Potential Vorticity (PV) as a Tool in Forecasting

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Potential Vorticity (PV) as a Tool in Forecasting

• Michael J. Brennan, Gary M. Lackmann, and Kelly
  M. Mahoney
• Department of Marine, Earth, and Atmospheric
  Sciences
• CSTAR Workshop
• 6 October 2005

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PV in Forecasting

• Initial attempts to integrate PV into the
  forecast process focused on upper-tropospheric
  dynamics and jet streaks
• These features can be easily understood using
  traditional tools like pressure level
  maps/analyses, QG diagnostics, etc.
• We advocate using PV to understand and track the
  impact of latent heating in the lower- and
  mid-troposphere
• Not as easily seen in traditional framework
• To take full advantage of this, forecasters need
  to be familiar with the PV framework
• Extratropical cyclone structure in terms of PV
• Impact of latent heating on PV distribution

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Why should Forecasters use PV?

• PV not conserved in the presence of latent
  heating
• PV useful to identify features driven by latent
  heating (e.g., convective scheme activity) in NWP
  models
• Model QPF ? low predictability relative to other
  parameters
• QPF errors feed back to dynamics via latent
  heating ? model forecast sensitive to accuracy of
  upstream QPF
• PV provides forecasters a means with which
  forecasters can
• Track impacts of latent heating in real
  atmosphere and model forecasts
• Identify situations where model guidance can be
  adjusted when latent heating has been
  misrepresented due to poor model QPF

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Potential Vorticity

• PV is the product of the
• Absolute vorticity
• Static stability

• High values of PV associated with
• Cyclonic flow
• High static stability
• Low tropopause
• Upper trough
• Low values of PV associated with
• Anticyclonic flow
• Low static stability
• High tropopause
• Upper ridge

Figures from Thorpe (1985) for Northern Hemisphere
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Principles of PV Thinking

• Conservation Principle
• PV conserved for adiabatic, frictionless flow
  (Hoskins et al. 1985)?
• PV NOT conserved for diabatic processes, like
  latent heating!
• Invertibility Principle
• PV can be inverted to recover balanced
  meteorological fields (height, wind, etc.,
  Hoskins et al. 1985, Davis and Emanuel 1991)?
• Links synoptic and dynamic meteorology
• Allows one to quantify impact of specific PV
  feature on rest of atmosphere

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Cyclones and PV

• Cyclogenesis is viewed as mutual amplification of
  PV anomalies on the upper and lower boundaries

Upper boundary anomaly
Circulation induced at upper boundary
Circulation induced at lower boundary
Lower boundary anomaly
Hoskins et al., (1985)?

• Upper-level anomalys imparts circulation at the
  surface
• Positive anomaly forms there ? its circulation
  reaches to the tropopause
• Circulation from one anomaly amplifies the other

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PV and Latent Heating

• PV generated below level of maximum heating
• Warming increases static stability
• Pressure falls ? convergence ? increases absolute
  vorticity

PV-
PV

• Opposite occurs above level of maximum heating
  where PV is reduced
• PV growth rate determined by vertical gradient of
  LHR

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PV and Latent Heating

• In the presence of vertical wind shear the PV
  redistribution occurs along the absolute
  vorticity vector (Raymond 1992)?

PV
Q
PV
Raymond (1992)?
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Conceptual Model
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The whole picture
Upper-level diabatic minimum
Upper-Level maximum
Surface warm anomaly
Diabatic lower-tropospheric maximum
Reed et al. (1992)?
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Major PV features in a mature extratropical
cyclone

• And their counterparts in the traditional QG
  framework
• Upper-tropospheric PV maximum
• Upper trough
• 2. Surface ? maximum
• Surface cyclone
• 3. Lower-tropospheric diabatic PV maximum
• Height (pressure) falls due to latent heating
• 4. Upper-tropospheric diabatic PV minimum
• Downstream ridging aloft due to latent heating

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Case Studies

• Diabatic PV maxima in the lower-troposphere can
  impact
• Extratropical cyclones
• Low-level jets
• Moisture transport
• Moisture transport in extratropical cyclones
• 2425 January 2000
• 2. Low-level jet enhancement in high-wind event
• 1 December 2004
• 3. Coastal extratropical cyclogenesis
• 17 February 2004

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Moisture Transport

• Diabatic PV maxima in lower-troposphere alter
  flow in region of high moisture content,
  impacting moisture transport
• Extratropical cyclones (e.g., Whitaker et al.
  1988, Brennan and Lackmann 2005)?
• Along cold fronts (e.g., Lackmann 2002)?
• Pineapple Express events (e.g., Lackmann and
  Gyakum 1999)?
• In Jan. 2000 case, precipitation early on 24 Jan.
  generated lower-tropospheric PV maximum
• Precipitation was unforecasted by Eta model
• Eta failed to generate PV maximum and inland
  moisture transport into region of heavy snowfall

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2425 Jan. 2000

• Compare model analysis to 24-h Eta forecast at 00
  UTC 25 January 2000

1.5 PVU maximum
No PV maximum
Significant inland moisture flux
Weak/no moisture flux
RUC Analysis 900700 mb PV, 700-mb moisture flux,
700-mb wind
24-h Eta Forecast 900700 mb PV, 700-mb moisture
flux, 700-mb wind
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Jan. 2000 Snowstorm Case

• Model failure to capture latent heating with IP
  IP feature apparent in lower-tropospheric PV
• Comparing model PV forecast to analyses could
  increase forecasters recognition of
  consequences of unforecasted latent heating
• Lower-tropospheric moisture transport feedbacks
  may be common to major model QPF failures

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Low-Level Jets

• Diabatic PV maxima can significantly strengthen
  low-level jets impacting transport of high
  momentum air to surface
• Case of 1 Dec 2004
• Strong low-level jet developed over the eastern
  US
• 12Z IAD sounding gt 90 kt winds near 800 mb
• Widespread wind damage from Mid-Atlantic to New
  England
• 2 fatalities, 5 injuries, 1.5 million in
  property damage (NCDC)?

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Radar and 900700 mb PV
2-km NOWRAD and RUC analysis 09 UTC
2-km NOWRAD and RUC analysis 12 UTC

• 1.25 PVU maximum develops along in wake of
  precipitation shield over Mid-Atlantic

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900700-mb PV and Low-Level Jet

• Low-level jet strengthens to 70 kt. on eastern
  flank of PV maximum
• Cyclonic circulation associated with PV maximum
  likely contributed to strength of jet
• Contribution can be as much as 40 (Lackmann
  2002)?

RUC analysis 09 UTC 900700-mb PV 850-mb wind and
isotachs (kt)?
RUC analysis 12 UTC 900700-mb PV 850-mb wind and
isotachs (kt)?
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LLJ Case

• Plotting 900700-mb layer PV indicated presence
  of diabatically enhanced LLJ
• Real-time evaluation of model QPF and PV could
  allow anticipation of over/underestimate of LLJ
  enhancement
• Separate question To what extent will LLJ winds
  mix to surface??

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Coastal Cyclogenesis

• 17 February 2004
• Real-time awareness that cyclogenesis in Eta
  model was tied to convective precipitation
• Sensitivity tested by running Workstation Eta
  model varying CP scheme (Betts-Miller-Janjic and
  Kain-Fritsch)?
• Initialized at same time with identical model
  configuration
• Results show very different cyclone evolution and
  precipitation patterns

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Workstation Eta Forecasts
900700-mb PV Sea-level pressure
Convective Precip

• 3 convective precip maxima
• 3 PV maxima
• 3 surface lows

• 1 continuous convective precip. maximum
• Broad surface low

BMJ CP Scheme
KF CP Scheme

• Convective precipitation generates low-level PV
  maxima and surface low centers offshore
• 3 low centers in BMJ run ? one with each area of
  convective precipitation
• 1 low center in KF run ? continuous line of
  convective precipitation

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Coastal Cyclogenesis

• Plotting 900700-mb layer PV with convective
  precipitation identified SLP minima linked to CP
  scheme activity
• With all due respect to CP schemes, these
  features should be interpreted as having a lower
  level of certainty

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Conclusions

• PV can be utilized by operational forecasters to
  track impact of latent heating in real atmosphere
  and NWP models
• PV thinking can be used to adjust model guidance
  in cases of misrepresented latent heating
• Lower-tropospheric PV maxima provide a means to
  recognize the impact of latent heating on
• Moisture transport
• Cyclogenesis
• Low-level jets

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Forecasting Tools

• Evaluate model performance
• Compare model QPF to observations, radar, and
  satellite to look for areas of misplaced or
  erroneous latent heating
• Compare diabatially generated PV features in
  model forecasts to high-frequency analyses
• Use PV thinking to adjust model guidance by
  understanding impact of latent heating on
  moisture transport, cyclogenesis, low-level jets

AWIPS Procedure (developed at NWS
Raleigh)? Overlay QPF (total and/or convective)
Lower-tropospheric PV and wind Sea level pressure
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Acknowledgements

• CSTAR Grants NA-07WA0206 and NA03NWS4680007
• NCEP provided Workstation Eta model and initial
  condition data for Feb. 17 2004 case simulations
• Jonathan Blaes of NWS Raleigh assisted with AWIPS
  procedure development
• Much of meteorological data provided by Unidata

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References

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  influence of incipient latent heat release on the
  precipitation distribution of the 24?25 January
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