**3. Synopses of Typhoon cases**

the formation process of TCs in the western North Pacific (WNP). Over 63% of the studied TCs have multiple MCS convection appearing at the formation process, and 35% have the single MCS appearing at the beginning of the formation process. About 90% of cases with a shorter time period of formation process (within 6 h) have single MCS [1]. The physical processes underthe single MCS and multiple MCS convection are found to include dissipation of convection leading to new eruptions of deep convection located near the edge of the dissipating convection core, ingestion of nearby convection, merging of multi-MCSs into single MCS, and merging of deep convection within the MCS associated with the aggregation

An MCS is organized as convective cloud clusters, and TC formation process is due to increasing organizition of MCS, which is caused by accumulation of mesoscale vorticity [2]. Convective activities resulted from thermodynamic and/or dynamic effects that affect the evolution of early development of TCs [3, 4]. They play critical roles in the formation process on TCs; furthermore, the local warming and diabatic heating associated with these convective activities also help to spin up the circulation and generate the warm core structure. How the pattern of MCS convection affects the rate of TC formation and early development is the focus of this chapter. In this chapter, two TC cases are studied and their synopses are first provided before their rates of development are discussed. The model simulations of the two cases are validated in terms of synoptic development and convective episodes. The convection types in the model are separated into convective and stratiform types with their respective vertical heating profiles. Then the heating associated with the MCSs and its effect on TC development are analyzed based on previous theories [5] and through Eliassen–Palm (EP) flux analysis. Then, the rates of development in the two TC cases are discussed based on the convective energy consumption point of view. In regard to this view, convection development and maintenance consume convective available potential energy (CAPE) in their local environment. It gradually makes the local environment less conducive for further convection development, which may affect the temporal evolution of TC formation when the surface vorticity is still below the threshold for tropical storm. However, it humidifies the middle and upper troposphere, and then gradually builds up the value of CAPE again in the

The numerical experiments in this study include two case studies: Typhoon (TY) Ketsana (2003) with the development of multiple MCSs and Typhoon Dan (1999) with only one MCS involved during its formation. These experiments are then examined to study the difference in the formation time and energy consumption in the two cases. In the simulations for both typhoon cases, the same set of nested domain settings is applied, and the same satellite datasets have been assimilated via the Weather Research and Forecasting Data Assimilation (WRFDA)

of vorticity in fast formation process.

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

local environment until new deep convection bursts up.

**2. Typhoon cases and numerical simulations**

system.

#### **3.1. Synopsis of Typhoon Ketsana (2003)**

In the middle of October 2003, Typhoon Ketsana initially developed from a disturbance embedded in a reversed-oriented monsoon trough between Luzon and Guam (about 1296 km east of Luzon Island) on October 15, 2003 (**Figure 1**). The monsoon trough provided a favorable large-scale environment with high humidity and abundant low-level cyclonic vorticity for TC formation. For several days, the system remained disorganized while drifting to the west–northwest due to weak steering currents, south of the subtropical ridge. The disturbance developed into a tropical depression at 1200 UTC on October 18 (taken as formation time), and on October 19, the Japan Meteorological Agency (JMA) upgraded the depression to tropical storm, and by that time the storm had begun drifting to the northeast. Throughout the days of October 20-21, movement was slow with only weak northeasterly steering currents controlling Typhoon Ketsana, although the intensification was not so slow. With favorable outflow, Ketsana quickly intensified and was upgraded into a typhoon at 1200 UTC on October 20. After an eye formed, the slow motion continued throughout the day as did intensification. By 1200 UTC on October 21, the intensity had reached the lifetime peak intensity of 125 kt. Ketsana started weakening at 1800 UTC on October 22, with the intensity falling to 115 kt.

At 0000 UTC on October 23, Typhoon Ketsana moved slowly northeastward, but a weakening trend had set in and began to accelerate the next day, with evidence of a mass of stratocumulus cloud to the northwest of Ketsana, showing the presence of colder, drier, and more stable air. At 0000 UTC on October 24, Ketsana was beginning extratropical transition. Drier air had penetrated into the circulation and went northeastward into the westerlies with doubled forward speed.

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

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

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

then a typhoon the next day. It reached a peak intensity of 110 kt (56.9 m/s) when affecting the Northern Luzon coast on October 5. TY Dan then moved over the South China Sea and weakened when it entered an increased vertical wind shear environment. It slowed down the next day and abruptly turned northward on October 7 with slight re-intensification. It eventually made a landfall near Xiamen, Fujian, China on October 9 and then weakened overland. Dan turned to the northeast and weakened to a tropical depression before it moved over the Yellow Sea later on October 10.
