Mutual Amplification Between Upper and LowerLevel Cyclonic Potential Vorticity in the Deepening of a

1
Mutual Amplification Between Upper and
Lower-Level Cyclonic Potential Vorticity in the
Deepening of a Extratropical Cyclone A Case Study

• Holly A. Anderson
• Synoptic-Dynamic Meteorology
• Dr. Cunningham
• December 6, 2007

2
Objectives

• An extratropical cyclone that formed on November
  20, 2006 off the east coast of the United States
  is analyzed within a PV and isentropic framework.
  
• This project seeks to describe the synoptic
  conditions leading to the development of the
  extratropical cyclone, as well as provide a study
  of the evolution of the associated potential
  vorticity fields as the upper-level cyclonic PV
  anomalies and the surface level cyclone mutually
  affect each other.

3
Case Study November 21-22, 2006

• A developing coastal extratropical storm brought
  freezing temperatures, heavy rains, frozen
  precipitation, and erosion to the east coast.
• Knocked down power lines and caused power outages
  to many along the east coast
• Rainfall totals
• Georgetown, DE 1.93 (49 mm) 22 Nov
• Newark, NJ 1.26 (32 mm) 23 Nov
• Kennedy Airport, NY - 1.56 (40 mm) 23 Nov
• Shoreham, NY 2.83 (72 mm) 23 Nov
• Bridgeport, CT 1.84 (47 mm) 23 Nov
• Hingham, MA 4.76 (121 mm) 24 Nov

4
November 21, 2006

• Snow flurries were reported by the National
  Weather Service as far south as central Florida
  in Orange, Lake, Seminole, and Volusia counties
  Florida as the state endured unusually early cold
  temperatures.

5
Charleston, South Carolina

• The first record of thundersnow in local history.
• Earliest winter season calendar date on which
  snow has been reported at KCHS.

NWS PNS - http//www.erh.noaa.gov/chs/text/PNSCHS_
11222006.txt
6
21 November 2006 12Z
7
Isentropic Potential Vorticity A Review

• IPV is conserved following 3-D, adiabatic,
  inviscid flow along an isentrope. By plotting
  Ertel PV on an isentropic surface, we can see a
  conserved quantity along a surface of another
  conserved quantity.
• Values of PV less than 2 PVU are typically
  tropospheric. Stratospheric values of PV are
  typically greater than 4 PVU, where 1 PVU 10-6
  m2s-1Kkg-1 (Bresky and Colucci, 1996).
• For this study, the tropopause is defined by the
  1.5 PVU surface.
• PV values of 1.5-4 PVU define the dynamic
  tropopause (Bresky and Colucci, 1996).

8
Data and Methodology

• NCAR Computational and Information Systems
  Laboratory (CISL) Data for Atmospheric and
  Geosciences Research
• NCEP Eta model data
• 40 km resolution
• 3 hour time intervals
• 26 pressure levels
• Analyzed in both Unidata Integrated Data Viewer
  (IDV) and GrADS
• For this project, plan view plots of IPV at the
  310K isentrope were plotted.
• In addition, PV cross-sections were calculated
  using centered finite differencing at pressure
  levels from 1000-50 hPa.
• By looking at plan views and vertical
  cross-sections, the interaction between the
  upper-level PV anomalies and the surface cyclone
  can be studied. The evolution in the position of
  the dynamic tropopause can also be noted.

9
21 November 00Z
10
21 November 12Z
11
22 November 00Z
12
22 November 12Z
13
23 November 00Z
14
Isotachs (shaded) and Heights (contours)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
PV and Cyclogenesis

• Positive PV on the 310K surface indicate the
  positive PV field in the upper-troposphere.
• The upper anomaly induces cyclonic motion that
  extends down into the troposphere.
• The upper level cyclonic motion acts to amplify
  the cyclonic motion at the surface, strengthening
  the surface cyclone.
• In turn, the surface cyclone amplifies the
  upper-level cyclonic motion.
• As long as they tilt westward with height,
  cyclonic motions at upper and lower levels
  mutually amplify one another in a feedback
  mechanism.
• Therefore, we expect to see higher PV values in
  the upper atmosphere as the surface cyclone
  deepens.
• Tropopause depression is noted by upper-level PV
  anomalies.

22
21 November 00Z

• Isobars indicate the cyclone is forming.
• The upper PV field is to the west of the
  low-level cyclone.

23
21 November 12Z

• The upper PV anomaly blob is tightening as the
  MSLP isobars tighten, indicating that both fields
  are strengthening and amplifying each other. The
  IPV at 310K has obtained a value of 5.5 PVU and
  is directly west of the surface cyclone.

24
21 November 12Z

• Location of the cross-section from 32N, 85W to
  32N, 71W

25
21 November 12Z

• As this cross-section cuts through the center of
  the cyclone, we see cyclonic motion from the
  meridional isotachs.
• Isentropes also bend upwards into the positive PV
  anomaly.
• Tropopause undulations are also seen.
• Tropopausal values of PV are shown close to the
  surface.

26
22 November 00Z

• The PV blob is advecting southeastward. The
  highest PVU value is 6 PVU, well into
  stratospheric values. The cyclone has achieved a
  low pressure of less than 1005 hPa.

27
22 November 00Z

• Location of the cross-section from 27N, 84W to
  27N, 76W

28
22 November 00Z

• Cyclonic motion is seen surrounding this PV
  anomaly.
• The tropopause is low in the atmosphere, at
  approximately 500-550 hPa. This means
  stratospheric values are the lowest. This
  corresponds to what we expect as the cyclone
  intensifies.

29
22 November 12Z

• The PV blob is at its tightest gradient, as is
  the surface low. The surface low pressure is less
  than 1002 hPa.
• The highest PV values are southwest of the
  surface cyclone.

30
22 November 12Z

• Location of the cross-section from 31N, 81W to
  31N, 78W

31
22 November 12Z

• The 310K isentrope is clearly within
  stratospheric values and is found around the 400
  hPa level.
• The tropopause is higher at this time, at
  approximately 475 hPa.
• The low-level PV anomaly is most likely due to
  diabatic heating at the surface.

32
23 November 00Z

• By 00Z on the 23 November, the upper-PV blob is
  becoming less tilted westward with height. The
  IPV field at 310K has weakened considerably. PV
  values have lowered to more tropospheric values.
  Likewise, the cyclone has diminished in strength,
  as shown by higher pressure values and a smaller
  pressure gradient.

33
23 November 00Z

• Location of the cross-section from 81W, 31N to
  77W, 31N through the southwestern part of the
  surface cyclone

34
23 November 00Z

• Stratospheric values are back to the 225-250 hPa
  level.
• The isentropes are also vertically stacked.
• Cyclonic motion is visible from the isotachs of
  meridional wind.
• PV of 1.5 PVU are at higher pressures, indicating
  the dynamic troposphere is very close to the
  ground.

35
23 November 12Z

• The low continues to move off to the northeast,
  tracking alongside the east coast. PV values
  continue to diminish and the blob weakens as
  the PV anomaly is no longer tilted westward with
  height.

36
24 November 00Z

• The PV blob decays to tropospheric values as
  the weakening cyclone moves northeastward off the
  coast.
• At this point, the upper-level PV blob is
  beginning to tilt westward with height once
  again. As the cyclone moves off the coast, it
  will bomb and strengthen once again.

37
Concluding Remarks

• Upper-level PV anomalies, noted by cyclonic
  motion, are amplified by lower-level cyclonic
  motion when they are tilted westward with height.
• We can see a clear relationship between the
  strength of the surface cyclone and the PV values
  of upper-level anomalies.
• Though this cyclone was not the strongest at this
  time, we can see how the upper-level PV anomalies
  and the surface low mutually amplified one
  another until they became stacked vertically with
  height and therefore decayed.

38
References

• Jonathan E. Martin, 2006 Mid-Latitude
  Atmospheric Dynamics A First Course, The
  University of Wisconsin-Madison.
• Davis, C. A., and K. A. Emanuel, 1991 Potential
  vorticity diagnostics of cyclogenesis. Mon. Wea.
  Rev., 119, 1929-1953.
• Bresky, Wayne C. and Stephen J. Colucci, 1996 A
  Forecast and Analyzed Cyclogenesis Event
  Diagnosed with Potential vorticity. Mon. Wea.
  Rev., 124, 22272244.