Analysis of the Factors that Led to Uncertainty of 1 Track Forecast of Typhoon Krosa (2019) by 101-2 Member Ensemble Forecast Experiments Using 3 NICAM

Abstract

Krosa would go through to the east of Japan without making landfall (Fig 1i).

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In the cases when two or more TCs closely coexist, they interact with each 115 other (a.k.a. "Fujiwhara effect"; Fujiwhara 1921Fujiwhara , 1923. Brand (1970) showed 116 that the interaction characteristics depend on the separation distance between 117 such TCs; rotating cyclonically within each other when the separation distance 118 is less than 750 NM (~1390 km) and attracting each other when the separation 119 distance is less than 400 NM (~740 km). Peng and Reynolds (2005) showed 120 that an interaction can occur even when the separation distance is 1861 km.    The best track data of the Regional Specialized Meteorological Center (RSMC;  Table 1), which were taken from the International Grand Global  for JMA data, the other center's data was regridded to the same resolution, 1.25 182 Table  1 8 × 1.25 deg. Where cov is the covariance between two arguments and σ is the standard 204 deviation. In the best-worst comparison, the best 20% and worst 20% ensemble    To examine the mechanism behind this forecast sensitivity, the best 20% and 241 worst 20% members (20 members for both) in terms of Krosa's track forecast 242 errors at 1200 UTC 11 August were selected and compared with each other.

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The best (worst) members had a forecast track error of less than 600 km 244 (more than 1700 km) at 1200 UTC 11 August.   are also shown. At FT = 1 h (Fig. 7a), the low SLP area associated with Krosa 303 for the worst members located slightly to the west of that of the best members.

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Corresponding to this westward shift, the existence frequency of Krosa's center in the east (west) of 142°E was less (more) with the worst members than with 306 the best members, whereas the frequency was almost the same in the south-307 north direction for both members. It is worth noting that simulated Krosa's 308 centers are biased to the northwest of the best track even in the best members.  The 680-m contour of Z925 was separated in the composite of the best member 327 (Fig. 8a); however, it was connected to Lekima in the composite of the worst 328 members (Fig. 8b). The differences in Z925 (Fig. 8c) indicate that there was an 329 anticyclonic anomaly north of Krosa's center with the best members in in the eastern semicircle (Fig. 8g). The lagged correlation analysis (Fig. 8h) 338 showed the consistent results; the 10-m wind speed is higher (lower) when the 339 track forecast error at 1200 UTC 11 August is larger (smaller). These results 340 indicate that the Krosa in the worst members had a larger circulation than that in 341 the best members.

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Although apparent differences in the vortex structure of Krosa could be found 343 between the best and worst members, a very little difference in Lekima's 344 structure was found (Fig.9). The anomaly of 10 degrees east of Lekima's center 345 can be related to the differences in Krosa's position and size. The Z925 was 346 higher in the best members (Fig. 9c). This result is consistent with the lagged 347 correlation analysis (Fig. 9d). The confluence region of Krosa's northerly and 348 Lekima's southerly was shifted to the west more with the worst members than 349 with the best members. The lagged correlation analysis indicates this feature by 350 the dipole structure east of the TC center (Fig. 9h), whereas the difference 351 between the best and worst members showed negative anomaly only there 352 (Fig. 9g).   Krosa's center is lower (Fig. 10a) and 10-m wind speed of the outer region, 356 especially from east to south, is larger (Fig. 10b)  Krosa's size is larger in the members with larger track error. The negative 359 correlation for SLP (Fig. 10c)     However, analyzing the mechanism in detail using operational model data is 414 beyond the aim of this paper, as the archived data resolution is much coarser 415 than the actual model resolution except for ECMWF (Table 1). Figure 15 shows 416 the spaghetti diagram of Z500 for the best and worst members for each    The difference in the composite fields between the best and worst members 448 indicates that Krosa had a larger vortex size with the worst members than with 449 the best members at the initial conditions. However, little differences were found