Influence of Okhotsk Sea Ice Distribution on a Snowstorm Associated with an Explosive Cyclone in Hokkaido, Japan

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1 1 Influence of Okhotsk Sea Ice Distribution on a Snowstorm Associated with an Explosive Cyclone in Hokkaido, Japan Tetsuya Kawano and Ryuichi Kawamura Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, Japan Abstract To investigate the influence of the distribution of sea ice in the Sea of Okhotsk on the behavior of a severe snowstorm, which occurred in Hokkaido, Japan, on March 13 and which was associated with an explosive cyclone, three WRF simulations with realistic, reduced, and enhanced sea ice-cover were carried out. A comparison among these experiments reveals that the extent of the sea ice influenced low-level temperatures and winds to the rear of the cyclone center during the development of the explosive cyclone over the Sea of Okhotsk. Sea ice insulates the ocean from heat exchange with the atmosphere. As a result, when the Okhotsk sea ice extent reaches Hokkaido Island, cold air masses from the north traverse the island without first being heated by the ocean. The consequent temperature reduction produces a lowlevel higher pressure region to the rear of the cyclone center. As a result, a large geopotential gradient is generated just to the rear of the cyclone center, and low-level winds are intensified within this region. Therefore, the Okhotsk sea ice extent reaching Hokkaido Island plays a significant role in lowering temperatures and intensifying winds in the island. (Citation: Kawano, T., and R. Kawamura, 1: Influence of Okhotsk sea ice distribution on a snowstorm associated with an explosive cyclone in Hokkaido, Japan. SOLA, 1, 1 5, doi: 1.151/sola.1-1.) 1. Introduction Explosive cyclones, extratropical cyclones that develop rapidly in the cold season, cause heavy snowfall and strong winds over large areas. The impact of these systems on daily life can be enormous, with disruption including damage to electrical power cables; cancellation of flights, ships, and trains; and traffic jams. Because of its location, Hokkaido Island, Japan (Fig. 1), is frequently subjected to explosive cyclones (e.g., Chen et al. 199; Yoshida and Asuma ; Yoshiike and Kawamura 9). In particular, the eastern part of Hokkaido Island is often battered by snowstorms associated with extreme cyclones that develop over the Sea of Okhotsk (hereafter, referred to as SOKH), resulting in severe disasters. By better understanding the behavior of such storms, we can establish disaster prevention strategies and improve the accuracy of cold-season weather forecasting. Because sea ice insulates the ocean from heat exchange with the atmosphere (e.g., Iwamoto et al. 1; Inoue et al. 5), the large seasonal and interanuual variability of the Okhotsk sea ice distribution influences local- to large-scale atmospheric phenomena. Nagata and Ikawa (19) showed that the Okhotsk sea ice influenced the formation of convergent clouds west of Hokkaido Island using a numerical model. Okubo and Mannoji (199) showed that the use of a realistic sea ice distribution improved the prediction of near-surface winds on Hokkaido Island when using the Japan Spectral Model (Segami et al. 199). Honda et al. (1999) performed numerical simulations with small and large ice distributions using an atmospheric general circulation model (AGCM). They showed that circulation anomaly patterns caused by sea Corresponding author: Tetsuya Kawano, Department of Earth and Planetary Sciences, Kyushu University, 7 Motooka Nishi-ku, Fukuoka, 1395, Japan. kawano.tetsuya.9@m.kyushu-u.ac.jp. Fig. 1. Topographic map of Japan and its surroundings with zoomed map of Hokkaido Island (inset). Colors indicate altitude (m). Rectangles labeled D1, D, D3, and D show the calculation domains. Superimposed on the map are the NCEP-FNL cyclone track (black line), the CNTL-simulated track (red line), and the LIC-simulated track (blue line) from 6 UTC 1 March 13 to 6 UTC 3 March 13, obtained from the results in D1. ice differences extend to North America through the Bering Sea. Using a reanalysis data set and an AGCM, Mesquita et al. (11) showed that the sea ice extent over the SOKH impacts storm tracks not only in the North Pacific but also in the Atlantic. This study investigates the influence of Okhotsk sea ice on the snowstorm behavior related to explosive cyclones. For the purpose of disaster prevention on Hokkaido Island, it is very important to better understand the impact of this sea ice on the snowstorm feature. Here, we present a case study of a snowstorm that caused a severe disaster in the eastern part of the island on March 13.. Case overview An extratropical cyclone occurred in the northeastern part of China at UTC February 13. The cyclone rapidly developed and became an explosive cyclone as it passed through the Sea of Japan and over Hokkaido Island (Figs. 1 and ). In this study, the definition of explosive cyclones is the one used by Yoshiike and Kawamura (9). By the morning of March 13, it had reached the SOKH. The minimum central pressure of 96 hpa was recorded at 1 UTC March 13; the inhabitants of the island thus experienced a dramatic change in weather conditions, from calm to severe. In Fig., we illustrate the evolution of 5-hPa and 5-hPa geopotential heights; sea level pressure (SLP); and 95-hPa winds, based on the National Centers for Environmental Prediction (NCEP) Final Operational Global Analysis (FNL) data. At 1 UTC 1 March 13, an upper-level trough was located just west of the cyclone. The feature exhibits a structure of baroclinic waves The Author(s) 1. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution. International (CC BY.) license (

2 Kawano and Kawamura, Influence of Okhotsk Sea Ice Distribution on a Snowstorm Maximum instantaneous wind speed (m s -1 ) (a) Omu (b) Tokoro (c) Betsukai Temperature ( C) Fig.. (a, b, c) 5-hPa (color shading; m) and 5-hPa (contours; m) geopotential heights and (d, e, f) 95-hPa wind speed (color shading; m s 1 ) and sea level pressure (contours; hpa) in the NCEP FNL data at (a, d) 1 UTC 1 March 13, (b, e) UTC March 13 and (c, f) 1 UTC March 13. Fig. 3. Observed evolution of the maximum instantaneous wind speed (red lines; m s 1 ) and near-surface temperature (black lines; C) from 6 UTC 1 March 13 to 6 UTC 3 March 13 at (a) Omu, (b) Tokoro, and (c) Betsukai. in the developing stage (Fig. a). By UTC March 13, the cyclone had reached the SOKH and evolved into a quasibarotropic structure (Fig. b). The SLP distribution associated with the cyclone varied significantly over time and exhibited an axis-asymmetric structure (Figs. d, e, and f). The SLP gradient around the cyclone center was small while the cyclone passed across Hokkaido Island but enlarged after the center reached the SOKH. As shown in Figs. d, e, and f, the strong low-level wind field also exhibits an axis-asymmetry, reflecting the pressure distribution. At 1 UTC 1 March 13, during the early stage of cyclone development, the region with the strongest low-level winds was located south to southeast of the cyclone, far from its center (Fig. d). Twelve hours later, low-level winds surrounding the center from southwest to northeast rapidly intensified (Fig. e). When the cyclone developed furiously over the SOKH, one of the strongest wind regions was located just to the rear of the cyclone center, over the eastern part of Hokkaido (Fig. f). In contrast, the strong wind region in front of the cyclone moved eastward, far away from the center. The high contrast between regions of strong and weak winds caused abrupt wind changes on Hokkaido Island after the passage of the cyclone center. In Fig. 3, we illustrate the evolution of the maximum instantaneous wind speed (MIWS) and temperature near the surface as observed at Omu, Tokoro, and Betsukai (see Fig. 1). All stations recorded an MIWS peak simultaneous with the abrupt temperature drops. The enhanced MIWS caused a sudden whiteout in the region, leading to fatal accidents. 3. Model and experimental design Numerical experiments were performed using the Weather Research and Forecasting (WRF) model (Skamarock et al. ) version To capture mesoscale to synoptic-scale phenomena, calculations were performed on four domains with two-way nesting (Fig. 1). The horizontal grid spacings of the four domains were 7 km (D1), 9 km (D), 3 km (D3), and 1 km (D). The pressure at the top of the model is 5 hpa, and the stretched grid contains vertical levels. The surface physics and planetary boundary layer schemes implemented were the unified Noah land-surface (Chen and Dudhia 1) and Yonsei-University (Hong et al. 6; Hong 1) schemes, respectively. Shortwave and longwave radiation processes were modeled by the Dudhia Shortwave (Dudhia SW; Dudhia 199) and Rapid Radiative Transfer Model Longwave (RRTM LW; Mlawer et al. 1997) schemes, respectively. The Milbrandt-Yau two-moment microphysics scheme (Milbrandt and Yau 5) was used in all domains. The Kain-Fritsch cumulus parameterization scheme (Kain and Fritsch 199; 1993) was also adopted in D1 and D. Initial and lateral boundary conditions, sea surface temperatures, and sea ice extent were obtained from the NCEP FNL data. D1 and D were simulated from 6 UTC 1 March 13 to 6 UTC 3 March 13 ( hours), while D3 and D were simulated from UTC March 13 to UTC 3 March 13 ( hours). This control simulation is hereafter referred to as CNTL. Two additional experiments with reduced and enhanced sea ice extent in the SOKH were performed to investigate the influence of variable sea ice distribution on the snowstorm related to the explosive cyclone. These modified sea ice distributions were determined based on daily mean ice concentration in the National Oceanic and Atmospheric Administration (NOAA) Optimum Interpolation Sea Surface Temperature dataset (OISST; Reynolds et al. 7). As illustrated in Figs. a, b, and c, the OISST ice concentration data show that the interannual variation in the distribution of sea ice in the SOKH is very large. Figures a and c are examples of particularly light and extensive sea ice cover on March in 6 and 1, respectively. Compared with these sea ice conditions, the distribution of sea ice on March 13, when the cyclone passed over the SOKH, is of medium size, with the

3 3 Fig.. (a, b, c) Daily-mean OISST ice concentration (%) on (a) March 6, (b) March 13 and (c) March 1; (d, e, f) sea ice distribution for (d) LIC, (e) CNTL and (f) HIC. southernmost edge of the sea ice reaching Hokkaido Island (Figs. b and e). We performed two additional experiments to examine the influence of differences in the surface heat fluxes from ice-covered and ice-free regions on the snowstorm behavior. One is a simulation with light ice cover (hereafter referred to as LIC; Fig. d) in the SOKH, the other is a simulation with heavy ice cover (hereafter referred to as HIC; Fig. f).. Results Although the CNTL-simulated cyclone moves eastward more slowly than the real cyclone did, the simulated track broadly agrees with that of the cyclone in the NCEP FNL data (Fig. 1). Synoptic features of the cyclone evolution are well reproduced in CNTL, though additional deepening is seen in the simulated cyclone. As shown in Figs. 5a, 5b, and 5c, CNTL reproduces the strong wind distribution around the cyclone and its evolution very well, which is focused on in this study. Figures 5d, 5e, and 5f show the evolution of precipitation around the cyclone center. The main areas of precipitation are located within strong wind regions during the developing stage of the cyclone. In the early developing stage, two heavy precipitation regions are located in the north and east of the cyclone center. Some of this precipitation fell over the eastern part of Hokkaido Island (Fig. 5d). The region of precipitation developed a commashaped structure in the SOKH as the cyclone evolved. At UTC March 13, there are only weak winds and no precipitation over the majority of the island (Fig. 5e). Twelve hours later, when Fig. 5. (a, b, c) CNTL-simulated 95-hPa wind speed (color shading; m s 1 ) and sea level pressure (contours; hpa) and (d, e, f) hourly-accumulated precipitation (color shading; mm) at (a, d) 1 UTC 1 March 13, (b, e) UTC March 13 and (c, f) 1 UTC March 13. The variables simulated in D1 are displayed in all panels. Vectors show 95-hPa winds. A blue rectangle in (a) shows the zoomed area displayed in (d, e, f). the center reaches the SOKH, strong winds and precipitation hit the island, especially in its eastern regions (Fig. 5f). A similar evolution of precipitation was observed by conventional radar of the Japan Meteorological Agency (not shown). This sudden increase in precipitation and wind was responsible for the abrupt change in weather from calm to severe observed in Hokkaido. The evolution of the CNTL-simulated -m temperatures and 1-m wind speeds at Omu, Tokoro, and Betsukai is shown in Fig. 6. While these simulated records are delayed relative to the observations because of the slow eastward movement of the simulated cyclone, and although there is a bias to the lower temperatures (about C) found at all points in the CNTL simulation (Figs. 3 and 6), the patterns of evolution of the near-surface temperatures and winds, which are the foci of this study, are well reproduced. The difference in the tracks between the CNTL-simulated and LIC-simulated cyclones is barely noticeable (Fig. 1). In addition, the central pressure evolution of the LIC-simulated cyclone agrees well with that of the CNTL-simulated cyclone (not shown). Another comparison between the CNTL and LIC experiments, however, demonstrates that the distribution of sea ice in the SOKH influences near-surface temperatures and winds on Hokkaido Island (Fig. 6). Although, before the arrival of the cyclone center at Omu, Tokoro and Betsukai, the -m temperatures and 1-m winds at each point are very similar in LIC and CNTL, differences between the experiments arise after the passage of the cyclone. The -m temperature and 1-m wind values determined in CNTL are at most ~ C lower and ~ m s 1 stronger than those

4 Kawano and Kawamura, Influence of Okhotsk Sea Ice Distribution on a Snowstorm Fig. 6. Simulated evolution of wind at a height of 1 m (red lines; m s 1 ) and temperature at a height of m (blue lines; C) from 6 UTC 1 March 13 to 6 UTC 3 March 13 at (a) Omu, (b) Tokoro, and (c) Betsukai. The variables simulated in D are displayed in all panels. Bold and thin lines show the results of CNTL and LIC, respectively. in LIC, respectively. This indicates that an extensive ice distribution lowers temperatures near the surface and enhances low-level winds to the rear of the cyclone center while the cyclone is located in the SOKH. Although the -m temperatures and 1-m winds simulated in D are shown in Fig. 6, those simulated in D3 and D also exhibit similar features. Figure 7 shows sensible heat flux from the surface; 95-hPa temperature, geopotential and wind speed for CNTL; and the differences between CNTL and LIC at the time when the cyclone developed over the SOKH. In the CNTL run, the ice-free (icecovered) region within the SOKH is a source (sink) of heat for the low-level atmosphere (Fig. 7a). As a result, the air mass moving southward through the Okhotsk ice-covered region from the north is not heated (Fig. 7b). These features are clearly demonstrated by the differences in sensible heat flux and 95-hPa temperature between CNTL and LIC (Figs. 7e and 7f). As a result, lower temperatures are found around Hokkaido Island in CNTL. The temperature differences between CNTL and LIC influence low-level geopotential and wind fields. Because the temperature change occurs only in low levels, lower temperatures in CNTL produce a low-level high-pressure region, located to the rear of the cyclone center (Figs. 7c and 7g). In addition, the differences in positions and pressures between the centers of the CNTLsimulated and LIC-simulated cyclones are small. Therefore, the geopotential gradient from the center to rear is enhanced in CNTL. Consequently, low-level winds are intensified in the area with the increased geopotential gradient, located just within the strong wind region (Figs. 7d and 7h). Although the HIC experiment was carried out, the differences in temperatures and winds near the surface between CNTL and HIC were small (Supplementary Fig. S1). This also indicates that the Okhotsk ice-covered region northeast of Hokkaido Island is of importance in lowering temperatures and intensifying winds on the island. Fig. 7. (a) Sensible heat flux from the surface (W m ), (b) 95-hPa temperature ( C), (c) 95-hPa geopotential (m s ), and (d) 95-hPa wind speed (m s 1 ) for CNTL at 1 UTC March 13; differences in (e) sensible heat flux from the surface (W m ), (f) 95-hPa temperature ( C), (g) 95-hPa geopotential (m s ) and (h) 95-hPa wind speed between CNTL and LIC at 1 UTC March 13. Vectors in (a) indicate 1-m winds for CNTL. Green lines in (a) and (e) indicate outlines of sea ice for CNTL and LIC, respectively. The variables simulated in D1 are displayed in all panels. 5. Discussion and conclusions We performed simulations with the WRF model using realistic (CNTL), reduced (LIC), and enhanced (HIC) sea ice distributions in the SOKH to investigate the influence of sea ice extent on the snowstorm associated with an explosive cyclone that developed over the SOKH on March 13. The CNTL simulation reproduced well the synoptic-scale structure and evolution of the cyclone in the NCEP FNL data. In addition, the CNTL-simulated evolution of near-surface temperatures and winds in Hokkaido agreed well with observations. A comparison between CNTL and LIC clearly demonstrates that the distribution of sea ice in the SOKH influenced nearsurface temperatures and winds in Hokkaido after the passage of the cyclone. Under the closed-sea conditions (i.e., CNTL and HIC), in which sea ice extends southward to Hokkaido Island itself, the ice-covered region within the SOKH is not able to function as a heat source for cold air masses passing over the region from the north. These cold air masses therefore reach Hokkaido Island without being heated, resulting in a reduction of low-level temperatures in the area. A low-level high-pressure region is consequently generated around the island, creating a large geopo-

5 5 tential gradient, and thus results in the intensification of low-level winds in the region. Matsuzawa and Takeuchi () showed that continuous snowdrifting occurs when temperatures are lower than 5 C and wind speeds exceed 11 m s 1 (see their Fig. 5). If this threshold is exceeded, a violent snowstorm with severe snowdrifting will occur. Our study therefore indicates that Hokkaido is in great danger of a snowstorm disasters caused by cyclones developing over a largely ice-covered SOKH. Given that the seasonal variability of the sea ice distribution in the SOKH is larger than the interannual variability, with sea ice extent peaking from late February to early March every year, we should pay particular attention to snowstorms associated with extratropical cyclones developing over the SOKH in this period. The influence of the distribution of sea ice in the SOKH on the snowstorm related to the explosive cyclone is likely insensible to the track of the cyclone as it passes over the SOKH, because changes in low-level temperature and wind occur on the scale of about 1 km (Figs. 7f, 7g, and 7h). Therefore, the snowstorm feature illustrated in this study must be also found in other cases associated with explosive cyclones developing over the SOKH. This study suggests that the Okhotsk ice distribution is one of important factors modulating the intensity of snowstorms associated with cyclones that develop furiously over the sea, irrespective of the cyclone track. In this study, the sea ice distribution is adopted as a fixed lower boundary condition. However, we should also consider an interactive atmosphere-ocean-ice system. While this study demonstrates that the distribution of sea ice in the SOKH influences snowstorms associated with extratropical cyclones developing over the sea, the cyclone most likely, in turn, affects the sea ice distribution. In addition, given that sea ice works not only as an insulator during heat exchange between the atmosphere and ocean, but also forms strongly baroclinic regions between its edge and the ocean, this baroclinicity most likely impacts cyclone development. Further investigation of this interactive atmosphereocean-ice system is required to better understand the relationship between sea ice and snowstorms related to explosive cyclones. Acknowledgements The authors thank two anonymous reviewers for their constructive comments that improved the manuscript significantly. This research was supported by JSPS KAKENHI Grant Numbers JP53 and JPH16. Edited by: T. 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Manuscript received 31 August 17, accepted 9 November 17 SOLA: jstage. jst. go. jp/browse/sola/