Advances in Understanding Extratropical John R. Gyakum Transition (ET) IWTC-VIII Topic 4.2 Julia Keller Department of Atmospheric and Oceanic Deutscher Wetterdienst Sciences Research and Development McGill University Numerical Models 805 Sherbrooke Street West Frankfurter Str. 135 Montreal, QC H3A 0B9 63067 Offenbach Canada Germany Working group: Sim Aberson, Heather Archambault, Lance F. Bosart, Chris Davis, Fermin Elizaga, Clark Evans, Jenni Evans, Chris Fogarty, Christian Grams, Kyle Griffin, Pat Harr, Bob Hart, Naoko Kitabatake, Matthew Kucas, Shawn Milrad, Ron McTaggart-Cowan, Florian Pantillon, João Rafael Dias Pinto, Elizabeth Ritchie, Kimberly Wood
Outline of presentation Introduction/Motivation Classification of extratropical transition Climatology Downstream impacts Forecasting of extratropical transition Processes during extratropical transition Hybrid systems Summary and recommendations for future research directions 2
Introduction/Motivation Since IWTC-VII, there has been increasing research on the complex interactions of the transitioning TC (tropical cyclone) and the extratropical westerlies ETs are often associated with explosive reintensification and/or copious rains ET modification of mid-latitude flow may trigger high-impact weather systems downstream Such a modification often coincides with reduced predictability in precisely those regions impacted by these high-impact weather systems 3
Introduction/Motiva tion (from Jones et al. 2003): Tracks of all tropical cyclones that underwent extratropical transition during 1970 99. (a) Western North Pacific. Tracks of tropical cyclones defined to be extratropical in JTWC best-track data. (b) Southwest Pacific [data as in (a)] and southeast Indian Ocean [tracks of tropical cyclones that accelerated toward the southeast under the influence of a midlatitude frontal system and maintained gales into midlatitudes, the socalled captured cyclones in Foley and Hanstrum (1994); best-track data taken from http://www.australiasevereweather.com/cyclone s/history.htm]. (c) North Atlantic. Tracks of tropical cyclones defined to be extratropical in National Hurricane Center best-track data. 4
Introduction/Motivation Hurricane Sandy and its ET 5
Classification of extratropical transition (from Atallah and Bosart 2003): The 850 700-hPa PV (shaded in cool colors every 0.2 PVU starting at 0.8 PVU) and wind (dark barbs, kt convention) and 300 200-hPa PV (shaded in warm colors every 2 PVU starting at 2 PVU) and wind (white barbs, kt convention) from Eta initialization for (a) 0000 UTC 16 Sep, (b) 1200 UTC 16 Sep, (c) 0000 UTC 17 Sep, and (d) 1200 UTC 17 Sep 1999. (Floyd 1999) 6
Classification of extratropical transition (Floyd 1999) (from Jones et al. 2003): Cyclone phase space diagrams for (a), (b) the extratropical transition of Hurricane Floyd in 1999. [(a), (b) Taken from Hart (2003) http://eyewall.met.psu.e du/cyclonephase.] 7
Classification of extratropical transition Based on environment Based on Cyclone Phase Space (CPS) pathway (Kofron et al. 2010b) 8
Climatology Eastern North Pacific and North Atlantic Ocean (Wood and Ritchie 2014) Average CPS frequency per TC during 2001-2010 in the (a) ENP (150 TCs) and (b) ATL (174 TCs). 9
Climatology Western North Pacific Ocean (Kitabatake 2011) June August October 26-year monthly mean field of Eady growth rate (contour, day -1 Kitabatake (2011) ), SST (Color, deg C), and the ET completion location (circle). Eady growth rate(hoskins and Valdes 1990; a parameter indicating baroclinic development of disturbance): 0.31 f N du dz In August, TCs tend to move to higher latitude in a weakly baroclinic environment above cooler sea surface before ET completion In October, TCs tend to complete ET in lower latitude in a strongly baroclinic environment above warmer sea surface 10
Climatology Southwest Indian Ocean (Griffin and Bosart 2014) Summary of TC and ET events in the SWIO west of 90 E by (a) TC season and (b) month, 1989-2013. Full height of bar represents TC events, with lower (blue) portion of bar represent the number of TCs undergoing ET. In (a), year on chart refers to year season ended. In (b), crossmonth events sorted by which month contained the end of TC life. Lower (blue) portion of bar represents the number of TCs beginning ET that month. 11
Climatology North Atlantic Ocean (Hart) Frequency of occurrence from 1871 2012 of TC XC proximity within 1500km. a) TC location. b) Extratropical cyclone location (as defined by 500mb closed center). 12
Climatology South Atlantic Ocean (Pinto) Simplified life cycle of two South Atlantic tropical storms. Anita (2010) was the only documented cyclone to undergo ET. Images based on infrared GOES-12 satellite imagery. 13
Downstream impacts Precipitation (Ritchie et al. 2011) Average rainfall swath (mm) for the two recurving TC groups identified in Ritchie et al. (2011) and their representative members. 14
Downstream impacts Precipitation (Milrad et al. 2013) For Frances (2004), (c) NARR ageostrophic frontogenesis as in panel (a), but total from t = -3 h to t = +3 h, and (d) CaPA total precipitation (mm, shaded) for t = - 6 h to t = + 6 h and NARR SLP (hpa, solid black) at t = 0 h. The black ovals denote the area of enhanced precipitation in the SLRV. In both (c) and (d), Montreal s Trudeau International Airport (CYUL) is marked with a blue star and Quebec s Jean-Lesage International Airport (CYQB) is marked with a black star. Bottom: Also for Frances (2004), (e) cross-section of ageostrophic frontogenesis (shaded) and equivalent potential temperature (K, red contours), at t = 0 h. The cross-sectional area is denoted by the black line in panel (c) and the black star indicates the center of the SLRV. (f) NARR sounding at t = 0 h at the location of the black star in panel (e). Plotted are temperature ( C, red), dew point ( C, blue), and winds (kts, black barbs). 15
Downstream impacts Precipitation (Milrad et al. 2013) Schematic showing the processes involved in producing enhanced precipitation during ET cases in the SLRV. The red circles and dotted lines indicate two common storm locations and tracks. The blue and purple arrows represent the geostrophic and pressure driven channeled surface wind, respectively. The black lines approximate isobars, with the identified pressure gradient threshold for pressure-driven channeling written in purple. The yellow ellipses are typical regions of ageostrophic frontogenesis and precipitation enhancement within the SLRV. 16
Downstream impacts Rossby wave trains (Archambault et al. 2014a) Composite analyses showing (a) strong (N = 54) and (b) weak (N = 54) TC extratropical flow interactions at the time of maximum interaction. Analyses show 500-hPa ascent (dashed green, every 2 x 10-3 hpa/s, negative values only), precipitable water (shaded according to gray scale, mm), and 200-hPa PV (blue, every 1 PVU), irrotational wind (vectors, >2 m/s; purple vectors, >8 m/s), negative PV advection by the irrotational wind (dashed red, every 2 PVU/day starting at 2 PVU/day), and wind speed (shaded according to color bar, m /s)). The star denotes the point of maximum interaction. The TC symbol denotes the composite TC position. 17
Downstream impacts Rossby wave trains (Quinting and Jones, 2014) Anomaly of RWP occurrence frequency [%] (a) and RWP amplitude [m/s] (b) from June to November climatology for western North Pacific TCs. Values that are statistically significant at the 95% confidence level are hatched. Black horizontal bar marks the range of longitudes of TCs at ET-time. White circle marks the mean longitude of all TCs at ETtime. 280 cases were used. 18
Downstream impacts Critical bifurcation point in the upper-level extratropical flow that governs the downstream impact of an ET (Grams et al. 2013) Trough-relative stream function (shaded every 0.5x10 6 m 2 /s) vertically averaged from 100 to 300 hpa and associated troughrelative non-divergent wind vectors (scale for wind speed in m/s below the colour bar). Contours: ur + vr = 0, 10 m/s (white solid). Time shown is 0600 UTC 1 October 2008. Tracks are shown relative to the estimated zonal trough propagation speed of 0.5 /h and centred on the TC position (open square) at the time shown. A geographical coordinate system centred on the bifurcation point is used. 19
Downstream impacts Importance of upstream middle-latitude flow (McTaggart- Cowan et al. 2001; Corboseiro et al. 2012) Tracks of Hurricane Earl in 5-day integrations starting at 1200 UTC 1 September (left panel, colour-coded for jet wind speeds as shown in the right panel legend). The potential temperature on the dynamic tropopause (2 PVU surface) at 1200 UTC 6 September are shaded according to the color bar. Initial wind speeds in the jet and their relation to Earl's final intensity are shown in the right panel. 20
Forecasting of extratropical transition Communication and outreach (Fogarty) Example for new warning product during ET 21
Processes during extratropical transition Inner core structure and intensity change (Aberson) Composites of Airborne Doppler wind analyses and dropwind sonde data at 7 and 0.5 km altitude from NOAA aircraft for Hurricane Arthur. Best track for Arthur from NHC. 22
Processes during extratropical transition Interaction with middle latitudes (Grams et al. 2013) Figure 5 from Grams et al. (2013a). 3-D view of ET Jangmi at 00 UTC 30 Sep 2008 (left) and 12 UTC 01 Oct Sep 2008 (right). 1.5 PVU PV surface (blue shading),320 K surface of Θe (transparent grey shading), representing 3-D baroclinic zone. v = 60m s -1 (green shading), highlighting upper-level midlatitude jet. Θ at 990 hpa (shading at bottom, brown colours >300 K, green 290 K,blue <280 K) and geopotential at 990 hpa (black contours, every 250 gpm). Paths of representative trajectories (every 100th, starting 28/12UTC ending 30/12UTC (left) and 30/12UTC 02/12UTC (right)) coloured by PV of air parcel moving along trajectory. Low-PV air (PV<0.6 PVU) grey shades, higher PV values (PV>0.6 PVU) in red shades (legend bottom right). 23
Processes during extratropical transition Interaction with middle latitudes (Griffin and Bosart 2013) Plot of 250 hpa isotachs (shaded per color bar; m s -1 ), 1000-500 hpa thickness (dashed contours; dam; 540 and below in blue), 500 hpa upward vertical motion (purple contours every 5 μb s -1 ), and mean sea level pressure (black contours every 4 hpa) for (a) 1200 UTC 8 March, (b) 0000 UTC 9 March, and (c) 1200 UTC 9 March 1990. 24
Processes during extratropical transition ET processes that occur without a predecessor TC (Davis et al. 2013) (a) and (b) Relative vorticity (10-5 /s) and streamlines and 850 hpa. C1 refers to cyclone 1, TCS025 refers to the remnants of TCS025; (c) Temperature (K) at 850 hpa. Arrows denote orientation of strongest flow. 25
Processes during extratropical transition The correct phasing of Helene with the Rossby wavetrain was crucial to both its correct track, and for the enhanced probability of developing the Mediterranean cyclone (Medicane). (Pantillon et al. 2013) Link between Hurricane Helene (2006) and a Mediterranean tropical-like cyclone (Medicane) in the operational ECMWF ensemble forecast, partitioned in 2 clusters. Tracks of Helene until 1200 UTC 26 September 2006 and tracks of the Medicane on 26 September 2006; tracks in the ECMWF analysis (solid black line), deterministic forecasts (dashed black lines), and ensemble forecasts (shading) initialized at 0000 UTC from 20 to 23 September 2006. Ensemble members that predict the Medicane are in color (red and blue of 22 and 23 September 2006, respectively). Dots indicate the position of Helene at 0000 UTC. 26
Hybrid systems Mediterranean Sea (Elizaga) MODIS image of the tropical-like Mediterranean cyclone on 19 th November 2013 at 15:30 UTC. The centre of the cyclone is just off the Sardinia coast. Cyclone phase space diagram from UKMET model. It shows the evolution of the Mediterranean storm between 18th November 00UTC and 21st November 00UTC. Source: Florida State University 27
Summary New methods of classification 1. Differences in ET environments 2. The CPS pathway and the isentropic PV Climatologies 1. Southwestern Pacific Ocean 2. Eastern North Pacific Ocean 3. Improved understanding of South Atlantic ET 4. North Atlantic interactions with extratropical cyclones. 28
Downstream impacts 1. Precipitation structures Summary 2. Amplification of middle-latitude Rossby wave trains 3. Sensitivity to phasing between the transitioning TC and the pre-existing middle-latitude wave train Forecasting 1. Assimilation of aircraft reconnaissance data have been shown to improve forecasts of ET s inner-core structure 29
Processes Summary 1. Advection of potential vorticity (PV) by the nondivergent wind 2. Diabatic PV modifications 3. Warm conveyor belt (WCB) ascent in a baroclinic zone Hybrid systems 1. Mediterranean cyclone with tropical characteristics 30
Recommendations for future research directions 1. The need to develop an objective classifier of ET, so that climatological studies and intensity estimations may be standardized. 2. The need to conduct more systematic studies of physical processes to determine the relative position of the ET and mid-latitude bifurcation points, and how well these are represented in NWP models. 31
Recommendations for future research directions 3.Additional insight is needed on how a TC may alter the midlatitude environment in a manner that facilitates its own reintensification, and the TC s role in diminishing predictability. 4.Future field campaigns are needed to document the role of condensational heating and its representation in numerical models during ET. 32
Discussion questions * Should we achieve a unification in how ET is defined, operationally and in climatological studies? Is the CPS the way to go, or do we need an additional classifier that captures the environmental factors and the midlatitude impact? * How do we achieve a better understanding the role of diabatic processes and the occurrence of bifurcation points during ET? How do these processes affect the synoptic situation and predictability in downstream midlatitudes? 33
Thank you! 34