A 40-year climatology of extratropical transition in the eastern North Pacific continued (and some other stuff)

Transcription

1 GOES-EAST image of Hurricane Dora, 1445 UTC 21 July 2011 A 40-year climatology of extratropical transition in the eastern North Pacific continued (and some other stuff) Elizabeth A. Ritchie and Kimberly M. Wood & David Kofron and Steven Felker and Scott Tyo The University of Arizona

2 Extratropical Transition 19 UTC 16 August UTC 18 August UTC 20 August UTC 21 August 2009 Extratropical Transition = decay of a tropical cyclone in conjunction with a mid-latitude feature (trough, preexisting cyclone, etc.) into an extratropical cyclone

3 Extratropical Transition All TC Tracks Guillermo Extratropical Transition Tracks Guillermo

4 Extratropical Transition

5 Extratropical Transition EOF 1 38% variance 44% 53% 19% variance EOF 2

6 Dissipaters vs. Intensifiers NOGAPS 1 Analyses from 2003 to re-curving TCs Separate into two classes: Intensifiers Dissipaters Define Reintensification: ΔP drop of 3 hpa P i, j is sea level pressure at the (i, j) point on grid. 1 P = n m n m i= 1 j= 1 P i P, j central Central pressure change relative to environmental pressure Use open wave definition for ET time (Demirci et al. 2007) (Kofron et al. 2010a,b)

7 Previous Work All 82 cases: Isentropic Potential Vorticity averaged within 500-km of TC center (Kofron et al. 2010a,b)

8 IPV Post-ET reintensification cases w/ an upstream interaction (Cold-core evolution): Post-ET dissipation: (Kofron et al. 2010a,b)

9 Previous Work Isentropic Potential Vorticity averaged within 500-km of TC center IPV [PVU] Average 330-K IPV 330-K IPV Minimum Centered NW Trough Cold-Core NW Trough Warm Seclusion NE Trough Post-ET Dissipater Non-ET Time [hr]

10 Intensifiers Eastern North Pacific Dissipaters Dissipaters Jimena 2009 Rick 2009 Dora 2011 Blas 2004 Dissipate Dissipate Dissipate Dissipate Otis 2005 Juliette 2007 Kiko 2007 Norbert 2008 Dissipate Maintain/Dissipate Dissipate Maintain/Intensify

11 Intensifiers Eastern North Pacific Dissipaters Intensifiers John 1994 Guillermo 1997 Ignacio 1997 Intensify Maintains Intensify Blanca 2009 Dolores2009 Guillermo 2009 Ignacio 2009 Intensify Maintains Intensify Intensify **Based on NHC Best Track data

12 Hurricane Guillermo 2009

13 Hurricane Guillermo 2009

14 Hurricane Guillermo mb

15 Hurricane Guillermo hPa Geopotential heights & SLP (shaded < 1008) Aug Z Aug Z Aug Z Aug Z Aug Z Aug Z

16 Hurricane Guillermo /00Z 21/00Z 22/00Z 400 hpa 500 hpa 700 hpa

17 Hurricane Guillermo 2009

18 Mean Fields ( ) August September October 200 hpa 500 hpa 700 hpa

19 SST fields September 2009

20 First Summary Continued discussion of ET in the eastern North Pacific Structural characteristics of ET are difficult to extract because of the dominance of the structure of the large-scale circulation pattern PV cross-sections indicate confinement of positive PV to upper levels of the atmosphere possibly due to presence of very strong and deep subtropical ridge in lower atmosphere. masks any low-level circulation development that would be captured in the area-averaged PV fields Continue to analyze structural characteristics using NOGAPS analyses find other ways to characterize the low-level structure Extend all tracks to at least 72 h past ET time using 700-hPa vorticity and other indicators of low-level circulation.

21 Predicting ET outcome Dominantly a two-phase process (Klein et al., 2000) Transformation phase Re-intensification phase (Demirci et al. 2007) (Felker et al. 2011) Individual storms often classified as Intensifiers or Dissipaters (e.g. Demirci et al., 2007; Kofron et al., 2010) Difficult to distinguish differences in storm structure of Intensifiers and Dissipaters during phase 1 (transformation) Spatial relationships between storms and the midlatitude environment can be used to differentiate Intensifiers and Dissipaters (Harr et al., 2000; Ritchie and Elsberry, 2007) Here we use these differences in mid-latitude spatial patterns to predict post-extratropical transition outcomes

22 Predicting ET outcome Classifying Storms: Intensifier 108 West Pacific storms from 2000 through 2008 ET time defined as when storm becomes open wave at 500mb at 20m contour level (as in Demirci et al., 2007) Intensifier if central surface pressure of storm dropped more than 3mb between ET +/- 6 hours and ET+54 +/- 18 hours Otherwise classified as dissipater Resulted in 53 intensifiers and 55 dissipaters overall ET-72 Dissipater ET ET+72

23 Surface Pressure Evolution Means

24 Classifying Storms - SVM Use 850-hPa potential temperature FNL analyses, 60 latitude x 50 longitude centered on TC center (3111 data points [features] per time frame) Data at 6 hour intervals from 72 hours before ET to 72 hours after ET Number of features reduced to 20 using Correlation-based Feature Selection (CFS) method (Hall, 1999) to prevent overtraining Features are those selected as discriminating the two classes of ET Chosen features are shown in white: Around the TC Deep in midlatitude trough

25 Classifying Storms - SVM Use 20 selected features at each relative ET time as SVM inputs Iterate through SVM parameters and use K-fold cross validation on testing set (80%) to choose the best set of parameters for generalization Re-run SVM on test set (20%) using established parameters to get true system performance

26 Classifying SVM Performance ET-72 ET-48 ET-24 ET ET+24 ET+48

27 Project Overview Over 75% (17 out of 21) of test storms were correctly classified at 72 hours prior to ET ( 3 dissipaters and 1 intensifier were misclassified) Features of the upstream trough were found to be especially influential in classifying storms as intensifiers or dissipaters Classification performance tends to decrease as ET time approaches, suggesting that storms exhibit very similar spatial patterns right before the transition process

28 Thank you