**4. MCSs analysis**

penetrated into the circulation and went northeastward into the westerlies with doubled

Typhoon Dan (1999) first developed over the Philippine Sea at 1200 UTC on October 1, 1999 to the east of Island Luzon. The Joint Typhoon Warning Center (JTWC) issued a TC formation alert at 0230 UTC on October 2. When deep convection was seen to build over the low-level circulation center from the south near 1500 UTC on October 2, the first warning of the TC was issued by the JTWC. The system further developed into a tropical depression about 1140 km east–northeast of Manila on October 3 (taken as formation time at 1200 UTC on October 3, 1999) and then moved westward (**Figure 2**). Dan intensified very fast to a tropical storm and

**Figure 1.** Best track of Typhoon Ketsana (2003) from the JTWC (adapted from Ref. [6]).

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

**Figure 2.** Best track of Typhoon Dan (1999) from the JTWC (adapted from Ref. [7]).

**3.2. Synopsis of Typhoon Dan (1999)**

forward speed.

One of the characteristics of TC formation in the monsoon trough is frequent development of MCSs due to low-level convergence that enhances convection. The definition of MCS in Ref. [8] (i.e., area >1000 km2 within the brightness isotherm −52°C) is applied. During the 48 h (1200 UTC, October 16–18) prior to Typhoon Ketsana's formation, five MCSs are observed. The first two developed on October 16 and early October 17, respectively. Later, MCS3 and MCS4 developed almost simultaneously near 1500 UTC on October 17, but then dissipated (**Fig‐ ure 3**). The fifth MCS5 developed at 0600 UTC on October 18 near the low-level circulation center, and led to the formation of the typhoon 6 h later.

**Figure 3.** Six-hourly IR1 satellite images from 1200 UTC on October 17, 2003 to 1800 UTC on October 18, 2003. The contour is TB of −75°C, and the black dot is the best-track location of Ketsana's formation (adapted from Ref. [10]).

**Figure 4** shows the development process of the formation of Typhoon Dan. Since October 1, 1999, there were many weak tropical convective clusters formed and maintained. At 1200 UTC on October 2, one of the cloud clusters started to develop and kept expanding to form a MCS at 0000 UTC on October 3 and then further developed to a tropical depression at 1200 UTC the same day. In contrast to Typhoon Ketsana, there was only one MCS that appeared during the formation process of Dan. While Lee et al. [1] identified that many of the TC cases with single MCS during formation were associated with easterly wave, filtered low-level winds (based on a simple running mean technique with similar low-pass effect of 3–8 days as in Ref. [9]) do not reveal wave activity during the formation of Typhoon Dan. Thus, Typhoon Dan is also classified as a typical monsoon trough formation. The focus here is the single MCS configuration associated with Typhoon Dan's formation in contrast to that of Typhoon Ketsana, with both cases embedded in similar environmental setting.

**Figure 4.** MCSs involved in the formation process of Typhoon Dan at (a) 0500 UTC on October 1, (b) 1200 UTC on October 2, (c) 0000 UTC on October 3, and (d) 1200 UTC on October 3, 1999. Raw pixel values have been shown. (Source: JMA GMS-5 infrared channel-1 data).

#### **5. Model validation for Typhoon Dan**

#### **5.1. Synoptic flow**

When comparing the NCEP FNL operational analysis data with the WRF simulation of Typhoon Dan in the large domain, it can be seen that the large-scale circulation has been simulated well. In the FNL analysis, there are two cyclonic regions at about 130°E and 140°E (**Figure 5**). The WRF model mainly developed the incipient vortex circulation at 130°E that eventually became Typhoon Dan. Such consistency with the analysis at the low and mid (not shown) levels provides the conditions for the right timing of TC formation in the model. At the upper level, the simulated subtropical high is located north of where Typhoon Dan is developing, with the maximum high pressure center east of the Taiwan island. This is well verified by the FNL analysis (**Figure 6**), and the system provides good outflow for the formation of Typhoon Dan during its development. The simulated formation position (based on identified near-surface circulation center) and early westward motion of Typhoon Dan match with the best track very well (**Figure 7**), with the formation only a small distance east of the actual position.

**Figure 4** shows the development process of the formation of Typhoon Dan. Since October 1, 1999, there were many weak tropical convective clusters formed and maintained. At 1200 UTC on October 2, one of the cloud clusters started to develop and kept expanding to form a MCS at 0000 UTC on October 3 and then further developed to a tropical depression at 1200 UTC the same day. In contrast to Typhoon Ketsana, there was only one MCS that appeared during the formation process of Dan. While Lee et al. [1] identified that many of the TC cases with single MCS during formation were associated with easterly wave, filtered low-level winds (based on a simple running mean technique with similar low-pass effect of 3–8 days as in Ref. [9]) do not reveal wave activity during the formation of Typhoon Dan. Thus, Typhoon Dan is also classified as a typical monsoon trough formation. The focus here is the single MCS configuration associated with Typhoon Dan's formation in contrast to that of Typhoon Ketsana, with

**Figure 4.** MCSs involved in the formation process of Typhoon Dan at (a) 0500 UTC on October 1, (b) 1200 UTC on October 2, (c) 0000 UTC on October 3, and (d) 1200 UTC on October 3, 1999. Raw pixel values have been shown.

When comparing the NCEP FNL operational analysis data with the WRF simulation of Typhoon Dan in the large domain, it can be seen that the large-scale circulation has been

both cases embedded in similar environmental setting.

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

(Source: JMA GMS-5 infrared channel-1 data).

**5.1. Synoptic flow**

**5. Model validation for Typhoon Dan**

**Figure 5.** Simulated (left) and FNL analyses (right) of 850 hPa geopotential height (m) and wind barbs at 2000 UTC on October 3, 1999.

**Figure 6.** Simulated (left) and FNL analyses (right) of 300 hPa geopotential height (m) and wind barbs at 2000 UTC on October 3, 1999.

**Figure 7.** Simulated (red) and JMA best track (blue) of Typhoon Dan (upper) and Ketsana (lower) during 0000 UTC, October 3–6, 1999 and 1200 UTC, October 17–20, 2003 (best track has been extended after October 20, 2003 for Ketsana).

The model validation for Typhoon Ketsana has been presented in detail in Ref. [10], and thus not repeated here. The WRF model applied here is the same version as that used in Ref. [10]. The simulation of Typhoon Ketsana in this study well reproduced the reversed oriented monsoon trough in October 2003 as well as the convection episodes of all MCSs associated with Typhoon Ketsana's formation. The simulated formation position of Ketsana is southwest of the best-track position (**Figure 7**); however, the weak steering flow during the early development has been well simulated.

#### **5.2. Evolution of MCS and intensity**

In Ref. [8], the area-average observed TB from satellite images shows a minimum at around 2100 UTC on October 17, 2003 that is associated with MCS3 and MCS4, and then there is another major decrease before formation associated with MCS5 (**Figure 5b**of Ref. [10]). Simulated radar reflectivity is used to examine convection activity in the model. The time series of simulated radar reflectivity has a local maximum at the same time of occurrence of MCS3 and MCS4. It then increases rapidly 6 h before Ketsana's formation, which is due to convective bursts within MCS5 and is consistent with the observed variation of the area-average TB. The simulated positions of these MCSs also match with those in satellite images: MCS3 was east of the lowlevel circulation center, and later MCS5 developed at a similar position (**Figure 8**). In terms of intensity, it can be seen that the simulated storm intensifies at the similar rate as observation before formation, however, becoming too intense in the rest of the simulation (**Figure 9**).

**Figure 8.** Simulated radar reflectivity (dbZ) of Typhoon Ketsana at (a) 1500 UTC on October 17 and (b) 0600 UTC on October 18, 1999.

**Figure 7.** Simulated (red) and JMA best track (blue) of Typhoon Dan (upper) and Ketsana (lower) during 0000 UTC, October 3–6, 1999 and 1200 UTC, October 17–20, 2003 (best track has been extended after October 20, 2003 for Ketsana).

The model validation for Typhoon Ketsana has been presented in detail in Ref. [10], and thus not repeated here. The WRF model applied here is the same version as that used in Ref. [10]. The simulation of Typhoon Ketsana in this study well reproduced the reversed oriented monsoon trough in October 2003 as well as the convection episodes of all MCSs associated with Typhoon Ketsana's formation. The simulated formation position of Ketsana is southwest of the best-track position (**Figure 7**); however, the weak steering flow during the early

In Ref. [8], the area-average observed TB from satellite images shows a minimum at around 2100 UTC on October 17, 2003 that is associated with MCS3 and MCS4, and then there is another major decrease before formation associated with MCS5 (**Figure 5b**of Ref. [10]). Simulated radar

development has been well simulated.

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

**5.2. Evolution of MCS and intensity**

**Figure 9.** Simulated (red) and JMA best-track minimum surface pressure (hPa) of Typhoon Ketsana during 0000 UTC, October 17–20, 2003.

**Figure 10.** Simulated mean sea-level pressure, 850-hPa wind barbs, and maximum radar reflectivity at 0300 UTC on October 2 (a) and 0700 UTC on October 2, 1999 (b).

The simulated MCS activities of Typhoon Dan are also examined via the simulated radar reflectivity. In early October 2, 1999, the model simulated patches of convection on the eastern side of the broad cyclonic circulation within the monsoon trough (**Figure 10**). A few hours later, the convection organized into a MCS northeast of the circulation center. Such convection persisted when the incipient vortex moved northwestward and intensified. At the beginning of formation, there were still some weak convective cloud clusters that developed slowly until the only MCS formed. In the early formation stage, the model has not captured the initial rate of intensification very well. In the simulation, the surface pressure only started to drop from about 0000 UTC on October 3 (**Figure 11**). Nevertheless, the convection pattern that developed from the single MCS has been reproduced well in the model. From October 3, the simulated intensification rate was similar to that in the best track, and by the end of simulation the TC was more intense than that Typhoon Dan actually attained.

**Figure 11.** Six-hourly time series of simulated (red) MSLP (hPa) and that in JMA best track (blue) from 1200 UTC on October 2, 1999.
