Potential Vorticity (PV) as a Tool in Potential Vorticity (PV) as a Tool in ForecastingForecasting

Michael J. Brennan, Gary M. Lackmann, and Kelly M. MahoneyMichael J. Brennan, Gary M. Lackmann, and Kelly M. MahoneyDepartment of Marine, Earth, and Atmospheric SciencesDepartment of Marine, Earth, and Atmospheric Sciences

CSTAR WorkshopCSTAR Workshop6 October 20056 October 2005

PV in ForecastingPV 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

Why should Forecasters use PV?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

p

fgPV )(

Potential VorticityPotential Vorticity

• 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

• PV is the product of the – Absolute vorticity

– Static stability

Figures from Thorpe (1985) for Northern Hemisphere

Principles of PV ThinkingPrinciples 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 quantify impact of specific PV feature on

rest of atmosphere

Cyclones and PVCyclones and PV• Cyclogenesis is viewed as mutual amplification of PV

anomalies on the upper and lower boundaries

–Upper-level anomaly’s imparts circulation at the surface–Positive anomaly forms there its circulation reaches to the tropopause

–Circulation from one anomaly amplifies the other

Upper boundary anomaly

Circulation induced at

lower boundary

Lower boundary anomaly

Circulation induced at

upper boundary

Hoskins et al., (1985)

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

PV+PV+

PV-PV-

PV and Latent HeatingPV and Latent Heating

• Opposite occurs above level of maximum heating where PV is reduced

• PV growth rate determined by vertical gradient of LHR

PV and Latent HeatingPV and Latent Heating• In the presence of vertical wind shear the PV redistribution

occurs along the absolute vorticity vector (Raymond 1992)

Raymond (1992)

+PV

–PVQ

The whole picture

Surface warm anomalyDiabatic lower-

tropospheric maximum

Upper-Level maximum

Upper-level diabatic minimum

Reed et al. (1992)

Major PV features in a mature Major PV features in a mature extratropical cycloneextratropical cyclone

• And their counterparts in the traditional QG framework

1. Upper-tropospheric PV maximum• Upper trough

2. Surface θ 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

Case StudiesCase Studies

• Diabatic PV maxima in the lower-troposphere can impact

– Extratropical cyclones– Low-level jets– Moisture transport

1. Moisture transport in extratropical cyclones• 24–25 January 2000

2. Low-level jet enhancement in high-wind event• 1 December 2004

3. Coastal extratropical cyclogenesis• 17 February 2004

Moisture TransportMoisture 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

24–25 Jan. 200024–25 Jan. 2000• Compare model analysis to 24-h Eta forecast

at 00 UTC 25 January 2000

RUC AnalysisRUC Analysis

900900––700 mb PV, 700-mb moisture flux, 700 mb PV, 700-mb moisture flux, 700-mb wind700-mb wind

24-h Eta Forecast 24-h Eta Forecast

900900––700 mb PV, 700-mb moisture flux, 700 mb PV, 700-mb moisture flux, 700-mb wind700-mb wind

1.5 PVU maximum No PV maximumSignificant inland moisture flux

Weak/no moisture flux

Jan. 2000 Snowstorm CaseJan. 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

Low-Level JetsLow-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 > 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)

Radar and 900Radar and 900––700 mb PV700 mb PV2-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

900900––700-mb PV and Low-Level Jet700-mb PV and Low-Level Jet

RUC analysis 09 UTC

900–700-mb PV

850-mb wind and isotachs (kt)

• 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 12 UTC

900–700-mb PV

850-mb wind and isotachs (kt)

LLJ CaseLLJ Case

• Plotting 900–700-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??

Coastal CyclogenesisCoastal 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-Janjić and Kain-Fritsch)– Initialized at same time with identical model configuration– Results show very different cyclone evolution and

precipitation patterns

Workstation Eta ForecastsWorkstation Eta Forecasts900–700-mb PV Sea-level pressure Convective Precip

BMJ CP Scheme KF CP Scheme

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

•1 continuous convective precip. maximum•Broad surface low

• 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

Coastal CyclogenesisCoastal Cyclogenesis

• Plotting 900–700-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

ConclusionsConclusions

• 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

Forecasting ToolsForecasting 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)

OverlayQPF (total and/or convective)

Lower-tropospheric PV and wind

Sea level pressure

AcknowledgementsAcknowledgements

• 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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