Master's Research Presentation

Master's Research Presentation

The Role of Inertial Instability inStratosphere-Troposphere Exchange andthe Generation of Inertial Flare-ups inMidlatitude CyclonesShellie RoweAOS Department Seminar6 March 2013

• GoalsOverview• Dynamical/Theoretical Background• Recent Research (Knox and Schultz 2006,Schumacher et al. 2010)• Methodology - UWNMS, Vis5d, GEMPAK• Case Studies - 6 February 2008, 20-21 February2011, 22 April 2005• Conclusions• Future work: Expand on this topic and apply it totropical cyclones

Goals• Evaluate negative EPV anomalies on theanticyclonic shear side of the westerly jet• Understand the origin of these negative EPVanomalies• Determine how an STE mechanism is createdby these inertially unstable regions• Investigate the nature of “inertial flare-ups”

Inertial Instability Theory• Inertial instability represents a large departure fromgeostrophic balance• Inertial instability may also be viewed as a keyadjustment process which restores a stable angularmomentum profile (geostrophic balance)• Inertial instability is possible wherever theanticyclonic relative vorticity rivals the Coriolisparameter in magnitude (Knox, 2003)• It is characterized by anomalous negative PV

Inertially UnstableInertially Stable

Moist Potential Vorticity• The concept of MPV has been used in studies ofConditional Symmetric Instability (CSI) inbaroclinic systems since CSI was first introducedas a potential mechanism for the generation offrontal rainbands (Bennetts and Hoskins 1979,Emanual, 1979)• In the aforementioned research it was shown thatMPV < 0 is an adequate condition for twodimensionalfrictionless CSI

Stratosphere-Troposphere Exchange (STE)


Turbulent Kinetic Energy (TKE)Turbulent kinetic energy is a measure ofthe intensity of turbulence in units ofm 2 /s 2 and is defined as:

Richardson Number (Ri)TKE and Ri are related by the mechanicalgeneration of turbulence by wind shear andthe buoyant generation of turbulence.The flow is dynamically unstable forflows with Ri < 0.25

Connection with Previous Work• Schultz and Knox (2006)• During maintenance of precipitation bands, areasof inertial instability were present in the regionsof the precipitation, suggesting a possiblemechanism for the banding• The release of this instability produced enoughvertical motion to lift parcels to their LCL,resulting in cloud bands associated with theprecipitation

Connection with Previous Work• Schumacher et al. (2010)• Snow bands occurred downstream of complexterrain on the anticyclonic shear side of the jet• Conditional instability along with negativepotential vorticity (dry symmetric instability),and negative absolute vorticity (inertialinstability) were present

Methodology• The three cases studies that were selectedrepresent a variety of midlatitude systems inwhich upper level inertial instability and STEis the dynamical focal point• Understand how inertial instability cangenerate STE• Investigate the cause of “inertial flare-ups”

What is an inertial flare-up?• A distinct strengthening of the jet coreassociated with anomalous inertial instabilityon the anticyclonic shear side of the jet• Local jet accelerations occur in the presence ofnegative EPV air• A momentum surge that often has a negativeEPV at its core

What causes an Inertial Flare-up?a) Conservation of AngularMomentum• An inertial flare-up occurs when convection ina poleward upglide region transports air withnegative EPV (unstable updraft) and higherangular momentum (from a lower latitude) intothe jet• In the midlatitudes a poleward displacement of2 o latitude provides an extra ~20 m/s westerlyanomaly

What causes an Inertial Flare-up?b) Geostrophic Adjustment• If the PGF is stronger than the Coriolis forcethe atmosphere is not in geostrophic balance• A stronger pressure gradient will increase windspeeds• Therefore in an inertially unstable state thePGF is increasing and this accelerates the jet(geostrophic adjustment)

Geostrophic Adjustment• An imbalance of forcespromotes divergence of airat points 4 (right entranceregion)• Air in the middle of the jetstreak is nearly geostrophicagain• Divergence of air at point 1(left exit region)• These two areas ofdivergence (1 and 4) areassociated with upwellingand intensification ofsurface lows

Inertial Flare-up• A region of inertial instability on theanticyclonic shear side of the jet can enhancethis area of divergence particularly in the rightjet entrance region or left exit region, resultingin a locally-enhanced circulation and an“inertial flare-up”

Methodology• Each of the three cases featured is modeled withthe UWNMS, utilizing 2.5° ECMWF data• The resolution for each case is specified to be20 km x 20 km x 300 m, with a grid volume of152x152x60 points• Model top = 16 km• Vis5d = visualization tool used to analyze modeloutput• Supplementary 80 km resolution ETA reanalysisvisualized in GEMPAK

Three cases that illustrate inertialinstability-facilitated STE• Case 1 – 6-8 February 2008 (Surface lowdevelops in right jet entrance region)• Case 2 – 22 April 2005 (Surface low developsinbetween left jet exit and right jet entranceof two moderate jets)• Case 3 – 20 February 2011 (Surface lowdevelops in left jet exit region)

Case 1 (surface low is equatorwardof the jet entrance region)6-8 February 2008‣ Central Wisconsin snow event 6 February 2008‣ An impressive polar jet with winds in excess of 70m/s coupled with large negative EPV values whichdropped to less than -3 PVU (1.0 x 10 -6 m 2 s -1 K kg -1 )at a height of 12 km on the anticyclonic shear side ofthe polar westerly jet

Surface low is equatorward of the jet entrance regionETA 80 km reanalysisLL

LETA 80 km Reanalysis

Richardson # blue .25TKE contoured .5 m 2 /s 2

Case 1 Animation

60 m/s at 1400 UT

75 m/s at 1700 UT

80 m/s at 1730 UT

80 m/s at 1800 UT

65 m/s at 0030 UT

Case 1 Conclusions• Low speed air is lifted to the base of thestratosphere from the surface by strong plumes ofvertical motion• Inertial instability forms on the anticyclonic shearside of the westerly jet, subsequently increasingthe strength of the jet• Mixed tropospheric and stratospheric air wrapspoleward and downward around the jet inaccelerated meridional circulations, yielding STE

Case 2 (between two jets, i.e.,equatorward of one jet entrance andpoleward of the other jet exit)22 April 2005‣ Central Wisconsin/northern Illinois earlyspring storm 22 April 2005‣ 30 m/s jet core wind speed maxima andnegative EPV values exceeding -1 PVU onthe anticyclonic shear side of the westerly jet

Between two jets, i.e., equatorward of one jetentrance and poleward of the other jet exitETA 80 km reanalysisLLL

ETA 80 km ReanalysisL

Richardson # blue .25TKE contoured .3 m 2 /s 2

Case 2 Animation

15 m/s at 1630 UT

36 m/s at 2000 UT

36 m/s at 2030 UT

30 m/s at 2100 UT

Case 2 Conclusions• Inertial instability flare-ups: lifting to the base ofthe stratosphere from the surface by strongplumes of vertical motion, accelerating a localmeridional circulation• These inertial instability flare-ups form on theanticyclonic shear side of the westerly jet,subsequently increasing the strength of the jet• Mixed stratospheric / tropospheric air wrapspoleward and downward around these flare-upsbringing stratospheric air into the troposphere

Case 3 (surface low is poleward ofthe jet exit region)20 February 2011‣Surface low pressure developed in the centralplains‣ Negative EPV values of -1.5 PVU at an altitudeof 11 km in the precipitation-affected regions‣ Westerly jet speeds not as impressive as Case #1but EPV anomalies strengthened core wind speeds

Surface low is poleward of the jet exit regionETA 80 km reanalysisLLL

LETA 80 km Reanalysis

Richardson # blue .25TKE contoured .3 m 2 /s 2

Case 3 Animation

36 m/s at 1500 UT

39 m/s at 1530 UT

21 m/s at 1800Z UT

51 m/s at 2300 UT

Case 3 Conclusions• Negative EVP lifted to the base of thestratosphere from the surface by strong plumesof vertical motion• Inertial instability flare-ups form on theanticyclonic shear side of the westerly jet,subsequently increasing the strength of the jet• Stratospheric air wraps poleward anddownward in these flare-ups, bringingstratospheric air into the troposphere

Summary• Anomalous negative EPV is lifted through thetroposphere and deposited on the anticyclonicshear side of the polar westerly jet.• These areas of inertial instability disturb the baseof the stratosphere, causing a STE mechanism.• An inertial flare-up is generated as a suddenmomentum surge, that has negative EPV at itscore, tightens isotachs and increases jet core windspeeds through conservation of angularmomentum and geostrophic adjustment

Case 1 Case 2Case 3

Can this idea be applied to TropicalCyclones?• Wu and Kurihara (1996) Negative PV anomaliesat upper levels can affect storm movement• Hitchman et al. (2004) High PV can wrap itselfaround a convective complex and contribute tothe decay of the storm• Y. Wang (2008) Strong outer spiral rainbands maylimit the intensity of tropical cyclone• Knowing this…..can stratospheric high PV dry airbe brought into the cyclone through an inertialinstability process and stall intensification?

Typhoon Roke 16 km

Typhoon Roke 25 km

Typhoon Talas 16 km

Typhoon Talas 25 km

Future Work• Categorizing STE in different jet configurationsand quantifying STE with tracers in the UWNMS• Understand STE with respect to tropical-extratropicalcoupling• Understand how intrusions of dry, high-PVstratospheric air modulate tropical cyclones intransition• Analyze purely tropical cyclones to perhaps seethe same dynamical features

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