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Global Guide to Tropical Cyclone Forecasting:
CHAPTER 2: TROPICAL CYCLONE STRUCTURE

2.5  EXTRATROPICAL TRANSITION

Tropical cyclones that penetrate into the middle latitudes pose special forecasting problems. Over three-fourths of tropical cyclones crossing 30° lat undergo "extratropical dissipation" and weaken as they move over land or colder water or encounter the vertical wind shear associated with mid-latitude baroclinity. As cooler air enters the western portions of the circulation, the tropical cyclone may expand (Section 2.4) and spread high seas and moderate winds over a much larger area, while still retaining a concentrated core with active convection. Eventually, the convection may either cease entirely or be displaced well east of the center and extratropical dissipation begins. Dissipating tropical cyclones, especially those with a large circulation, retain winds of well over gale force aloft within 1-2 km of the surface for several days. Any process which causes vertical mixing (convection or passage over rough terrain) can therefore result in strong surface winds.

The remaining one-fourth of tropical cyclones passing into the middle latitudes interact directly with a frontal zone and undergo extratropical transformation to produce an extratropical cyclone with far different dynamics and distribution of wind and rainfall (Brand and Guard, 1978; Muramatsu, 1985). The differences in characteristics between a tropical and extratropical cyclone are summarised in Fig. 2.14 and Table 2.2.

Figure 2.14: Schematic of the surface temperature and wind fields associated with a tropical cyclone (left) and an autumn marine extratropical cyclone, such as might result from an extratropical transformation of the tropical cyclone (right), adapted from Mook (1955). The structure of the extratropical cyclone and the sizes of both cyclone types can vary considerably from this example.

During transformation, the cyclone may be a confusing and rapidly changing mixture of tropical and extratropical characteristics. Sekioka (1970) describes transformation types as "compound" or "complex" based on low-level synoptic structure. Compound extratropical transformation occurs when the tropical cyclone merges with a pre-existing extratropical cyclone, which usually intensifies because of the additional diabatic heat and moisture. Complex extratropical transformation occurs when a tropical cyclone approaches a frontal zone and induces a new low-pressure wave on the front. The tropical cyclone then typically accelerates into this wave and a single extratropical cyclone results. The structure changes associated with both of these processes are described in Section 2.5.3.

Extratropical transformations have received little attention from the tropical cyclone community. Rapidly developing marine extratropical lows, or bombs (Sanders and Gyakum, 1980), have received much more attention. These systems develop in a baroclinic environment, but their mature state is strongly influenced by surface fluxes and diabatic processes and they have some structural features in common with tropical cyclones. Newton and Holopainen (1990) provide a thorough review of their structure and dynamics. Some of this knowledge may be applicable to transforming tropical cyclones as well, so that blending of tropical and extratropical forecasting methodologies might be beneficial.

2.5.1 Climatology

Climatologies of extratropical transformations which are of use to forecasters are not generally available. A goal of the IWTCs is to encourage tabulation of transformation events in different basins using standardised definitions and procedures.

Climatologies may be either geographical or synoptic. A geographical climatology of the number of tropical cyclones entering the mid-latitudes by basin is provided in Table 2.3. Synoptic climatologies, such as Table 2.4 (from Brand and Guard, 1978), are more useful to forecasters since they quantify such issues as the length of various periods of the cyclone lifecycle and frequency of transformations by type. This climatology, based on only a single year of data, could be used as a starting point for tabulations in any basin by those who are familiar with the subtleties of the records. Other basins will certainly differ quantitatively and may have other patterns besides compound and complex as well, but these will only become known through systematic archival and summary in each basin. Such climatologies can then serve as the starting point for specific forecasting rules and guidelines.

2.5.2 Analysis Procedures

Tropical cyclone remnants should be tracked as long as possible. Visible satellite imagery is especially useful for analysing these systems, as the low-level circulation center (LLCC) can be followed even if the convection has dissipated or is no longer organised. It is also important to follow the mid-tropospheric remnants (which may follow a different course than the LLCC) because they contain abundant moisture. Heavy rain can quickly develop if a tropical cyclone remnant is drawn into an extratropical weather system, even if the tropical cyclone's low-level circulation has become insignificant by itself.

2.5.2.1 Synoptic Analysis

Extratropical cyclogenesis occurs as low-level ascent associated with warm advection becomes superimposed with upper-level ascent associated with cyclonic vorticity advection. In individual cases, either the low-level or upper-level component may dominate. Foley (1989) cites cases of both types. Unfortunately, descriptions of extratropical transformation have been almost entirely from conventional analyses rather than satellite imagery, so the suggested analysis procedures are also weighted quite heavily to traditional synoptic meteorology methods. This is unsatisfactory given the lack of conventional data over the oceans (although it is possible to prepare somewhat useful analyses using only surface and upper-troposphere data).

Muramatsu (1985) suggested that the extratropical transformation encompassed several of the following features:

1. The temperature and moisture fields develop distinct asymmetries;

2. The organised core convection becomes disrupted and disappears;

3. The circulation in middle and upper levels becomes disrupted and weakens;

4. The major precipitation bands shifts towards the front eastward quadrant;

5. Dry, cold air intrudes into the circulation from the front;

6. The area of gale force winds expands and becomes highly asymmetric.

Analysis and forecasting of extratropical transition should take such features into account.

Figure 2.15: Suggested symbols for composite charts of extratropical transformation and heavy rainfall associated with poleward moving tropical cyclones

Figure 2.16: Composite chart using the symbology of Fig. 2.15 for the remnants of Hurricane Agnes on 1200 UTC 20 June 1972.

Figure 2.17: Composite chart using the symbology of Fig. 2.15 for the remnants of Hurricane Agnes on 1200 UTC 21 June 1972.

Figure 2.18: Composite chart using the symbology of Fig. 2.15 for the remnants of Hurricane Agnes on 1200 UTC 22 June 1972.

Figure 2.19: Composite chart using the symbology of Fig. 2.15 for 0300 UTC 15 October 1954, as Hurricane Hazel approaches the middle Atlantic coast of the United States ahead of an extremely vigorous baroclinic trough.

Figure 2.20: Composite chart using the symbology of Fig. 2.15 for 1500 UTC 15 October. Hazel has been transformed into a major extratropical low. Winds of 25 m/sec or more are occurring in the shaded areas (adapted from Mook, 1955).

Fig. 2.15: suggests symbols that can be used to highlight low-level and upper-level features of interest for monitoring extratropical transitions. These features are also important for forecasting the occurrence of heavy rain, as described in Section 2.6. Selected features from the low-level analysis (surface or 850 hPa), mid-level analysis (500 hPa), and upper level analysis (200, 250, or 300 hPa), which are associated with vertical motions or thermal and vorticity advection, can be identified and plotted together on a "composite chart." The degree to which features are becoming aligned in the vertical can be readily seen, and the composite chart itself becomes a compact archival form for synoptic information about the case.

Examples of composite charts for a complex transformation with slight redevelopment (Agnes, 1972) and a compound transformation with explosive redevelopment (Hazel, 1954) are shown in Figs. 2.16-2.18 and 2.19-2.20, respectively. The Agnes (1972) transformation did not result in a strong extratropical low but did cause widespread heavy rain and severe flooding (Di Mego and Bosart, 1982a,b; see Section 2.6).

Agnes is initially dissipating over land within an upper-level ridge (Fig. 2.16). One day later, a front and upper trough advance into the region (Fig. 2.17). The front subsequently merges with Agnes (Fig. 2.18), which is then centred beneath an upper trough. The Hazel (1954) transformation was far more rapid and violent. A rapidly advancing front and upper trough with a very strong jet stream overtake severe hurricane Hazel as it accelerates northward (Fig. 2.19) and a severe extratropical cyclone with a highly asymmetric wind distribution results within 12 h (Fig. 2.20).

Figure 2.21: Satellite image patterns associated with extratropical cyclogenesis (from Newton and Halopainan, 1990). Dashed lines indicate cirrostratus cloud decks, cross-hatching frontal clouds, and dots low-middle level cloud decks.

2.5.2.2 Satellite Image Interpretation

Unfortunately, there are no widely used satellite image classification technique in the manner of Dvorak (1975, 1984) exists for extratropical transformation, although some cases have been examined by Sekioka (1970) and Herbert and Poteat (1975). Guidelines for interpreting imagery associated with extratropical cyclogenesis are available and are provided in Fig. 2.21. The initial development signature (Fig. 2.21a) is called a "baroclinic leaf"; it is a region of solid middle- to high-level cloudiness parallel to the surface front and high-level jet, with the incipient surface low located near its equatorward upstream corner. As the extratropical low develops, the leaf deforms into a comma cloud made up of multi-layer cloud with considerable variability (Figs. 2.21b-c).

2.5.3 Prediction

2.5.3.1 Numerical Models

Research numerical models have successfully simulated both marine extratropical cyclogenesis and extratropical transformation of tropical cyclones. Unlike tropical cyclone intensity, the horizontal resolution requirement should be within reach of regional models and the highest resolution global models. However, at IWTC-II (1990), it was indicated that the performance of operational NWP models on extratropical cyclones has tended to be poor. Analysis uncertainties may be responsible, as may an inadequate treatment of boundary layer and diabatic processes. If the latter error source dominates (as some studies of rapid extratropical cyclogenesis seem to indicate), model physics packages may be improved sufficiently to provide good guidance. In any case, numerical model performance should be noted and recorded in connection with potential cases of extratropical transformation so that strengths and weaknesses can be reliably identified.

2.5.3.2 Empirical Methods

No clearly documented empirical methods for forecasting extratropical transition exist. A suggested decision aid is provided in Fig. 2.22. It is patterned after those used for intensity change and rainfall and has received very little testing. Care thus needs to be taken with its application, and local documentation of the results is strongly recommended. One example of a controversial hypothesis in Fig. 2.22 is that strong redevelopment during extratropical transition will not occur without favourable upper-level conditions. Although this may be true, it has not been adequately tested either empirically or theoretically and forecaster prudence is recommended.

Contents Chapter 2.6