SECTION 5. FACTORS AFFECTING INTENSITY

Many of the physical processes associated with TC intensification and decay are difficult to observe and poorly understood. Nevertheless, it is possible to determine whether some of these physical processes encourage or discourage TC intensification (Appendix C).

In regard to practical approaches to intensity prediction, forecasters are generally limited to analysis of synoptic patterns. A few of the observable synoptic patterns are discussed in this section.

As viewed by satellites (and satellite cloud-tracked winds and water-vapor-tracked winds), upper-troposphere circulation patterns associated with intensification are the easiest to identify because the upper tropospheric clouds usually obscure shallower clouds. These upper-troposphere patterns can define the outflow at the top of the TC, which indicates mass removal from the center of the storm (Fig. 6.2).

In general, a mesoscale anticyclone is located directly over or near the center of the TC. This represents the location in the upper wind field where there is a buildup of mass from rising convective motion within the center of the cyclone.

On the other hand, another larger, synoptic scale anticyclone is often found that pre-existed within the vicinity of the intensifying TC. The location of this larger anticyclone can vary depending upon many environmental factors, including the lower-level forcing mechanisms that are helping to create the cyclone.

The relative location of this large-scale anticyclone with respect to the TC will help dictate the direction of outflow patterns around the cyclone. These outflow patterns can be identified from satellite infrared cloud imagery, especially when the imagery is animated. These outflow patterns can be classified into one of three basic categories depending on the number of channels:

(1). Single-channel Outflow. The single-channel outflow may be divided into two subcategories based on direction of the outflow channel. Tropical cyclones with single-channel poleward outflow pattern (Fig. 6.3) generally intensify at an average maximum rate of 15 to 20 kt/6 hr. Tropical cyclones with single-channel equatorward outflow pattern (Fig. 6.4) generally intensify at an average maximum rate of 25 to 28 kt/6 hr.

(2). Dual-channel Outflow. Tropical cyclones with a dual-channel outflow pattern (Fig. 6.5) generally intensify at an average maximum rate of 35 kt/6 hr.

(3). No Outflow Channel. Tropical cyclones with little outflow (Fig. 6.6) generally intensify at a very slow rate as they are unable to evacuate mass.

A strong upper level (250-200 hPa) cyclonic circulation to the north or northwest of a TC, namely the tropical upper tropospheric trough (TUTT or TUTT Cell), is a common occurrence during July and August in the northern Pacific. Sadler (1976) found that this type of upper-level circulation pattern is favorable for vigorous outflow to the north (Fig. 6.7a). In addition, this pattern generally occurs as the cyclone nears the western edge of the subtropical ridge where enhanced equatorward outflow is common (Fig. 6.7b). The combined effects of the northward and southward outflow often lead to rapid deepening.

There are a few other observable phenomena that can affect TC intensity:

(1). Cumulus convection. Satellite cloud imagery can show whether convection is increasing or decreasing, and whether the TC cloud patterns become more or less organized. The convective activity implies the stage of TC development. Therefore, cumulus convection should be monitored continuously by the forecaster.

(2). Sea surface temperature (SST). A SST of 26ºC is generally considered to be the minimum for TC formation. Anomalously high SST can cause more heat and moisture flux from the ocean to the atmosphere. This condition favors further development of the TC (Holliday and Thompson, 1979; Merrill,1987). Table 6.7 indicates that rapid deepening is more likely once the SST is higher than 28.5ºC. Climatological data show that the primary rapid-deepening area in the Pacific is the east Philippine Sea region (7-23ºN, 123-160ºE).

(3). Vertical wind shear. Weak vertical wind shear (e.g., the vector difference of horizontal wind vectors at the sruface and 300-hPa is less than about 15 kt and 45º angle, for a TC that is located south of the subtropical ridge) aids TC intensification while strong vertical wind shear inhibits TC intensification.

(4). Low-level circulations. Low-level cyclonic circulations are favorable regions for TC intensification. The summer monsoon trough in the western North Pacific is an area where low level cyclonic circulations are abundant. The Earth's rotation (i.e., Coriolis effect) can also contribute to cyclonic circulation.

(5). Low-level convergence. Low-level convergence zones such as the Inter-Tropical Convergence Zone (ITCZ), are suitable areas for TC intensification.

(6). Land, coast, and mountain effects. These effects can be quite complex (Merrill, 1987). In general, a TC that moves over land decays. A TC decays much faster when it passes over mountainous regions, such as Taiwan or Luzon, than it does over flat land. Also, a TC often re-intensifies when it re-enters a marine area. Consult local rules of thumb for details on specific regions. For the Philippines, see Shoemaker (1991); for Taiwan, see Brand and Blelloch (1973).

(7). Tropical cyclone transformation. A TC that enters into the mid-latitudes either decays rapidly or transforms into an extratropical cyclone. A decaying TC may still produce heavy rain, especially when it moves over mountainous areas. When a transformation from TC to extratropical cyclone occurs, forecast responsibility is transferred from the TC forecast center to another forecast office.

***** End of SECTION 5 *****

Main

Chapter 6

Section 4 Section 6

Section 5