MainSpecial terms are frequently used to describe a tropical cyclone's motion and its large-scale environmental pattern on a synoptic weather chart. The following list of definitions, although not exhaustive, provides a baseline of operational TC motion terminology.
2.1 Straight Moving Tropical Cyclones
Straight moving and non-recurving TCs usually move toward northwest direction or about 260-320° (Fig. 4.4), throughout their lifetimes (Hodanish, 1991). These TCs are typically embedded in the deep easterly flow equatorward of a well-established subtropical anticyclone.
2.2 Recurving Tropical Cyclones
Tropical Cyclone recurvature is the change in TC track from westward and poleward to eastward and poleward. Recurving TC is defined as poleward movement of a TC from the deep easterlies of the tropics into the strong upper tropospheric westerlies of the mid-latitudes. This motion is generally associated with TCs that move northwestward around the western portion of the subtropical ridge and then turn poleward and eastward under the influence of the mid-latitude westerlies. Recurving TCs can be further classified with respect to how rapidly they recurve.
a. Sharply Recurving Cyclones - Sharply recurving cyclones are those which change rapidly from a westerly or northwesterly course to a northeasterly or easterly one (Fig. 4.5).
b. Slowly Recurving Cyclones - Slowly recurving cyclones are those which change slowly from a westerly or northwesterly course to a northeasterly or easterly one (Fig. 4.5). The term broad recurvature is also used for these tracks.
2.3 Left (NH) Or Right (SH) Turning Tropical Cyclones
A left (northern hemisphere) or right (southern hemisphere) turning TC is one which is on a poleward course but instead of continuing to recurve poleward and eastward, it turns back to the left or right and resumes a poleward and westward course (Fig. 4.5).
A stair-step occurs when a TC embedded in the mean flow associated with the subtropical ridge moves poleward for a short period of time then resumes a westward track. Stair-steps have been observed to occur as weak shortwave troughs transit along the poleward side of the subtropical ridge.
2.4 Erratic Moving Tropical Cyclones.
Erratic moving or looping TCs are storms that move at speeds less than three knots and have significant changes in direction over short periods of time. These significant changes in storm direction are at times characterized by looping or circular direction changes (Fig. 4.5).
2.5 Equatorward of the Ridge
Tropical cyclones moving in this environment are embedded in the deep easterlies equatorward of the latitude of the subtropical ridge line (Fig. 4.6). These storms typically track west-northwestward (NH)/west-southwestward (SH).
2.6 On the Ridge
Tropical cyclones moving in this environment are embedded in the weak steering flow associated with breaks in the subtropical ridge (Fig. 4.6). These storms are typically located near the mean latitude of the subtropical ridge and move slowly in response to the weaker winds associated with the ridge line and breaks in the ridge. Erratic or looping motion is common in this environment.
2.7 Poleward of the Ridge
Tropical cyclones moving in this environment are embedded in the mid-latitude westerlies poleward of the mean latitude of the subtropical ridge (Fig. 4.6). These storms are typically accelerating under the influence of the westerlies.
2.8 Binary Interaction
Occasionally two TCs will come in close proximity (within about 10 degrees latitude) to one another and rotate about their common mass center. This type of interaction is often referred to as the Fujiwhara effect.
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Main
Chapter 4
Section 2