Resolution Impact on Rapid Intensification and Structure Change 1 of Super Typhoon Hagibis (2019)

Typhoon Hagibis was a large and intense tropical cyclone that had significant societal impacts in Japan. It went through a period of explosive rapid intensification (RI), with an increase of maximum wind speed from 60 kt to 160 kt in 24 h, immediately followed by a secondary eyewall formation (SEF) and an eyewall replacement cycle (ERC). Operational forecasts from COAMPS-TC (Coupled Ocean/Atmosphere Mesoscale Prediction System – Tropical Cyclone) failed to capture Hagibis’ explosive RI, peak intensity, and the associated inner- core structural evolution. Four COAMPS-TC sensitivity experiments, initialized at 1200 UTC 5 Oct. 2019, were conducted to study the impact of horizontal resolution on prediction of Typhoon 31 Hagibis’ RI and structure. Results indicate that rapid intensification of the storm to Category 4 32 intensity can be simulated with the finest grid spacing at 4-km, but use of 1.33-km for the finest 33 grid spacing facilitates more realistic prediction of the explosive intensification rate, Category 5 34 peak intensity, and small inner core accompanying the RI. Our sensitivity experiments indicate that realistic simulation of Hagibis’ SEF/ERC requires a very intense storm with a small inner core 36 as a prerequisite for its occurrence; therefore the finest grid spacing at 1.33-km is a necessary but 37 not sufficient to capture the SEF/ERC. The simulation of the RI and SEF/ERC is also sensitive to 38 the resolution of the outermost grid, which has impacts on the storm’s moisture distribution by 39 modulating the flow of moist air from the deep tropics into the TC. While these results have implications for the grid configuration of operational models like COAMPS-TC, additional work 41 is needed to gain systematic understanding of the physical processes associated with simulation of 42 explosive RI and SEF/ERC.

Forecast System (HWRF) regional dynamical model did the best in terms of intensifying the storm 77 at early lead times, but still only reached a peak intensity of 120 kt. The CTCX regional dynamical  Fig. 1 clearly show a sharp increase in intensity during the explosive RI, followed 83 by a sudden decrease in intensity that accompanied the ERC, and subsequent re-intensification. 84 Intensification of TCs is challenging to predict, and RI is even more difficult to capture 85 due to its sudden onset and rapid evolution. Various dynamic and thermodynamic processes are 86 believed to play important roles in TC intensification. Emanuel (1986Emanuel ( , 1994Emanuel ( , 2003 proposed the preserving Rossby wave energy in the TC core region and fine enough grid spacing (≤ 3 km) to 95 resolve convection.

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A secondary (concentric) eyewall, often identified as a secondary convective ring with a 97 secondary tangential wind maximum outside the primary inner eyewall, is one of the important 98 characteristics in intense TCs (Wang et. al 2016). Despite various hypotheses that attempt to 99 explain secondary eyewall formation (SEF), it remains elusive why hurricanes develop secondary 100 eyewalls and ERC. Montgomery and Kallenbach (1997) suggested that vortex Rossby waves may 101 contribute to SEF. Zhu et al. (2004) showed that an outer spiral rainband becomes a concentric 102 secondary eyewall as Hurricane Bonnie (1998) moved from a high-to a weak-sheared environment.  Most of those SEFs studied occurred as/after the TC reached peak intensity. (COAMPS-TC ®2 ), using grid spacing as small as 1.33 km in the region containing the TC inner 120 core. We also performed COAMPS-TC forecast experiments in which the changed the outer grid 121 spacing from 36 km to 12 km, in order to better resolve the environmental flow around the storm.

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Our overall goal was to accurately simulate the time-evolution of Hagibis, starting at the tropical 123 depression stage, continuing through the RI interval, and ending after the completion of the ERC.

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The objectives of this study are to (i) examine the impacts of horizontal grid spacing on track and 125 intensity forecasts for Typhoon Hagibis; (ii) assess the roles of the finest-resolution moving-nested 126 grid and the fixed outer coarse mesh on the storm's intensification and inner-core structure 127 changes; (iii) evaluate Hagibis's predicted structure during the RI period and ERC w.r.t. the  (Table 1).

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All the experiments used a fixed large outer grid (with the same domain, 8640 x 6480 km) and at 143 least one storm-following moving nested grid. The experiment Q36km3 used the operational grid 144 configuration, consisting of a fixed outer grid at 36 km grid spacing and two storm-following 145 nested grids at 12 km and 4 km grid spacing. Experiment Q36km4 was configured like Q36km3, 146 except it used an additional storm-following nested grid at 1.33 km grid spacing. As shown in 147 Table 1, the addition of the 1.33 km nested grid in Q36km4 is quite expensive, with a 148 computational cost for the Q36km4 run that is 6.5 times that of Q36km3. Experiment Q12km2 149 utilized a fixed 12 km outer grid and a storm-following 4 km nested grid. Replacing the 36 km 150 outer grid with a 12 km outer grid results in a computational cost for Q12km2 that is 3.6 times that     (Fig. 2c). The storm developed in a moist environment, with the 850 hPa relative humidity over 188 90% within the inner core of the storm (Fig. 2d).  only extend to a radius of 25 km; such winds extend between 2 and 3 times as far in Q36km3. 269 These results indicate that the forecast inner core structure is sensitive to model grid spacing in the 270 inner-core region, as we anticipated would be the case. However the inner core structure also 271 appears to be sensitive to the grid spacing of the outer mesh, given that Q36km4 and Q12km3 272 differ only in grid spacing in that part of the model domain.     It is important to note the nature of the radial profile of the low-level tangential winds at 361 60 h and 72 h in Fig. 9. Beyond 40 km radius, Q12km3 shows a much more gradual decrease in 362 tangential wind speed with radius than Q36km3 and Q36km4. A broad area of relatively constant 363 10-m winds located outside the inner core in the Q12km3 experiment at 72 h can also be seen in 364 Fig. 6d, differing markedly from the 72 h wind fields from Q36km3 (Fig. 6a) and Q36km4 (Fig.   365   6b). This broadening of the wind field outside the inner core seen in Q12km3 is a precursor to 366 SEF, following the sequence described by Huang et al. (2012). The state of the radial profile of 367 the tangential winds at the end of the RI period appears to be a key factor governing which 368 COAMPS-TC simulations undergo SEF and which do not.

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Another noteworthy aspect of the simulations represented in Fig. 9 is the depth and

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To further examine the time-evolution of the azimuthal mean tangential wind field (at 1 393 km altitude) and diabatic heating / ascent (at 6.5 km altitude), radius-time plots are shown for 394 experiments Q36km3 (Fig. 11a), Q36km4 (Fig. 11b), and Q12km3 (Fig. 11c). In Q36km3, there   Hagibis is still relatively weak at 12-h lead time in the forecast, with MWS at 49 kt and MSLP at 461 988 hPa. Convective bands are primarily found in the south and southwestern quadrants (Fig. 14a).

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From 24 to 36 h lead time (Figs. 14b,c), an eyewall forms as the inner core becomes better 463 organized and Hagibis rapidly intensifies from Category 1 to 3, with a drop in MSLP from 975 to 464 955 hPa. During this time, the outer convective bands in the simulation are mostly in the 465 southwest quadrant, similar to the convective distribution shown in the satellite imagery (Fig. 5b,c).

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From 36 to 48 h, Hagibis continues to rapidly intensify in the Q12km3 experiment, attaining an 467 MSLP of 918 hPa and MWS of 131 kt at 48h (Fig. 14d). At 48 h, the simulation shows a small-

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By the 72 h lead time (Fig. 14f), Hagibis starts to weaken with an MSLP 900 hPa and MWS 472 of 136 kt, and the formative secondary eyewall is apparent in the simulated radar reflectivity, 473 which is similar to the satellite observation shown in Fig. 5f. The inner eyewall weakens as the 474 outer eyewall contracts to a smaller radius at 84 h (Fig. 14g), which is similar to Fig. 5g. Hagibis 475 continues to weaken in the simulation with an MSLP of 915 hPa and MWS of 99 kt and its inner 476 eyewall starts to dissipate at 96 h (Fig. 14h), which is similar to Fig. 5h. The forecast storm 477 weakens further and has an MSLP of 929 hPa and MWS of 99 kt at 108 h, and its inner eyewall 478 dissipates almost completely (Fig. 14i).

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The storm structure variations seen from the simulated radar reflectivity from experiment intensity. The storm then went through an ERC that resulted in a slightly weaker storm (145 k, 75 491 m s -1 ), but with a larger inner core (15 n mi, 28 km RMW). It is very challenging to simulate this 492 type of storm evolution (RI followed by an ERC) with a regional dynamical tropical cyclone  In summary, we use the Hagibis case study to gain a better understanding of the relationship 541 of RI and SEF/ERC to horizontal grid spacing. We found that the storm can develop to Category 542 4 with the finest grid spacing at 4-km, though with a much slower intensification rate than observed 543 and an inner core that is too big. That means that simulation at 4-km grid spacing is not sufficient 544 to resolve the small inner core at a horizontal scale of ~10 km. The SEF/ERC occurs only when 545 the inner core is quite small, which is only possible with the grid spacing at 1.33 km. Therefore 546 the 4-km grid spacing is capable of producing an RI, but it is not enough for the subsequent ERC.

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The 1.33 km grid spacing is a necessary condition to resolve a small inner core to set the stage for 548 SEF/ERC, but it is not sufficient condition for happening of SEF/ERC.    LGEM are statistical intensity forecast models, and the ICNW consensus is the average intensity forecast 680 considering a set of models.