The Impacts on Extended-Range Predictability of Midlatitude Weather Patterns due to Recurving Tropical Cyclones

Transcription

1 The Impacts on Extended-Range Predictability of Midlatitude Weather Patterns due to Recurving Tropical Cyclones Patrick A. Harr and Heather M. Archambault Naval Postgraduate School, Monterey, CA Hurricane Maria TY Nabi TY Saola Achknowledgment: Office of Naval Research, Marine Meteorology

2 Composite flow evolution associated with TC recurvature N = hPa PV (blue, PVU), meridional wind anomalies (shaded, m s 1 ), and regions where anomalies are significant at 99% confidence level (black)

3 Operational GEFS uncertainty with TC Malakas (2010) Initialization time Blue: Verifying forecast Red: Ensemble forecast mean Green: Ensemble forecast members (N = 20)

4 Operational GEFS uncertainty with TC Malakas (2010) F+24 h Blue: Verifying forecast Red: Ensemble forecast mean Green: Ensemble forecast members (N = 20)

5 Operational GEFS uncertainty with TC Malakas (2010) F+48 h Blue: Verifying forecast Red: Ensemble forecast mean Green: Ensemble forecast members (N = 20)

6 Operational GEFS uncertainty with TC Malakas (2010) F+72 h Blue: Verifying forecast Red: Ensemble forecast mean Green: Ensemble forecast members (N = 20)

7 Operational GEFS uncertainty with TC Malakas (2010) F+96 h Blue: Verifying forecast Red: Ensemble forecast mean Green: Ensemble forecast members (N = 20)

8 Operational GEFS uncertainty with TC Malakas (2010) F+120 h Blue: Verifying forecast Red: Ensemble forecast mean Green: Ensemble forecast members (N = 20)

9 Monthly distribution of TC recurvature Recurvature: 292 of 801 WNP TCs; 36.5% % that recurve Archambault et al. 2013

10 Recurving western North Pacific TC Tracks: N = 292 KEY: TD TS TY EC Mean TC recurvature point: 24.9 N, E

11 ESRL/PSD second-generation reforecast data (Hamill et al. 2013) Reforecasts of 2012 version of NCEP Global Ensemble Forecasting System (GEFS) initialized with 0000 UTC CFSR initial conditions for 1985 present Ensemble contains 10 perturbed members plus a control run Members are perturbed using ensemble transform with rescaling technique T254 (~0.5 ) horizontal resolution; 42 vertical levels Continuous, homogenous dataset ideal for systematically examining predictability of largescale flow surrounding TC recurvature

12 Methodology Compute root mean-square errors (RMSEs), anomaly correlations (ACs) and ensemble spread (standard deviation) for 500-hPa geopotential height reforecasts verifying during western North Pacific TC recurvature between (N = 224) May November Western North Pacific (WPAC) North Pacific (NPAC) Northern Hemisphere (NH) Compare against climatology of reforecast RMSEs, ACs, and ensemble spread to test whether skill and uncertainty during recurvature are significantly different from climatology

13 Domains for computing forecast skill and uncertainty statistics Western North Pacific

14 Domains for computing forecast skill and uncertainty statistics North Pacific

15 Domains for computing forecast skill and uncertainty statistics Northern Hemisphere

16 Climo minus recurving TC RMSE: West Pacific 8-day forecast verifying 4 days past recurvature (m)

17 Climo minus recurving TC RMSE significance: West Pacific > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Decreased Skill Increased Skill

18 Climo minus recurving TC RMSE significance: West Pacific 8-day forecast verifying 4 days past recurvature > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Decreased Skill Increased Skill

19 Climo minus recurving TC RMSE significance: West Pacific Forecasts initialized before recurvature and verifying before recurvature have increased skill Forecasts initialized just before recurvature and verifying after recurvature have degraded skill > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Decreased Skill Increased Skill

20 Increased skill Decreased skill > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Decreased Skill Increased Skill

21 Increased skill Decreased skill > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Decreased Skill Increased Skill

22 Decreased skill > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Increased Skill Decreased Skill

23 Decreased skill at longer lead times > 97.5% Significance (negative tail) > 97.5% Significance (positive tail) Increased Skill Decreased Skill

24 Recurving TC Forecast - Analysis + 24 h NPAC

25 Recurving TC Forecast - Analysis + 48 h NPAC

26 Recurving TC Forecast - Analysis + 72 h NPAC

27 Recurving TC Forecast - Analysis + 96 h NPAC

28 Recurving TC Forecast - Analysis h NPAC

29 Recurving TC Forecast - Analysis h NPAC

30 Recurving TC Forecast - Analysis h Flow pattern of N. America is under amplified NHem

31 Conclusions In a climatological sense, recurvature of WPAC TCs impacts midlatitude predictability Decrease in skill and increase in spread (not shown) for forecasts initialized prior to recurvature but verifying after recurvature Increase in skill and decrease in spread (not shown) for forecasts that verify prior to recurvature Decreased skill after recurvature mainly at longer forecast intervals (beyond 5 days) Of the three regions examined, the North Pacific region is most sensitive to decreases in skill and increases in spread. Tropical cyclone recurvature coincides with a transition from a high to low predictability regime as defined by statistically significant decreased skill and increased spread