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 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 09/30/21 12:36 PM UTC 1780 MONTHLY WEATHER REVIEW VOLUME 140 cycles of the local wind accelerations. As possible cau- ses, Part I has suggested diurnally varying cool-air in- trusion 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 sim- ulation data are indispensable. Therefore, this study investigates the diurnal varia- tions 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 east- erly 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 night- time (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). Unauthenticated | Downloaded 09/30/21 12:36 PM UTC JUNE 2012 S H I M A D A E T A L . 1781 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 varia- tion in the SLP difference is represented in the simula- tion. 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 dur- ing the entire simulation period on an average.