1. Michael S. Fischer and Brian H. Tang
The Influence of an Upper Tropospheric Potential Vorticity Anomaly on Rapid Tropical Cyclogenesis
Michael S. Fischer and Brian H. Tang

2. Motivation
Nearly half of all Atlantic genesis events form in an environment characterized by upper tropospheric forcing for ascent (McTaggart-Cowan et al. 2013)
Numerical modeling studies have demonstrated how an upper-tropospheric PV anomaly can result in TC genesis (Montgomery and Farrell 1993, Davis and Bosart 2003)
Why do we care about an upper-tropospheric PV anomaly resulting in TC genesis? It’s a very common genesis pathway.
Recent studies have demonstrated that in the Atlantic basin, a genesis pathway characterized by an upper-tropospheric PV anomaly, is quite common with nearly half of all genesis events originating from such a pathway.
Numerical modeling studies have provided insight into the physical mechanisms by which an upper-tropospheric PV anomaly can result in TC genesis.
However, the configurations of an upper-tropospheric PV anomaly resulting in rapid rates of TC genesis have not been clearly distinguished.
Background
Methodology
Results
Conclusions

3. Motivation
Nearly half of all Atlantic genesis events form in an environment characterized by upper tropospheric forcing for ascent (McTaggart-Cowan et al. 2013)
Numerical modeling studies have demonstrated how an upper-tropospheric PV anomaly can result in TC genesis (Montgomery and Farrell 1993, Davis and Bosart 2003)
Why do we care about an upper-tropospheric PV anomaly resulting in TC genesis? It’s a very common genesis pathway.
Recent studies have demonstrated that in the Atlantic basin, a genesis pathway characterized by an upper-tropospheric PV anomaly, is quite common with nearly half of all genesis events originating from such a pathway.
Numerical modeling studies have provided insight into the physical mechanisms by which an upper-tropospheric PV anomaly can result in TC genesis.
However, the configurations of an upper-tropospheric PV anomaly resulting in rapid rates of TC genesis have not been clearly distinguished.
Background
Methodology
Results
Conclusions

4. Data and Definitions
Used ERA-Interim, HURDAT, and GridSat IR imagery to analyze Atlantic TCs between 1980–2014
TCs were binned into three groups, dependent upon the genesis rate
The TC genesis rate is defined by the initial 24-hr change in maximum sustained winds (ΔVmax)
Background
Methodology
Results
Conclusions

5. Data and Definitions
Used ERA-Interim, HURDAT, and GridSat IR imagery to analyze Atlantic TCs between 1980–2014
TCs were binned into three groups, dependent upon the genesis rate
The TC genesis rate is defined by the initial 24-hr change in maximum sustained winds (ΔVmax)
Rapid Tropical Cyclogenesis (RTCG): ΔVmax ≥ 25 kt
Slow Tropical Cyclogenesis (STCG): 10 kt ≤ ΔVmax < 25 kt
Neutral Tropical Cyclogenesis (NTCG): -5 kt ≤ ΔVmax < 10 kt
Background
Methodology
Results
Conclusions

6. Methodology
A new TC/disturbance center was assigned based upon the 850-hPa relative vorticity centroid
Two subgroups were established based upon the upper-tropospheric flow regime: High-PV, Low-PV
High-PV: a PV anomaly exists on the 350-K isentropic surface ≥ 0.5 PVU within 500 km of the TC genesis location
0.5-PVU anomaly
Two different upper-tropospheric flow regimes were noted in the Atlantic basin
Upper trough present
Upper level anticyclone dominates
The 350 K isentropic sfc is a good proxy for the subtropical tropopause
500-km radius
Background
Methodology
Results
Conclusions

7. Genesis Rate Distribution (kt)
For the purpose of this talk, I will be focusing on RTCG and NTCG events, as these two groups have the most pronounced differences
STCG events are basically a blend of the RTCG and NTCG composites
Transition to convective structure of these
Background
Methodology
Results
Conclusions

8. Infrared Brightness Temperature (K)
Storms with more rapid rates of genesis feature more symmetric convection, even at the time of genesis
RTCG:
NTCG:
RTCG - NTCG:
t = 0 hr
Shear Direction
In all these plots, the brightness temperatures are shear-relative, with the shear vector pointing to the right side of the screen, as denoted by the black arrow.
t = 24 hr
Background
Methodology
Results
Conclusions

9. Vertical Wind Shear Calculated within a 200–800-km annulus
The distribution of vertical wind shear in RTCG events is shifted toward lower values than NTCG events
The differences of the distributions are statistically significant at the 95% level
The distribution of vertical wind shear is shifted toward lower values in RTCG events.
The mid-level shear is best correlated to the genesis rate in High-PV events.
Other environmental parameters do not show a strong correlation to the genesis rate, such as sea surface temperature and environmental moisture.
Background
Methodology
Results
Conclusions

10. 200-hPa Composite Winds (m s-1)
RTCG events feature a sharper trough than NTCG events, with stronger upstream ridging
RTCG:
NTCG:
t = -24 hr
Since the trough is upstream of the TC, this suggests the trough may be providing forcing for ascent due to positive vorticity advection
t = 24 hr
Background
Methodology
Results
Conclusions

11. 200-hPa Composite Winds (m s-1)
RTCG events feature a sharper trough than NTCG events, with stronger upstream ridging
RTCG:
NTCG:
t = -24 hr
Since the trough is upstream of the TC, this suggests the trough may be providing forcing for ascent due to positive vorticity advection
t = 24 hr
Background
Methodology
Results
Conclusions

12. Forcing for Ascent
Can use a simplified version of the Sutcliffe-Trenberth form of the QG Omega Equation, similar to Bracken and Bosart (2000)
Wind shear (Vt) is calculated between 500 and 200 hPa
Layer mean relative vorticity (ζavg) is calculated between 300 and 250 hPa
These levels were chosen to highlight forcing for ascent provided by the upper-tropospheric PV anomaly
Background
Methodology
Results
Conclusions

13. 300–250-hPa Mean Relative Vorticity (x104 s-1)
The upper-tropospheric PV anomaly remains upshear of the TC in the RTCG composite, while the NTCG composite loses the upshear tilt immediately after genesis
RTCG:
NTCG:
t = -24 hr
The individual components used to diagnose the QG forcing ascent are plotted here.
t = 24 hr
Shear Direction:
Background
Methodology
Results
Conclusions

14. Upshear semicircle averaged forcing for ascent (x10-10 s-2)
RTCG:
NTCG:
Upshear semicircle averaged forcing for ascent (x10-10 s-2)
NTCG events depict forcing for descent over the TC center following genesis
Since the two genesis rates groups have similar values of moisture in the inner-core region, this suggests RTCG events experience more diabatic heating through latent heat release
Background
Methodology
Results
Conclusions

15. Upshear semicircle averaged forcing for ascent (x10-10 s-2)
RTCG:
NTCG:
Upshear semicircle averaged forcing for ascent (x10-10 s-2)
NTCG events depict forcing for descent over the TC center following genesis
Since the two genesis rates groups have similar values of moisture in the inner-core region, this suggests RTCG events experience more diabatic heating through latent heat release
Background
Methodology
Results
Conclusions

16. Upshear semicircle averaged forcing for ascent (x10-10 s-2)
RTCG:
NTCG:
Upshear semicircle averaged forcing for ascent (x10-10 s-2)
NTCG events depict forcing for descent over the TC center following genesis
500-hPa vertical velocity (hPa day-1)
ERA-Interim vertical velocity values closely resemble the diagnosed forcing for ascent
Since the two genesis rates groups have similar values of moisture in the inner-core region, this suggests RTCG events experience more diabatic heating through latent heat release
Background
Methodology
Results
Conclusions

17. RTCG - NTCG IR Brightness: RTCG: NTCG: Background Methodology Results
Since the two genesis rates groups have similar values of moisture in the inner-core region, this suggests RTCG events experience more diabatic heating through latent heat release
Shear Direction
Background
Methodology
Results
Conclusions

18. PV Anomaly Structure
Vertical cross-sections can be taken along the shear vector to assess the depth and tilt of the PV anomaly
In addition to forcing for ascent, the structure and temporal evolution of the upper-tropospheric PV anomaly itself will be analyzed.
Vertical cross-sections were taken along the shear vector within 1,000 km of the TC location.
Shear Direction
Background
Methodology
Results
Conclusions

19. PV Anomaly (PVU) RTCG: NTCG: 30 hours prior to TC genesis
RTCG events feature stronger and vertically deeper anomalies
24 hours following TC genesis
PV anomalies in RTCG events are still tilted against the shear, while NTCG events have a neutral tilt
Background
Methodology
Results
Conclusions

20. PV Anomaly (PVU) RTCG: NTCG: 30 hours prior to TC genesis
RTCG events feature stronger and vertically deeper anomalies
24 hours following TC genesis
PV anomalies in RTCG events are still tilted against the shear, while NTCG events have a neutral tilt
Background
Methodology
Results
Conclusions

21. Upshear PV Anomaly RTCG: NTCG:
300–250-hPa layer mean within upshear semicircle
The upper-tropospheric PV anomaly in RTCG events remains upshear for a greater duration than NTCG events
The position of the upshear tilt of the upper level PV anomaly can be seen in a Hovmoller of the upshear quadrant.
Background
Methodology
Results
Conclusions

22. Upshear PV Anomaly RTCG: NTCG:
300–250-hPa layer mean within upshear semicircle
The upper-tropospheric PV anomaly in RTCG events remains upshear for a greater duration than NTCG events
This is correlates well with the period where forcing for ascent sharply decreases over the TC inner core
Background
Methodology
Results
Conclusions

23. Upshear PV Anomaly RTCG: NTCG:
300–250-hPa layer mean within upshear semicircle
The upper-tropospheric PV anomaly in RTCG events remains upshear for a greater duration than NTCG events
RTCG upper-tropospheric PV anomaly is effectively phase-locked to newly formed TC
Phase-locking of the PV anomaly in RTCG events allows for a greater duration of forcing for ascent and moist baroclinic instability
Background
Methodology
Results
Conclusions

24. Conclusions
Conclusions
The presence of an upper-tropospheric PV anomaly nearby a tropical disturbance can provide a pathway for quick TC spin-up
RTCG events are associated with stronger and deeper PV anomalies that become phase-locked with the newly-formed TC
Greater magnitude and duration of quasigeostrophic forcing for ascent, especially upshear of the TC
Stronger vertical velocities and more symmetric convection
Vertical redistribution of PV through diabatic heating
To summarize, the goal of this study was to identify which configurations of an upper tropospheric PV anomaly were favorable for increased rates of TC genesis.
The differences in the temporal evolution of the structure and position the upper-tropospheric PV anomalies plays a vital role
Note that these findings match the simulations of Montgomery and Farrell (1993) and Davis and Bosart (2003)
Background
Methodology
Results
Conclusions

25. Extra Slides Conclusions Background Methodology Results Conclusions
To summarize, the goal of this study was to identify which configurations of an upper tropospheric PV anomaly were favorable for increased rates of TC genesis.
The differences in the temporal evolution of the structure and position the upper-tropospheric PV anomalies plays a vital role
Note that these findings match the simulations of Montgomery and Farrell (1993) and Davis and Bosart (2003)
Background
Methodology
Results
Conclusions

26. Storm Tracks RTCG STCG NTCG Conclusions Background Methodology Results
To summarize, the goal of this study was to identify which configurations of an upper tropospheric PV anomaly were favorable for increased rates of TC genesis.
The differences in the temporal evolution of the structure and position the upper-tropospheric PV anomalies plays a vital role
Note that these findings match the simulations of Montgomery and Farrell (1993) and Davis and Bosart (2003)
Background
Methodology
Results
Conclusions