Upper-Level Precursors Associated with Subtropical Cyclone Formation

in the North Atlantic

Alicia M. Bentley, Lance F. Bosart, and Daniel KeyserUniversity at Albany, SUNY

38th Annual Northeastern Storm ConferenceRutland, VT

9 March 2013

Subtropical Storm Sean8 November 2011

28N

68W72W 64W

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)

Subtropical CyclonesDiabatic

Energy Sources

BaroclinicEnergy Sources

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

TCs Subtropical cyclones

Frontal cyclones

Motivation

• 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

• Adapt Davis (2010) methodology for STC identification– Equations and schematics

• Case Study: STC Sean (2011) – Track– Anticyclonic wave breaking (AWB) precursor event– Tropical transition (TT)– Application of adapted Davis (2010) methodology for STC

identification

• Discussion and Conclusions

Outline

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

Adapt Davis (2010) Methodology

absolute vorticity

425 hPa

Potential temperature anomaly Length of 6° of latitude

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

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

Lower-tropospheric baroclinic processes (PV1)

200 hPa

925 hPa

Adapt Davis (2010) Methodology

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

• Midtroposphericlatent heat release:(interior PV anomaly)

500 hPa

Lower-tropospheric baroclinic processes (PV1)

200 hPa

925 hPa

Adapt Davis (2010) Methodology

Midtroposphericlatent heat release

(PV2)

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

• Midtroposphericlatent 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-troposphericdynamical 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

Midtroposphericlatent heat release

(PV2)

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

• Midtroposphericlatent heat release:(interior PV anomaly)

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

500 hPa

Lower-tropospheric baroclinic processes (PV1)

200 hPa

925 hPa

Adapt Davis (2010) Methodology

Midtroposphericlatent heat release

(PV2)

Upper-troposphericdynamical processes

(PV3)

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

• Midtroposphericlatent heat release:(interior PV anomaly)

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

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

500 hPa

Lower-tropospheric baroclinic processes (PV1)

200 hPa

925 hPa

Midtroposphericlatent heat release

(PV2)

300 hPa

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

• Midtroposphericlatent heat release:(interior PV anomaly)

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

• Vertical wind shear

Vertical wind shear

Adapt Davis (2010) Methodology

Upper-troposphericdynamical processes

(PV3)

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

Case Study

• STC Sean (2011)(6 November – 12 November)

– Track– AWB precursor event– Tropical transition (TT)– Time series of PV1–PV3

and PV1/PV2

Images created using 0.5° Global Forecast System (GFS) analyses

(Dynamic Tropopause: 1.0° GFS analyses)

Image courtesy of the National Climatic Data Center

Subtropical Storm Sean0600 UTC 8 Nov

Officially tracked by NHC as an “extratropical low

pressure system”0000 UTC 6 Nov

Tropical Storm Sean1800 UTC 8 Nov

TS Sean reclassified asan “extratropical low

pressure system”0000 UTC 12 Nov

Tropical cycloneSubtropical cycloneExtratropical cyclone / Remnant low

STC Sean (2011): Track

STC Sean (2011): Upper-level Precursors

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

1200 UTC 31 October 2011

STC Sean (2011): Upper-level Precursors

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

T1

1200 UTC 31 October 2011

AWB

STC Sean (2011): Upper-level Precursors

1200 UTC 1 November 2011

T1

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

AWB

STC Sean (2011): Upper-level Precursors

1200 UTC 2 November 2011

T1

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

T1

STC Sean (2011): Upper-level Precursors

1200 UTC 3 November 2011

T1

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 3 November 2011

T1

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 4 November 2011

T1

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 5 November 2011

T1

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 6 November 2011

T1

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 7 November 2011

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 8 November 2011

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 9 November 2011

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

1200 UTC 10 November 2011

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

STC Sean (2011): Upper-level Precursors

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

1200 UTC 11 November 2011

STC Sean (2011): Upper-level Precursors

T2

Dynamic tropopause (DT, 1.5-PVU surface) potential temperature (shaded, K) and wind (barbs, kts), 925–850-hPa layer-averaged cyclonic relative vorticity (black contours every 0.5 × 10−4 s−1)

1200 UTC 12 November 2011

Position of cross section

STC Sean (2011): Tropical Transition

PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10−3 hPa s−1), and winds (barbs, kts)

0000 UTC 6 November 2011

33.5°N,70°W 23.5°N,70°WL

Position of cross section

STC Sean (2011): Tropical Transition

PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10−3 hPa s−1), and winds (barbs, kts)

1200 UTC 7 November 2011

32.5°N,69°W 22.5°N,69°WL

Position of cross section

STC Sean (2011): Tropical Transition

PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10−3 hPa s−1), and winds (barbs, kts)

0000 UTC 9 November 2011

33°N,70°W 23°N,70°WL

Position of cross section

STC Sean (2011): Tropical Transition1200 UTC 10 November 2011

PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10−3 hPa s−1), and winds (barbs, kts)

25°N,71°W35°N,71°WL

Position of cross section

STC Sean (2011): Tropical Transition0000 UTC 12 November 2011

PV (shaded, PVU), potential temperature (solid black every 3 K), upward vertical motion (dashed blue every 3 × 10−3 hPa s−1), and winds (barbs, kts)

41°N,59.5°W 31°N,59.5°WL

STC Sean (2011): Adapted Davis (2010)

PV metrics and vertical wind shear

values calculated from the 0.5° Climate Forecast System

Reanalysis V2 dataset

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

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 2011925–300-hPa vertical wind shear:

24.6 m s−1

T1

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV3

PV2PV1

PV1PV2

PV

1/P

V2 P

VU

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 2011925–300-hPa vertical wind shear:

13.6 m s−1

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV

1/P

V2 P

VU

PV3

PV2PV1

PV1PV2

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 2011925–300-hPa vertical wind shear:

10.3 m s−1

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV

1/P

V2 P

VU

PV3

PV2PV1

PV1PV2

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 2011925–300-hPa vertical wind shear:

13.9 m s−1

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV

1/P

V2 P

VU

PV3

PV2PV1

PV1PV2

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 2011925–300-hPa vertical wind shear:

12.9 m s−1

T2

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV

1/P

V2 P

VU

PV3

PV2PV1

PV1PV2

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

T2

1200 UTC 11 November 2011

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV

1/P

V2 P

VU

PV3

PV2PV1

PV1PV2

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

T2

1200 UTC 12 November 2011

STC Sean (2011): Adapted Davis (2010)

6 N

ov

8 N

ov

10 N

ov

7 N

ov

9 N

ov

11 N

ov

12 N

ov

PV

1/P

V2 P

VU

PV3

PV2PV1

PV1PV2

Conclusions

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

• STCs can form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events

• Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes to the evolution of STC Sean (2011)

• Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during TT of STC Sean (2011)

Questions? ambentley@albany.edu

Special Thanks: Kyle MacRitchie and Matthew Janiga

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

• STCs can form beneath intrusions of midlatitude PV streamers into the subtropics associated with AWB events

• Davis (2010) methodology adapted to quantify the relative contributions of lower-tropospheric baroclinic processes, midtropospheric condensation heating, and upper-tropospheric dynamical processes to the evolution of STC Sean (2011)

• Upper-tropospheric PV reduced and lower-tropospheric PV enhanced during TT of STC Sean (2011)