Download - Upper-Level Precursors Associated with Subtropical Cyclone Formation in the North Atlantic
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? [email protected]
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)