Upper-Level Precursors Associated with Subtropical Cyclone Formation
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Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic. Subtropical Storm Sean 8 November 2011. 28N. 72W. 68W. 6 4 W. Alicia M. Bentley, Daniel Keyser, and Lance F. Bosart University at Albany, SUNY 16 th Cyclone Workshop 27 September 2013

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Upper-Level Precursors Associated with Subtropical Cyclone Formation

in the North Atlantic

Subtropical Storm Sean

8 November 2011

28N

72W

68W

64W

Alicia M. Bentley, Daniel Keyser, and Lance F. Bosart

University at Albany, SUNY

16th Cyclone Workshop

27 September 2013

Research support provided by NSF Grant AGS-0935830


Subtropical Cyclones Formation

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)


Subtropical Cyclones Formation

DiabaticEnergySources

TCs

Subtropical cyclones

Frontal cyclones

BaroclinicEnergy Sources

Adapted from Fig. 9 in Beven (2012)30th Conference on Hurricanes and Tropical Meteorology


Motivation Formation

  • There is currently no objective set of characteristics used to define subtropical cyclones (STCs)

  • The hybrid nature of STCs makes them likely candidates to become tropical cyclones (TCs) via the tropical transition (TT) process

  • Few studies address the relationship between STCs, TC development, and high-impact weather events


Outline Formation

  • Adapt Davis (2010) methodology for STC identification

  • Refine objective STC identification technique and apply to North Atlantic baroclinically influenced tropical cyclogenesis cases to construct STC climatology (1979–2010)

  • Perform a cyclone-relative composite analysis of the upper-tropospheric features linked to STC formation

  • Discussion and Conclusions


Adapt Davis (2010) Methodology Formation

  • Davis (2010) methodology:

    • Based on Ertel potential vorticity (PV)

    • Formulated in terms of two PV metrics that quantify the relative contributions of baroclinicprocesses 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 Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

425 hPa

Potential temperature anomaly

Length of 6° of latitude

absolute vorticity


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PVanomaly)

425 hPa

Potential temperature anomaly

Length of 6° of latitude

absolute vorticity

Ertel PV anomaly

PV1/PV2 : measure of the contribution of lower-troposphericbaroclinic processes relative to the contribution of condensation heating


Adapt Davis (2010) Methodology Formation

200 hPa

925 hPa


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

200 hPa

925 hPa

Lower-tropospheric baroclinic processes (PV1)


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PV anomaly)

200 hPa

925 hPa

Lower-tropospheric baroclinic processes (PV1)


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PV anomaly)

200 hPa

500 hPa

Midtroposphericlatent heat release (PV2)

925 hPa

Lower-tropospheric baroclinic processes (PV1)

PV1/PV2 : measure of the contribution of lower-tropospheric baroclinic processes relative to the contribution of condensation heating


Adapt Davis (2010) Methodology Formation

  • Introduce additional metric to diagnose upper-tropospheric dynamical processes

  • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly)

Ertel PV anomaly

300 hPa

Length of 6° of latitude


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PV anomaly)

200 hPa

500 hPa

Midtroposphericlatent heat release (PV2)

925 hPa

Lower-tropospheric baroclinic processes (PV1)

PV1/PV2 : measure of the contribution of lower-troposphericbaroclinic processes relative to the contribution of condensation heating


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PV anomaly)

  • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly)

Upper-troposphericdynamical processes(PV3)

200 hPa

500 hPa

Midtroposphericlatent heat release (PV2)

925 hPa

Lower-tropospheric baroclinic processes (PV1)

PV1/PV2 : measure of the contribution of lower-troposphericbaroclinic processes relative to the contribution of condensation heating


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PV anomaly)

  • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly)

Upper-troposphericdynamical processes(PV3)

200 hPa

500 hPa

Midtroposphericlatent heat release (PV2)

925 hPa

Lower-tropospheric baroclinic processes (PV1)


Adapt Davis (2010) Methodology Formation

  • Lower-troposphericbaroclinic processes:(near-surface potentialtemperature anomaly)

  • Midtroposphericlatent heat release:(interior PV anomaly)

  • Upper-troposphericdynamical processes:(upper-tropospheric PV anomaly)

Upper-troposphericdynamical processes(PV3)

200 hPa

500 hPa

Midtroposphericlatent heat release (PV2)

925 hPa

Lower-tropospheric baroclinic processes (PV1)

PV3/PV2 : measure of the contribution of upper-troposphericdynamical processes relative to the contribution of condensation heating


STC Identification Formation

  • Apply adapted Davis (2010) methodology to a subset of North Atlantic baroclinically influenced tropical cyclogenesis cases identified in McTaggart-Cowan et al. (2013)

  • IBTrACS (v03r03)

  • North Atlantic tropical cyclogenesis cases1948–2010: 816 cases1979–2010: 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


STC Identification Formation

N = 460

Strong TT

Weak TT

Trough induced

Low-level baroclinicNonbaroclinic


STC Identification Formation

N = 460

Strong TT

Weak TT

Trough induced

Low-level baroclinicNonbaroclinic

Category Description

Strong TT Upper-level disturbance without strong lower-level thermal gradients

Weak TT Upper-level disturbance with moderate lower-level thermal gradients

Trough induced Upper-level disturbance without appreciable lower-level thermal gradients

Low-level baroclinic Strong lower-level thermal gradients without an upper-level disturbance

Nonbaroclinic No appreciable baroclinic influences


STC Identification Formation

N = 460

Strong TT

Weak TT

Trough induced

Low-level baroclinicNonbaroclinic

Category Description

Strong TT Upper-level disturbance without strong lower-level thermal gradients

Weak TT Upper-level disturbance with moderate lower-level thermal gradients

Trough induced Upper-level disturbance without appreciable lower-level thermal gradients

Low-level baroclinic Strong lower-level thermal gradients without an upper-level disturbance

Nonbaroclinic No appreciable baroclinic influences


STC Identification Formation

N = 460

Strong TT

Weak TT

Trough induced

Low-level baroclinicNonbaroclinic

Category Description

Strong TT Upper-level disturbance without strong lower-level thermal gradients

Weak TT Upper-level disturbance with moderate lower-level thermal gradients

Trough induced Upper-level disturbance without appreciable lower-level thermal gradients

Low-level baroclinic Strong lower-level thermal gradients without an upper-level disturbance

Nonbaroclinic No appreciable baroclinic influences


STC Identification Formation

N = 460

Strong TT

Weak TT

Trough induced

Low-level baroclinicNonbaroclinic


STC Identification Formation

N = 222

Strong TT

Weak TT

Trough induced

Low-level baroclinicNonbaroclinic


STC Identification Formation

  • Identify STC signature in PV metrics to refine objective identification technique

  • Determine the time and position when individual cyclones became STCs using objective identification technique to construct STC climatology (1979–2010)

  • Perform a cyclone-relative composite analysis to document the structure, motion, and evolution of the upper-tropospheric features linked to STC formation


STC Identification Formation

  • Identify STC signature in PV metrics to refine objective identification technique

  • Determine the time and position when individual cyclones became STCs using objective identification technique to construct STC climatology (1979–2010)

  • Perform a cyclone-relative composite analysis to document the structure, motion, and evolution of the upper-tropospheric features linked to STC formation


STC Identification Formation

STC Sean (2011)

1745 UTC 7 November 2011

PV3/PV2

PVU

EX LOSTC TS EX

Image courtesy of NASA Goddard MODIS Rapid Response Team

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

1745 UTC 7 November 2011

PV3/PV2

PVU

EX LOSTC TS EX

Image courtesy of NASA Goddard MODIS Rapid Response Team

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

Criteria:

PV3/PV2

PVU

EX LOSTC TS EX

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

Criteria:

  • Slope of PV3/PV2 < 0 at t = 0 h, 6 h

PV3/PV2

PVU

EX LOSTC TS EX

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

Criteria:

  • Slope of PV3/PV2 < 0 at t = 0 h, 6 h

  • PV3 > 0 and PV2 > 0 at t = 0 h

PV3/PV2

PVU

EX LOSTC TS EX

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

Criteria:

  • Slope of PV3/PV2 < 0 at t = 0 h, 6 h

  • PV3 > 0 and PV2 > 0 at t = 0 h

  • Slope of PV3 < 0 at t = 6h, 12 h

PV3/PV2

PVU

EX LOSTC TS EX

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

Criteria:

  • Slope of PV3/PV2 < 0 at t = 0 h, 6 h

  • PV3 > 0 and PV2 > 0 at t = 0 h

  • Slope of PV3 < 0 at t = 6h, 12 h

  • Not a hurricane

  • Not a tropical storm

  • Not a tropical depression for ≥ 12 h

PV3/PV2

PVU

EX LOSTC TS EX

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

PV1 PV2 PV3 PV3/PV2


STC Identification Formation

STC Sean (2011)

Criteria:

  • Slope of PV3/PV2 < 0 at t = 0 h, 6 h

  • PV3 > 0 and PV2 > 0 at t = 0 h

  • Slope of PV3 < 0 at t = 6h, 12 h

  • Not a hurricane

  • Not a tropical storm

  • Not a tropical depression for ≥ 12 h

PV3/PV2

PVU

EX LOSTC TS EX

9 Nov

7 Nov

8 Nov

10 Nov

12 Nov

11 Nov

= STC identified

PV1 PV2 PV3 PV3/PV2


STC Climatology ( Formation 1979–2010)

N = 105

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


STC Climatology ( Formation 1979–2010)

N = 105

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


STC Climatology ( Formation 1979–2010)

N = 34

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


STC Climatology ( Formation 1979–2010)

N = 56

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


STC Climatology ( Formation 1979–2010)

N = 15

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


STC Climatology ( Formation 1979–2010)

N = 125

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


STC Climatology ( Formation 1979–2010)

N = 105

~3 STCs/year

Strong TT

Weak TT

Trough Induced

Average Strong TT

Average Weak TT

Average Trough Induced

Average STC


Conclusions Formation

  • STCs have characteristics of both tropical and extratropicalcyclones and are likely candidates to become TCs via TT

  • Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes during baroclinically influenced tropical cyclogenesis events

  • Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during STC formation

  • Cyclone-relative composite analysis revealed that STCs form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events


Questions? Formation [email protected]

  • STCs have characteristics of both tropical and extratropicalcyclones and are likely candidates to become TCs via TT

  • Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes during baroclinically influenced tropical cyclogenesis events

  • Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during STC formation

  • Cyclone-relative composite analysis revealed that STCs form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events

Special Thanks: Chris Davis and Ron McTaggart-Cowan


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