Potential Vorticity (PV) Thinking in Potential Vorticity (PV) Thinking in Operations: The Utility of Operations: The Utility of

NonconservationNonconservation

Michael J. Brennan*, Gary M. Lackmann, and Kelly M. Michael J. Brennan*, Gary M. Lackmann, and Kelly M. MahoneyMahoney

Department of Marine, Earth, and Atmospheric Sciences, North Carolina State Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North CarolinaUniversity, Raleigh, North Carolina

IntroductionIntroduction

• PV can potentially be used by operational forecasters as PV can potentially be used by operational forecasters as a tool in the forecast processa tool in the forecast process

• PV hasn’t gained widespread acceptance in the PV hasn’t gained widespread acceptance in the operational forecasting communityoperational forecasting community

• Goals of the paperGoals of the paper Provide examples of situations where PV diagnosis adds Provide examples of situations where PV diagnosis adds

value to traditional forecast methodsvalue to traditional forecast methods

Provide specific suggestions for diagnostic PV tools for use Provide specific suggestions for diagnostic PV tools for use in forecastingin forecasting

Overview of PV ConceptsOverview of PV Concepts

PV principlesPV principles• PV is proportional to the product of absolute vorticity PV is proportional to the product of absolute vorticity

and static stabilityand static stability PV = (1/PV = (1/ρρ))ηη ∙ ▼ ∙ ▼θθ

• In the Northern Hemisphere positive PV anomalies are In the Northern Hemisphere positive PV anomalies are associated with positive relative vorticity and high associated with positive relative vorticity and high values of static stabilityvalues of static stability

• PV is conserved in frictionless, adiabatic flowPV is conserved in frictionless, adiabatic flow

• PV is not conserved in diabatic processesPV is not conserved in diabatic processes

Overview of PV ConceptsOverview of PV Concepts• PV is invertiblePV is invertible

Piecewise inversions can be used to quantify effects of Piecewise inversions can be used to quantify effects of heating and friction influenced developments of specific heating and friction influenced developments of specific anomalies by solving a boundary value problemanomalies by solving a boundary value problem

• Davis, C.A., and K.A. Emanuel, 1991: Potential vorticity diagnostics of cyclogenesis. Davis, C.A., and K.A. Emanuel, 1991: Potential vorticity diagnostics of cyclogenesis. Mon. Wea. Rev., 119, 1929-1953Mon. Wea. Rev., 119, 1929-1953

Not practical for operational forecastersNot practical for operational forecasters

• Cyclogenesis can occur when a cyclonic circulation Cyclogenesis can occur when a cyclonic circulation associated with an upper-level +PV anomaly extends to associated with an upper-level +PV anomaly extends to the surface creating a surface warm anomaly through the surface creating a surface warm anomaly through advectionadvection

• Diabatic PV anomalies can play a large role in moisture Diabatic PV anomalies can play a large role in moisture transport and precipitation distributiontransport and precipitation distribution

Case Study ExamplesCase Study Examples

Moisture transport in ETCMoisture transport in ETC• East coast cyclone of January 24-25, 2000 was poorly East coast cyclone of January 24-25, 2000 was poorly

forecast by NWP modelsforecast by NWP models

• A piecewise Ertel PV inversion was used to show that a A piecewise Ertel PV inversion was used to show that a lower-tropospheric PV maximum, generated by LH in an lower-tropospheric PV maximum, generated by LH in an “incipient precipitation” (IP) area, was critical in “incipient precipitation” (IP) area, was critical in moisture transport for the heavy precipitation areamoisture transport for the heavy precipitation area

• Models did not forecast the IP and therefore were unable Models did not forecast the IP and therefore were unable to generate the PV maximum responsible for the to generate the PV maximum responsible for the moisture transportmoisture transport

A) RUC analysis 0000UTC Jan. 25, 2000 SLP (solid), 800mb wind barbs, 900-700mb PV (shaded PVU). B) 24 hr ETA forecast model. C) RUC SLP (solid), 800mb wind barbs, 800mb moisture flux (shaded g/kg m/s). D) 24 hr ETA forecast

Case Study ExamplesCase Study Examples Moisture Transport in ETC cont…Moisture Transport in ETC cont…

• ETA model did not forecast the PV maximum and ETA model did not forecast the PV maximum and showed little moisture transport compared to RUC showed little moisture transport compared to RUC analysisanalysis

• The PV inversion showed that the PV maximum was The PV inversion showed that the PV maximum was responsible for 20-25kt of the onshore flow, enhancing responsible for 20-25kt of the onshore flow, enhancing the moisture fluxthe moisture flux

• How can this be used?How can this be used? Overlay lower-tropospheric PV from high-frequency analyses with Overlay lower-tropospheric PV from high-frequency analyses with

radar imagery to model QPF and PV forecastsradar imagery to model QPF and PV forecasts

Evaluate PV forecasts in NWP to diagnose errorsEvaluate PV forecasts in NWP to diagnose errors

This example shows that a low level PV max can increase the This example shows that a low level PV max can increase the onshore flow poleward of its location causing precipitation to onshore flow poleward of its location causing precipitation to spread farther inland than predicted in NWP modelsspread farther inland than predicted in NWP models

Case Study ExamplesCase Study Examples

Coastal extratropical cyclogenesisCoastal extratropical cyclogenesis• PV provides a conceptual tool for diagnosing PV provides a conceptual tool for diagnosing

extratropical coastal cyclogenesisextratropical coastal cyclogenesis

• Mahoney and Lackmann found that the model predicted Mahoney and Lackmann found that the model predicted cyclones and fronts were associated with the Betts-cyclones and fronts were associated with the Betts-Miller-Janjic (BMJ) CP schemeMiller-Janjic (BMJ) CP scheme

• In this case PV was a useful tracer for monitoring the In this case PV was a useful tracer for monitoring the influence of CP scheme-driven diabatic processes in influence of CP scheme-driven diabatic processes in model forecastsmodel forecasts

• Workstation version ETA model forecasts using the BMJ Workstation version ETA model forecasts using the BMJ CP scheme and the Kain-Fritsch (KP) CP scheme were CP scheme and the Kain-Fritsch (KP) CP scheme were performedperformed

a) 30 hr forecast BMJ, 900-700mb PV (shaded, PVU), SLP (dashed), 3-h convective precip (solid, mm). b) as in a except 36 hr

c,d) as in a,b except for KP scheme

Case Study ExamplesCase Study Examples Coastal Coastal extratropicalextratropical cyclogenesis cont… cyclogenesis cont…

• Results of the forecasts show that the character of the Results of the forecasts show that the character of the cyclone was very sensitive to the CP scheme usedcyclone was very sensitive to the CP scheme used

• The differences in model runs can lead to changes in The differences in model runs can lead to changes in moisture transport and thermal advectionmoisture transport and thermal advection

Both are important to forecasters.Both are important to forecasters.

• How can this be used?How can this be used? PV can be used as an indicator of intense CP scheme activityPV can be used as an indicator of intense CP scheme activity

PV maxima can continue after convective precipitation has ended PV maxima can continue after convective precipitation has ended and serve as a tracer of the influence of earlier latent heatingand serve as a tracer of the influence of earlier latent heating

Forecasters should be aware of possible intense CP scheme Forecasters should be aware of possible intense CP scheme influence when heavy convective precipitation is collocated with influence when heavy convective precipitation is collocated with new lower-tropospheric PV maximanew lower-tropospheric PV maxima

Case study ExamplesCase study Examples

Low-level jet enhancementLow-level jet enhancement• January 13-14, 2005: a slow cold front moved across January 13-14, 2005: a slow cold front moved across

central and eastern U.S. producing precipitation totals central and eastern U.S. producing precipitation totals over 25mm, high winds, hail, and tornadoesover 25mm, high winds, hail, and tornadoes

• At 0600Z a PV maximum developed along a cold At 0600Z a PV maximum developed along a cold frontal rain band, and an LLJ was East of the PV frontal rain band, and an LLJ was East of the PV maximummaximum

• By 1200Z the PV maximum increased to 1.5 PVU in the By 1200Z the PV maximum increased to 1.5 PVU in the heaviest precipitation area and the LLJ increased to heaviest precipitation area and the LLJ increased to 80kt over southern Maine80kt over southern Maine

0600 Jan. 14 2005 900-700mb PV (solid, PVU) and 2km radar mosaic reflectivity

1200 Jan. 14 2005 900-700mb PV (solid, PVU) and 2km radar mosaic reflectivity

0600 Jan. 14, 2005 900-700mb PV (shaded, PVU), 850mb wind barbs, and 850mb isotachs (dashed)

1200 Jan. 14, 2005 900-700mb PV (shaded, PVU), 850mb wind barbs, and 850mb isotachs (dashed)

Case Study ExamplesCase Study Examples

Low-level jet enhancement cont…Low-level jet enhancement cont…• A PV budget showed that latent heating associated with A PV budget showed that latent heating associated with

the precipitation aided the LL PV maximum developmentthe precipitation aided the LL PV maximum development

• A quasigeostrophic PV inversion indicated that over 40% A quasigeostrophic PV inversion indicated that over 40% of the LLJ was due to the presence of the LL PV anomalyof the LLJ was due to the presence of the LL PV anomaly

• How can this be used?How can this be used? Overlaying LL PV, winds, and radar imagery can identify situations Overlaying LL PV, winds, and radar imagery can identify situations

when a PV maximum associated with LHR is enhancing a LLJwhen a PV maximum associated with LHR is enhancing a LLJ

Comparing model QPF to short term observations and examining Comparing model QPF to short term observations and examining PV distribution can help forecasters determine if models are over or PV distribution can help forecasters determine if models are over or under-estimating LLJ intensityunder-estimating LLJ intensity

Operational ToolsOperational Tools New “procedure” for the National Weather Service’s New “procedure” for the National Weather Service’s

Advanced Weather Interactive Processing System (AWIPS)Advanced Weather Interactive Processing System (AWIPS)

• Introduced to promote the use of PV in identifying impacts of latent Introduced to promote the use of PV in identifying impacts of latent heating in model forecastsheating in model forecasts

• Allows forecasters to examine model forecasts of SLP, LL PV and winds, Allows forecasters to examine model forecasts of SLP, LL PV and winds, and total and convective precipitationand total and convective precipitation

• Allows users to identify the impact of latent heating on atmospheric Allows users to identify the impact of latent heating on atmospheric dynamicsdynamics

Met Office developmentsMet Office developments

• Forecasters directly alter model generated PV fields, then use PV Forecasters directly alter model generated PV fields, then use PV inversion to obtain other fieldsinversion to obtain other fields

• Forecasters can use PV framework to adjust NWP model guidance Forecasters can use PV framework to adjust NWP model guidance which can improve lead times for short and medium-range forecastswhich can improve lead times for short and medium-range forecasts

AWIPS: 900-700mb PV (Shaded, PVU), total precip. (blue contours), convective precip. (red contours), 850mb wind barbs, and SLP (white contours)

ConclusionsConclusions

• Using PV to identify and diagnose diabatically produced Using PV to identify and diagnose diabatically produced LL cyclonic PV anomalies gives forecasters a new motive LL cyclonic PV anomalies gives forecasters a new motive to incorporate PV framework into their forecast methodsto incorporate PV framework into their forecast methods

• PV can be used as a tool along with ensemble forecasts PV can be used as a tool along with ensemble forecasts to give more confidence in forecasts of fields with high to give more confidence in forecasts of fields with high uncertainties (i.e. QPF)uncertainties (i.e. QPF)

• This forecasting method may gain more acceptance in This forecasting method may gain more acceptance in operational meteorology with further research, operational meteorology with further research, enhanced graphical displays, and training modulesenhanced graphical displays, and training modules