Subtropical and Hybrid Systems subtropical and hybrid systems ... the cyclone as it heads toward ......

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 November 2010; 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

November 2010 IWTCVII; La Réunion

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 quasi‐ geostrophic 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 warm‐ core 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


Developers Nondevelopers 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 ridge‐ relative coordinates while CUTOFF and TROUGH are in disturbance‐relative coordinates.

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

development


(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).

(d) GOES‐12 infrared imagery (shaded according to the colorbar in°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.



Results of 56‐ensemble member simulations indicate lows predictability of intensity forecasts.

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.



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.



• 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.



• 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.



• 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!


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