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

Mesoscale simulations of dynamical factors discriminatingbetween a tornado outbreak and non-event over the southeast USPart I: 84±48 hour precursors

J. M. Egentowich, M. L. Kaplan, Y.-L. Lin, and A. J. Riordan

With 28 Figures

Received December 23, 1999Revised January 16, 2000

Summary

Observational analysis and mesoscale numerical simula-tions are in agreement concerning key dynamical processeswhich occurred over Mexico and the Gulf of Mexico 84hours prior to the 1988 Raleigh (RDU), NC tornadooutbreak. The subtropical jet (STJ) over northern Mexicoand its associated transverse ageostrophic circulation forcedair down the eastern side of the Sierra Madre Mountainscreating adiabatic warming due to compressional heating.Along with this warm air, a low-level trough of lowpressure formed and a low-level jet (LLJ) developed overthe western Gulf of Mexico. This LLJ began the processthat transported very warm and potential vorticity (PV) richair from the Mexican plateau to the Carolina Piedmont.

The low-level PV maximum over central NC at the timeof the tornado was a coherent entity traceable back 84 hoursto the Mexican plateau. Over the Mexican plateau, the STJtransported the PV rich air southward then down to themidlevels. There was substantial heating over the plateauproducing a deep well-mixed layer and a mountain-plainssolenoid. An area of strong vertical convergence developedin the 500±600 hPa layer which increased the thermalgradient and maintained the PV. This mid-level PV wastransported to the low-levels by a hydrostatic mountainwave. As the PV maxima moved down the lee of themountains it increased due to strong static stability, tiltingand frictional effects. Finally, the PV maxima moved alongthe Gulf Coast and up the East Coast to central NC.

1. Introduction

The Raleigh, NC tornado of 28 November 1988was an extraordinary event, which has stimulated

a great deal of interest from the weatherforecasting and research communities. Thistornado occurred in the middle of the night inlate November with an unusual F4 (for NC)intensity. This devastating tornado was on theground for 83 miles resulting in 4 deaths, 154injuries, and an estimated 77.2 million dollars inproperty damage (NOAA 1988). It occurred inthe vicinity of the polar jet (PJ) entrance region,not the exit region, where previous jet dynamicalparadigms have focused (e.g. Kaplan and Paine,1977; Uccellini and Johnson, 1979; Kocin et al.,1985, 1986; Zack and Kaplan, 1987). The severeweather at Raleigh was part of a larger outbreaknear the PJ entrance region, which producednumerous tornadoes over NC and VA. TheNational Weather Service didn't issuing anysevere weather watch or warning prior toobservations of severe weather, in part, becausethe severe weather predictive indices based upontypical jet exit region dynamics were unremark-able (Gonski et al., 1989). Also, the severeweather occurred within a region of negative500 mb vorticity advection and weak low-levelwarm advection thus minimizing typical quasi-geostrophic (QG) indicators of severe weatherpotential (Gonski, 1989). The inability ofstandard indices/dynamical paradigms to indicateat least modest tornadic potential for such an

Meteorol. Atmos. Phys. 74, 129±157 (2000)

extreme event is reasonable because most of oursevere weather prediction paradigms were impli-citly based on the PJ streak exit region model.The ambiguity of the Raleigh tornado case studyindicates that more research is needed to under-stand the mechanisms that produce severeweather in the southeast US.

Upper level jet streaks and their associatedtransverse ageostrophic circulations are often thedriving force for surface weather. The transversecirculations about the jet streak develop so theatmosphere maintains a balance between themass and momentum ®elds in an effort tomaintain a QG state. The upper-level jet streakand its associated transverse ageostrophic circu-lation are coupled to the low-level circulation(Uccellini and Johnson, 1979).

A potential complicating issue to the Uccelliniand Johnson paradigm (1979) proposed byUccellini and Kocin (1987) and applied inKaplan et al. (1998) is the juxtapositioning ofthe PJ streak and subtropical jet (STJ) streak(Fig. 1). The two transverse ageostrophic circu-lations, associated with the jet streaks, phase ina manner which produces an area of intenseand deep vertical motion; the direct circulation,associated with the PJ entrance region, phaseswith the indirect circulation accompanying theSTJ exit region. Between the two upper-level jetstreaks there is con¯uence in the low levels,ascent through the column, and di¯uence in theupper levels. With both jet streaks producingdivergence aloft over the same area one would

intuitively expect a substantial amount of massremoved from the column of air.

Kaplan et al. (1998) proposed a new paradigm,with three primary concepts, for severe weatherdevelopment over the southeast US. First, thejuxtapositioning of the PJ entrance region and theSTJ exit region and their transverse secondarycirculations create deep QG vertical motions.Second, geostrophic adjustment of the mass ®eldto the wind ®eld occurs near the continental airand polar air interface, i.e., low-level frontsaccompany the return branch of the STJ streakand return branch of the PJ streak resulting inan unbalanced mid-tropospheric mesoscale jetstreak or jetlet. Third, a parcel undergoesstretching and tilting as it is caught up in thethermally direct unbalanced circulation in theright front quadrant of the geostrophicallyadjusting jetlet.

The secondary circulations associated with thePJ and STJ transport air from very differentsource regions, producing a strong cross-streamfrontal zone, and an unbalanced, mid-level jetlet.The jetlet is unbalanced because it is constantlyaccelerating under the STJ exit region where itnormally would be decelerating. The exit regionof the developing jetlet is a region of extremethermal wind imbalance since vertical motionsare no longer balanced, i.e., accelerations occurwhere the Uccellini and Johnson (1979) para-digm proposes decelerating ¯ow. The unbalancedjetlet, and ascent, extends over the tropical aircreating a longer and more intense destabiliza-

Fig. 1. Schematics depicting ahorizontal and b vertical struc-tures of the transverse ageo-strophic cicrulations about thepolar and subtropical jet streaksfrom Kaplan et al. (1998)

130 J. M. Egentowich et al.

tion and the formation of severe weather to theright of the QG-balanced PJ. That is because airparcels converging into the unbalanced region canbe more rapidly destabilized as they have largerconvective available potential energy (CAPE).

It is assumed that a favorable large-scalehydrostatic environment, which is rich in rota-tional energy (cyclonic vorticity), is necessaryfor convective storms to produce focused vorticessuch as tornadoes. There are four ways in whichthe environment can contribute rotational en-ergy to intensifying convection: (1) through theconvergence of vorticity, (2) vertical wind shearsthat enhance the tilting of horizontal vorticityinto the vertical, (3) baroclinically-generatedvorticity (solenoidal maxima), and (4) friction-ally generated vorticity. A hydrostatic environ-ment rich in potential vorticity (PV), i.e., theproduct of the vertical vorticity and staticstability, is likely to be an environment whichfavors the intensi®cation of supercell convectioninto rotating convection because of all four ofthese forcing mechanisms.

First, by de®nition, a signi®cant magnitude ofenvironmental PV cannot be separated from themagnitude of the vertical component of vorticity.Second, strong horizontal wind shears accom-panying vertical vorticity maxima imply verticalshears as they often occur near thermal bound-aries or highly con¯uent circulations. Therefore,high vorticity often implies high vertical shear.Third, thermal boundaries, fronts, and highlycon¯uent circulations are typically baroclinic/solenoidal in structure. Strong vertical potentialtemperature gradients are typically associatedwith large horizontal gradients of potential tem-perature near fronts and, hence near solenoids.Fourth, friction may generate new PV. Therefore,PV is an excellent tracer for the potentialrotational energy if it can be combined with afavorable environment for the generation andmaintenance of convection.

Gyakum et al. (1995) related the diabaticproduction of PV to rapid surface cyclogenesis.Gyakum found horizontal gradients of grid-scalelatent heating produced a mid-tropospheric PVanomaly that was related to rapid surfacefrontogenesis. Also, surface sensible heatingcreated a boundary layer anomaly, which inter-acted with the mid- and upper-level PV maximaresulting in rapid surface cyclogenesis. Finally,

this study concluded that surface frontogenesisproduced a vertical wind shear enhancing thediabatic generation of PV through tilting. Zehn-der and Keyser (1991) investigated rapid cyclo-genesis in the absence of diabatic effects. Theyfound the interaction between upper- and lower-level disturbances characterized by nonuniformPV results in a signi®cant increase in relativevorticity at the surface.

Potential vorticity in the midlevels (in isobariccoordinates) may be estimated by:

PV � ÿ @�@p��p � f �|��{z��}

1

ÿk̂ � @~Vh

@p�rp�|��{z��}

2

: �1�

Term 1 represents the contribution due toabsolute vorticity on a pressure surface in astatically stable environment. Term 2 representsthe contribution due to the tilting of isentropicsurfaces and is expected to be important in anarea of an intense baroclinic zone. PV is oftenestimated using only term 1; however, in thisstudy term 2 is essential to understanding theunderlying physical processes that separate asevere weather event from a non-severe event.Term 1 is typically associated with a balancedQG fold as in a nonsevere weather event whileterm 2 is associated with unbalanced tilting andis more important in a severe weather event. Thegeneration of mesoscale PV may be examined inisentropic coordinates using the Lagrangian PVequation (Gidel and Shapiro, 1979):

d

dtÿ�� f � @�

@p

� �� ÿ�� f � @

@p

d�

dt

� �|��{z��}

1

ÿ @�

@p

� �k̂ � r� �~F|��{z��}

2

� @�

@p

� �k̂ � r�

d�

dt

� �� @

~V

@�|��{z��}3

:

�2�Term one relates to the production or destructionof PV due to vertical gradients of diabaticheating with isentropic absolute vorticity. Thisterm is often called the stability change termsince it is related to the changes in static stability

Dynamical factors discriminating between a tornado outbreak and non-event over the southeast US ± Part I 131

@�@p

due to heating (or cooling) across a layer.

Term two represents the change in PV resultingfrom frictional stresses (F). Term three representsthe change in PV resulting from the tilting of thehorizontal component of vorticity into the verticalthrough horizontal diabatic heating. In the low-levels, terms 2 and 3 are the main mechanism forPV production and maintenance.

In this paper, we will explore the relationshipbetween the existence of the STJ and its effectson the lower environment (associated mass andmomentum adjustments, development of a LLJ,and low-level PV) 84±48 hours before thedevelopment of severe weather (tornado eventover central NC on 28 November 1988). Also,we will compare the environment of the RDUtornado event to a synoptically similar eventwhere severe weather was forecast but nonedeveloped over central NC (25 January 1990). Inthe two companion papers our investigation ofthe pre-tornadic environment will contract inboth temporal and spatial resolution. Section 2will compare the synoptic situations of the eventand non-event cases. Section 3 will describe themesoscale model used to simulate the earlyenvironment. Section 4 deals with the origins ofairmasses at different levels over central NC atthe time of the event and non-event. Section 5examines the numerical simulations of these twocases. In Section 6 we will investigate the originsof low-level PV. Finally, in Section 7 we willsummarize and present our conclusions.

2. Event and non-event comparison

In this series of papers, we compare themesoscale environment of a tornadic weatherevent of 28 November 1988 at 0600 UTC(referred to as event) to that of a non-event of25 January 1990 (referred to as non-event). In thelater case, the Raleigh NWS of®ce (Gonski,personal communication, 1998) expected severeweather to develop over central NC at 1800 UTC(designated as the onset of the non-event). Theconvective outlook and second day severe out-look were forecasting severe thunderstormsacross the Piedmont, with possible isolatedtornadoes over GA. AT 1437 UTC, the NationalSevere Storms Forecast Center (Kansas City)issued a severe thunderstorm watch for central

GA, SC and NC valid from 1400 to 2000 UTC.The window of the non-event ends when thefront moves over the Piedmont near 0000 UTC26 January 1990. Since the actual time of thenon-event is ambiguous we will examine thenon-event over a period of time starting at 1800UTC 25 and ending 0000 UTC 26 January 1990.Since the synoptic situations were so similar andyet there were dramatic differences in theoutcome, there must be some crucial and subtledifference(s) that greatly enhanced the potentialfor severe weather. A direct comparison betweenthese cases should highlight these differences. Inthe event case, no severe weather was forecastedin NC yet an F4 tornado originated near RDUand traveled northeast. In the non-event case,severe weather was expected by the local NWSof®ce and from the National Severe StormPrediction Center but did not materialize overNC.

We perform an in-depth comparison betweenthe event and the expected onset time of the non-evnt cases. Comparisons back 6, 24 and 48 hoursare synoptically similar in many ways but 72hours prior to event there are striking synopticdifferences. For the sake of brevity we willhighlight the comprison using the NWS analysissix hours before each event: 0000 UTC, 28November 1988 to 1200 UTC, 25 January 1990.

In both cases, the 200 hPa analysis (Fig. 2a, b)depict a deep trough of low pressure extendingnorth-south from MN to western TX. Theorientation of the troughs is the same for thetwo cases. The PJ extends from eastern TX overthe Appalachian Mountains and northeast toQuebec. Over the southeastern states, the direc-tions of observed winds are displaced to the rightof the height contours indicating supergeo-strophic speeds. Also, the winds are strongerover the southeastern states (due to the STJ) inthe event case. The temperature pattern is verysimilar between the two cases. The event caseSTJ and PJ are clearly depicted in Fig. 2c, a crosssection based on observations.

The 500 hPa analysis (Fig. 3a, b) depicts a deeptrough of low heights over the central part of thecontinent with a pronounced low over MI andWI. The orientation of the trough of low heightsextends back over the Big Bend region of TX.The jet extends from northeast TX over WV andinto Canada. In both cases, the wind speed and


direction, over the southeastern states are similar.The height gradient is slightly weaker over MI toOH in the 1990 case translating to a slightlyweaker mid-level jet over the Piedmont. Also,the temperature pattern is very similar in bothcases.

The 850 hPa analysis (Fig. 4a, b) depicts a lowover the northern Midwest with a troughextending over the TN valley and to centralLA. In the 1988 case, a low is beginning to cutoff over northern WI with a minimum heightof 128 dm. In the 1990 case, the low has cut

Fig. 2. a NWS 200 hPa analysis of isotachs (knots) and vectors, temperature (C) and height (dm) and c observation derivedcross section from Evansville, IN (EVV) to Jacksonville, FL (JAX), isotachs (dashed line, msÿ1) and � (solid line, K) valid at0000 UTC 28 November 1988; b NWS 200 hPa analysis of isotachs and vectors (knots), temperature (C) and height (dm) andd observationally derived cross section from Peoria, IL (PIA) to Cape Kennedy, FL (XMR) isotachs (solid line, msÿ1) and �(dashed line, K) valid at 1200 UTC 25 January 1990


off over northern IL with a minimum heightof 128 dm. The band of maximum wind isorientated from southwest to northeast over thesoutheastern US. The temperature ®eld is similarin both the cases except that temperatures were2±4 �C warmer in the 1988 case over thesoutheast US.

The 0035 UTC 28 November 1988 and 1235UTC 25 January 1990 radar summaries (Fig. 5a,b) have many similarities. There is a line of rainand thunderstorms along the Carolina coast and aprecipitation free region over the Piedmont. Aline of heavy thunderstorms is over the SC andGA border, moving into central NC at 26 to

31 msÿ1. However, the height of convection issigni®cantly higher in the 1988 case, 14 kmversus 8.1 km. In addition, the event case hasmore clusters of enhanced re¯ectivity indicatingmore convective instability.

3. Model summary

Due to the lack of high-resolution observationaldata, numerical simulations must be employed tounderstand the environments prior to the eventand non-event. The MASS model used in thisstudy is a modi®ed version of the modeloriginally developed by Kaplan et al. (1982).

Fig. 3. NWS 500 hPa analysis of wind vectors,temperture (C), and height (dm) valid at a 0000UTC 28 November 1988, and b 1200 UTC 25January 1990


The numerical experiments used in this series ofpapers are summarized in Table 1. The gridresolution and coverage are shown in Figs. 6aand b. The model speci®cs are summarized in thefollowing (MESO 1995):

Model numerics:

1. Hydrostatic primitive equation model (3-Dequations for u, v, T, q & p),

2. Vertical resolution is 37 sigma layers for the72 km run and 40 layers for the 24 & 12 kmrun,

3. Energy absorbing upper sponge layer,4. Terrain following sigma-p coordinate system,

5. A 30 second short timestep for the gravitywave mode (forward-backward scheme),

6. A 90 second long timestep for the slowadvection mode (split-explicit time marchingintegration using the Adams-Bashforthscheme).

Initialization scheme:

1. A ®rst guess using NMC/NCAR reanalyzedgridded data (2:5� 2:5 lat/long grid & 17levels),

2. Reanalysis using 3-D OI scheme (Daley 1991),3. High resolution average terrain using one pass

9 point smoother,

Fig. 4. NWS 850 hPa analysis of wind vectors,temperature (C), and height (dm) valid at a0000 UTC 28 November 1988, and b 1200UTC 25 January 1990


4. Ehanced moisture analysis through syntheticRH retrieval scheme,

5. Weekly averaged 1� 1 latitude/longitude seasurface temperature data,

6. Anderson level II land use classi®cationscheme,

7. Climatological subsoil moisture database,

8. Normalized difference vegetation index(NVDI).

Planetary boundary layer physics:

1. A modi®ed Blackadar high resolution PBLscheme (Zang and Anthes 1982),

Fig. 5. NWS radar summaries valid at a 0035 UTC 28 November 1988, and b 1235 UTC 25 January 1990

Table 1. The MASS modeling simulations performed in the study

Initialization Duration Resolution Matrix size Modi®cations(UTC) (hours) (km) (x, y, z)

1200 UTC 24 Nov 88 24 24 170, 130, 40 None0000 UTC 25 Nov 88 72 72 130, 110, 37 None0000 UTC 22 Jan 90 24 24 170, 130, 40 None1200 UTC 22 Jan 90 72 72 130, 110, 37 None0000 UTC 25 Nov 88 36 24 170, 130, 40 None1200 UTC 23 Jan 90 36 24 170, 130, 40 No latent heating1200 UTC 23 Jan 90 36 24 170, 130, 40 None0000 UTC 26 Nov 88 36 24 170, 130, 40 No latent heating0000 UTC 26 Nov 88 36 24 170, 130, 40 None1200 UTC 24 Jan 90 36 24 170, 130, 40 No latent heating1200 UTC 24 Jan 90 36 24 170, 130, 40 None0000 UTC 27 Nov 88 36 24 205, 155, 40 No latent heating0000 UTC 27 Nov 88 36 24 205, 155, 40 None1200 UTC 27 Nov 88 24 24 205, 155, 40 Yes, MDR data1200 UTC 27 Nov 88 24 12 205, 155, 40 None1200 UTC 24 Jan 90 36 24 205, 155, 40 None1200 UTC 24 Jan 90 36 24 205, 155, 40 No latent heating0000 UTC 25 Jan 90 24 24 205, 155, 40 Yes, MDR data0000 UTC 25 Jan 90 24 12 205, 155, 40 None


2. A surface energy budget based on the Noilhanand Planton scheme (1989),

3. Soil hydrology based on the Mahrt and Panscheme (1984).

Moisture physics:

1. Grid scale bulk parameterization of cloudwater & ice, rainwater and snow (Zang 1989,Lin et al. 1983),

2. Sub-grid scale Kuo-MESO convective para-meterization scheme.

The NMC/NCAR reanalysis project (Kalnayet al., 1996) provided the ®rst guess ®elds forproducing the database which is used to initializethe numerical model. The dataset is reanalyzedusing radiosonde ballon data and aviation surfacedata sets from the hourly observational networkusing a 3D-Optimal Interpolation (3DOI) scheme(Daley, 1991). The grid resolutions are 72 km, 24km, or 12 km, centered at 60�N. Simulations arecentered over 36�N so the actual resolution,taking map distortion into account, is much ®ner(57 km, 19 km, or 9.5 km).

Fig. 6. Area of integration for simulations usedin this study; a depicts area of 72 km �130�110�and 24 km �170�130� simulations, and bdepicts area of 24 km �205�155� and 12 km�205�155� simulations


Fig. 7. Trajectories constructedfrom the 72 km simulations forparcels ending at 700 hPa overRDU. Station plots contain pres-sure (hPa) and temperature (C).Displayed wind vectors depicttotal wind �msÿ1�. Parcels initi-alized at a 0600 UTC 28 andended at 0900 UTC 25 Novem-ber 1988, and b 2100 UTC 22and ended at 1800 UTC 25January 1990

Fig. 7. Continued. Valid at 2100UTC 22 January 1990 to 1800UTC 25 January 1990


In addition, three-dimensional parcel trajec-tories used throughout these papers are derivedfrom a Mesoscale Atmospheric SimulationSystem Trajectory (MASSTRAJ) software pack-age (Rozumalski, 1997). MASSTRAJ usesmodel-simulated mass and momentum ®elds toretrace or forecast the path of an air parcel.

4. Airmass origin: 72 hour back trajectories

Three-dimensional parcel trajectories are con-structed from the 72 km and 72 h simulations toidentify the airmass origin. Parcels end overcentral NC at the time of the event and theexpected time of the non-event. In the event case,there are three airstreams ¯owing into RDU. Theupper-level ¯ow (not shown) originates suthwestof the Baja of California. The mid-level ¯ow(Fig. 7a) originates near Guadalajara. The low-level ¯ow (not shown) originates near theCayman Islands. In the non-event case there aretwo ¯ows into RDU. The upper- and low-level¯ows (not shown) originate from the south-western US. The mid-level ¯ow (Fig. 7b)originates in the Mexican Gulf.

The mid-level trajectories highlight an impor-tant difference between the cases (in the eventcase, the movement of warm air off the Mexicanplateau). The event case 700 mb air parceloriginates off Guadalajara at 677 mb and movesover the Sierra Madre Orientals, descends(warming by adiabatic compression), movesover the Gulf, and ®nally ascends towardRDU (Fig. 7a). When it arrives over central NCit is unseasonably warm (with a temperature of4 �C). By contrast, the non-event case parceloriginates near the Yucatan Peninsula, circlesthe Gulf and ascends over the southeast US toRDU (Fig. 7b). The air parcel is unseasonablywarm; however, it is cooler than the eventcase.

In summary, in the event case, the upper- andmid-level airmasses originate over Mexico andthe low-level airmass originates over the Gulf ofMexico. In contrast, the high pressure overMexico dominates the non-event case ¯ow; thusthe upper-level air parcels travel across thesouthern US. The low- and mid-level airmassesoriginate over the Gulf of Mexico. It will beshown in the next section that the event casetrajectories are dominated by the STJ.

5. Simulated jet development

5.1 Jet development of the event case

The numerical simulations depict strong sub-tropical jetogenesis over northern MX and TXduring the 0000 and 1200 UTC 25 November1988 time period (Fig. 8a, b). The area of jetdevelopment is associated with low- to middle-tropospheric cold advection over the RockyMountains leading to height falls (north) andheight rises (south) above the surface warmed airon the leeside of the Mexican plateau. Thicknessadvection for the 700±300 hPa layer is ploted(Fig. 9). The jetogenesis is associated with themaximum differential thickness advection. Par-cel trajectories through the jet streak verify theexistence of the expected transverse ageostrophiccirculations about the jet (Fig. 10a). Throughoutthe trajectory period, the jet core is located overTX (Fig. 10b, c). The parcel originates upstreamof the developing jet, accelertes, ascends andde¯ects to the left when it enters the jet(thermally direct circulation). The parcel thendecelerates, de¯ects to the right and descendswhen it exits the jet (thermally indirect circula-tion), which is consistent with an idealized jetstreak (Keyser and Shapiro, 1986).

The STJ's entrance region creates integratedmass ¯ux divergence over eastern MX and thesouthwestern Gulf. In response to the upper-leveldivergence, the 1470 and 1500 m isoheights (Fig.11) move southward, which indicates heightfalls. The 12 to 18 h forecasts depict a low-leveltrough developing over eastern MX and thewestern Gulf. Also, the lower tropospheric ¯owaccelerates towards the northeast over the openwaters of the Gulf. At 0600 UTC on 25November 1988 the low-level ¯ow off the TXcoast is from the southwest at 5ÿ 10 msÿ1 (Fig.11a) and by 1800 UTC on the 25th (Fig. 11b), thelow-level ¯ow is about 15 msÿ1. Similarly, the700 hPa level (not shown) offshore low-level ¯owincreases from 5ÿ10 msÿ1 to 15ÿ20 msÿ1. TheLLJ is a low-level response to the upper-tropo-spheric ageostrophic circulation and the low-level trough. The accelerating LLJ transports thehot air from the Mexican plateau region to theeastern Gulf of Mexico coastal states by 1200UTC 26 November 1988. It will be shown thatthis hot air then becomes available to produce a


strong midlevel jet and convective instabilityduring periods of geostrophic adjustment aheadof the polar front accompanying the PJ.

The signi®cance of these dynamics can be bestappreciated by examining a 72-hour backwardtrajectory (Fig. 12). The trajectory ends at 700hPa, near RDU at 0000 UTC 28 November 1988.The parcel originates over the Mexican plateau,moves over the Gulf then takes a northeastwardturn (point A) and begins to ascend in response tothe increasing northward-directed LLJ (approxi-

mately 1800 UTC 25 November 1988). Thissignals the important role of the STJ in thetransport of southern Mexican air towards thesoutheastern US.

In summary, the transverse ageostrophiccirculation associated with the STJ removesmass from the air column, which creates ascentover the western Gulf of Mexico. Thus, a low-level trough and a LLJ develop over the westernGulf. The LLJ transports the hot air from theMexican plateau to the southeast US.

Fig. 8. Rawinsonde-derived analyses of 200 hPawind vectors, temperature (dotted lines, C), andheight (solid lines, dm) valid at a 0000 UTC 25,and b 1200 UC 25 November 1988


5.2 Jet development of the non-event case

There is strong PJ development (at 300 hPa) from1800 UTC 22 to 18 UTC 23 January 1990 (Fig.13a, b) over CO, KS, OK and MO. The PJdevelopment also appears on the 200 hPa level.Again, the maximum jet development is associatedwith the maximum height falls and cold air advec-tion. Throughout the entire period, there is an upper-level ridge over MX so the STJ does not developover MX during the ®rst 24 hours of the simulation;i.e., the wind speed varies by only 5 msÿ1.

We examine the 850 hPa level over the westernGulf of Mexico for any LLJ development. Figure14a and b depict the 850 hPa level winds valid at1800 UTC 22 and 1800 UTC 23 January 1990.At 1800 UTC 22 January 1990 the low-level ¯owis anticyclonic and is centered over the Gulf ofMexico. Over the southern US the ¯ow ispredominantly west to east with the exceptionof a small area of anticyclonic ¯ow over a weakridge over MS. By 1800 UTC on the 23rd, anupper-level jet begins to develop over CO, KS,OK, and MO (Fig. 13b). The thermally indirectcirculation about the jet exit region induces alow-level ¯ow from the south over AL and GA.In addition, the jet entrance region has athermally direct circultion, which creates a low-level northerly ¯ow over CO and NM with thewinds accelerting to 10ÿ15 msÿ1. A LLJ doesnot develop over the western Gulf of Mexico.

The difference between the cases in the jetdevelopment and synoptic situation is apparent

when comparing the 850 hPa tempertures. Figure15a and b depicts the 850 hPa temperature,pressure, and wind valid 1200 UTC 23 January1990 and 0000 UTC 26 November 1988,respectively. Over the western Gulf, in the eventcase, the winds are from the southwest and thetemperatures are 18 to 20 �C facilitating leecyclogenesis. In the non-event the winds over thewestern Gulf coast are from the east and tem-peratures are 12 to 14 �C.

In summary, the non-event case has a differentsynoptic pattern, upper-level jet development andthus different low-level responses. At this time(48±72 hours before the expected event), the ¯owis zonal. A jet streak develops over northern TXstretching into MO (farther to the north than theevent case). In the upper-levels, there is a weakridge over MX. The ridge does not allow the STJto move anywhere near the Gulf of Mexicoinhibiting any LLJ development and the transportof the hot air from over the Mexican plateautowards the US Gulf Coast. Without the hotMexican air over the southeastern US, the mid-level, cross-stream height gradient is weaker sothe midlevel jet does not develop above the warmmoist tropical air.

6. Potential vorticity

In this study, we investigate low-level PVproduction (Eq. 2), which is largely the productof diabatic processes normally considered smalland neglected. We investigate the origins and

Fig. 9. MASS simulated, 24 km mesh, 700±300hPa, 1 h thickness advection (solid lines denotepositive value and dashed lines denote negativevalues, mhÿ1) valid at 0600 UTC 25 November1988


transport of low-level PV because low-level PVis related to surface mesolow development andsevere weather.

6.1. Origins of low-level potential vorticity 72hours before event and non-event

For the event case, we examine the origins of thelow-level PV maximum ending over central NC atthe time of the tornado outbreak. Figure 16 depictsthe 900 hPa PV maxima every 12 hours for the 72hours before the event and non-event. In the eventcase (Fig. 16a±e), the PV maximum starts over theTX coast (just west of Houston) and moves east.

When the maximum reaches the Alabama Gulfcoast it turns to the northeast and moves over thecentral NC at the time of the tornado. The non-event case (Fig. 16f±j) has several PV maximaoriginating near the TX/OK border and movingacross central AR and TN. These PV maximaweaken over time. A ®nal PV maximum developsover central MS (associated with convection) at1200 UTC 25 January 1990 and tracks to theAppalachians. Figure 17 depicts the 72-hour pathof the low-level PV maximum. The low-level PV istraced using the widely accepted 1 PV unit��10ÿ6 K hPaÿ1sÿ1� demarcating air of strato-spheric origin (Reed, 1955; Whitaker et al., 1988).The low-level PV maximum over central NC tracesback to an area south of BRO. At the onset of thesimulations (0600 UTC 25 November 1988) PVmaxima are in the lee of the mountains over twoareas: near Monterrey, MX (MMTY) and nearDRT. The PV maximum near MMTY movesnortheastward over Corpus Christi, TX (CRP)where it moves in close proximity to another PVmaximum that formed near DRT. Parcel trajec-tories con®rm the relative strength and movementof the PV maxima. The PV maximum originatingnear BRO is the dominant PV maximum.

We also examine the origins of the low-levelPV maxima for the non-event case (Fig. 16f±j).There is a low-level PV maximum over theAppalachians, but not over central NC. The PVmaximum traces back 21 hours into northernMS, then weakens until the track is not de®nable.Also, several PV maxima move east from the leeside of the Rocky Mountains across OK, AR andTN to the Appalachians. The 72-hour low-levelPV maximum tracks are depicted in Fig. 17.

In summary, there are two signi®cant differ-ences between these cases. First, the event casePV maximum is a coherent and traceable entityover 72 hours. In the non-event case PVmaximum is not a coherent entity. Second, thenon-event case PV maximum appears to origi-nate over the Rocky Mountains in CO. In theevent case, the low-level PV maximum originatesmuch farther to the south, over MX.

6.2 Generation of low-level potential vorticity 72hours before event and non-event

We investigate the origins of the low-level PVsouth of BRO. A mountain wave exists after

Fig. 10. MASS simulated, 24 km mesh a trajectoryinitialized at 0400 UTC 25 and ended at 0000 UTC 26November 1988. Station plots contain pressure (hPa),temperature (C), and total wind speed �msÿ1�. Displayedwind vectors depict total wind; b 250 hPa isotachs (shadedat intervals of 5 for speeds greater than 50msÿ1) valid at1500 UTC 26, and c 250 hPa isotachs (shaded at intervalsof 5 for speeds greater than 50msÿ1) valid at 2100 UTC 26November 1988


0600 UTC 25 November 1988 on the east side ofthe Sierra Madre Mountains (Fig. 18) evident inthe low-level folding of the isentropes. Also, theFroude number over the Mexican plateau(upstream) was 1.03, which belongs to a ¯owregime conductive to producing strong down-slope winds (Lin and Wang, 1996). The wavesgenerated in the lee of the Sierra Madremountains (wave breaking region) transport PV

to the low-levels. The 0.5 (PV units) contourextended from the 500±600 hPa level over themountains to the 900 hPa level along the coast.

The mid-level PV maximum is transporteddown the mountains but the next question iswhere does the mid-level PV originate. AT 1500UTC 24 November 1988 (Fig. 19) there is a largeupper-level PV maximum (> 4.5 PV units at 200hPa) over TX and northern MX. The 1 PV unit

Fig. 11. MASS simulated, 24 km mesh, 850 hPawind vectors, temperature (dotted lines, C), andheight (solid lines, dm) valid at a 0600 UTC, andb 1800 UTC 25 November 1988


contour extended as far south as the southern tipof the Baja of California. As a STJ streak moveseastward over the west coast of MX its exitregion and associated thermally indirect ageo-strophic circulation (indicated by the ageostrophicwind directed to the right of the stream)approaches the upper-level PV maximum. Figure20 (valid 1800 UTC 24 November 1988) is across section extending across the STJ exitregion and the Mexican plateau depicting thethermally indirect ageostrophic circulation asso-ciated with the STJ streak. The thermally indirectageostrophic circulation creates sinking (A inFig. 20) in the right exit region of the jet streak(over the Mexican plateau) facilitating thedownward transport of PV and ascent in the leftexit region (B in Fig. 20). Descent in the rightexit region of the jet streak core transports a lobeof upper-level PV down to the middle troposphere(500±600 hPa). This situation is considerablydifferent from the isolated jet/front system usedby Danielsen (1968) to develop his conceptualmodel of the downward transport of PV in the left

entrance region of the jet. Danielsen concludedthe greatest production of PV is associated withstrong static stability found in the left jetentrance region where sinking was also occurringso downward transport of PV would mainlyoccur in that area. The conceptual model did notinclude the interaction of several jet streaks. Inthe event case, there are two jet streaks within theSTJ; the ®rst jet streak (located over TX and theGulf Coast States) facilitates PV production andtransports the PV southward. Then the secondSTJ streak and its associated thermally indirectcirculation transports the PV to the mid-levelsover the Mexican plateau. The downward trans-port of PV is another way the STJ is critical forthe future development of severe weather overthe southeast US.

The PV is transported to the mid-levels untilapproxiately 2000 UTC 24 November 1988. Atthe same time, there is substantial heating overthe Mexican plateau. By 0000 UTC 25 Novem-ber 1988 (Fig. 21), there is a deep and warm dryadiabatic layer (potential temperature of 310 K

Fig. 12. Trajectory constructed from the 72 kmmesh MASS simulation initialized at 0000 UTC25 and ended on 0000 UTC 28 November 1988.Station plots contain pressure (hPa), tempera-ture (C), and displayed wind vectors depict totalwind


up to � 650 hPa) over MX, which is being forcedby large surface sensible heat ¯uxes resultingfrom clear skies, minimal soil moisture andarid conditions. We will refer to the area withinthe box as the PV generation area (Fig. 17).

By 2100 UTC, a mountain-plains solenoid(MPS) (Tripoli and Cotton, 1989) produces awell mixed, elevated boundary layer. In theeastern half of the PV generation area, strongascent occurs in the low levels and descent

Fig. 13. MASS simulated, 24 km mesh, 300 hPa windvectors, temperature (dashed lines, C), and height (solidlines, dm) valid at a 1800 UTC 22 January 1990, and b1800 UTC 23 January 1990

Fig. 14. MASS simulated, 24 km mesh, 850 hPa windvectors, temperature (dashed lines, C), and height (solidlines, dm) valid at a 1800 UTC 22 January 1990, and b1800 UTC 23 January 1990


occurs in the mid and upper levels. At 0100 UTC25 November 1988 (Fig. 22) these two ageo-strophic circulations create an area of verticalconvergence in the 500±600 hPa layer over theeastern half of the PV-generation area (Fig. 17).

This convergence perturbs the potential tempera-ture ®eld, increases the thermal gradient (staticstability) and generates PV in the 500±600 hPalayer. The relative contribution of each term inthe PV (Eq. 2) is evaluated at 0100 UTC aboutthe 625 mb level in the PV generation area in amanor analogous to Kaplan and Karyampudi(1992b). Data used in the calculations are takenfrom parcel trajectory output with the exceptionof the gradient ®eld, which is derived using acentered ®nite differencing scheme calculatedfrom trajectories surrounding the center trajec-tory point. Term 1 dominates the production ofPV (contributing 78%) by the dramatic increasein static stability associated with the intensediabatically-forced thermal gradient (and verticalfrontogenesis) above the dry adiabatic layer.Surface sensible heating is the main contributionto the diabatically-forced thermal layer. Thismid-level PV maximum moves eastward and by0600 UTC it is over the Sierra Madre Orientals.Later, during the 0600±1200 UTC nocturnalperiod, a hydrostatic mountain wave develops inthe lee of the mountains (Fig. 18) facilitating thedownward transport of the midlevel PV gener-ated earlier. Model-generated soundings indicatean inversion develops due to radiational coolingover the coastal area in the early morning hours.Thus, the low-level PV moves down the lee ofthe mountains and increases due to strong staticstability in the presence of nocturnal surfacecooling.

For the non-event case we again examine the¯ow on the east side of the Sierra MadreMountains near BRO (Fig. 23). Throughout thesimulations there were no low-, mid-, or upper-level PV maxima over the coastal region. In thenon-event case, the low-level wind ®eld over theGulf of Mexico was from the east thus nomountain waves developed near BRO. Also,there was not a STJ to facilitate the downwardtransport of mid- or upper-level PV . Addition-ally, cross sections depict very little daytimeheating over the Mexican plateau (Fig. 24). Theeasterly low-level wind advects moisture rich airover the Mexican plateau producing low-levelclouds thus minimizing the solar radiation andsurface warming. The advection of relativelywarm, moist air off the Gulf reduces the intensityof the nocturnal inversion because of reducedradiational cooling, hence, less PV production

Fig. 15. MASS simulated, 24 km mesh, 850 hPa windvectors, temperature (dashed lines, C), and height (solidlines, dm) valid at a 1200 UTC 23 January 1990, and b0000 UTC 26 November 1988


Fig. 16. MASS simulated, 24 km mesh 900 hPaPV (shaded greater than 1 PV unit by 0.5) validat a 0600 UTC 26, b 1800 UTC 26, c 0600 UTC27, d 1800 UTC 27, e 0600 UTC 28 November1988, f 1800 UTC 23, g 0600 UTC 24, h 1800UTC 24, i 0600 UTC 25, and j 0000 UTC 26January 1990

Fig. 17. Depiction of 900 hPa PV maximumtrack using 24 km mesh MASS simulation data,solid line is event case and dashed line is non-event case


occurs as the stability is weaker than in the eventcase. Throughout the evening, a shallow surfaceinversion develops over the mountains increas-ing low-level PV over this region due to theincreasing static stability. Also, we examine thePV maximum east of the Rocky Mountains over

northern TX and OK. The PJ induces waves inthe lee of the mountains, which transports PVdownward.

Next, we evaluate the relative contribution ofeach term in Eq. 2 as the parcel descends theleeward mountain slope. In the event case, the

Fig. 18. MASS simulated, 24 km mesh, crosssection extending from 25N, 101W to 23.5N,96W. Includes wind vectors, � (solid lines, K),and PV (dashed lines, contoured by 0.5 andshaded greater than 1 PV unit) valid at 1200UTC 25 November 1988

Fig. 19. MASS simulated, 24 km mesh, 200 hPaPV (dashed line, contoured by 0.5 PV units),wind speed (solid line, contoured at 40msÿ1)and ageostrophic wind vectors valid at 1500UTC 24 November 1988


total PV increased 412%, from 0.7665 (PV units)to 3.159 (PV units). Term one contributes 30.6%to the producton of PV by the change in staticstability associated with the developing low-levelradiation inversion over the coastal region. Also,vorticity increases due to vortex tube stretchingas the air volume advects down the mountains.Term two contributes 31.9% to the change in PV

resulting from horizontal gradients of diabaticheating within regions of wind covariances. The910 hPa PV maxima is within the inversionhence it is affected by friction. Term threecontributes 37.5% to the production of PVresulting from the tilting of the horizontalcomponent of vorticity into the vertical throughhorizontal diabatic heating. The low-level

Fig. 20. MASS simulated, 24 km mesh, crosssection extending from 30.5N, 110W to 22N,96.7W. Includes ageostrophic wind vectors, �(solid line, K), wind speed (thick solid line,contoured at 25 and 30msÿ1) and PV (dashedlines, contoured by 0.5 PV unit) valid at 1800UTC 24 November 1988

Fig. 21. MASS simulated, 24 km mesh, crosssection extending from 26.5N, 110W to 22.2N,92.5W. Includes ageostrophic wind vectors, �(solid lines, K), and PV (dashed lines, con-toured by 0.5 and shaded greater than 1 PVunit) valid at 0000 UTC 25 November 1988


vertical wind shear and vertical potential tem-perature gradient advect down the mountains(and is tilted) generating PV . The diabaticforcing mechanisms in the PV equation (Eq. 2)that are typically neglected are very important inthe low-level generation of PV .

In the non-event case, the parcel trajectory isdifferent from the event case ascending the SierraMadre Orientals then circling (clockwise) the

mountains. As the parcel ascends the lee slope ofthe Sierra Madre Mountains, the total PVdecreases from 0.273 PV units at 982 hPa to0.173 PV units at 788 hPa. The decrease of low-level PV is due to a production of anticyclonicvorticity. Our calculations from model outputindicate that term one produces 69% of the totalanticyclonic vorticity. Term one represents theproduction of anticyclonic PV by vortex tube

Fig. 22. MASS simulated, 24 km mesh, crosssection extending from 26.5N, 110W to 22.2N,92.5W. Includes ageostrophic wind vectors, �(solid lines, K), and PV (dashed lines, con-toured by 0.5 and shaded greater than 1 PVunit) valid at 0100 UTC 25 November 1988

Fig. 23. MASS simulated, 24 km mesh, crosssection extending from 25N, 110W to 23.5N,96W. Includes wind vectors, � (solid lines, K),and PV (dashed lines, contoured by 0.5 andshaded greater than 1 PV unit) valid at 0900UTC 23 January 1990


contracting as the air volume moves up themountains. Term two contributes 13.8% to theproduction of anticyclonic PV resulting fromhorizontal gradients of diabatic heating withinregions of wind covariances. The frictionalcomponent consists of effects from barotropicsurface drag and the baroclinic effect of mixing.Term three represents a smaller amount (17.2%)of anticyclonic PV production as the low-levelvertical wind shear and vertical potential tem-perature gradient are advected up the mountains.Here, the synoptic ¯ow is transporting the parcelsin a clockwise direction over the mountains(rightward-directed KE). We hypothesis thatthe large scale mixing is bringing down therightward directed momentum overwhelmingthe effects of surface drag and producing anti-cyclonic PV .

6.3 Maintenance of low-level potential vorticity

We investigate the transport and maintenance ofthe low-level PV as it moves north along the TXcoast. First, we examine the possibility that thecontinuous downward transport of the upper-level PV maintains the low-level PV . Weproduce numerous cross sections through thelow-level PV maximum as it propagates up theTX coast. Throughout this period, there is no

downward transport of the upper-level PV .Supporting the idea that the PV is generatedand maintained in the low levels.

We examine other potential energy sources(wind shear gradient, potential temperaturegradient, and surface convergence) and theirrelationship to low-level PV . These energysources all act to produce and/or maintain PVvia Term 3 in Eq. (2). The ®gures valid 1200UTC on 25 November 1988, depict: 900 hPa PVand the gradient of the magnitude of wind shearover the 925 to 850 hPa layer (Fig. 25a), 900 hPaPV and surface streamlines (Fig. 25b), and 900hPa PV and surface potential temperature (Fig.25c). Each PV maximum (> 1 PV unit) isassociated with a strong vertical and horizontalwind shear gradient, localized areas of stronggradients of potential temperature, and a con-vergence area. The low-level PV maxima areassociated with these energy sources throughoutthe simulation. In addition, the strength of thevertical and horizontal wind shear associatedwith the LLJ is closely corelated to the low-levelPV ± as the shear weakens so does the PV . So, itappears a combination of strong vertical andhorizontal wind shear, surface convergence and athermal gradient are essential ingredients for theproduction or maintenance of low-level PV .Low-level vertical and horizontal wind shear

Fig. 24. MASS simulated, 24 km mesh, crosssection extending from 26.5N, 110W to 22.2N,92.5W. Includes ageostrophic wind vectors, �(solid lines, K), and PV (dashed lines, con-toured by 0.5 and shaded greater than 1 PVunit) valid at 0000 UTC 23 January 1990


associated with the LLJ is the most likely transportand maintenance mechanism of PV since the LLJis tied to the persistent STJ.

Next, we investigate the transport and main-tenance of low-level PV for the non-event case.Depictions of the 900 hPa PV and the magnitudeof wind shear, potential temperature gradient, andsurface convergence show a different relationshipthan the event case. The depictions valid 0000UTC on 23 January 1990 (Fig. 26a, b, c) have nosigni®cant areas of PV . Over GA, SC and NCthere is strong vertical and horizontal wind shear.However, there is no signi®cant convergence anda weak potential temperature gradient thus nolow-level PV maximum.

6.4. Lagrangian investigation of low-levelpotential vorticity

In the ®nal PV analysis of the event case, weswitch to a Lagrangian frame of reference. Welocate the PV maximum south of BRO and senda parcel through that area at 0600 UTC 25November 1988. The parcel originates over theSierra Madre Mountains at 0300 UTC, descendsover the coastal area, moves northward, moves inproximity with another low-level PV maximum(originating from the mountains near DRT) andcontinues to move northeastward to LA (Fig. 27).The PV maximum originating over MX is thedominant feature. In this trajectory, the parcel'smotion vector is almost exactly the same as thePV maximum's vector so the parcel's movementand values listed in Table 2 can be considered avolume encompassing the PV maximum. ThePV associated with the parcel increased as itdescended to the 910 mb level due to staticstability, vortex tube stretching, tilting, andfrictional effects. As the PV maximum (parcel)moves northward along the coast it is maintainedby convergence, a thermal gradient, and stronglow-level wind shear associated wih the LLJ. Asthe LLJ's shear ®eld weakens with time so doesthe PV maximum. At approximately 0300 UTC

Fig. 25. MASS simulated, 24 km mesh, valid at 1200 UTC25 November 1988 of PV (contoured (dashed lines) andshaded greater than 1 PV unit) and a magnitude of shear(solid lines, msÿ1) over the 925 to 850 hPa layer, b surfacestreamlines, and c surface theta (solid lines, K)


26 November 1988, the parcel enters an area ofconvection in eastern TX; consequently, the PVmaximum increases from latent heat energy.Figure 27 depicts the parcel trajectory and 900hPa latent heating ��Chÿ1� at 0400 UTC 26November 1988. The greatest PV increase (from0.8989 to 1.705 PV units) occurs when the parcelmoves through a latent heating maximum�> 40�Chÿ1� at 0400 UTC. Sensible heating isaso examind, but is negligible (maximum valueof 0:11 Chÿ1). So, the low-level PV increase isdirectly related to latent heating.

In summary, south of BRO, the PV is initiallytransported downward to the mid-levels by theageostrophic circulation associated with the STJ,maintained by the mountain-plains solenoid, thentransported to the low-levels by an ensemble ofmountain waves. Also, diabatic effects accom-panying increased static stability, vortex tubestretching, tilting and frictional effects generatePV . The PV is transported by the LLJ andmaintained by low-level wind shear and latentheating. The low-level PV maximum originateseast of the mountains near BRO, moves north-east, follows the coastline along the Gulf Statesand up to RDU at the time of the tornado.

In the ®nal PV analysis of the non-event casewe again examine parcel trajectories. Parcels onthe leeward side of the mountains all originateover the low coastal area of northeastern MX,ascend over the Sierra Madre Orientals thenmove clockwise back over MMTY some 24hours later (Fig. 28, Table 4). The synoptic ¯owand parcel movement is very different from theevent case. In the non-event case, the parcelascends (cools) the leeward side of the SierraMadre Mountains then descends (warms) overthe MX plateau. In the lee of the mountains,there is little low-level PV most likely becausethe east to west ¯ow does not facilitate thedevelopment of mountain waves or a surfaceinversion, which facilitates low-level PV . Overthe parcel's path, there is no convection (latentheating) over the Mexican plateau. As the parcel

Fig. 26. MASS simulated, 24 km mesh, valid at 0000 UTC23 January 1990 of 900 hPa PV (contoured (dashed lines)and shaded greater than 1 PV unit) and a magnitude ofshear �msÿ1� over the 925 to 850 hPa layer, b surfacestreamlines, and c surface theta (K)


moves across the Mexican plateau through theevening hours a weak radiation inversion devel-ops, which increases the low-level stabilitygenerating small amounts of low-level PV . Weakdaytime heating also acts to increase the low-level stability, which maintains the weak, low-level PV . The most signi®cant aspect of the

parcel trajectory is that the parcel and anyassociated PV remains over MX. Without theSTJ and its associated LLJ the warm, PV rich airis not transported to the north. In summary, veryweak, low-level PV is generated over the MXplateau; however, there is not a transportmechanism (LLJ) to move the PV over the US.

Table 2. Trajectory initiated at 0300 UTC 25 November 1988, passed through the 910 hPa PV maximum (25.7N, 99.8W) at0600 UTC 25 November 1988 and ended at 0500 UTC 26 November 1988. Trajectory data shown here is every 2 hours and wasderived from hourly data of a 24 km MASS simulation. The following abbreviations are de®ned: Latitude (LAT), Longitude(LON), Pressure (PRS), Temperature (TMP), and Potential Vorticity (PV).

Time (UTC) LAT (�N) LON (�W) PRS (hPa) TMP (�C) PV (PV units)

25/0400 25.25 100.57 765.3 11.7 .766525/0600 25.33 100 910 23.8 3.15925/0800 25.94 99.52 915.5 22.9 2.40825/1000 26.09 99.2 914.6 23.3 2.13125/1200 26.15 98.84 919.4 23.57 1.97125/1400 26.3 98.48 922.6 23.8 1.69525/1600 26.6 98.02 908.8 22.53 1.40225/1800 27.05 97.53 890.6 20.82 1.09425/2000 27.54 96.99 876.5 19.32 0.95925/2200 28.09 96.41 866.1 18.1 0.865726/0000 28.67 95.82 844.8 16.36 0.898926/0200 29.37 95.24 844.6 16.17 1.093226/0400 30.16 94.57 837.9 15.27 1.705

Fig. 27. MASS simulated, 24 km mesh,trajectory initialized at 0600 UTC 25 and endedat 0600 UTC 26 November 1988 plotted overlatent heating due to convection (contoured by10 Chrÿ1) at 900 hPa. Station plots containpressure (hPa), temperature (C), and total windspeed �msÿ1�. Displayed wind vectors depicttotal wind


7. Conclusion

Observational analysis and mesoscale numericalsimulations are in agreement concerning keydynamical processes which occurred over MXand the Gulf of Mexico 72 hours prior to the1988 Raleigh, NC, tornado outbreak. Strong coldair advection over NM and northwestern TXintensi®ed the height gradient (PGF) and, in turn,intensi®ed the STJ streak over TX and MX.Initially, the STJ exit region and its associatedtransverse ageostrophic circulation created sink-ing over the Mexican plateau. Air was forceddown the eastern side of the Sierra MadreMountains, which warmed due to adiabatic(compressional) heating. Along with this warmair, a low-level trough of low pressure developedover the western Gulf of Mexico. The STJ rightentrance region moved over the western Gulf ofMexico, which helped create upper-level diver-gence, upper-level mass removal and ascent inthe air column. The low-level convergence,ascent throughout the levels and velocity diver-gence aloft helped develop the surface troughand the associated LLJ. This LLJ began theprocess to transport warm low-level air from theMexican plateau to the Piedmont.

The non-event case had a different synopticpattern. Seventy-two hours before the expectedevent, the upper-level ¯ow was zonal over theUS with a ridge over MX. A PJ developedover northern TX, OK and MO. The upper-levelridge prevented the development of a STJ overMexico. During the early period, no transport

Fig. 28. MASS simulated, 24 km mesh, trajectoryinitialized at 1800 UTC 22 and ended at 1800 UTC 23January 1990. Station plots contain pressure (mb), tem-perature (C), and total wind speed �msÿ1�. Displayed windvectors depict total wind

Table 3. Same as Table 2 except trajectory initiated at 1800 UTC 22 January 1990 and ended at 1800 UTC 23 January 1990.

Time (UTC) LAT (�N) LON (�W) PRS (hPa) TMP (�C) PV (PV units)

22/1400 25.6 99.67 982.3 13.2 0.27322/1600 25.6 99.91 970.8 13.1 0.10022/1800 25.5 100.2 900.9 14.6 0.06022/2000 25.5 100.5 811.8 13.1 0.14022/2200 25.5 100.9 793.0 14.5 0.10723/0000 25.5 101.2 787.9 14.1 0.17323/0200 25.8 101.4 797.9 16.0 0.30223/0400 26.2 101.4 789.6 15.0 0.29223/0600 26.5 101.4 806.8 16.4 0.48923/0800 26.86 101.2 819.6 17.6 0.44123/1000 27.2 100.9 839.6 18.9 0.38523/1200 27.6 100.7 840.3 18.5 0.46123/1400 27.9 100.2 821.0 16.5 0.35723/1600 28.0 99.66 800.9 14.3 0.353


mechanism developed to move the hot, dry airoff the Mexican plateau and over the Gulf ofMexico. There was one very important differencebetween the cases ± the air over the Gulf wasmuch cooler than the event case (generally 4 to 6C cooler).

The event case PV was generated in the upperlevels, over Mexico, transported down to thelower levels, then advected over central NCarriving 72 hours later at the time of a tornadicoutbreak. Initially, there were two jet streaksembedded within the STJ. The ®rst streak(located over TX and the Gulf Coast States)facilitated PV production and transported the PVsouthward. The second streak and its associatedthermally indirect circulation transported thePV down to the mid levels �� 500 hPa� over theMexican plateau. At the same time, heating overthe Mexican plateau producing a deep and warmdry adiabatic layer over MX, which was forcedby large sensible heat ¯uxes resulting from clearskies, minimal soil moisture and arid conditions.A MPS developed over the Mexican plateau,which created ascent to 550 hPa. An area ofvertical convergence was created in the 500±600hPa layer increasing the thermal gradient (staticstability) and generating PV . The increase instatic stability was primarily responsible for theincrease in mid-level PV . This midlevel PVmaximum then moved eastward and a hydro-static mountain wave transported the PV down-ward near the Mexican coast. As the PVmaximum moved downward, it increased dueto increasing static stability, tilting of horizontalvorticity into the vertical and frictional effects.The low-level PV moved along the Gulf Coastand ®nally north along the East Coast to centralNC at the time of the tornado outbreak. Thelow-level PV maximum was maintained andincreased by a combination of low-level windshear, latent heating, frictional effects andsensible heating.

In the non-event, the STJ was absent, thus PVwas not transported downward to the midlevels.The low-level wind ®eld over the western Gulf ofMexico was from the east advecting clouds overthe Mexican plateau minimizing surface heatingand inhibiting a deep dry adiabatic layer. Theeasterly low-level ¯ow prevented the develop-ment of mountain waves southwest of BROinhibiting the downward transport of mid-level

PV . In addition, the non-event case did not havea LLJ over the western Gulf of Mexico totransport northward any low-level PV .

There were two distinct differences betweenthe cases. First, the PV maximum was a coherentand traceable entity over 72 hours in the eventcase while in the non-event case the PVmaximum was not a coherent, long-lasting entity.Second, the event case PV maximum originated� 1500 km to the south than that of the non-event case.

Acknowledgements

The ®rst author would like to thank the Air Force Instituteof Technology for the opportunity to pursue my advanceddegree. The authors wish to thank Drs Robert Rozumalskiand Kenneth Waight III of MESO Inc. for access to andhelp with the MASS model.

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Corresponding authors' address: Dr. Yuh-Lang Lin,Department of Marine, Earth and Atmospheric Sciences,North Carolina State University, Raleigh, NC 27695-8208(E-Mail: [email protected])


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