186 SOLA, 2017, Vol. 13, 186−191, doi:10.2151/sola.2017-034

AbstractThe resolution dependency of simulated stratosphere-to-tro-

posphere transport (STT), associated with mid-latitude closed cyclones at mid- and upper-troposphere in early spring around Japan, was investigated using the Nonhydrostatic ICosahedral Atmospheric Model (NICAM), with three horizontal resolutions (~220, 56, and 14 km). We defined “STT-producing cyclone” as the developed mid-latitude cyclones that have isolated low pres-sure systems with closed contours at mid- and upper-troposphere, generated by the deepening of a trough. The STT-producing cy-clone itself, with a large spatial scale, is well-simulated from three horizontal resolution experiments. As the horizontal resolution increased, the baroclinic zone on the rear side of the surface cold front became sharper along the southern areas of the upper low, bringing stratospheric dry air down to the troposphere. This com-prises a sharp and deep intrusion of dry air into the troposphere, with increased dry air along the jet-axis. While the sufficient resolution for accurate estimation remains unclear, these results suggest that the minimum horizontal resolution for reproducing the sharp and deep intrusion from the stratosphere to the tropo-sphere during the STT-producing cyclone events is approximately 14−56 km.

(Citation: Yamashita, Y., M. Takigawa, K. Ishijima, H. Aki-yoshi, C. Kodama, H. Yashiro, and M. Satoh, 2017: Resolution dependency of numerically simulated stratosphere-to-troposphere transport associated with mid-latitude closed cyclones in early spring around Japan. SOLA, 13, 186−191, doi:10.2151/sola.2017-034.)

1. Introduction

The stratosphere-troposphere exchange (STE) was investigat-ed, with both observations and global model simulations, and the annual mean global STE mass flux was estimated at ~1010 kg s−1 (e.g., Appenzeller et al. 1996b; Gettelman et al. 1997; Olsen et al. 2004; Schoeberl 2004; Stevenson et al. 2006). The mass flux of STE is larger in the Northern Hemisphere (NH) than in the South-ern Hemisphere (e.g., Olsen et al. 2013; Yang et al. 2016). Tro-posphere-to-stratosphere transport is dominant in low-latitudes, while stratosphere-to-troposphere transport (STT) is dominant in the mid- and high-latitudes, with the maximum being along the poleward edge of subtropical-jets (e.g., Holton et al. 1995; Yang et al. 2016). The STT usually occurs with a cross-tropopause flow, induced by the tropopause folding caused by the developed mid-latitude cyclones, accompanied by dry airstream from upper to lower levels (e.g., Johnson and Viezee 1981; Cooper et al.

2001). However, many other processes (e.g., breaking gravity waves, mesoscale convective systems) are related to the STT (e.g., Stohl et al. 2003). Tropopause folding is often formed during the development of a cut-off low (e.g., Stohl et al. 2003), which is a subset of the mid-latitude cyclones.

Deep STT events are those where air parcels are rapidly trans-ported down from the stratosphere to levels below 700 hPa and around planetary boundary layer (PBL), and are likely associated with deep tropopause folding events through mid-latitude cy-clones (e.g., Wernli and Bourqui 2002). This sometimes includes an irreversible exchange from the stratosphere to the troposphere, with the transport of potentially ozone-rich air into the PBL (e.g., Stohl et al. 2003).

The downward mass flux during deep STTs has geographical characteristics, with distinct maxima in the North Pacific and North Atlantic storm-track regions, the Mediterranean, and moun-tainous areas (i.e., the Rockies, the Himalayas, and the Tibetan Plateau) during the NH winter and spring (e.g., Sprenger and Wernli 2003; Škerlak et al. 2014). The STT over each region has different tropospheric impacts through various transport processes (e.g., Škerlak et al. 2014; Yang et al. 2016). Therefore, it is im-portant to determine the spatial distribution of STT and its process for each region, in order to understand the regional characteristics of STT. Many previous studies have focused on STT events in North America using both observations and global models with a horizontal resolution of 50−300 km (Lefohn et al. 2001, 2011; Fiore et al. 2002; Langford et al. 2009; Bourqui and Trepanier 2010; Lin et al. 2012). Some studies have focused on STT events in the Mediterranean, using a global model with a relatively low resolution (~300 km; e.g., Akritidis et al. 2016), as well as on events in Europe, using global models with relatively higher resolutions (~100 km), and regional models with a resolution of ~14 km (e.g., Hoffmann et al. 2016). Relatively fewer studies have focused on East Asia, including those using observational analysis (Büker et al. 2005), a trajectory model (Tsutsumi et al. 1998), and a regional model with a resolution of ~20 km (Wu and Lü 2016).

The deep tropopause folding events involve small-scale fila-mental structures, with a horizontal scale of less than 100 km (e.g., Appenzeller et al. 1996a). The STE mass flux during these events is potentially underestimated by numerical models with relatively low horizontal resolution (~50−300 km). Consistently, the mass exchange over the Pacific and Atlantic storm-track regions in high horizontal resolution models (~20 km) is larger in magnitude than that in low resolution models (~180 km) below 400 hPa, due to the inability to simulate filamental structures along with the storms in low resolution model (Mizuta and Yoshimura 2009).

In the area around Japan, storm-track activities are more frequent in early spring than in midwinter (e.g., Nishii et al. 2009), and the deep tropopause folding events by mid-latitude cyclones sometimes occur in March (early spring; e.g., Itoh and Narazaki 2016). It should be noted that the behavior of mid- latitude cyclones, such as the development of a cut-off low, is related to their interaction with the hemispheric scale planetary

Resolution Dependency of Numerically Simulated Stratosphere-to-Troposphere Transport Associated with Mid-Latitude Closed Cyclones

in Early Spring around Japan

Yousuke Yamashita1, Masayuki Takigawa1, Kentaro Ishijima1, Hideharu Akiyoshi2, Chihiro Kodama1, Hisashi Yashiro3, and Masaki Satoh4, 1

1Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan2National Institute for Environmental Studies, Tsukuba, Japan

3RIKEN Advanced Institute for Computational Science (AICS), Kobe, Japan4Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Japan

Corresponding author: Yousuke Yamashita, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa, 236-0001, Japan. E-mail: yyousuke@jamstec.go. jp. ©2017, the Meteorological Society of Japan.

187Yamashita et al., Resolution Dependency of Stratosphere-to-Troposphere Transport

contrast, these events occurred approximately once in 2 years for MIROC5.0-CCM, ERA-interim, and JRA-55. The events occurred twice for EXP14, but this represented only one-month data and the result is of low confidence.

3. Results

As an example of STT-producing cyclone events around Japan, Fig. 1a shows the observed water vapor image of a closed low over the eastward part of Hokkaido, while Fig. 1b shows a snapshot of the 500-hPa geopotential height (black contours), relative humidity (RH; shadings), and a 200-hPa horizontal wind (green vectors) for JRA-55. Figure 1a shows the extension of the dry airstream from western Japan to the east of the low center along the jet-axis as the thick green vectors from Fig. 1b. Moreover, there was a wet area to the southeast of the low center, with relatively wet air from the north to the west of the low center. Furthermore, Fig. 1a shows the spiral structure of dry airstream towards the low center, corresponding to the dry airstream of JRA-55 from east to west via north of the low center (Fig. 1b). However, the spiral structure immediately around the low center could not be seen due to the coarse resolution of JRA-55 with a 1.25° × 1.25° longitude-latitude grid.

Figures 1c, 1d, and 1e show snapshots of each STT-producing cyclone event for EXP220, EXP56, and EXP14. The most devel-oped event was chosen based on the condition of a minimum of 500-hPa geopotential height at the low center. The widespread dry air (blue) to the southwest of the low center can be seen in EXP220. In contrast, EXP56 and EXP14 showed a sharpened dry airstream along the jet-axis at approximately 5400 m. Moreover, three experiments reproduced the wet airstream from southeast to west of the low center via the north, with the sharpened structure being visible to the northwest of the low center in EXP56 and EXP14. It is noteworthy that the spiral structure of dry airstream toward the low center becomes sharp in EXP14.

Figure 2 shows the composite mean of STT-producing cyclone events for the potential temperature (black contours), location of the tropopause (green crosses), and relative humidity (RH; shadings). The days of composites were defined such that the STT-producing cyclones were located in the most southward region when criteria (1) and (2) of Subsection 2.2 are satisfied. The definition of the tropopause is the same as that by Schoeberl (2004): The tropopause is located at the 3.5 potential vorticity unit (PVU) surface, except where it exceeds the height of the lapse rate tropopause (LRT) of the World Meteorological Organization definition, while the 380-K surface is used if the tropopause levels for both 3.5 PVU and LRT, are above the 380-K surface.

The distribution of potential temperature for all three resolu-tions indicated relatively low values at 40°N−50°N, with 400− 850 hPa around the low center. This suggests that even low- resolution models can reproduce the cold core structure of STT- producing cyclones, which have a large spatial scale. Above the cold core, the tropopause extends down to levels lower than those in surrounding areas, with low RH air intruding from the strato-sphere to the troposphere along the southern edge of the cold core

waves (e.g., trough/ridge of planetary wave, blocking; e.g., Nakamura et al. 1997). Hence, the global model is required to sim-ulate the mid-latitude cyclones. In this study, we performed global numerical simulations with three horizontal resolutions (~220, 56, and 14 km) to study the resolution dependency of mass transport of STT associated with the tropopause folding events by well- developed mid-latitude cyclones in March around Japan.

2. Data and methods

2.1 DataWe used the output data from three simulations with different

resolutions (220, 56, and 14 km), performed with NICAM (Tomita and Satoh 2004; Satoh et al. 2008, 2014), which was used for the transport of aerosols and gases (e.g., Niwa et al. 2011; Goto et al. 2015). Simulations included 78 vertical layers from the surface to ~50 km altitude. The vertical resolution around the tropopause was ~0.4 km. The experimental conditions of greenhouse gas concentrations were fixed to the year 2000, and the sea-surface temperature was fixed to the calendar month climatology for 1979−1999. A spin-up run was performed for 10 years using the 220-km simulation (hereafter EXP220). The three resolution calculations followed from the final date of the spin-up run, with integration periods of 10 years for EXP220, 3 years for the 56-km simulation (EXP56), and 1 month for the 14-km simulation (EXP14).

We also used output data from the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) chemistry-climate model (CCM). The latest CCM was developed from version 5.0 of the Model for Interdisciplinary Research on Climate (MIROC). This CCM includes the detailed stratospheric chemistry scheme of version 3.2 of MIROC-CCM (Akiyoshi et al. 2009, 2016; Morgenstern et al. 2017). The latest MIROC5.0-CCM well reproduces the observed seasonal cycle of ozone. The horizontal resolution is T42 (~300 km), with 34 vertical layers from the surface to ~80 km in altitude, while the vertical resolution around the tropopause is ~1.3−2.2 km. The experimental conditions were fixed to the year 2000, with calcula-tions performed for 100 years, following a 10-year spin-up run.

For observational data, we used 1.0° × 1.0° longitude-latitude grid data of the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) for 38 years, from 1979 to 2016 (Dee et al. 2011), and 1.25° × 1.25° longitude- latitude grid data of the Japanese 55-year Reanalysis (JRA-55) dataset (Kobayashi et al. 2015; provided from the Japan Meteoro-logical Agency) for 59 years, from 1958 to 2016.

2.2 Definition of STT-producing cyclone events around JapanSince the STT usually occurs with tropopause folding by the

developed mid-latitude cyclones, we defined “STT-producing cyclones” as the developed mid-latitude cyclones that have an isolated low-pressure system, generated by the deepening of a mid- and upper tropospheric trough occurring upstream of Japan; that is, (1) the daily mean 500-hPa geopotential height over 20°N−50°N, 135°E on the westward (upstream) of northern Japan and over western Japan is below 5100 m in March (except for 1st March and 31st March), and (2) the centers of closed circles of geopotential height exist in 40°N−50°N, 130°E−140°E at both 300-hPa and 500-hPa levels, located along the Sea-of-Japan side of Hokkaido (northern Japan). Criterion (2) was required because (1) includes open troughs as well as closed cyclonic circulations. The definition of the STT-producing cyclone in this study meets the cut-off low condition for the deepening of an upper trough with a well-developed closed circulation. However, it does not include the condition for detaching from the basic westerly current to be located on the equatorward side of this current (e.g., Nieto et al. 2005). We converted the numerical data to the same horizon-tal resolution (~1° × 1° longitude-latitude resolution) before event detection.

The STT-producing cyclone events occurred approximately once per year, in March, for EXP220 and EXP56 (Table 1). In

Table 1. Frequency of the STT-producing cyclone events in March (times/month). Data of free runs extend for 10 years for EXP220, 3 years for EXP56, 1 month for EXP14, and 100 years for MIROC5.0-CCM. Reanalysis data are 38 years (1979 to 2016) for ERA-interim and 59 years (1958 to 2016) for JRA-55.

Freq.

NICAM EXP220NICAM EXP56NICAM EXP14MIROC5.0-CCMERA-interimJRA-55

1.11.32.00.40.50.6

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(around 35°N−40°N). With the increase in horizontal resolution, the dry air intrusion became sharper, and the minimum value of RH decreased. The low RH area (i.e., lower than 0.5) reached ~800 hPa for EXP56 and EXP14. The descent area of the dry air corresponded to the area of the sharpened dry airstream along the jet-axis in EXP56 and EXP14 (Figs. 1d and 1e).

Near the descent area of the dry air around 35°N, a baroclinic zone became clearer as the horizontal resolution increased. For example, the baroclinic zone near the southern edge of 35°N (horizontal magenta line) was approximately 8° latitudinal range (27°N−35°N) of 285−300 K at ~750-hPa level for EXP220, while it was approximately 7° (28°N−35°N) at ~750-hPa for EXP56, and approximately 4° (31°N−35°N) at ~650-hPa for EXP14. This baroclinic zone was located on the rear side of the cold front (see Fig. 3). It should be noted that the similar resolution dependency

with the sharp dry air intrusion in the narrow baroclinic zone is derived from each composite member (not shown).

Figure 3 shows the composite mean STE mass flux, derived from the addition of mass flux across the 380-K level to mass tendency in the lowermost stratosphere between the 380-K level and the troposphere, as proposed by Appenzeller et al. (1996b) and Schoeberl (2004). The mass flux across the 380-K level and the mass tendency in the lowermost stratosphere were averaged with the 20-day boxcar filter before the STE mass flux calculation, similar to Schoeberl (2004). This procedure possibly captures other events one week before/after the events. Since the exper-imental period of EXP14 was too short to derive the STE mass flux statistically, the results are not shown here. The green lines in Fig. 3 indicate the tropopause height with contours for every 50-hPa level. Tropopause folding down to the 350-hPa level was

(a) Himawari-8

(d) EXP56 (e) EXP14(c) EXP220

(b) JRA-55

Fig. 1. (a) Himawari-8 water vapor image at 00:00 UTC on 16 April 2016 (from http://www.jma.go.jp). (b) Snapshot at 00:00 UTC on 16 April 2016 for JRA-55, and (c−e) Snapshots of the most developed single event for (c) EXP220, (d) EXP56, and (e) EXP14. (b−e) show 500-hPa RH (shading), 500-hPa geopotential height (black lines; thin lines every 60 m and thick lines every 120 m), 200-hPa horizontal wind (green vectors; thin vectors indicating 30− 50 m s−1, thick vectors indicating larger than 50 m s−1), and sea level pressure (gray lines; thin lines every 4 hPa, thick lines every 20 hPa; labels at 980 hPa and 1000 hPa).

(a) EXP220 (c) EXP14(b) EXP56

Fig. 2. Latitude-height cross-section averaged over 120°E−150°E around the low center using the composite mean of the STT-producing cyclone events for (a) EXP220, (b) EXP56, and (c) EXP14. Shading indicates RH (scale shown at the bottom of the panel), black contours denote potential temperature (thin lines every 3 K, thick lines every 15 K), green crosses denote the location of the tropopause, while horizontal magenta lines indicate the latitudinal range of 285−300 K along the southern edge of 35°N (i.e., the baroclinic zone).

189Yamashita et al., Resolution Dependency of Stratosphere-to-Troposphere Transport

simulated on the southwest side of the low center for EXP220. Correspondingly, the downward STE mass flux was strong to the south and west of the low center. Black broken lines denote 850-hPa equivalent potential temperature, denoting the mean location of the front lines. The area of the downward STE mass flux was located around the baroclinic zone along the northern edge of the cold front. Tropopause folding was also observed for EXP56, being down to the 400-hPa level around the low center, causing a strong downward flux over the southern edge of the low center and on the rear side of the cold front. The values of STE mass flux averaged over 110°E−160°E, 30°N−45°N were −1.3 and −2.5 g cm−2 day−1 for EXP220 and EXP56, respectively, with that for EXP56 being approximately twice that of EXP220.

4. Summary and discussion

The resolution dependency of STT by STT-producing cyclone events in March around Japan has been investigated using three horizontal resolutions (220, 56, and 14 km) of a global model (NICAM). As the horizontal resolution increased, the baroclinic zone on the southern side of the low center became sharper, there-by bringing more stratospheric dry air down to the troposphere, and increasing the sharpness and depth of the dry air intrusion in the troposphere. This was accompanied by an increase in the STE mass flux. Based on these results, the sufficient resolution for an accurate estimation remains unclear. However, 14−56 km resolu-tion is required to reproduce the observed STT processes during

the STT-producing cyclone events, since their processes have sharp and deep downward air intrusion along the sharp baroclinic zone, with the intrusion processes including a horizontal scale of less than 100 km. The coarser resolution models underestimated the STE mass flux in the troposphere.

MIROC5.0-CCM has a coarser horizontal resolution (~300 km) than EXP220, with the baroclinic zone at the southern edge of low center being less distinct than that in EXP220 (not shown). Figure 4a shows that the downward STE mass flux was relatively large along the southwestward edge of the low center. The value of STE mass flux averaged over 110°E−160°E, 30°N− 45°N was −1.1 g cm−2 day−1, which was slightly lower than that of EXP220. In MIROC5.0-CCM, the ozone mixing ratio can be calculated. Therefore, we were able to directly estimate the ozone flux of STE with the ozone mixing ratio at 380 K and the STE mass flux. Figure 4b shows that the ozone mixing ratio was relatively large to the northwest of the low center, indicating that the downward ozone flux of STE was stronger in the low center and the area to its southwest. The value of the ozone flux of STE averaged over 110°E−160°E, 30°N−45°N was −0.50 × 10−6 g cm−2 day−1. Due to the coarser horizontal resolution, we speculated that the flux values of MIROC5.0-CCM would be underestimated. This underestimation of the ozone flux of STE during the STT-producing cyclone events would lead to the under-estimation of the episodic increases in the surface ozone level around Japan, downstream of the ozone intrusion area.

Some CCM projections indicated an increase in the STT of ozone due to the combination of stratospheric ozone recovery in

(a) EXP220 (b) EXP56

(b)

Fig. 3. Composite mean of STE mass flux for STT-producing cyclone events (shading; blue = downward, red = upward). Scale is shown at the bottom of the panel with a unit of g cm−2 day−1. Black solid lines indicate 500-hPa geopotential height (contours of 5100 m, 5400 m, and 5700 m are shown), defining the low center. Black broken lines denote 850-hPa equivalent potential temperature (thin lines every 3 K, thick lines every 15 K), denoting the mean loca-tion of the front lines. Green lines indicate the tropopause height (every 50 hPa).

Fig. 4. (a) Same as Fig. 3, but for the results of MIROC5.0-CCM. (b) Ozone flux of STE for MIROC5.0-CCM (shading; scale shown at the bottom of the panel with a unit of ×10−6 g cm−2 day−1), with 500-hPa geopotential height (black solid lines) and ozone mixing ratio at 380-K level (black broken lines; thin line every 50 ppbv, thick line every 200 ppbv) shown.

190 SOLA, 2017, Vol. 13, 186−191, doi:10.2151/sola.2017-034

low halogen concentrations and an accelerated Brewer-Dobson circulation in warmer climates (e.g., Hegglin and Shepherd 2009). Using the massive computational resources of the future, long-term simulations of high-resolution CCM are needed to clarify the present and future projection of ozone flux of STE and its impact on the surface.

Acknowledgements

The authors thank anonymous reviewers for their helpful comments and Drs. Kazuyuki Miyazaki, Tatsuya Seiki, and Yohei Yamada for many helpful comments and technical supports. This work is supported by the FLAGSHIP2020, MEXT within the priority study 4 (Advancement of meteorological and global environmental predictions utilizing observational “Big Data”), KAKENHI (JP16K16186), MEXT, and the Environment Research and Technology Development Fund, Ministry of Environment, Japan (2–1709). The NICAM calculation was performed using the K computer at the RIKEN AICS through the HPCI System Research project (Project ID: hp160231, hp170232). The MIROC 5.0-CCM calculation was performed using NEC SX-ACE com-puter at the NIES.

Edited by: H. Mukougawa

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Manuscript received 23 August 2017, accepted 20 September 2017SOLA: https://www. jstage. jst.go. jp/browse/sola/