool air reaching the west exit encounters warm air over the Japan Sea. During the daytime, higher air temperature inside the strait and shallow layer of the cool air reduce the pressure gradient in the west exit. Thus, the degree of inflow of the low-level cool layer determines the low-level horizontal pressure gradient within and adjacent to the strait and the bay. The depth of the cool layer over the study area undergoes a similar change to those shown in Fig. 11. Figure 13 shows height maps of an isentropic surface of 290 K. At 0500 JST 8 June (Fig. 13a), the isentropic surface with a height of 700–800 m covers the eastern half of the study area to the west exit. The cool-air out- FIG. 12. Time series of magnitudes of pressure gradient in (a) the flow from the west exit corresponds to the area in which east inlet, (b) Mutsu Bay, and (c) the west exit. Gray (black) solid line indicates gradient magnitude of SLP (hydrostatic pressure the height of the isentropic surface decreases from 700 from surface up to 700 m). Variations in the 290-K potential tem- to400m.At1800JST8June(Fig.13b),theisentropic perature height are also shown. The sampling locations are de- surface higher than 600 m is confined to the region, noted in Fig. 8.

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FIG. 14. Backward trajectories projected on a horizontal surface. The trajectories originate from the starting points (large circles) of a 150-m height at 0600 (black) and 1900 (gray) JST 8 Jun 2003. Namely, the trajectories pass through the east inlet, Mutsu Bay, and the west exit at around 0500 and 1800 JST 8 Jun 2003, which are the same as the time when the other analyses are performed. Small circles indicate sampling locations every 10 min.

(Fig. 14) on the assumption of steady flow. Although these trajectories are derived from the interpolated data used in Fig. 8, they originate from downwind of the three locations at a height of 150 m and are assumed to be on the isentropic surface at the starting points. We have confirmed FIG. 13. Height maps of an isentropic surface of 290 K at (a) that the Bernoulli function is fairly constant along the 0500 JST and (b) 1800 JST 8 Jun 2003. Black shade indicates areas trajectories, and this fact indicates that the application of in which the isentropic surface is not defined. (2) is reasonable. Figure 15 shows the variations in wind speed, the height of the air parcel, and air temperature 4. Flows passing through the three locations along the trajectories. In all cases during the daytime and nighttime in Fig. 15, wind speed and air temperature cor- a. Trajectory analysis relate inversely with height. The air parcel height as- We have presented diurnal variations in temperature cends (descends) upwind (downwind) of the three and depth of the cool-air layer and the resulting varia- locations, and temperature decreases (increases) due to tion in horizontal pressure gradient and subsidence of adiabatic cooling (heating). These variations are consis- the air. In this subsection, we approximately examine tent with the mountain wave regime of a gap wind shown the correspondences of the cool air subsidence and wind in Gabersˇek and Durran (2004) and indicate the impor- acceleration via the Bernoulli equation for steady in- tance of the downslope effect of the cool-air layer. viscid compressible flow (Gabersˇek and Durran 2004): We next look into the diurnal variation in the pa- rameters shown in Fig. 15. In the west exit (Figs. 15a,c), y2 rapid subsidence occurs to the west of 140.28E or after B 5 1 c T 1 gz, (2) 2 p exiting the strait for both times. In the east exit (Figs. 15a,c), subsidence begins from a more easterly location where y is wind speed, cp is the specific heat of air at (141.28E) at 1800 JST 8 June than that (141.18E) at constant pressure, T is the air temperature, g is the 0500 JST 8 June. This result is consistent with the fact gravitational acceleration, and z is the height. The that the cool air is confined to more eastern area dur- Bernoulli function B is conserved along a trajectory for ing the daytime than during the nighttime, as shown in dry adiabatic motion or for motion on the isentropic Fig. 11. In Mutsu Bay, subsidence mainly occurs to the surface. This equation indicates that the trajectories west of 141.08E at 0500 JST 8 June (Fig. 15b). However, at must be descending when the wind speed and temper- 1800 JST 8 June (Fig. 15d), the subsidence is enhanced ature increase downstream. over Mutsu Bay due to the developed mixed layer over The three terms on the right-hand side of (2) are com- the upstream heated land. Thus, we can confirm that the puted along the trajectories passing through the three lo- variations of the terms of (2) reflect diurnal variation of cations at around 0500 JST 8 June and 1800 JST 8 June the inflowing the cool layer.

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FIG. 15. Variations in wind speed (black line with circles), height (dark gray line with triangles), and air temperature (light gray line with diamonds) along the trajectories (Fig. 14) in the vicinity of (a),(c) the east inlet and the west exit and (b),(d) Mutsu Bay at around (a),(b) 0500 and (c),(d) 1800 JST 8 Jun 2003. b. Momentum balance Meanwhile, the southeasterly wind blowing into Mutsu Bay is accelerated in the along-wind direction as it We look into low-level horizontal momentum balance passes over the land and the eastern bay (141.48–140.88E; by reassembling the terms of the horizontal momentum Fig. 16a), and is deflected by the southward-directed equations: pressure gradient force. In the west exit (140.18–140.48E), significant accelerations by the large along-wind pres- Dv 1 sure gradient forces (Figs. 16a and 17a) are confirmed, as 52 $p 2 f k 3 v 1 F, (3) Dt r mentioned in Fig. 11a. The pressure gradient force first acts southwestward along the direction of the strait and where v is the horizontal wind vector, p is pressure, r is produces the acceleration in the crosswind direction the air density, f is the Coriolis parameter, k is a unit (Fig. 17c). However, after exiting the strait (139.48– vector in the vertical direction, and F is the residual. The 140.08E), the Coriolis force, which is enhanced by the term on the left-hand side is the material derivative of accelerated wind in the along-wind direction, acts north- the wind vector or the Lagrangian acceleration, and the ward (Figs. 16a and 17c), enabling the easterly winds to first and second terms on the right-hand side represent extend westward. At the same time, the anticyclonic the pressure gradient and Coriolis forces, respectively. curvature of the strong winds due to the Coriolis force is The third term on the right-hand side includes the other confirmed while the pressure gradient force decreases effects as residual. Figure 16 shows hourly mean mo- during 0000–0900 JST (Fig. 3a,e). The importance of the mentum balances at 150 m at 0500 and 1800 JST during Coriolis force for the anticyclonic curvature of the gap the4days(0900JST6June–0800JST10June),and outflow is reported by Steenburgh et al. (1998). Fig. 17 shows the along- and crosswind balances on the The dominant terms at 1800 JST (Figs. 16b and 17e–h) defined wind routes (Fig. 8). In Fig. 17, the crosswind are nearly the same as those at 0500 JST; however, components and the resulting Coriolis forces in the along- changes in magnitude of the dominant terms produce wind direction are zero at all points on the routes. The diurnal variations. The acceleration and southward de- residual is relatively small compared with other dominant flection of the wind inflowing from the east inlet are terms. enhanced (Figs. 16b and 17e,g) owing to the increased At 0500 JST (Figs. 16a and 17a–d), the wind acceler- pressure gradient force toward the lee of Mt. Osorezan. ation in the east inlet of the strait is induced by the Enhanced along-wind acceleration and southward de- along-wind pressure gradient force along from 141.28 flection are also true to the wind in Mutsu Bay (Figs. 17f,h). to 140.98E. The flow then turns counterclockwise from In the west exit, the acceleration in the along-wind di- 141.28 to 140.58E owing to the pressure gradient force rection becomes weaker at 1800 JST (Fig. 17e) than that directed to the lee of Mt. Osorezan (Figs. 16a and 17c). at 0500 JST (Fig. 17a), as shown in Fig. 11. After the The pressure gradient force is induced by the high air wind exits the strait, nearly geostrophic balance is at- temperature in the lee of Mt. Osorezan and cool air tained (140.18–139.48E; Figs. 16b and 17g). This transi- damming along the Hokkaido coast (Figs. 7a and 9b). tion is induced by the southward-directed pressure

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FIG. 16. Hourly mean momentum balances at (a) 0500 and (b) 1800 JST at a height of 150 m from 0900 JST 6 Jun to 0800 JST 10 Jun 2003. The vectors represent Lagrangian acceleration (red), pressure gradient force (blue), Coriolis force (green), and the residual (gray). Black arrows and brown contours indicate the horizontal winds and the pressure (0.2-hPa interval), respectively, at 150-m height. For clarity, the vectors are plotted at 0.128 intervals over the sea surface and in the low altitudes. gradient forces due to the warm air advected from the frequently occurs during the summer in this study area. Tsugaru Peninsula and the cool air from the strait. This We summarize the structures of the low-level winds in nearly geostrophic balance to the west of the strait during the study area for nighttime and daytime and the diurnal the daytime is a major difference from that during the variations in the locally strong winds at the three key nighttime. locations, as shown in Fig. 18. The easterly or southeasterly low-level wind splits into the two flows owing to the terrestrial blockings of the 5. Summary and discussion Shimokita Mountains and Mt. Osorezan and enters the Using a numerical model simulation of the low-level Tsugaru Strait from its east inlet and Mutsu Bay from a easterly wind blowing through the Tsugaru Strait and southeast low-level gap. The wind from east of the strait Mutsu Bay on 5–10 June 2003, we have investigated gradually shifts to the south in the lee of Mt. Osorezan diurnal variations in the locally strong winds specific to with the low pressure induced by daytime warming. The the sustained cool easterly wind condition. This condi- southeasterly wind passes through Mutsu Bay, and the tion, which is induced by the developed Okhotsk high, minor extension from Mutsu Bay merges with the flow

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FIG. 17. Hourly mean momentum balances in the (a),(b),(e),(f) along- and (c),(d),(g),(h) crosswind directions on the defined routes in Fig. 8 at (a)–(d) 0500 and (e)–(h) 1800 JST at 150 m. The data are sampled along the (left) northern route and (right) southern route. The color representation is the same as that in Fig. 16. from the east inlet. The wind then blows out from the over the upstream land. Moreover, the diurnal thermal west exit. Corresponding to these wind routes, the cool cycle over the upstream land has an important effect on air flows into the strait and the bay from the east with the wind. During the daytime, the well-mixed cool layer diurnal variation. During the nighttime, the cool air develops over the land up to 500 m, which enhances the in the well-mixed layer up to 500 m passes completely shallowing of the cool layer over the bay and accelerates through the strait, the bay, and the terrestrial gaps west- the wind. During the nighttime, the cool-air layer passes ward to the Japan Sea. During the daytime, this cool-air completely through Mutsu Bay, and the wind speed layer becomes shallow (;300 m) and the core of the cool increases with slight subsidence in the western side of air retreats from the west. These diurnal variations in the the bay. The winds are significantly accelerated in the west cool-air layer induce those in the horizontal pressure exit due to the along-wind pressure gradient force because gradient. In the process of the low-level cool-air inflow, of a rapid drop in pressure in the west exit. During the locally strong winds are generated in the east inlet of the daytime, the accelerated winds extend southwestward strait, in Mutsu Bay, and in the west exit of the strait. along the direction of the strait below a height of 300 m in In the east inlet, the inflowing winds are accelerated nearly geostrophic balance. The wind acceleration is en- by a large pressure gradient force during the daytime hanced during the nighttime when the cool air reaches when the inflowing cool air encounters warm air inside the west exit. Afterward, the extension of easterly winds the strait. The descent of the cool air occurs more sig- undergoes a slight anticyclonic rotation because of the nificantly during the nighttime than during the daytime large Coriolis force and the reduced pressure gradient in passing through the east inlet, contributing to wind force. acceleration. In Mutsu Bay, the southeasterly winds are This study and Part I emphasize the important role of accelerated down the pressure gradient force in passing the Tsugaru Strait and Mutsu Bay as conduits of the

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and meteorology groups of Tohoku University for their valuable advice. The authors acknowledge the National Center for Atmospheric Research for provid- ing access to the MM5 model. The MM5 simulation is performed using the supercomputer system SX-9 of Cyber Science Center, Tohoku University. We downloaded the SeaWinds/QuikSCAT, SeaWinds/ ADEOS2, and Pathﬁnder SST data from the NASA Physical Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory. The GPV and weather observation station data were provided by the Japan Meteorological Agency. The buoy observations in Mutsu Bay were provided by Aomori Prefectural Fisheries Research Center Aquaculture Institute. This study is supported by the Grant-in-Aid for Young Scien- tists (B) and the Research Program on Climate Change Adaptation of the Japanese Ministry of Education, Cul- ture, Sports, Science, and Technology.

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