Munehiko Yamaguchi Typhoon Research Department, Meteorological Research Institute of the Japan Meteorological Agency 9:00 – 12: (Thr) Topic No. 7 Ensemble Initial Perturbations and Their Growth for Tropical Cyclone Forecasts Tropical Cyclone Ensemble Forecast Nanjing, China

ECMWF (50 members) Sinlaku initiated at 12UTC 10 Sep Dolphin initiated at 00UTC 13 Dec Some contradictions of ensemble spread among EPSs Japan Philippines Taiwan NCEP (20 members) Black line: Best track Grey lines: Ensemble member

What controls the ensemble spread of tracks? 1.Methods: Different methods of creating initial perturbations may control it, resulting in different growth of the perturbations. 2.Amplitudes: Initial amplitudes of the perturbations may affect the size of the ensemble spread, especially in the early forecast stage. ECMWFNCEPJMA MethodSV methodEnsemble Transform SV method Amplitudes are determined in each NWP center in a statistical way

Results 1.Perturbation structure and amplitudes are quite different among the NWP centers 2.Those differences cause the different modification of TC advection flows 3.Baroclinic energy conversion within a vortex leads to the modification of the advection flows 4.Differences of the ensemble spread of tracks among NWP centers are attributed to the growth of the perturbation and the initial amplitude

Best track and intensity of Typhoon Sinlaku by RSMC Tokyo-Typhoon Center Best track Intensity MSLP (hPa)

Synopsis in a before-recurvature stage 500 hPa Streamfunction (solid line) and Pacific High (bold line) Sinlaku was located west of the Pacific High. Sinlaku slowly moved northward by the steering flow associated with the Pacific High.

Vertical profile of perturbation Kinetic Energy Perturbation kinetic energy at each vertical level is averaged over the all ensemble members over a 2000 km x 2000 km domain centered on Sinlaku JMA ECMWF NCEP

Horizontal distribution of vertically averaged perturbation KE in a before-recurvature stage ECMWFNCEPJMA 2000km The horizontal distributions are created by averaging the energy over the all ensemble members and over the all vertical levels in a storm relative coordinate.

Radial profile of the axisymmetric tangential wind ECMWFNCEP Verification at 850 hPa in the before recurvature stage Black: Non-perturbed (Control) member Green: Perturbed members

Decomposition of flows in the vicinity of TCs Background flows associated with synoptic features Total flow Steering vector TC circulation itself Axisymmetric circulation Asymmetric circulation Asymmetric propagation vector Spatial Low-pass filter Cutoff wavelength=1200km Spatial Low-pass filter Cutoff wavelength=1200km Total flow minus Background flow HL

Distinctive feature of azimuthal wavenumber 1 perturbation Only azimuthal wavenumber 1 perturbation can create (advection) flows over the maximum vortex area. HL H H LL HH H L L L Azimuthal wavenumber 1 perturbation Azimuthal wavenumber 2 perturbation Azimuthal wavenumber 3 perturbation Advection flow canceled

Steering and asymmetric propagation vectors ECMWFNCEP Black: Non-perturbed (Control) member Green: Perturbed members Steering vector Asymmetric propagation vector

ECMWFNCEP T+0h T+12h T+48h Spread with time Does not spread with time

Dynamical mechanisms of the growth of the steering and asymmetric propagation vector ECMWF’s SV-based perturbations are found to grow through; 1.Baroclinic energy conversion within a vortex 2.Baroclinic energy conversion associated with mid- latitude waves 3.Barotropic energy conversion within a vortex Those features are less distinctive in the NCEP EPS

Baroclinic energy conversion within a vortex The dynamics of the baroclinic instability in the mid-latitude waves can be applied to a TC-like vortex in a cylindrical coordinate system. Certain latitude North pole Mid-latitude Certain radius TC center Tropical cyclone Streamfunction perturbation Temperature perturbation C C W W

ECMWF ensemble member 21 before recurvature Shade: Streamfunction perturbation Contour: Temperature perturbation ECMWF ensemble member 21 (500hPa) Schematic L L H +T -T

Wind perturbation at 500 hPa in storm relative coordinate Radial heat flux at r=500km T = 0 hour (initial time) 3000 km N W E S NWESN

Wind perturbation at 500 hPa in storm relative coordinate Radial heat flux at r=500km T = 6 hours 3000 km N W E S NWESN

Wind perturbation at 500 hPa in storm relative coordinate Radial heat flux at r=500km T = 12 hours 3000 km N W E S NWESN

Wind perturbation at 500 hPa in storm relative coordinate Radial heat flux at r=500km T = 18 hours 3000 km N W E S NWESN

day forecasts 3 day forecasts Relationship of spread of ensemble track forecasts between ECMWF and NCEP

1. Perturbation growth as well as the initial amplitude of the perturbations control the spread of ensemble track forecasts. 2. Baroclinic energy conversion plays a role in the modification of the track Summary Though the ECMWF initial perturbation amplitudes are small, the growth of the perturbations helps to obtain an appropriately large ensemble spread of tracks. Meanwhile, the relatively large amplitudes of initial perturbations seem to play a role in obtaining the ensemble spread of tracks in NCEP.