Tropical Cyclone TUTT interactions'

1
Tropical Cyclone - TUTT interactions.

• Mid-Term Presentation
• Mark Guishard
• Meteo 597C - Tropical Meteorology

2
Paper Reviews

• Classic Paper - Sadler,J.C., A role of the
  Tropical Upper Tropospheric Trough in Early
  Season Typhoon Development, Monthly Weather
  Review, July 1976, Vol. 104, pp.1266-1278
• Contemporary Paper - Molinari, J. Skubis, S.,
  Vollaro, D., Alsheimer, F., and Willoughby, H.E.,
  Potential Vorticity Analysis of Tropical Cyclone
  Intensification, J. of Atmos. Sciences, 1998,
  Vol. 55, pp. 2632-2644

3
Introduction
Sadler, 1976

• Fig 1 In the Western Pacific, storm outflow is
  impeded by the upper easterlies in early Tropical
  Season

4
Sadler, 1976

• Fig 2 Deep troughs provide a channel for
  outflow (as per Ramage, 1974 and Erickson and
  Winston, 1972)

5
Sadler, 1976

• This has long been recognized as a mechanism for
  tropical cyclone development in the West Pacific
  (Wilkie, 1964), and intensification in the
  Atlantic (Colón and Nightingale, 1963).
• During the early season, however, these deep
  troughs seldom extend into the area of concern
  (tropical northwest Pacific).
• Another path for the outflow was required to
  explain early season TC development...

6
TUTT
Sadler, 1976

• Fig 3 Tropical Upper Tropospheric Troughs -
  these can also supply a channel to the westerlies
  for the outflow.

7
Conceptual Synoptic Model (Fig 4)
Sadler, 1976
8
Conceptual Synoptic Model (Fig. 5)
Sadler, 1976
9
Results TUTT interaction examples
Sadler, 1976

• 1971 Pacific tropical season
• TUTT positioned in the northwest Pacific
• Resulting in the development of three successive
  typhoons
• Freda Gilda Harriet

10
Fig. 6 250mb synoptic streamline analysis
visible satellite images for West Pacific June
11/12
Sadler, 1976
Freda
11
Fig. 7 250mb synoptic streamline analysis
visible satellite images for West Pacific June
28/29
Sadler, 1976
Harriet
12
Molinari (1997)
Molinari, et al (1997)

• First a brief overview of Potential Vorticity,
  and some nomenclature
• PV 1/? ??/?z ?a
• Hydrostatic Ertel PV -g ??/?p ?a
• Useful as a meteorological tool - a conservative
  property that describes both Dynamics (by ?a) and
  Thermodynamics (by ??/?p)
• Here, ? - ?p/??, defined as a pseudo-density,
  so PV,
• ? g ?a ?-1

13
Potential Vorticity
Molinari, et al (1997)
PV1/? ??/?z ?a
? ??
z
?
Fig. 8 Conservation of potential vorticity along
isentropic surfaces.
14
PV Anomalies
Molinari, et al (1997)
?
Fig. 9 Cross section of isentropes (thin lines,
increasing values upwards) and tropopause (or
PV2 surface) - thick line. Idealised PV anomaly
- cyclonic on left, anti-cyclonic on right.
Isotachs of an induced flow solid into page,
dashed out of page.
15
Isentropic motion through PV anomalies
Molinari, et al (1997)
Fig. 10 Conservation of PV along isentropic flow
Blue arrow Isentropic up-gliding and spin
down Red Arrow Isentropic down-gliding and spin
up
?
16
Applications of PV
Molinari, et al (1997)

• Air with high values (gt1.5) of PV is (generally)
  dry and very stable, while low PV air is
  relatively moist and unstable.
• So, we can compare high PV with dry,
  stratospheric air, and low PV with moist,
  tropospheric air. And hence...
• dry intrusions of stratospheric air on, for
  example Satellite Ch.3 IR (water vapour) imagery
  may be treated as High PV anomalies.

17
PV and Water Vapour Imagery
Molinari, et al (1997)

• Fig. 6 - V PV maximum, H cloud head
• Very useful as an operational model diagnostic.
• Taken from Young, M., Application of Satellite
  imagery at the National Meteorological Centre of
  the UK Met Office, UK Met Office, Bracknell, UK

18
TC-PV interactions
Molinari, et al (1997)

• A TC approaches an upper PV anomaly
• Ascent is induced ahead of the upper trough (as
  we might expect from positive vorticity
  advection).
• -ve PV associated with the anticyclonic outflow
  butts heads with the TUTTs Upper PV anomaly...
• and the trough itself is deformed

Increased Shear as trough approaches
Upper ve PV anomaly
19
TC-PV interactions
Molinari, et al (1997)

• So, upon initial approach of the trough, overall
  shear is increased, and the TC is likely to
  weaken.
• The TC tends to prevent propagation of the upper
  PV anomaly directly across it, instead causing it
  to become almost a cut-off feature from the rest
  of the trough.
• The effect of this is the decrease the horizontal
  extent of the upper PV anomaly
• ...creating less positive upper PV (i.e. more
  negative and anticyclonic),which enhances the
  outflow and decreases the shear
• ...and allows re-intensification.

20
Fig. 8 PV surfaces associated with Hurricane
Danny (1985), plotted on cross sections of ?
levels vs. radius from the system centre (west to
east orientation). Based on ECMWF model data.
Dannys evolution (Molinari, et al, 1997)
Weakened Upper PV anomaly Enhanced
outflow Intensified vortex
21
Conclusions

• So, the conclusions of Sadler and Molinari et al
  suggest that TUTTs induce development and
  intensification of tropical cyclones by (at
  least) two mechanisms
• Sadler - provision of a channel to the
  westerlies for the outflow of a TC, thereby
  enhancing the in-up-out circulation by
  increasing upper divergence.
• Molinari et al - Interactions between the upper
  PV anomaly (associated with the TUTT) and the
  lower PV anomaly (associated with the TC) induce
  weakening initially, and then re-intensification.
  
• The exact nature of the interactions between the
  upper PV anomaly and the hurricane remains
  elusive because the tropical cyclone core is
  inadequately observed.

22
Appendix
Molinari, et al (1997)

• Eliassen-Palm Fluxes
• E-P Flux can be interpreted as effective
  (angular) momentum flux
• FL -r(?uL)vL, p??
• Where r radius,
• u, v are radial tangential velocity,
  respectively
• ? is Montgomery potential, ? p/? gz
• Subscript L denotes storm relative, and primes
  denote deviations from the azimuthal average.
• Cylindrical coordinates (r and ?) used
  throughout.

23
Appendix
Molinari, et al (1997)

• Eliassen-Palm Fluxes
• EP-fluxes can also be used to provide information
  about wave activity for quasi-geostrophic flows,
  with this application relying on the equality
  between the divergence of the flux and the eddy
  potential vorticity flux under the
  quasi-geostrophic approximation. See Lee and
  Leach (1996).
• Montgomery potential
• ? p/? gz (or ? ?p ?)
• ? represents an exact stream function on specific
  volume anomaly surfaces. This is also known as
  the Montgomery stream function and the Bernoulli
  function in the geostrophic approximation.

24
Appendix - Eliassen-Palm Flux
Molinari, et al (1997)

• Originally developed as a diagnostic tool for
  interaction between eddies and atmospheric
  zonal-mean flow.
• E-P flux vector used to represent eddy momentum
  heat transport.
• Eddy-inducing forcing is the divergence of E-P
  flux (?FL). ?FL and the eddy PV flux may be
  considered equivalent under the quasi-geostrophic
  approximation.
• Maximum ?FL implies maximum angular momentum
  spinup, and is found, in this case (Molinari, et
  al, 1997), in a narrow layer near the 350K
  potential temperature surface throughout the
  period examined.

25
Appendix - E-P Flux
Molinari, et al (1997)

• Transport of angular momentum associated with
  eddies to the mean flow may be approximated by
  divergence of the E-P flux(? FL).
• Furthermore, ? FL (r?)-1 can be used to
  represent the influence of eddy activity on mean
  tangential velocity.
• This quantity has been used previously by
  Molinari et al (1995) to represent ...the
  tangential velocity change as a result of PV flux
  by azimuthal eddies in Hurricane Elena
• Oh! Now I understand...