Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic Alicia M. Bentley University at Albany, SUNY Cyclone Research Group 8 July 2013 Research support provided by NSF Grant AGS

Subtropical Cyclones Operational Definition “A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones.” “Unlike tropical cyclones, subtropical cyclones derive a significant portion of their energy from baroclinic sources… often being associated with an upper-level low or trough.” − National Hurricane Center Online Glossary (2012)

Davis (2010) methodology: –Based on Ertel potential vorticity (PV) –Formulated in terms of two PV metrics that quantify the relative contributions of baroclinic processes and condensation heating to the evolution of individual cyclones Davis (2010) methodology is similar to Hart (2003) cyclone phase space diagrams Adapt Davis (2010) Methodology

Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Adapt Davis (2010) Methodology absolute vorticity 425 hPa Potential temperature anomaly Length of 6° of latitude

Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) Adapt Davis (2010) Methodology absolute vorticity 425 hPa Potential temperature anomaly Length of 6° of latitude Ertel PV anomaly PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

Adapt Davis (2010) Methodology 200 hPa 925 hPa

Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly)

Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly)

500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

Adapt Davis (2010) Methodology Introduce additional metric to diagnose upper-tropospheric dynamical processes Upper-tropospheric dynamical processes: (upper-tropospheric PV anomaly) Ertel PV anomaly 300 hPa Length of 6° of latitude

500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa Adapt Davis (2010) Methodology Upper- tropospheric dynamical processes (PV3) Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) Upper-tropospheric dynamical processes: (upper-tropospheric PV anomaly) PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa 300 hPa Lower-tropospheric baroclinic processes: (near-surface potential temperature anomaly) Midtropospheric latent heat release: (interior PV anomaly) Upper-tropospheric dynamical processes: (upper-tropospheric PV anomaly) Vertical wind shear Adapt Davis (2010) Methodology Upper- tropospheric dynamical processes (PV3) PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating

Case Study STC Sean (2011) (6 November – 12 November) –Track –Time series of PV1–PV3 and PV1/PV2 during tropical transition (TT) Images created using 0.5° Climate Forecast System Reanalysis v2 (CFSR v2) dataset

Image courtesy of the National Climatic Data Center Tropical cyclone Subtropical cyclone Extratropical cyclone / Remnant low STC Sean (2011): Track

STC Sean (2011): Adapted Davis (2010) PV metrics and vertical wind shear values calculated from 0.5° CFSR v2 dataset PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating 500 hPa Lower-tropospheric baroclinic processes (PV1) 200 hPa 925 hPa 300 hPa Vertical wind shear Upper- tropospheric dynamical processes (PV3)

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 6 November –300-hPa vertical wind shear: 24.6 m s −1 T1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV3 PV2 PV1 PV2 PV1/PV2 PVU

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 7 November –300-hPa vertical wind shear: 13.6 m s −1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV1 PV2 PV3 PV2 PV1

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 8 November –300-hPa vertical wind shear: 10.3 m s −1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1 PV2

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 9 November –300-hPa vertical wind shear: 13.9 m s −1 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 1200 UTC 10 November –300-hPa vertical wind shear: 12.9 m s −1 T2 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) Calculation Location 925–300-hPa vertical wind shear: 18.8 m s −1 T UTC 11 November 2011 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

DT potential temperature (shaded, K), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10 −4 s −1 ) 925–300-hPa vertical wind shear: 41.6 m s −1 Calculation Location T UTC 12 November 2011 STC Sean (2011): Adapted Davis (2010) 6 Nov8 Nov 10 Nov 7 Nov 9 Nov 11 Nov 12 Nov PV1/PV2 PVU PV3 PV1 PV2 PV1

STC Climatology (1979–2010) Apply adapted Davis (2010) methodology to North Atlantic tropical cyclogenesis cases identified in McTaggart-Cowan et al. (2013) IBTrACS (v03r03) North Atlantic cases 1948–2010: 816 cases 1979–2010: 460 cases (period of CFSR) 36 h backward trajectories obtained using a reverse steering flow calculation (McTaggart-Cowan et al. 2008) and added to IBTrACS “A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis”

STC Climatology (1979–2010) Development pathways: (1) strong TT, (2) weak TT, (3) trough-induced, (4) nonbaroclinic, and (5) low-level baroclinic n = 460

STC Climatology (1979–2010) The development pathway of a cyclone is determined by the “characteristics of the environmental state” 12 h prior to the cyclone’s first position in IBTrACS –Q metric (Q): the average convergence of the 400–200 hPa Q-vector (non-divergent) within 6° of the point of interest –Low-level thickness asymmetry (Th): the maximum 1000–700 hPa thickness values in two semi-circles within 10° of the point of interest

STC Climatology (1979–2010) The development pathway of a cyclone is determined by the “characteristics of the environmental state” 12 h prior to the cyclone’s first position in IBTrACS –Q metric (Q): the average convergence of the 400–200 hPa Q-vector (non-divergent) within 6° of the point of interest –Low-level thickness asymmetry (Th): the maximum 1000–700 hPa thickness values in two semi-circles within 10° of the point of interest

STC Climatology (1979–2010) Development pathways: (1) strong TT, (2) weak TT, (3) trough-induced, (4) nonbaroclinic, and (5) low-level baroclinic n = 460

STC Climatology (1979–2010) Development pathways: (1) strong TT, (2) weak TT, (3) trough-induced, n = 222

STC Climatology (1979–2010) Development pathways: (1) strong TT, (2) weak TT, (3) trough-induced, n = 621

STC Climatology (1979–2010) Development pathways: (1) strong TT, (2) weak TT, (3) trough-induced, n = 124

STC Climatology (1979–2010) Development pathways: (1) strong TT, (2) weak TT, (3) trough-induced, n = 361

STC Climatology (1979–2010) PV metric graphs, DT maps, and cross sections created for each cyclone

Future Work Determine the time/position when individual cyclones became STCs using the three aforementioned PV metrics –Possible that not all baroclinic developments with upper-tropospheric features became STCs during their lifecycle –Identify features/trends in PV metrics that are associated with STCs –Develop objective identification techniques Perform a cyclone-relative composite analysis for each of the aforementioned development pathways to document the structure, motion, and evolution of the upper-tropospheric features linked to STC formation

Future Work Determine the time/position when individual cyclones became STCs using the three aforementioned PV metrics –Possible that not all baroclinic developments with upper-tropospheric features became STCs during their lifecycle –Identify features/trends in PV metrics that are associated with STCs –Develop objective identification techniques Perform a cyclone-relative composite analysis for each of the aforementioned development pathways to document the structure, motion, and evolution of the upper-tropospheric features linked to STC formation

(1)Identify features/trends in PV metrics that are associated with STCs Future Work PV1 iiiiii PV2 iiiiii PV3 iiiiii PV1/PV2

(1)Identify features/trends in PV metrics that are associated with STCs Future Work PV1 iiiiii PV2 iiiiii PV3 iiiiii PV1/PV2

Future Work (1)Identify features/trends in PV metrics that are associated with STCs (2)Develop objective identification techniques PV1 iiiiii PV2 iiiiii PV3 iiiiii PV1/PV2

Future Work Slope PV2 iiiiii PV3 iiiiii (1)Identify features/trends in PV metrics that are associated with STCs (2)Develop objective identification techniques

Future Work PV3/PV2 Ratio PV2 iiiiii PV3 iiiiii PV3/PV2 iiiiii (1)Identify features/trends in PV metrics that are associated with STCs (2)Develop objective identification techniques

Future Work PV3/PV2 Ratio PV2 iiiiii PV3 iiiiii PV3/PV2 PV3/PV2 slope iiiiii (1)Identify features/trends in PV metrics that are associated with STCs (2)Develop objective identification techniques

Thoughts/Comments/Questions? PV3/PV2 Ratio PV2 iiiiii PV3 iiiiii PV3/PV2 PV3/PV2 slope iiiiii (1)Identify features/trends in PV metrics that are associated with STCs (2)Develop objective identification techniques