**6. Mesoscale heating and early development in Typhoon Ketsana**

#### **6.1. Axisymmetric structure**

**Figure 10.** Simulated mean sea-level pressure, 850-hPa wind barbs, and maximum radar reflectivity at 0300 UTC on

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

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

October 2 (a) and 0700 UTC on October 2, 1999 (b).

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

October 2, 1999.

was more intense than that Typhoon Dan actually attained.

It can be seen from the synopsis and model validation of Typhoon Ketsana that the MCS developments during its formation were highly spatially asymmetric with respect to the lowlevel circulation center. These MCSs would impose vorticity enhancement and heating effect to the system-scale vortex. Before such responses are analyzed, the axisymmetric structure in the simulation is first examined through the azimuthal mean of tangential and radial wind. The system center is taken as the surface circulation center.

On October 17, the axisymmetric structure of the simulated Typhoon Ketsana shows a broad low-level cyclonic circulation (**Figure 12a**). The maximum tangential wind is below 850 hPa and located between 150 and 200 km from the center. This broad circulation is consistent with the early MCS activities during that day when the first four MCSs developed more than 100 km away from the center. There was only a small region of inflow below 950 hPa, and thus the secondary circulation has not been set up in this stage.

**Figure 12.** Azimuthal average of tangential (shaded) and radial (contour) wind (m/s) at (a) 1500 UTC on October 17, (b) 0800 UTC on October 18, (c) 1000 UTC on October 18, and (d) 0600 UTC on October 19.

On October 18, when MCS5 developed near the system center, strong low-level tangential winds started to move inward within 50 km, and at the same time extended to the midlevels (**Figure 12b, c**). The eyewall structure has been established just before the formation time of 1200 UTC on October 18. The radial winds also reveal the secondary circulation of inflow below about 800 hPa and outflow above 150 hPa.

Less than a day after the formation time, a mature axisymmetric structure is observed in the simulation. The most intense tangential winds concentrated at about 850 hPa and have been extended from 100 to 200 km from the center (**Figure 12d**). The eyewall structure is clear, and the eye has been enlarged.

#### **6.2. Diabatic heating associated with the MCSs**

It is well known that convective-type and stratiform-type rain, besides their respectively unique microphysical nature, are associated with different diabatic heating profiles [11, 12]. The latter is important to the process of vorticity enhancement and warm core during TC development. Convection is associated with deep tropospheric heating, while stratiform rain is associated with upper-level heating but possibly low-level cooling due to evaporation.

**Figure 13.** Convective (red) and stratiform (green) rainfall in simulated Typhoon Ketsana at (a) 1200 UTC on October 17, (b) 1500 UTC on October 17, (c) 0800 UTC on October 18, and (d) 1200 UTC on October 18, 2003.

The method in Refs. [13] and [14] is used to separate the two types of rain in Typhoon Ketsana based on the simulated radar reflectivity. The identification of the convective rainfall has further confirmed the MCS activities during the typhoon's development. On October 17, 2003, among the larger stratiform rain patches, the convective rain concentrated first to the west of the low-level circulation center (which was near 130°E, 14°N), and a few hours later to the north and northeast (**Figure 13a, b**). This is consistent with the temporal evolution of the deep convection from MCS3 to MCS4.

1200 UTC on October 18. The radial winds also reveal the secondary circulation of inflow below

Less than a day after the formation time, a mature axisymmetric structure is observed in the simulation. The most intense tangential winds concentrated at about 850 hPa and have been extended from 100 to 200 km from the center (**Figure 12d**). The eyewall structure is clear, and

It is well known that convective-type and stratiform-type rain, besides their respectively unique microphysical nature, are associated with different diabatic heating profiles [11, 12]. The latter is important to the process of vorticity enhancement and warm core during TC development. Convection is associated with deep tropospheric heating, while stratiform rain is associated with upper-level heating but possibly low-level cooling due to evaporation.

**Figure 13.** Convective (red) and stratiform (green) rainfall in simulated Typhoon Ketsana at (a) 1200 UTC on October

The method in Refs. [13] and [14] is used to separate the two types of rain in Typhoon Ketsana based on the simulated radar reflectivity. The identification of the convective rainfall has further confirmed the MCS activities during the typhoon's development. On October 17, 2003,

17, (b) 1500 UTC on October 17, (c) 0800 UTC on October 18, and (d) 1200 UTC on October 18, 2003.

about 800 hPa and outflow above 150 hPa.

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

**6.2. Diabatic heating associated with the MCSs**

the eye has been enlarged.

In early October 18, 2003, deep convection occurred again but much closer to the low-level circulation center, which is associated with MCS5. The deep convection persisted until the formation of the cyclone (**Figure 13c, d**). On the other hand, the stratiform rain areas are much larger than the convective areas at all times, which is similar to the partition of other TC cases based on the observed rainfall [15].

When the simulated diabatic heating in WRF is examined, the maximum heating is identified to locate around the midlevel, with extension from about 850 hPa up to 200 hPa. The spatial distribution of maximum heating is confined to specific locations and thus attributed to the deep convection episodes. For example, the azimuthal average of diabatic heating during October 17 shows tropospheric heating positions mostly outside the radius of 50 km, which is likely due to the early MCS activities (**Figure 14a, b**). The maximum heating of these early episodes is of the order of 10−3 K/s.

**Figure 14.** Azimuthal average diabatic heating in simulated Typhoon Ketsana at (a) 1600 UTC on October 17, (b) 2300 UTC on October 17, (c) 0500 UTC on October 18, and (d) 0800 UTC on October 18, 2003. Note the changes in scales.

In early October 18, it can be seen that maximum heating occurs within 50 km that is associated with the last convection episode before the formation of Ketsana (**Figure 14c**). The heating location further moves inward to the circulation center and increases in magnitude about an order higher than the previous episodes (**Figure 14d**).

#### **6.3. Responses to diabatic heating**

Reference [5] analyzed the responses of a weak TC vortex to diabatic heating based on the analytical solution of the balanced vortex model, which leads to rapid development toward the steady state with a mature warm-core thermal structure. It was found that the responses depend on the radial location of the diabatic heating. In particular, three factors of static stability, baroclinity, and inertial stability are involved in critical formulating of the responses, especially the inertial stability that usually varies substantially with the radius in a TC vortex. Diabatic heating would be most effective when it is located within the high inertial stability region, which is inside the radius of maximum wind. When the heating position is outside the radius of maximum wind, rapid development is not likely. Thus, based on these theoretical results, the convection episodes associated with the MCSs in the case of Typhoon Ketsana are increasing in heating efficiency for forming the TC. The diabatic heating in the earlier MCSs is outside the inner core with high inertial stability, which is mostly inside radius of 50 km (**Figure 15a**). This is consistent with the simulated TC that shows only slowly intensifying lowlevel winds in the outer region. During early October 18, when the TC incipient vortex is becoming more intense, the high inertial stability region expands slightly (**Figure 15b**). When the last convection episode associated with MCS5 occurs, the heating sits mostly in that region and is thus able to spin-up the inner-core winds rapidly (**Figure 12c**) and lead to TC formation.

**Figure 15.** Inertial stability in the simulated Typhoon Ketsana at (a) 0600 UTC on October 17 and (b) 0800 UTC on October 18, 2003, which correspond to **Figure 14a, d**.

Another way to examine the effects of diabatic heating, especially that with azimuthally asymmetric distribution and thus possessing eddy heat flux, is through the EP fluxes [16–20]. The EP fluxes on isentropic coordinates are calculated and analyzed for the simulated Typhoon Ketsana. The EP flux vector consists of the horizontal and vertical components, which correspond to angular momentum flux and heat flux, respectively. The divergence of the flux vector indicates the flux gradients.

In early October 18, it can be seen that maximum heating occurs within 50 km that is associated with the last convection episode before the formation of Ketsana (**Figure 14c**). The heating location further moves inward to the circulation center and increases in magnitude about an

Reference [5] analyzed the responses of a weak TC vortex to diabatic heating based on the analytical solution of the balanced vortex model, which leads to rapid development toward the steady state with a mature warm-core thermal structure. It was found that the responses depend on the radial location of the diabatic heating. In particular, three factors of static stability, baroclinity, and inertial stability are involved in critical formulating of the responses, especially the inertial stability that usually varies substantially with the radius in a TC vortex. Diabatic heating would be most effective when it is located within the high inertial stability region, which is inside the radius of maximum wind. When the heating position is outside the radius of maximum wind, rapid development is not likely. Thus, based on these theoretical results, the convection episodes associated with the MCSs in the case of Typhoon Ketsana are increasing in heating efficiency for forming the TC. The diabatic heating in the earlier MCSs is outside the inner core with high inertial stability, which is mostly inside radius of 50 km (**Figure 15a**). This is consistent with the simulated TC that shows only slowly intensifying lowlevel winds in the outer region. During early October 18, when the TC incipient vortex is becoming more intense, the high inertial stability region expands slightly (**Figure 15b**). When the last convection episode associated with MCS5 occurs, the heating sits mostly in that region and is thus able to spin-up the inner-core winds rapidly (**Figure 12c**) and lead to TC formation.

**Figure 15.** Inertial stability in the simulated Typhoon Ketsana at (a) 0600 UTC on October 17 and (b) 0800 UTC on

Another way to examine the effects of diabatic heating, especially that with azimuthally asymmetric distribution and thus possessing eddy heat flux, is through the EP fluxes [16–20].

October 18, 2003, which correspond to **Figure 14a, d**.

order higher than the previous episodes (**Figure 14d**).

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

**6.3. Responses to diabatic heating**

During October 17, it can be seen that the EP fluxes with large divergences are mostly scattered at the large radii outside 200 km and concentrated at the lower levels (**Figure 16a**), which are associated with the heating of the earlier MCS convection episodes. The magnitudes of the low to midlevel flux vectors are larger outside 200 km compared with the inner core. The outward directions of these flux vectors indicate inward transport of angular momentum fluxes. There are also small upward components of these flux vectors, indicating inward heat fluxes.

**Figure 16.** Divergence (contour) of the EP flux vector (arrow) in simulated Typhoon Ketsana at (a) 1200 UTC on October 17 and (b) 1800 UTC on October 18, 2003.

The locations of the largest EP flux divergence remain in the outer region throughout October 17, and then move inward during early October 18. Sometimes, the EP flux vector directions show outward transport of angular momentum in the outer low to midlevel region, which is likely due to spinning up of the outer winds. Right after the formation of the typhoon, the EP flux divergence near the inner core is still strong (**Figure 16b**). Outside radius of 100 km strong magnitudes of outward EP flux vectors indicated inward angular momentum transport again when the core vortex of the typhoon develops rapidly.
