2Extratropical Transition A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones.This process is generally referred to as extratropical transition (ET).During ET a cyclone frequently acquires increased forward motion and sometimes intensify substantially, so that such systems pose a serious threat to land and maritime activities.Often poorly forecast by current day numerical models and associated with periods of poor synoptic predictability over a wide area downstream.Extratropical transition occurs in nearly every ocean basin that experiences tropical cyclones with the number of ET events following a distribution in time similar to that of the total number of tropical cyclone occurrences.The largest number of ET events occur in the western North Pacific while the North Atlantic basin contains the largest percentage of tropical cyclones that undergo ET with 45% of all tropical cyclones undergoing ET.
3The IssueTropical cyclones transform into extratropical cyclones as they move northward, usually between 30° and 40° latitude.Interaction with upper-level troughs or shortwaves in the westeries, and preexisting baroclinic zones is an important factor in ET.During extratropical transition, cyclones begin to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat from condensation (from convection near the center) to baroclinic processes.The low pressure system eventually loses its warm core and becomes a cold-core system. During this process, a cyclone in extratropical transition will invariably form or connect with nearby fronts and/or troughs. Due to this, the size of the system will usually appear to increase. After or during transition, the storm may re-strengthen, deriving energy from primarily baroclinic processes, aided by the release of latent heat.The cyclone will also distort in shape, becoming less symmetric with time, but sometimes retains a tight, tropical-like core.
4The Other Direction As Well! Less frequently, an extratropical cyclone can transit into a tropical cyclone if it reaches an area of ocean with warmer waters and an environment with less vertical wind shear.The process known as "tropical transition" involves the usually slow development of an extratropic cold core vortex into a tropical cyclone
5Big Impacts of ETSevere flooding associated with the ET of Tropical Storm Agnes [1972Hurricane Hazel (1954) resulted in 83 deaths in the Toronto area of southern Ontario, Canada. In the northwest Pacific, severe flooding and landslides have occurred in association with ET.An example is the ET of Tropical Storm Janis (1995) over Korea, in which at least 45 people died and people were left homeless.In one southwest Pacific ET event (Cyclone Bola) over 900 mm of rain fell over northern New Zealand).Another event brought winds gusting to 75 m s-1 to New Zealand's capital city, Wellington (Hill 1970 ), resulting in the loss of 51 lives when a ferry capsized.Extratropical transition has produced a number of weather-related disasters in eastern Australia, due to severe flooding, strong winds, and heavy seas [e.g., Cyclone Wanda in 1974).Tropical systems that reintensify after ET in the North Atlantic constitute a hazard for Canada [e.g., Hurricane Earl in 1998 and for northwest Europe. The extratropical system that developed from Hurricane Lili (1996) was responsible for seven deaths and substantial economic losses in Europe.Many of the largest NW windstorms are ET events (Columbus Day Storm, 1981 storm and others)
9A Few ReferencesSarah C. Jones, Patrick A. Harr, Jim Abraham, Lance F. Bosart, Peter J. Bowyer, Jenni L. Evans, Deborah E. Hanley, Barry N. Hanstrum, Robert E. Hart, François Lalaurette, Mark R. Sinclair, Roger K. Smith and Chris Thorncroft. 2003: The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions. Weather and Forecasting: Vol. 18, No. 6, pp. 1052–1092.Patrick A. Harr, Russell L. Elsberry and Timothy F. Hogan. 2000: Extratropical Transition of Tropical Cyclones over the Western North Pacific. Part II: The Impact of Midlatitude Circulation Characteristics. Monthly Weather Review: Vol. 128, No. 8, pp. 2634–2653.Patrick A. Harr and Russell L. Elsberry. 2000: Extratropical Transition of Tropical Cyclones over the Western North Pacific. Part I: Evolution of Structural Characteristics during the Transition Process. Monthly Weather Review: Vol. 128, No. 8, pp. 2613–2633.
16ET and PrecipitationDuring an ET event the precipitation expands poleward of the center and is typically maximum to the left (right) of the track in the Northern (Southern) Hemisphere. The change in the structure of the precipitation field from the more symmetric distribution in a tropical cyclone to the asymmetric distribution during ET can be attributed to increasing synoptic-scale forcing of vertical motion associated with midlatitude features such as upper-level PV anomalies or baroclinic zones.Ets have been associated with extraordinarily large precipitation amounts and associated flooding.
17Precipitation and ETDiMego and Bosart (1982a) diagnosed the contributions to the vertical motion during the ET of Agnes (1972) and showed how the forcing of vertical motion evolves from an almost symmetric forcing due to diabatic heating during the tropical phase to an asymmetric quasigeostrophic forcing during ET.A majority of the precipitation associated with ET occurs poleward of the center of the decaying tropical cyclone. Harr and Elsberry (2000) identified this as a region of warm frontogenesis north and east of the tropical cyclone center. Harr and Elsberry (2000) showed that the ascent and frontogenesis in the warm frontal region had a gentle upward slope. This suggests that warm, moist air in the southerly flow ahead of the tropical cyclone center ascends along the gently sloping warm front, allowing the region of precipitation to extend over a large area ahead of the tropical cyclone.
18Precipitation (in.) and cyclone track for the extratropical transition of (a) southwest Pacific Cyclone Audrey (1964) during the 72-h period ending 2300 UTC 14 Jan 1964 (taken from Bureau of Meteorology 1966 ) and (b) North Atlantic Hurricane Hazel (1954) during the 24-h period ending 0600 UTC 16 Oct 1954 [adapted from Palmén (1958) ]. Solid circles mark the track of Hurricane Hazel in 3-h increments from 0900 UTC 15 Oct to 0600 UTC 16 Oct 1954.
19ET and PredictabilityET events are often not predicted well by today’s synoptic models (e.g., GFS)ET events are often associated with periods of poor predictability over large areas downstream of the transition.
20Anomaly correlations for NOGAPS forecasts of 500-hPa heights over the North Pacific (20°–70°N, 120°E–120°W) during Aug Each panel represents a specific forecast interval, as labeled. The extratropical transition events that occurred during the month are marked in (a).
21The 500-hPa height (contours) and mean sea level pressure (shaded) from the NOGAPS model for Typhoon David (1997). Top row: analyses at 0000 UTC 18 Sep (left), 0000 UTC 19 Sep (middle), and 0000 UTC 20 Sep (right). Middle row: corresponding forecasts initialized at 0000 UTC 16 Sep. Bottom row: corresponding forecasts initialized at 0000 UTC 17 Sep
22European Centre for Medium Range Weather Forecasts (ECMWF) Hovmoeller plot for forecast from 9 September 2003, 12 UTC: root mean square difference (RMSD) of ensemble forecasts with perturbations averaged over 40° - 50° N for 200 hPa (left) and 500 hPa (right). The high values of RMSD spread downstream from Typhoon Maemi (black dot) at both levels.European Centre for Medium Range Weather Forecasts (ECMWF)Ensemble Prediction SystemEPS
23ET EnergeticsPalmén (1958) compared the ET of Hurricane Hazel with a typical extratropical cyclone in terms of their sources and sinks of energy. He found that an extraordinary amount of kinetic energy was exported to the midlatitude westerlies from the region of the decaying tropical cyclone, which led him to estimate that only two to three disturbances such as Hazel would provide the entire Northern Hemisphere north of 30°N with the kinetic energy sufficient to maintain the circulation against frictional dissipation.
24The Details of ETThe physical mechanisms associated with the transformation stage of the extratropical transition of a tropical cyclone were simulated with a mesoscale model by Ritchie and Elsberry (2001, MWR).There appears to be three steps in the transformation, which compares well with available observations.During step 1 of transformation when the tropical cyclone is just beginning to interact with the midlatitude baroclinic zone, the main environmental factor that affects the tropical cyclone structure appears to be the decreased sea surface temperature. The movement of the tropical cyclone over the lower sea surface temperatures results in reduced surface heat and moisture fluxes, which weakens the core convection and the intensity decreases.
25During step 2 of transformation, the low-level temperature gradient and vertical wind shear associated with the baroclinic zone begin to affect the tropical cyclone.Main structural changes include the development of cloud-free regions on the west side of the tropical cyclone, and an enhanced rain region to the northwest of the tropical cyclone center. Gradual erosion of the clouds and deep convection in the west through south sectors of the tropical cyclone appear to be from subsidence.
26Step 3. Even though the tropical cyclone circulation aloft has dissipated, a broad cyclonic circulation is maintained below 500 mb. Whereas some precipitation is associated with the remnants of the northern eyewall and some cloudiness to the north-northeast, the southern semicircle is almost completely clear of clouds and precipitation.
31Interaction With Midlatitude Troughs Correct phasing is crucial.If the TC is too far east with little upper support (and cold water) it dies.If TC too west, it is west of the upper support and is facing lots of cold dry air…it dies.Only for a critical ~5 deg swath does it have everything…upper support, ability to tap warm, moist air.
32Hurricane/ET Floyd 16-17 Sept 99 Colle (2003, MWR) studied this with a hig resolution MM5 simulation.
45More important than one might expect Nearly half of the Atlantic tropical cyclones from 2000 to 2003 depended on an extratropical precursor (26 out of 57).Many of these disturbances had a baroclinic origin and were initially considered cold-core systems.A fundamental dynamic and thermodynamic transformation of such disturbances was required to create a warm-core tropical cyclone. This process is referred to as tropical transition (TT).
46TTTropical cyclogenesis associated with extratropical precursors often takes place in environments that are initially sheared, contrary to conditions believed to allow tropical cyclone formation.The adverse effect of vertical wind shears exceeding 10–15 m s-1on the formation of low-latitude storms is well documented (DeMaria et al. 2001).However, a beneficial role of vertical shear, hypothesized to organize convection, was indicated by the statistical analysis of Bracken and Bosart (2000) for 24 developing cases in the northern Caribbean Sea.
47TTDavis and Bosart (1986, BAMS) showed that apparent that PV “debris” extruded from the midlatitude jet is common over the warm oceans of the subtropical Atlantic, even as far south as 15°N on occasion. In September 2001 alone, they counted 34 upper-level vorticity maxima (averaged over a 3° × 3° latitude–longitude box) greater than 10−5 s−1 persisting for at least 12 h while over ocean temperatures greater than 25°C.