Subtropical and Hybrid Systems IWTC VII Topic 1.6 John R. Gyakum Department of Atmospheric and Oceanic Sciences McGill University, Montreal Canada Working group: John L. Beven II, Lance F. Bosart, Fermin Elizaga, Jenni Evans, Thomas J. Galarneau, Jr., Mark Guishard, Bob Hart, Matthew Kucas, Jonathan Martin, Rachel G. Mauk, Ron McTaggart Cowan
Outline of discussion Introduction/Motivation Classifying subtropical and hybrid systems Examples Forecasting considerations Climatological analyses Recommendations for future research directions IWTC VII Topic 1.6 ; La Réunion
Introduction/Motivation Since IWTC VI, there has been increasing documentation and evidence in the refereed literature of extratropical processes occurring in association with named tropical cyclones. Such cases are classified as either subtropical or hybrid cyclones
Classifying subtropical and hybrid systems Hybrid cyclones have energetics and structures of both tropical and baroclinic cyclones Subtropical cyclones (STCs) possess attributes of both tropical cyclones (low level warm cores) and ordinary baroclinic (governed by quasigeostrophic processes) developments A Frontal hybrid is characterized by organized moist convection in the presence of surface frontal zones
Tools for classifying subtropical and hybrid systems Cyclone phase space (Hart 2003) objectively indicates a subtropical or hybrid cyclone to exhibit asymmetric, shallow warm or cold core structures A time evolution into a deep warm core structure shows a tropical transition (TT) An illustration of the utility of the Cyclone Phase space approach is shown Tropical Storm Karen s TT:
Cyclone phase space analysis of Karen (2001), utilizing the US National Weather Service Global Forecasting System analyses (adapted from Guishard et al. 2009; their Fig. 2) Initial development of Karen occurred on 10 October with a cold core in 600 300 hpa levels, becoming a deep symmetric warmcore system by 13 October.
Conceptualization of the life cycle of the TT process, as shown by Jon Martin:
McTaggart Cowan et al. (2008) TC development pathway study illustrates development locations by category Non Baroclinic: 40% Low level baroclinic: 13% Transient trough: 16% Trough induced: 3% Weak TT: 13% Strong TT: 15%
A global development pathways analysis of McTaggart Cowan et al. 2010 shows dramatic variability by basin
Galarneau s recent research investigates these genesis pathways with a focus on TCs that form in conjunction with extratropical precursors Composite at 12 h prior to TC genesis (for developers) or initial maximum intensity (for nondevelopers) of 300 hpa streamfunction (solid contours every 6.0 10 6 m 2 s 1 ), relative vorticity (shaded according to the colorbar in 10 5 s 1 ), and nondivergent wind (standard barbs in kt) for (a) developing CUTOFF, (b) nondeveloping CUTOFF, (c) developing TROUGH, (d) nondeveloping TROUGH, (e) developing FOLDOVER, and (f) nondeveloping FOLDOVER. Note that FOLDOVER composite is in ridgerelative coordinates while CUTOFF and TROUGH are in disturbance relative coordinates. Developers Nondevelopers
Examples (Delta 2005; Galarneau 2010 ) Baroclinic development process: Dynamic tropopause maps; potential temperature in K, according to colorbar; 925 850 hpa layer averaged relative vorticity (white contours; every 2.0 x 10 5 s 1 ) NHC declared genesis time is T 0:
TC Delta at 0600 UTC 22 Nov (T 12 h; Galarneau 2010) at initial warm core (a) 250 200 hpa PV (black contours at 1.0, 2.0, 3.0, 5.0, 7.0, and 10.0 PVU), 500 hpa ascent (solid blue contours every 3.0 10 3 hpa s 1 starting at 3.0 10 3 hpa s 1 ), PW (colorbar; mm), and 200 hpa irrotational wind (m s 1 ), (b) 300 hpa streamfunction (green contours every 6.0 10 6 m 2 s 1 ), 400 200 hpa layer mean Q vector div. (colorbar in 10 16 K m 2 s 1 ) and Q vectors (arrows in 10 12 K m 1 s 1 ), and 700 hpa Q vector divergence (dashed at 10.0, 7.0, 5.0, 3.0, 2.0, 1.0, 0.5, and 0.25 10 16 K m 2 s 1 ), (c) 900 hpa Petterssen frontogenesis [colorbar in K (100 km) 1 (3 h) 1 ], potential temperature (black contours every 2 C), wind (kt), and 950 850 hpa PV (solid blue contours every 0.2 PVU starting at 0.4 PVU). development (d) GOES 12 infrared imagery (shaded according to the colorbar IWTCVII; La in Réunion C) at 0600 UTC 22 November 2005. TC Delta is marked X.
Northern Hemisphere Blocking pattern From McTaggart Cowan et al. (2006) in their discussion of the TT process of Catarina (2004) off of the coast of Brazil; The TT process occurred in the Southern Hemisphere. The figure on the right is designed for Northern Hemisphere meteorologists!
Forecasting (New England Hurricane of 1938; Hart 2010) Results of 56 ensemble member simulations indicate lows predictability of track forecasts. a) 120hr WRF Simulations initialized on 1200 UTC 18th September using each of the 56 members of the 20CR V2 ensemble. The line color indicates minimum mean sea level pressure of the simulated TC at the respective simulation hour. The solid black line is the best track. Each ensemble member is labeled by its number in a circle at the 5 day mark, although that number is largely unreadable unless the figure is dramatically increased in size.
Forecasting (New England Hurricane of Results of 56 ensemble member simulations indicate lows predictability of intensity forecasts. 1938; Hart 2010) b) The pressure traces associated with each of the 56 simulations shown in (a). The solid gray shading is the standard error of the mean, the latter given by the dotted black line. A significant component (but not nearly all) of the great ensemble spread after the 22nd results from variability in the timing of landfall within the WRF ensemble.
Forecasting (New England Hurricane of 1938; Hart 2010) c) Cross section 1 (B vs. VTL) of the CPS (Hart 2003) for each of the 56 ensemble members in (a) and (b). Note the remarkable structural uncertainty of the cyclone as it heads toward and into New England, with nearly every type of synoptic scale cyclone present within the 56 member ensemble.
Climatological analyses; Mauk 2010; NCEP/NCAR reanalyses Consistent differences are found in CAPE and EL (equilibrium level) values between baroclinic (18) and tropical (3) systems: 21 TC events over the NE Atlantic from 1975 2009 (October through December)
Climatological analyses (Evans and Braun 2010) Mean monthly distribution of ST occurrences in the South Atlantic, the Tasman Sea, and the North Atlantic basins. The South Atlantic had a total of 63 ST developments, the Tasman Sea had a total of 111 ST developments, and the North Atlantic basin had a total of 197 ST developments. SH systems are larger (with respect to gale wind radius) with more background shear, and with less symmetry than those systems in the North Atlantic
Recommendations for future research directions Since there is presently no single set of objective criteria for cyclone type, a substantial contribution could be made to the operational forecast community by recommending a universal cyclone classification methodology, based on the latest research, operational forecasting capabilities, and availability of real time data
Recommendations for future research directions (Continued) Once a definition of STC is established, then an accepted, and maintained, database of STCs (based on reanalysis data) would allow for needed analysis and diagnosis of such STCs.
Recommendations for future research directions (Continued) In the absence of a more appropriate warning system, it may be preferable to continue to use TC warning systems for subtropical/hybrid systems. A comprehensive study addressing the quality of model performance in STC and hybrid systems evolution is needed.
Recommendations for future research directions (Continued) A study examining STC impacts on downstream predictability is needed, considering the significant interactions of STCs with extratropical flows. Studies are needed that evaluate the effectiveness of our current data collection and analysis systems, as they relate to STCs.
Recommendations for future research directions (Continued) Studies are needed that investigate possible improvements in predictability that could be obtained from environmental and/or targeted sampling programs. Continued research of TC development pathways are needed, since traditional analysis and numerical model forecast techniques capabilities may depend significantly on the subtropical or hybrid systems natures during the development processes.
Thank You!