1. Generation mechanism of strong winds in the left-rear quadrant of Typhoon MA-ON (2004) during its passage over the southern Kanto district, eastern Japan Wataru Mashiko (Meteorological Research Institute) Copyright of this photo; Yokohama Observatory (2004)

2. Typhoon MA-ON (2004) Rapid traslation speed; 70 km/h Central pressure at landfall; 950 hPa Stationary front existed to the south of eastern Japan Surface synoptic weather chart at 15 LST on 9 October 2004 Best track analysis Japan Meteorological Agency (JMA) Numerals on the right show MSLP (hPa)

3. Topography and geographical locations around the Kanto Plain, Eastern Japan Mt.Tanzawa Sagami Bay Tokyo Hiratsuka Kanto Plain Kanto Mountains The solid contour interval of elevation is 500 m, with dotted lines of 50 m and 250 m. The Kanto Mountains with more than 1000 m elevation run from north to south on the western side of Kanto Plain.

4. Surface wind and temperature during the passage of MA-ON Wind barb and flag represent 5m/s and 25m/s, respectively (10 min average). Solid line shows the track of MA-ON. Broken line shows the elevation of 250 m. 18:00 16:0017:0017:30 19:00

5. Soundings at Tsukuba The strong wind of 38.4 ms -1 (1-min average) was recorded when the wind direction started changing from north to northwest, which indicates that location of Hiratsuka was shifted to the rear side of MA-ON. The cold air with north-easterly winds was below 950 hPa. Surface time series of 1-min-average wind speeds, wind direction and pressure 38.4m/s 17:00 N E W Hiratsuka observatory (NIED)

6. What is the structure of landfalling MA-ON ? What is the generation mechanism of the strong winds in the left-rear quadrant of MA-ON in spite of its rapid translation ? Numerical simulation

7. Numerical model 2-way 2-nested mesh Relationship of nesting Design of model domains  2-way multi-nested movable mesh model (Mashiko and Muroi 2003) This model is based on operational nonhydrostatic model of Japan Meteorological Agency (JMA). Cold-rain explicit cloud microphysics ( no cumulous parameterization) A B

8. Wind speed before and after landfall 250 m Height1000 m Height 14:36 LST Before landfall 959 hPa 17:24 LST After landfall 964 hPa Strong winds on the left-rear side occurred at the low level after landfall.

9. Vertical cross section C-A ＡＣ Ａ Ｃ Ｃ Ｂ Vertical cross section A-B Wind Speed （ Arrows show horizontal wind ） EPT （ Arrows show horizontal wind ） Ａ Ｂ Ａ Ａ Ｂ Structure of landfalling MA-ON The flow of the cold air formed narrow channel between typhoon center and the mountain range. The typhoon with high EPT moved over the cold air at the low level in the Kanto plain. The strong wind area on the western side corresponds to the cold air quite well. EPT (Equivalent potential temperature) at a height of 250 m

10. When the storm center moved to the Sagami bay, the strong winds on the left-rear side occurred. The strong winds correspond to the outflow of the cold air quite well. Transition of wind velocity and potential temperature at a height of 250 m Potential TemperatureWind Speed

11. Strong winds in the left-rear quadrant of the typhoon (1724LST) The strong winds occurred over the Sagami bay, which corresponds to the outflow from the channel-like cold flow formed by the mountain range and typhoon center. Wind speed with SLP (contour) at a height of 250 m Potential temperature at a height of 250 m Sagami Bay

12. Trajectory analysis Backward; 48min, Forward; 12min, Starting time; 1724 LST Marker interval ３ min Origin Starting at 240 m above the sea surface Shaded in color is model orography. The parcels moved along the eastern side of the Kanto Mountains and accelerated southward. After the parcels passed near Mt. Tanzawa, they descended with diffluent flows toward the Sagami Bay. Mt. Tanzawa Sagami Bay TR-4

13. Time evolution of the height of isentropic (θ= 296 K ) with the trajectory of TR-4 When TR-4 accelerated descending and passed near Mt. Tanzawa at 1718 LST, large southward decline of isentropic exists there. (large southward PGF) 1712LST1718LST1724LST Dashed lines denote SLP with a contour interval of 4 hPa Mt. Tanzawa

14. Generation mechanism of the strong winds Horizontal momentum equation Acceleration Pressure gradient Corioli Friction (PGF) PG slope PG depth PG large d; potential temperature deficit, α; terrain slope, h; depth of the cold layer, θ 0 ; average PT for cold layer Parish and Cassano (2003), Mahrt (1982) PG slope ; Cold, stably stratified air over sloping terrain PG depth ; Variation in cold-layer thickness PG large ; Large scale pressure gradient

15. Schematics of PGF effects PG depth PG large PG slope Ground Cold layer Z Ground Cold layer Z Ground Cold layer θ1 ＜ θ2 Z or Mountains L H Strait PG slope ; Cold, stably stratified air over sloping terrain PG depth ; Variation in cold-layer thickness PG large ; Large scale pressure gradient

16. Along wind components of the momentum equation of TR-4 Shaded in color is model orography. Mt. Tanzawa Sagami Bay Pressure Gradient PGF = PG large + PG depth + PG slope TR-4 starts at 17:24 LST originating at 240 m above sea level.

17. Summary and Conclusion  The strong winds on the left-rear side of the storm occurred over the Sagami Bay, where is the exit of the channel-like cold flow at low level formed by the Mountain range and the typhoon center.  The strong winds are not only supported by the large-scale circulation, but also locally generated mesoscale forcing due to the variation in cold-layer thickness. The dynamics and structure of the strong winds can be identified as those of “gap flow”.

18. Comparison between simulation results and observation at Hiratsuka Hiratsuka observatory （ 139.346E ， 35.306N ） Model Result （ 139.352E ， 35.282N ）

19. Sensitivity experiment ① ; Orographic effect Removing eastern part of the Kanto mountains Model topography Control Run Wind speed at 250 m height ( 17:24 JST ) SLP 963.6hPa SLP 964.7hPa The strong winds on the left-rear side decreased. (This experiment means that PG depth and PG slope does not work.) Sensitivity experiment

20. ② ; Effect of low-level cold air The strong winds on the left-rear side decreased, and wind direction changed. (This experiment means that PG depth, PG slope, PG large does not work.) Changing the surface temperature to 26 ℃ PT Wind speed 963.0 hPa 963.6 hPa Control Run

21. Vertical cross-section of wind speed and PT along the flow The cold flow becomes shallower and accelerated to more than 60 m/s descending downward.