Main(An abridged version from Guard, 1977)
As a follow on to the George (1975) technique, Guard (1977) developed an operational forecast technique that uses the 200 hPa wind and 200 hPa synoptic pattern analyses to determine the likelihood of recurvature. The following is an abridged version of the Guard (1977) technique which is primarily for use in the western North Pacific.
Select between one of the two seasonal synoptic regimes, winter (Fig. A.1) or summer (Fig. A.2). Then identify the recurvature or nonrecurvature pattern within the regime from the following four major categories:
1. Winter Regime Recurvature Pattern
For recurvature to occur in this synoptic pattern (Fig. A.3) an increasing belt of westerly winds must be located to the west-northwest of the TC and have a direct link with the upper-levels of the TC. If these criteria are not met, recurvature is unlikely.
2. Winter Regime Nonrecurvature Pattern
Figure A.4 depicts the typical winter regime nonrecurvature pattern. There are conditions where there appears to be a direct link between the westerlies and the TC outflow, but recurvature does not occur. One such condition is illustrated in Figure A.4. If the axis of a mid-latitude trough is stationary, and is more than 2000 km west of the TC, recurvature will not occur. This is most common in December and early January when a long wave trough is quasi-stationary over or near India. The trough in this position allows the mid-tropospheric subtropical ridge to exist, without interruption, well into Asia; TCs will move toward the west, south of the ridge, and dissipate over land.
The second condition occurs when a TC, still well south of the axis of the mid-tropospheric subtropical ridge (STR), collides with upper level westerlies. When this occurs an upper tropospheric trough is induced near the intersection of the westerlies and the TC's upper level outflow. Under such situations the mid-tropospheric STR is observed to build southward, west of the TC. This southward shift of the STR produces strong vertical wind shear beneath the induced upper level trough. The TC is then subjected to strong vertical wind shear and is reduced to a weak low-level circulation; it continues westward with the low level flow and the upper levels dissipate.
3. Summer Regime Recurvature Patterns
A synoptic study of the summer regime was conducted to identify relationships between upper tropospheric (200 hPa) flow and the movement of TCs. Figures A.5, A.6, A.7 and A.8 show typical recurvature patterns.
Figures A.5 and A.6 are characteristic synoptic patterns that result in TCs with a greater northward than westward component of movement. Under the influence of the pattern in Figure A.5 the TC would acquire a more eastward movement, and under Figure A.6 a more westward movement would be acquired. Although intense cyclonic cells within the East Asian Trough (EAT) are projected down to the middle troposphere, they frequently elude the sparse data at this level and the resulting analysis is not definitive. This is disastrous if one depends on the use of mid-tropospheric flow in steering TCs. If the EAT is relatively stationary, a TC moving toward it will eventually be subjected to northward steering currents. The northward displacement exhibited by the TC increases with the intensity of the EAT and decreases with distance from the EAT. This synoptic pattern is conducive to recurvature.
Figure A.7 illustrates another synoptic pattern which is conducive to recurvature. An anticyclone northeast of a TC can produce strong southeasterly flow. In such cases, TCs are observed to acquire a large northward component and move toward recurvature. If, however, the anticyclone to the northeast builds westward, the storm would acquire a more westward component (Fig. A.7). The overall speed of movement could remain the same, or even increase, but the storm will move more slowly toward stronger mid-latitude westerlies.
It was observed that during the transition seasons (spring and fall) TCs might initially be in a summer regime, but change to a winter regime. This most commonly occurred with TCs between 135E and 140E, and the Asian land mass. In this region (during transition seasons) the Tropical Upper Tropospheric Trough (TUTT) is weak and short wave troughs moving eastward from Asia can be quite strong. East of 135E to 140E, the TUTT is stronger and the short wave troughs are weaker. When a TC acquires a direct link between its outflow circulation and the mid-latitude westerlies during a transition season, the TC should be treated as a winter TC west of 135E to 140E and as a summer TC east of 135E to 140E.
Figure A.8 is an example of a synoptic pattern which may retard a TC's northward component of movement. An anticyclone to the west or northwest of a TC can produce flow with a southward component which will reduce the TC's northward component. Any southward flow toward the storm, regardless of the synoptic pattern producing it, may retard a TCs northward movement.
4. Summer Regime Nonrecurvature Pattern
Figure A.9 is indicative of a nonrecurvature situation. When the Asian upper level anticyclone remains east of a TC, the TC will exhibit a greater westward than northward component of movement, and recurvature will not occur. Such TCs commonly affect the southern Ryukyu Islands, Taiwan, and the People's Republic of China. The westward movement of the TC appears to be related to the strength of the 200 hPa winds at the eastern or southeastern periphery of the STR. If these winds are 10 to 20 knots (5.1 to 10.3 m/s) the TC will move toward the northwest into the ridge; if the winds are 20 to 40 knots (10.3 to 20.6 m/s) the TC will move toward the west-northwest; if the winds are 40 to 60 knots (20.6 to 30.9 m/s) the TC will move toward the west; and if the winds are greater than 60 knots (30.9 m/s) the TC will move south of west.
List of Figures of Appendix A
Figure A.1. A 200 hPa streamline analysis depicting a winter regime situation. Wind barbs are in knots. Major longwave axis may vary longitudinally. (After Guard, 1977)
Figure A.2. A 200 hPa streamline analysis depicting summer regime. TW is a trough in the westerlies (short dashed), STR is the subtropical ridge (heavy short dashed), SER is the subequatorial ridge (dotted), TUTT is the tropical upper tropospheric trough (long dashed), MPT is the Mid-Pacific Trough, and EAT is the East Asian Trough. (After Guard, 1977)
Figure A.3. A 200 hPa streamline analysis depicting a winter regime situation. Wind barbs are in knots and illustrate the direct link between the tropical cyclone upper level outflow circulation and the mid-latitude westerlies. (After Guard, 1977)
Figure A.4. A 200 hPa streamline analysis illustrating the primary winter regime non-recurvature situation. Fine dashed line represents the 500 hPa 588 decameter contour. Thick dashed line shows the tropical cyclone past (tropical storm symbol) and future (typhoon symbol) cyclone track. (After Guard, 1977)
Figure A.5. A 200 hPa streamline analysis of synoptic situation dictating north-ward movement of summer regime tropical cyclones. Dashed line shows past (tropical storm symbols) and future (typhoon symbols) cyclone movement. (After Guard, 1977)
Figure A.6. A 200 hPa streamline analysis depicting a synoptic situation under which a northward moving summer regime tropical cyclone would acquire a more westward component of movement. Dashed lines show the past (tropical storm symbols) and future (typhoon symbols) movement of the tropical cyclone. (After Guard, 1977)
Figure A.7. A 200 hPa streamline analysis depicting a synoptic condition under which a summer regime tropical cyclone would move toward recurvature. Dashed line shows past (tropical storm symbols) and future (typhoon symbols) cyclone movement. Wind barbs are in knots. (After Guard, 1977).
Figure A.8. A 200 hPa streamline analysis illustrating a synoptic situation conducive to reducing a summer regime tropical cyclone's northward component of motion. Tropical storm symbols indicate past cyclone movement and typhoon symbols indicate future cyclone movement. (After Guard, 1977)
Figure A.9. A 200 hPa streamline analysis illustrating a non-recurvature situation during a summer regime. Dashed line shows past (tropical storm symbols) and future (typhoon symbols) movement of tropical cyclone. Wind barbs are in knots. (After Guard, 1977)
***** End of APPENDIX A *****
Main
Chapter 4
Section 3
Appendix A