Extratropical transition of tropical cyclones: Operational forecasting challenges. Matthew Kucas Joint Typhoon Warning Center Pearl Harbor, HI, USA

Transcription

1 Extratropical transition of tropical cyclones: Operational forecasting challenges Matthew Kucas Joint Typhoon Warning Center Pearl Harbor, HI, USA

2 Outline Motivation for discussion Brief overview of tropical cyclone structure and dynamics Atlantic extratropical transition climatology Changes in the tropical cyclone and environment during extratropical transition Impacts on the midlatitude flow pattern Forecasting challenges and methods

3 Extratropical transition: Motivation for discussion Frequent occurrence Direct effects on trans-oceanic transport Impact on midlatitude development - implications for forecasting far downstream of the transition event Unique forecast challenges

4 Review of tropical cyclone (very) basic dynamics Typically form in the deep tropics H Latent heat transfer from warm sea surface diabatic heating in convection sustains warm core Strong secondary circulation drives inflow Cyclonic flow in lower levels, anticyclonic flow aloft (~200 hpa) Diameter: km L Subsident eye of diameter km in intense cyclones with surrounding, sloped eyewall; Convective rainbands outside eyewall Gradient wind balance; Nearly axisymmetric (Frank, 1977)

5 Extratropical transition in the Atlantic basin: climatology Hart and Evans, 2001: Nearly 50% of Atlantic tropical cyclones undergo extratropical transition (ET) Atlantic basin re-curving TCs ( ): 49% re-intensified, 42% weakened, 7% stable Cyclones forming in deep tropics (south of 20 N) more likely to re-intensify in transition Western Europe directly impacted by a transitioning tropical cyclone once every 1-2 years Transitions most frequent between 35 N and 45 N Latitude depends on steering features and location of tropical development region relative to mid-latitude baroclinic zone Occur at highest latitudes during season peak (August-September) Climate cycles (ENSO, NAO, PNA) impact on inter-annual ET climatology is minimal

6 Extratropical transition in the Atlantic basin: climatology Hart and Evans, 2001: Figure 2D Return period for tropical cyclones in extratropical phase based on National Hurricane Center best track data (passage within 300 km of any point)

7 Highlighting the extratropical transition of Hurricane Maria (2005) Steering ridge H Cyclone best track from Pasch and Blake, 2005, figure 1 Satellite imagery from NRL TC page:

8 Extratropical transition: Process Jones et al, 2003: Two stages of extratropical transition: Transition and extratropical. Cyclone may dissipate during transition stage unless interaction with pre-existing mid-latitude features occurs. Environmental characteristics Upper-level jet streak may form poleward of TC Increased vertical wind shear and baroclinicity Along-track sea surface temperatures (SST) decrease; strong SST gradient Strong moisture gradient Increasing Coriolis parameter Transition stage Changes in TC Track speed increases Asymmetries in wind, cloud, and precipitations fields Dry air intrusion lowlevel circulation center exposed Warm front forms - indicated by triangular delta rain region (observed by satellite) Warm core to cold core Interaction with pre-existing: upper-level vorticity anomaly extratropical cyclone or strong baroclinic zone Extratropical stage Resulting cyclone characteristics Vertical tilt with height Fronts seen in in-situ and satellite observations Greatly expanded wind, precipitation, and wave fields Wind field expands down-shear Precipitation shield moves left (poleward) of storm track Wave fields move with storm Conversion of mean flow potential to eddy kinetic energy

9 Extratropical transition: Re-intensification Midlatitude pattern and ocean temperatures strongly influence transition location, intensity change, motion (Hart and Evans, 2001) Upper-level outflow from tropical cyclone may enhance upstream midlatitude jet entrance region, followed by interaction between low level circulation and baroclinic zone (Klein et al., 2002) Warm air drawn poleward - enhance temperature gradient along baroclinic boundary positive contribution to baroclinic (Eady) growth rate at 700 hpa (Hart and Evans, 2001): 0.31fU z N Frontogenesis as temperature gradients rotated (Jones et al., 2003) Total precipitable water observed by passive microwave satellite sensor during the extratropical transition of Hurricane Maria (2005). Image courtesy Naval Research Lab. Increasing vertical wind shear erodes upper potential vorticity anomaly of cyclone phasing with upper level trough to Northwest can drive re-intensification (Jones et al., 2003)

10 Extratropical transition: Generalized patterns Harr et al., western North Pacific ET cases, Constructed composites of 500 hpa and surface pressure fields from NOGAPS global model analyses Two composite patterns associated with ET: Northwest and Northeast Northwest pattern: approaching midlatitude trough is dominant circulation intensification of cyclone during transition Northeast pattern: large, quasi-stationary circulation to northeast is dominant circulation no intensification of cyclone during transition Figure 1 from Harr et al., 2000: Composites of the two primary extratropical transition patterns showing 500 hpa heights (solid lines) and surface pressure (shaded in 4 hpa increments starting at 1004 hpa)

11 Impacts of extratropical transition on the midlatitude flow pattern Most significant when cyclone remains strong or re-intensifies during ET Effects may occur upstream, near, or downstream of transitioning cyclone generally up or downstream for strong ET, near for weak ET (Anwender et al., 2008) Rossby wave train may propagate rapidly downstream from ET point (Riemer et al., 2008) More pronounced perturbation in the downstream flow pattern when the Eady baroclinic growth rate is largest September/October (Reynolds et al., 2009) Extratropical transition events can significantly decrease medium-range predictive skill for downstream events in numerical models (Jones, et al., 2003) Downstream pattern can change even if tropical cyclone is fully captured by midlatitude jet (Jones et al., 2003)

12 Extratropical transition: Unique forecasting challenges 1) Predicting track Determine whether a tropical cyclone will re-curve poleward from the tropics and resultant acceleration Establish whether a re-curving cyclone will undergo extratropical transition or dissipate within tropical air mass 2) Determining the timing of ET 3) Setting forecast intensity and structure (wind and precipitation) 4) Analyzing position, intensity, and structure during transition

13 Forecasting extratropical transition: Track Joint Typhoon Warning Center official forecast (JTWC) JTWC consensus (CONW) models: Weber barotropic model (WBAI) Navy Operational Global Atmospheric Prediction System (NGPI) Global Forecasting System (AVNI) European Center for Medium Range Forecasting global model (ECM2) Japanese Global Spectral Model (JGSI) Tropical Cyclone Local Area Prediction System (TCLI) JTWC official and numerical model track forecasts and verifying best track (with TC symbols) for Super Typhoon 15W, Choi-Wan (2009) Geophysical Fluid Dynamics Model, Navy (GFNI) United Kingdom Meteorological Office global model (EGR2) Large spread in both direction and speed among numerical models possible in the days before ET - Uncertainty in anticipated evolution of the synoptic pattern - Pre-recurvature track uncertainty - both recurvature location / timing and ET evolution (Anwender et al., 2008)

14 Forecasting extratropical transition, Timing: Environmental indicators Upper-level flow baroclinic wave pattern, vertical wind shear, location of jet streak(s) Sea surface temperature (SST) absolute values and gradients Cloud patterns stratocumulus cloud decks poleward of baroclinic boundaries Diffluent jet entrance region Hurricane Maria (2005) Sharp SST gradient Upper level cloud drift winds from University of Wisconsin CIMSS archive: 5-day pentad SST analysis for 06 September 2005, from AOML SST Archive:

15 Forecasting extratropical transition: Timing: Model indicators Model flow fields hpa thickness fields Capture 500 hpa good indicator of capture by the mid-latitude flow Cyclone phase space products quantify: hpa thickness asymmetry (begin ET) Vertical derivative of cyclone-induced 500 mb height anomaly in the hpa layer - assume thermal wind balance to classify cyclone as warm or cold core (complete ET) (Hart, 2003) Model forecast capture of TY 02W (2007) in 500 hpa height field. 500 hpa heights (white lines), relative vorticity (solid color), and temperature (degrees C green dashed lines) from NOGAPS model shown here. Cyclone phase space analysis for Hurricane Maria (2005) from Florida State University example product archive:

16 Forecasting extratropical transition: Intensity and structure Intensity factors: vertical wind shear, sea surface temperatures, baroclinic influences, model development Wind field factors: Expansion on down-shear side of cyclone forecast often based on climatology, persistence, and model fields Multiple challenges: Numerical models: limited observations, assimilation and parameterization schemes vary, complex ocean / atmosphere interactions, use of synthetic observations (Jones et al., 2003) Deep layer shear from University of Wisconsin CIMSS archive: Increasing vertical wind shear and decreasing sea surface temperatures drive weakening prior to re-intensification Difficult to forecast amount and timing of weakening and re-intensification, especially several days in advance Final advisory graphic for Hurricane Maria (2005), courtesy Naval Research Lab.

17 Forecasting extratropical transition: Analysis during transition Along-track acceleration Less distinct center Wind field shift and expansion

18 Extratropical transition: Contribution to a local forecast strategy? Given the impact of ET on baroclinic development downstream may want to choose a model for local forecasting that fits the operational center s official forecast for track, intensity, and wind field evolution Research needed to test this hypothesis Inspection of ensemble model forecasts may provide more information on predictability in a given case For example: ECMWF the model of choice in this case?

19 References Agustí-Panareda, A., S.L. Gray, G.C. Craig, and C. Thorncroft, 2005: The extratropical transition of tropical cyclone Lili (1996) and its crucial contribution to a moderate extratropical development. Mon. Wea. Rev., 133, Anwender, D., P.A. Harr, and S.C. Jones, 2008: Predictability associated with the downstream impacts of the extratropical transition of tropical cyclones: Case studies. Mon. Wea. Rev., 136, Evans, J.L. and R.E. Hart, 2003: Objective indicators of the life cycle evolution of extratropical transition for Atlantic tropical cyclones. Mon. Wea. Rev., 131, Frank, W., The structure and energetics of the tropical cyclone I. Storm structure. Mon. Wea. Rev., 105, Jones, S.C., P.A. Harr, J. Abraham, L.F. Bosart, P.J. Bowyer, J.L. Evans, D.E. Hanley, B.N. Hanstrum, R.E. Hart, F. Lalaurette, M.R. Sinclair, R.K. Smith, C. Thorncroft, 2003: The extratropical transition of tropical cyclones: Forecast challenges, current understanding, and future directions. Wea. Forecasting,18, Harr, P.A. and J.M. Dea, 2009: Downstream development associated with the extratropical transition of tropical cyclones over the western North Pacific. Mon. Wea. Rev., 137, Harr P.A., D. Anwender, and S.C. Jones, 2008: Predictability associated with the downstream impacts of the extratropical transition of tropical cyclones: Methodology and a case study of Typhoon Nabi (2005). Mon. Wea. Rev., 136, Harr, P.A., R.L. Elsberry, and T.F. Hogan, 2000: Extratropical transition of tropical cyclones over the western North Pacific. Part II: The impact of midlatitude circulation characteristics. Mon. Wea. Rev., 128, Hart, R.E. and J.L. Evans, 2001: A climatology of the extratropical transition of Atlantic tropical cyclones. J. Climate, 14, Hart, R.E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev., 131, Hart, R.E., J.L. Evans, and C. Evans, 2006: Synoptic composites of the extratropical transition life cycle of North Atlantic tropical cyclones: Factors determining posttransition evolution. Mon. Wea. Rev., 134, Kitabatake, N., 2008: Extratropical transition of tropical cyclones in the western North Pacific: Their frontal evolution. Mon. Wea. Rev.,136, Klein, P.M., P.A. Harr, and R.L. Elsberry, 2002: Extratropical transition of western North Pacific tropical cyclones: Midlatitude and tropical cyclone contributions to reintensification. Mon. Wea. Rev., 130, Pasch, R.J. and E.S. Blake, 2005: Tropical cyclone report: Hurricane Maria. National Hurricane Center Tech. Rep., 9 pp. Reynolds, C.A., M.S. Peng, and J.-H. Chen, 2009: Recurving tropical cyclones: Singular vector sensitivity and downstream impacts. Mon. Wea. Rev., 137, Riemer, M. and S.C. Jones, 2010: The downstream impact of tropical cyclones on a developing baroclinic wave in idealized scenarios of extratropical transition. Quart. J. Roy. Meteor. Soc., 136, Riemer, M., S.C. Jones, and C.A. Davis, 2008: The impact of extratropical transition on the downstream flow: An idealized modeling study with a straight jet. Quart. J. Roy. Meteor. Soc., 134, Ritchie, E.A. and R.L. Elsberry, 2007: Simulations of the extratropical transition of tropical cyclones: Phasing between the upper-level trough and tropical cyclones. Mon. Wea. Rev., 135,

20 Joint Typhoon Warning Center Located in Pearl Harbor, HI Established originally located in Guam Approximately 25 staff from Naval Maritime Forecast Center (Navy) and 17 th Operation Weather Squadron (Air Force) Analysis and forecasting of tropical cyclone activity for US Government in Pacific and Indian Oceans Genesis potential bulletins Tropical cyclone formation alerts Tropical cyclone warnings