**7. CAPE and the rate of TC formation**

While both typhoons Dan (1999) and Ketsana (2003) formed in similar location of the WNP and the same month of year, their rates of formation were very different. Typhoon Dan developed from a disturbance to tropical depression in about 1 day; however, Typhoon Ketsana took 2 days to form from the initial MCS convection. What factors lead to these different rates of TC development? In this section, this question is examined through the CAPE consumption and recovery point of view.

**Figure 17.** Simulated surface-based CAPE for Typhoon Dan at (a) 0300 UTC on October 2, (b) 1200 UTC on October 2, (c) 1500 UTC on October 2, (d) 0000 UTC on October 3 (e) 0600 UTC on October 3, and (f)1200 UTC on October 3, 1999.

When Typhoon Dan started to develop on October 2, 1999, the CAPE in the region was quite low (**Figure 17a**). The only source of high CAPE was from the southwest of the incipient circulation center. This region of high CAPE persisted throughout the day of October 2, 1999 to support the convection development of the MCS (**Figure 17b**). After the convection further developed near the circulation center, the CAPE was lowered in the region (**Figure 17c**). When the disturbance attained near-tropical low intensity at 0000 UTC on October 3, the low-level circulation center migrated to the northwest into a region still of quite low CAPE (**Fig‐ ure 17d**). In other words, throughout the early development of Typhoon Dan, the CAPE with high value in that area was enough to support the development of one MCS, which was close to the low-level circulation center to establish the surface winds effectively. Thus, multiple MCSs development before formation has not been observed in this case. However, the CAPE recovered rapidly after the formation of Typhoon Dan (**Figure 17e, f**), which was good to support further intensification of the storm. This can also be identified in the area-average CAPE in **Figure 18** (the control experiment), which shows the CAPE consumption from 1200 UTC on October 2 to 0000 UTC on October 3, and then the rapid increase thereafter.

**7. CAPE and the rate of TC formation**

126 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

and recovery point of view.

While both typhoons Dan (1999) and Ketsana (2003) formed in similar location of the WNP and the same month of year, their rates of formation were very different. Typhoon Dan developed from a disturbance to tropical depression in about 1 day; however, Typhoon Ketsana took 2 days to form from the initial MCS convection. What factors lead to these different rates of TC development? In this section, this question is examined through the CAPE consumption

**Figure 17.** Simulated surface-based CAPE for Typhoon Dan at (a) 0300 UTC on October 2, (b) 1200 UTC on October 2, (c) 1500 UTC on October 2, (d) 0000 UTC on October 3 (e) 0600 UTC on October 3, and (f)1200 UTC on October 3, 1999.

When Typhoon Dan started to develop on October 2, 1999, the CAPE in the region was quite low (**Figure 17a**). The only source of high CAPE was from the southwest of the incipient circulation center. This region of high CAPE persisted throughout the day of October 2, 1999 to support the convection development of the MCS (**Figure 17b**). After the convection further developed near the circulation center, the CAPE was lowered in the region (**Figure 17c**). When the disturbance attained near-tropical low intensity at 0000 UTC on October 3, the low-level

**Figure 18.** Area-averaged (13–17°N, 127–132°E) surface-based CAPE during formation of Typhoon Dan in the control WRF experiment (black) and sensitivity test (green).

On the other hand, the formation of Typhoon Ketsana (2003) experienced multiple cycles of CAPE consumption and recovery associated with the MCS episodes of convection. After the first two MCSs dissipated in early October 17, the CAPE on the western side of the incipient vortex was clearly lowered (**Figure 19a**). The CAPE did not recover much, and thus the two later MCSs on the same day actually developed out of only moderate values of CAPE. Their consumption of CAPE further lowered the values on the west and north sides of circulation (**Figure 19b**). At about 0600 UTC on October 18, high CAPE values entered the core region of the vortex, which supported the last (fifth) MCS to develop near the center and led to the formation of Typhoon Ketsana at 1200 UTC on October 18 (**Figure 19c, d**). The last MCS dissipated near 0000 UTC on October 19 (**Figure 19e**) that remained with low CAPE values in the core. After that the TC vortex sustained and high CAPE values reappeared associated with the eyewall convection (**Figure 19f**).

Therefore, the development of Typhoon Ketsana during the 2 days before formation was much driven by convection and thus related to the variation of CAPE in the area of development. The convection episodes associated with the MCSs spun up the winds first at the periphery and then in the core, and led to formation of the typhoon, because the four early MCSs formed and developed northwest of the core. This process was quite slow compared with the single MCS development in Typhoon Dan.

**Figure 19.** Simulated surface-based CAPE for Typhoon Ketsana at (a) 1200 UTC on October 17, (b) 0000 UTC on October 18, (c) 0600 UTC on October 18, (d) 1200 UTC on October 18, (e) 0000 UTC on October 19, and (f) 0700 UTC on October 19, 2003.

**Figure 20.** Area-averaged (13–17°N, 127–132°E) surface-based CAPE during formation of Typhoon Ketsana in the control WRF experiment (green), sensitivity test with strengthened MCS4 (black) and weakened MCS4 (yellow).

Since the circulation associated with Typhoon Ketsana's formation did not move much, the area-average CAPE time series also shows the consumption and recovery cycle on the day of 1200 UTC, October 17–18 (**Figure 20**, control experiment). In order to study the MCS activities of Ketsana, Lu et al. [10] designed three sensitivity experiments on the impacts of MCSs on the early intensification rate. The same set of experiments is applied here to analyze the CAPE consumption and recovery cycles. Base on satellite images, the MCS3, MCS4, and MCS5 regions are identified as (12.8–15.8**°**N, 127.8–130.8**°**E), (15.8–18.8**°**N, 130.8–133.8**°**E), and (128– 178**°**N, 127.8–130.8**°**E), respectively. In the sensitivity experiment 1, the area-averaged relative humidity in the MCS3 region between 200 and 700 hPa at 1200 UTC on October 17, 2003, which was higher than that around MCS4, is assimilated into the MCS4 region within the 6-h assimilation window (similarly for later experiments), in order to strengthen the intensity of MCS4. In experiment 2, MCS4 is weakened, and only 60% of the 200–700 hPa relative humidity in the control experiment is retained and then assimilated at 1200 UTC on October 17, 2003.

The sensitivity experiments in **Figure 20** show different CAPE consumption and recovery cycles of Typhoon Ketsana. When convection in the MCS4 is stronger (black line), the CAPE drops much more than in the control experiment during late October 17. However, the recovery of CAPE is quite efficient after, and by 0600 UTC on October 18, the average CAPE value is not much lower than that in the control experiment. The early intensification rate of Ketsana is actually faster than that of the control in this experiment. On the other hand, when convection in the MCS4 is weaker (yellow line), the average CAPE consumed is less initially, then builds up slightly, and goes through another cycle before the TC formation. Although the CAPE level near formation time in this experiment is similar to that in the control, the weakened MCSs lead to a slower formation and early intensification rate.

Similar experiment as in the second sensitivity experiment of Typhoon Ketsana was performed for Typhoon Dan (green line in **Figure 18**). With a weakened MCS, CAPE consumption before Dan's formation is much smaller than that in the control. Due to the weaker convection, the early intensification rate of the simulated typhoon is also slower. After that, CAPE recovery is still quite efficient in the environment, but has not increased to the same level as in the control experiment. Therefore, from the comparison of the two typhoon cases here, it can be concluded that the rate of TC formation directly depends on how much each convection episode associated with MCS contributes to spinning up the low-level vorticity, which has been pointed out in previous studies. On the other hand, it can also be seen that how CAPE recovers in the developing TC depends on the earlier convection episodes, but not only on the large-scale environment. The control by this factor on the pace of TC's early development deserves more case studies.

#### **8. Discussion and conclusions**

**Figure 19.** Simulated surface-based CAPE for Typhoon Ketsana at (a) 1200 UTC on October 17, (b) 0000 UTC on October 18, (c) 0600 UTC on October 18, (d) 1200 UTC on October 18, (e) 0000 UTC on October 19, and (f) 0700 UTC on

128 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

**Figure 20.** Area-averaged (13–17°N, 127–132°E) surface-based CAPE during formation of Typhoon Ketsana in the con-

trol WRF experiment (green), sensitivity test with strengthened MCS4 (black) and weakened MCS4 (yellow).

October 19, 2003.

It has been observed that the time for the tropical disturbances to develop into tropical storm varies with quite a large range, and is dependent on the synoptic pattern. For example, in analyzing the MCS activities during TC formations in the WNP, Lee et al. [1] found that for formations with a single MCS, some of them associated with easterly waves usually take an average of 13 h to form a TC. In contrast, for monsoon trough-related formations, there are usually more than two MCSs, and the average time to form a TC is about 1 day. Very often, more than one MCS coexist at the same time within the monsoon trough. Whereas recent studies on TC formations, such as Refs. [10, 21, 22], focused on contributions from convectivescale systems such as vortical hot towers (VHTs) and convectively induced vorticity anomalies (CVAs), their interactions with mesoscale circulations, and mechanisms to generate the core surface vortex, less discussion was on the relationship between convection patterns and development times of TCs. The numerical simulations of typhoons Dan and Ketsana here illustrate that the pace of early development of a TC depends much on the convection configuration, namely, whether it is a single MCS or interacting multiple MCSs.

It might seem to be counterintuitive that the multiple MCS convection pattern is not speeding up the TC development process due to spinning up of larger relative vorticity within more areas in the TC, but rather slowing the process down compared with the single MCS pattern. Nevertheless, this phenomenon may be explained from the point of view of competing for convective resources. Whenever deep convection occurs, the environmental CAPE is consumed. Descent of cold and dry air left by heavy precipitation is unfavorable for new convection development, which has to wait until the environmental CAPE increases again by heat and moisture fluxes from the ocean surface. When Ref. [21] analyzed the simulated formation of Hurricane Dolly (2008), similar processes were found under the TC system scale, namely, there were episodes of deep convection during the development of Hurricane Dolly, but in between there was a period when the CAPE was minimum. This interpretation is consistent with the simulation for Typhoon Ketsana, especially during the development of MCS3 and MCS4. These two MCSs developed at about the same time with a large area of convection, but did not lead to TC formation right after. The reason may be that MCS4 to the east was consuming part of the CAPE within the TC area, making the MCS3 to the west less conducive to developing more deep convection and subsequent low-level vorticity generation. In fact, MCS3 dissipated first, and the system intensification slowed down before another MCS developed in the core region.

It is interesting that this kind of CAPE argument is also applicable to the convective scales, as has been briefly discussed in Ref. [21]. Various convection processes, such as waxes and wanes, merging and splitting within each MCS are common during the early development of TC. That is, these processes represent the variability of the convective-scale VHT and CVA activities of the MCS, which is likely also due to the consumption and recovery cycles of CAPE. One point to note is that since the single-MCS convection pattern has been identified as the most common one at the end of early development, that is, for the TC to enter the intensification phase, the convection within that single MCS has to be long-lasting enough for the system to intensify to a sustainable level. In other words, there may be a hypothesis that the CAPE consumption and recovery cycles to sustain convection are more efficient under convective scales compared with mesoscales. This may be due to the fact that the areas for the surface fluxes to keep the lowlevel atmosphere warm and moist are simply much smaller, and/or that merging of the VTHs/ CVAs is effective to sustain deep convection, especially when these systems are moving toward the TC center under the system-scale convergent flow. However, these assertions have to be verified by further investigations.

In the case that merging of MCSs occurs, the resulted mesoscale vortex may lead to faster system intensification rather than the retardation as obtained in the earlier simulations [23– 25]. More numerical studies on TC cases with explicit MCS merging have to be conducted to validate such hypothesis.
