the diurnal evolution of cold fronts in the …roger/...morning glories generated by cold fronts...

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETYQ. J. R. Meteorol. Soc. 00: 1–16 (2008)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/qj.000

The Diurnal Evolution of Cold Fronts in the AustralianSubtropics

Gerald L. Thomsena∗ Michael J. Reederb and Roger K. Smitha

aMeteorological Institute, University of Munich, GermanybSchool of Mathematical Sciences, Monash University, Australia

Abstract:The effect of the diurnal cycle on the evolution of cold fronts in the Australian subtropics is investigated in two high-resolutionnumerical simulations. The simulations are made using the Pennsylvania State University/National Center for Atmospheric ResearchMesoscale Model (MM5) with the initial and boundary conditions taken from the operational analyses of the European Centre forMedium Range Weather Forecasts. These simulations are compared with the observations of two cold fronts taken during the CentralAustralian Fronts Experiment in 1991. The simulations show a number of features that have been suspected, but never confirmedor quantified. (i) Although the wind field in the boundary layer is frontogenetic, daytime turbulent mixing is strongly frontolytic,which accounts for the weakening and deceleration of the fronts during the late morning and afternoon when convective mixing inthe boundary layer is most vigorous. (ii) When the mixing subsides in the early evening, the low-level winds increase along with thedeformation and convergence, leading to a strengthening and acceleration of the fronts. (iii) Bore-like disturbances are generatedduring the early hours of the morning as the nocturnal inversion strengthens. These bores propagate ahead of the front, developinga series of large-amplitude waves at their leading edge.Copyright c© 2008 Royal Meteorological Society

KEY WORDS Bore; Cold Front; Diurnal Cycle; Morning Glory; Sea Breeze

Received June 2, 2008; Revised ; Accepted

1 Introduction

Except during the warmest months (December to March),mid-latitude cold fronts in the Southern Hemisphere reg-ularly extend far in to the subtropics (Reeder and Smith1998). These subtropical incursions are most commonduring the dry season, but become less frequent as thecontinent warms and the mean subtropical ridge axis andmid-latitude westerlies migrate polewards. In the decadecommencing 1988, a series of four, relatively modest fieldexperiments were conducted in central and northern Aus-tralia to investigate the structure and behaviour of conti-nental cold fronts as they advance into the subtropics. Intotal, fifteen fronts were documented in some detail duringthese experiments (Smith and Ridley 1990, Smith et al.1995, Reeder et al. 1996, Deslandes et al. 1999, Reeder etal. 2000; Preissler et al. 2002), with the principal findingsas follows.

• Cold fronts in the Australian subtropics are gener-ally no deeper than about 1 km and they advanceinto a convectively well-mixed boundary layer, typ-ically 3-4 km deep, which, during the night, overliesa strong, shallow radiation inversion.• The fronts weaken and decelerate after sunrise as

the mixing in the boundary layer intensifies.

∗Correspondence to: Michael J. Reeder. School of Math-ematical Sciences, Monash University, Australia. E-mail:[email protected]

• The fronts strengthen and accelerate in the eveningas the boundary layer turbulence weakens: of the 15fronts observed in during the field experiments, 13crossed central Australia during the evening or earlyhours of the morning.

• As the fronts approach the northern coastline ofthe continent they invariably generate propagatingbores in the evening or early hours of the morn-ing. These bores propagate ahead of the front onthe strong surface-based nocturnal inversion or onthe cool air advected inland by the sea breeze. Overland, the bores generally dissipate before noon, bywhich time the stable layer is destroyed by con-vective mixing, but they decay much more slowlyonce over the water. If the air is moist enough, thebore waves generate lines of low cloud, known inAustralia as morning glories, in this case southerlymorning glories as there are other ways that borescan be generated over northeastern Australia (Smith1988).

The present paper aims to explain these observationsand to quantify the effects of the diurnal cycle within theconvective boundary layer on their dynamics. A furtheraim is to examine the extent to which the fine-scalestructure of fronts, including the bores they generate,can be forecast. Our study is focussed on numericalsimulations of two cold fronts observed during the 1991Central Australian Fronts Experiment (hereafter referredto as CAFE91). These fronts are selected as they are

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2 G. L. THOMSEN, M. J. REEDER AND R. K. SMITH

typical of the fifteen observed and they are relativelywell documented. Even so, despite the additional fielddata, the observational data over central and northernAustralia are still comparatively sparse and the fronts arenot sampled well enough to provide a complete pictureof their structure and evolution. Therefore, our approachis to use the numerical simulations to fill the gap andprovide a more complete picture of the fronts and the bore-like disturbances that they generate on spatial scales muchfiner than the observational network. (See Smith et al.1995 for further information on the observational networkdeployed for CAFE91.)

The only previous investigations into the effect ofthe diurnal cycle on the dynamics of cold fronts thatwe are aware of have been idealised and focussed onthe midlatitudes (Pagowski and Taylor 1998, Reeder andTory 2005). One other study by Thomsen and Smith(2006) examined the generation of propagating bores bycold fronts. They analysed simulations of two southerlymorning glories generated by cold fronts observed duringthe Gulf Lines EXperiment (GLEX) in 2002. (The resultsof GLEX are reported by Goler et al., 2006, Smith etal., 2006 and Weinzierl et al. 2007.) The present studydiffers from that by Thomsen and Smith by its focus onthe dynamics of cold fronts in the Australian subtropics,whereas Thomsen and Smith were concerned mainly withthe prediction and evolution of the bore associated withthe southerly morning glory. Consequently, the presentwork compliments that by Thomsen and Smith.

The simulations described are made using the Penn-sylvania State University/National Center for Atmos-pheric Research Mesoscale Model (MM5), with the ini-tial and boundary conditions provided by the operationalanalyses from the European Centre for Medium RangeWeather Forecasts (ECMWF). The horizontal grid reso-lution of the inner domain is 3 km, which is much smallerthan that currently used for numerical weather prediction.Apart from Thomsen and Smith (2006), all previous mod-elling work on subtropical cold fronts has been based onoperational numerical weather prediction output with hor-izontal resolutions no better than 50 km (Smith et al. 1995,Deslandes et al. 1999, Reeder et al. 2000; Preissler et al.2002).

The paper is structured as follows. Section 2 sum-marises the numerical model and initial conditions. Sec-tions 3 and 4 discuss the numerical simulations of thetwo best observed cold fronts during CAFE91, comparingthe simulations against the available observations. Bothcases are relatively well documented examples of noctur-nal frontogensis and the associated generation of propa-gating bores. The first case, Event 1, is included as it pro-vides a clear example of how fronts in the Australian sub-tropics weaken and stall during the day, but develop sharpsurface wind changes after sunset. Although the tempera-ture contrast is relatively weak in Event 1, the cold frontgenerated a bore which separated from the front. In con-trast, the second case, Event 2, is a strong nocturnal frontmarked by sharp temperature falls. It is included as the

bore generated remains attached to the cold front through-out most of the early morning. Once the front begins toweaken, the bore propagates far ahead of the original coldfront and develops into the principal wind change. Thestructure of the bores in each event is discussed in Section5, while some conclusions are drawn in Section 6.

2 MM5

MM5 is described in detail by Grell et al. (1995). Themodel is nonhydrostatic and uses a finite differenceapproximation to the equations with σ = (p− pt)/(ps −pt) as the vertical coordinate, where p is pressure, ps isthe surface pressure and pt the top pressure, set here to100 mb. There are 23 σ-levels with 15 levels below 3 km.The latter are located at heights of approximately, 0, 40,80, 150, 230, 300, 380, 540, 700, 870, 1200, 1550, 1900,2300 and 2750 m, giving a relatively high resolution in theboundary layer.

The calculations are made on two nested, horizontaldomains. The locations of these domains are plotted inFig. 1. The outer domain has 221 × 221 grid points witha horizontal resolution of 9 km and the inner domainhas 361 × 361 points with a horizontal resolution of3 km. The land use and orography are taken from theMM5 data set and have a 5’ resolution for the outerdomain and 2’ resolution for the inner domain. Thetime step is 27 s for the outer domain and 9 s forthe inner domain. The Grell cumulus parameterizationscheme (Grell 1993) is implemented in the outer domain,but no cumulus scheme is used in the inner domain ascold fronts over central and northern Australia are mostlydry. The boundary-layer scheme as implemented in theNational Centers for Environmental Prediction (NCEP)Medium Range Forecast (MRF) system by Hong and Pan(1996) is selected for all domains. The Dudhia scheme(Dudhia 1989) is chosen to represent explicit moistureconversions, and a bucket scheme is used for the soilmoisture budget. A short and long-wave cloud and groundradiation scheme that takes account of diurnal variationsis used. A wave radiation scheme is applied at the upperboundary to inhibit the reflection of wave energy from themodel top. The initial conditions for the two cold frontsexamined here are taken from the ECMWF operationalanalyses at 1000 Eastern Standard Time (EST) on 9September and 16 September. (EST = UTC + 10 h.) Thesedata have a horizontal resolution of 0.25◦. In each case themodel is integrated forward for 48 h.

3 Event 1

Event 1 developed a sharp wind change during the nightof 9-10 September, arriving at Mount Isa, which is close tothe centre of both model domains, at about 0930 EST 10September. (All places mentioned in the text are markedin Fig. 2.) The passage of the front raised dust across theregion, which lasted throughout most of the morning andclosed the airport at Mt Isa. Despite the strength of thewind change, only weak temperature falls were recorded

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Q. J. R. Meteorol. Soc. 00: 1–16 (2008)DOI: 10.1002/qj

SUBTROPICAL COLD FRONTS 3

(a) (b)

Figure 1. Event 1. ECMWF analyses at (a) 1600 EST 9 September and (b) 1000 EST 10 September. MSLP (dashed lines), horizontal windvectors and virtual potential temperature (shaded) at 850 mb. Contour intervals are 3 mb and 4 K. The two boxes centred over northeastern

Australia show the outer and inner domains used in the simulations with MM5.

at the surface. The front weakened further and stalledduring the day. It subsequently reformed and accelerated,passing through Hughenden (144.2◦E, -20.8◦S) in theeastern most part of the domain at about 2330 EST 10September.

3.1 Synoptic environment for Event 1

The synoptic environment for Event 1 is illustrated in Fig.1, which shows the ECMWF analyses of mean sea levelpressure (MSLP) at 1600 EST 9 September and 1000 EST10 September. Horizontal wind vectors and the virtualpotential temperature at 850 mb are plotted also. In midafternoon (1600 EST) on 9 September, a low pressure sys-tem lying over the ocean just south of the continent is con-nected to a trough extending northward and westward. Thewarmest air lies along the northwestern part of the trough.This thermal ridge and its associated pressure trough willbe called simply the heat trough. An anticyclone is locatedto the west of the low and associated trough, and at lowlevels the wind is directed across the isobars towards thetrough. A ridge extends along the northeastern coastline,directing an onshore flow. The cold front (Event 1) ischaracterised by the largest virtual potential temperaturegradient and is oriented southeast to northwest from thenorthern part of the low and along the northeastern bound-ary of the high. This synoptic pattern, with the cold frontlying in the trough region between two anticyclones andrelatively far from the center of the low, is the predomi-nant synoptic environment for cold frontal passages acrossAustralia (Reeder and Smith 1998).

By mid-morning the following day (1000 EST 10September) the cold front has strengthened, the heat lowhas contracted, and they have both moved northeastward(Fig. 1b). Overnight strengthening, as illustrated in Fig.1, is the norm for fronts as they advance across theAustralian interior (Reeder and Smith 1998). The low isnow located in the southeastern corner of the continent

and the high has expanded across much of the westernand central parts of Australia. The configuration of thehigh and low directs southwesterly winds across much ofcentral Australia. On the northeastern side of the front theflow is northwesterly, while on the southwestern side it issouthwesterly.

Figure 2. Map of northeastern Australia marking the places men-tioned in the text.

3.2 Mesoscale structure and evolution of Event 1

The mesoscale structure and evolution of Event 1 in the24 hr period commencing 1500 EST 9 September isexamined now. The potential temperature, horizontal windvectors and cyclonic relative vorticity from the outer grid




of the MM5 simulation are shown in Fig. 3 at six hourlyintervals.

During the afternoon (1500 EST 9 September, Fig.3a), the dominant feature is the heat trough, which isoriented northwest to southeast and lies in the westernhalf of the domain. The wind change across the heattrough is relatively weak. The cold front, identified bythe line of strongest gradient in potential temperature, islocated in the southwestern corner of the model domainand is separate from the heat trough (see also Fig. 1a).In the northern part of the domain, coastal gradients inthe potential temperature mark the leading edges of thesea breeze fronts. At this time the sea breeze fronts areconfined to the coastal margin, with the exception ofthe east coast of the Cape York Peninsula where thebackground easterlies have advected the sea breeze frontinland.

Six hours later at 2100 EST (Fig. 3b) the front hasadvanced northeastwards into the model domain, althoughthe position of the trough axis, as defined by the maximumrelative cyclonic vorticity, has not moved appreciably.Nevertheless, the winds have strengthened significantlyboth ahead of and behind the trough, and the cold front andheat trough have become indistinguishable. To the southof the trough the wind is strong (typically 20 m s−1 on the0.955 σ surface) and southwesterly, while on the northernside it is weaker and northerly or northwesterly. Thesouthwesterlies behind the front are associated with theridging across the continent (Fig. 1a) and are accompaniedby the northeastward progression of the cooler air. At thistime, the sea breeze front has advanced about 150 kminland from the gulf coast.

By 0300 EST 10 September the cold front is wellmarked by the relatively strong gradient in potential tem-perature and a pronounced line of cyclonic relative vor-ticity, oriented northwest to southeast, in the southwesternpart of the domain (Fig. 3c). One of the most importantfeatures of the simulation is that the winds strengthenand turn anticyclonically throughout the night. The anticy-clonic rotation is especially pronounced in the cold air onthe southwestern side of the trough. The overnight acceler-ation of the low-level winds and subsequent intensificationof the cold front are evidently related to the way in whichthe boundary layer adjusts to large changes in the turbulentstress. Although not shown here, the winds in the bound-ary layer are sub-geostrophic during the day because ofthe stress associated with turbulent mixing. At night, oncethe (buoyantly-generated) turbulent mixing subsides, thebalance of forces in the boundary layer is dramaticallyaltered. In response, the wind rotates anticyclonically andair parcels accelerate down the pressure gradient towardsthe trough, strengthening the low-level flow. Inertial oscil-lations such as these are a common feature of the boundarylayer over the Australian interior (e.g. May 1995; Arnupand Reeder 2007).

The cold front continues to advance northwestward,and by 0900 EST (Fig. 3d) it is close to Mt Isa as observed.However, in the following 6 hours (1500 EST Fig. 3e),

the structure of the front changes dramatically. The low-level winds weaken as the daytime turbulent mixing inthe boundary layer increases. A curved line of cyclonicrelative vorticity still marks the leading edge of the coldfront, but the potential temperature gradients in the coldair are mostly weaker than at 0900 EST and the coldfront is stationary. The isentropes near the leading edgeof the front are more convoluted than they were six hoursearlier as a result of the vigorous daytime mixing and theunderlying topography. Consequently, in some localisedregions the horizontal potential temperature gradient is alittle stronger than before. By this time the heat troughhas become re-established in the westernmost part of thedomain.

3.3 Frontogenesis during Event 1

The material rate of change of the magnitude of the hor-izontal potential temperature gradient, |∇hθ|, was intro-duced by Petterssen (1936) as a measure of the tendencyof the atmosphere to develop fronts (see also the general-ization by Keyser et al. 1988) and is known as the fronto-genesis function. In sigma coordinates on the sphere, thefrontogenesis function can be expressed as

D

Dt|∇hθ| = − 1

2 |∇hθ| (D − E′ cos (2β))

− ∂θ∂σ

n · ∇hσ + n · ∇hQ(1)

where θ is the potential temperature, Q is the dia-batic heating rate, which includes radiative effects, latentheating, turbulent mixing and thermal diffusion, n =∇hθ/|∇hθ| is the unit vector pointing along the potentialtemperature gradient towards warmer air, D = ∂u/∂x+(∂ (vcosϕ) /∂y) /(cosϕ) is the horizontal divergence,E = ∂u/∂x− (cosϕ)∂ (v/cosϕ) /∂y is the stretchingdeformation, F = ∂v/∂x+ (cosϕ)∂ (u/cosϕ) /∂y is theshearing deformation, E′ =

√E2 + F 2 is the total defor-

mation, ϕ is the latitude, x is the arc length along a lineof constant latitude, y is the arc length along a line ofconstant longitude, α = tan−1 ((∂θ/∂x)/(∂θ/∂y)), δ =12 tan−1(E/F ) is the angle between the x axis and the axisof dilatation, and β = δ − α is the angle between the axisof dilatation and the isentropes.

Physically, the first term on the right hand side of Eq.(1) describes the contribution to material rate of changeof the horizontal potential temperature gradient from thecombined effects of convergence and deformation. Thesecond term represents the contribution due to the tilt-ing of vertical gradients of potential temperature into thehorizontal, while the third term is the contribution fromdifferential diabatic heating in the direction of the poten-tial temperature gradient.

Figures 4a and 4b show vertical cross sections ofthe frontogenesis function calculated on the outer domainat 2100 EST 9 September and 1500 EST 10 Septemberrespectively. These times correspond with Figs. 3b and 3eand are chosen to illustrate the differences in the structureand evolution of the cold front at night and during the day.




(a) (b)

(c) (d)

(e)

Figure 3. Event 1. Potential temperature and horizontal wind vectors on the σ = 0.955 surface from the MM5 outer domain at (a) 1500EST 9 September, (b) 2100 EST, (c) 0300 EST 10 September, (d) 0900 EST and (e) 1500 EST. Contour interval for potential temperatureis 2 K. The arrow below each panel indicates 10ms−1. Shading marks those regions where the magnitude of the cyclonic relative vorticityexceeds 7.5 × 10−5 s−1 on the same σ-surface. The thick lines in panels b and e mark the location of the vertical sections plotted in Fig.

4.




The location of the cross sections are marked on Figs. 3band 3e. Figures 4c and 4d show the contributions to thefrontogenesis function from the convergence and defor-mation terms at the two times. The tilting terms contributerelatively little to the frontogenesis function and are notplotted. The contribution by differential diabatic heatingto the frontogenesis function, plotted in Figs. 4e and 4f,shows a great deal more local variability than the defor-mation and convergence terms as it appears to be linkedmore closely to the variations in the topography, boundarylayer eddies and the thermodynamic and radiative effectsof cloud. For this reason, the fields of frontogenesis havebeen smoothed in the horizontal and vertical directionsusing a 9 point filter. Even after smoothing the fronto-genesis fields show considerable spatial variability as thepattern of underlying topography, the daytime turbulentmixing and the attempt of the model to produce boundarylayer rolls are imprinted strongly on the low-level windsand potential temperatures, and also because the calcu-lation of the frontogenesis function involves products ofdifferentiated quantities. The background field in all pan-els of Fig. 4 is the potential temperature.

During the late evening at 2100 EST, the surface windchange is located at about 600 km in the plane of thevertical cross section. The cold air is relatively deep. Forexample, the 306 K isentrope slopes uniformly from theleading edge of the surface front to a height of about 4 kmat the left hand edge of the cross section. The cold frontadvances into a deep well mixed layer overlying a veryshallow radiation inversion. At this time above the lowestfew hundred metres, the frontogenesis function is positivealong the leading edge of the cold front and in the warm airahead (Figs. 4a). The main region in which deformationand convergence contribute to frontogenesis is centred onthe cold front, but there are other localised regions (Figs.4c). Although not shown, the strong horizontal winds areconfined to the lowest 2 km and develop once the daytimemixing ceases. It is for this reason that cold fronts inthe Australian subtropics are shallow: cold fronts developwhere there is active frontogenesis, which, in this case, isin the lowest 2 km of the atmosphere.

On the following day at 1500 EST the cold frontand sea breeze front are located at about 400 km and 800km respectively in the plane of the vertical cross section.During the day, buoyantly-generated turbulent mixingproduces a broad heat trough between the cold front andthe sea breeze front that is relatively homogeneous inthe horizontal. The mixed layer ahead of the cold frontis about 3 km deep, while in the cold air to the southof the front the mixed layer is 2 km deep. At this time,the frontogenesis function is negative at the leading edgeof the front, although it becomes positive in the cold air(Figs. 4b). This pattern of frontolysis and frontogenesisaccounts for the weakening and deceleration of the coldfront during the day. A similar pattern occurs at the seabreeze front and is the reason why the front remainsfixed to the coast during the day. The main contributionsfrom deformation and convergence to the near-surfacefrontogenesis are found to the rear of the leading edge of

the cold front. While an increase in the turbulent stress inthe boundary layer increases the cross-isobaric flow, it alsodecelerates the flow, the net result being a weaker field ofdeformation and convergence (Figs. 4d). Deformation andconvergence are produced by the developing sea breezecirculation and are important also in strengthening the seabreeze front.

The differential diabatic heating term at the two timesare shown in Figs. 4e and f. At night, the contribution tothe frontogenesis function from differential diabatic heat-ing is mostly negative. The main exception to this patternis below the shallow cloud that forms in the prefrontal air.Here the longwave radiative emission by the cloud warmsthe air, while longwave radiative emission from the sur-face produces a shallow region of frontolysis in the lowestlayers of the model. During the day solar radiation heatsthe surface, which then warms the air close to the surface.Subsequently, this heated air is distributed through theboundary layer by turbulent mixing. To the extent that theradiative heating is homogeneous, the mixed layer warmsat a rate which is inversely proportional to its depth, andhence the pattern of daytime heating largely reflects theheight of the mixed layer. Consequently, the cold side ofthe front warms faster than the warm side as the mixedlayer is shallower in the cold air. This pattern of heatingand cooling is frontolytic also. These results are consistentwith those of Reeder and Tory (2005) who found that, ina version of the Eady problem which included a boundarylayer, differential heating produced by horizontal varia-tions in the mixed-layer depth was frontolytic during theday.

3.4 Comparison between the observations and simula-tion of Event 1 on the inner grid

The model calculations on the inner grid are comparednow with the observed structure and evolution of Event1 as it traversed northeastern Australia. Figure 5a showsa time-height cross section of potential temperature con-structed from radiosonde ascents made hourly at MountIsa around the time of passage of Event 1. Time in thisfigure runs from right to left so that, to the extent that aGalilean transformation from time to space is appropriate,the section approximates a vertical cross section throughthe front. Figure 5b shows the corresponding time-pseudoheight cross section of potential temperature at approx-imately the location of Mount Isa constructed from themodel simulation on the inner grid. (Pseudo-height isa pressure dependent height-like coordinate defined byHoskins and Bretherton 1972).

The model captures the key features in the observa-tions. In the early hours of the morning on 10 September,a shallow radiation inversion forms beneath a weakly sta-ble remnant of the mixed layer from the previous day.Above the remnant mixed layer lies the more stable freeatmosphere. Throughout the early hours of the morningthe stable layer deepens, although the stable layer in themodel is a little weaker than that observed. For exam-ple, at 0600 EST the observed stable layer is about 50 m




(a) (b)

(c) (d)

(e) (f)

Figure 4. Event 1. Vertical cross section of the potential temperature and contributions to the frontogenesis function from the MM5outer domain. Left hand side panels are at 2100 EST 9 September along the line marked in Fig. 3b. Right hand side panels are at 1500EST 10 September along the line marked in Fig. 3e. (a)-(b) Frontogenesis function D|∇hθ|/Dt. (c)-(d) Convergence and deformationterm − 1

2|∇hθ| (D − E′ cos (2β)). (e)-(f) Differential diabatic heating term n · ∇hQ. Contour interval for potential temperature is 2 K.

Contour interval for frontogenesis is 5 × 10−10 K m−1 s−1. In panels (e)-(f) the bold contours mark cloud.

deeper and the change in the potential temperature fromthe surface to the top of the stable layer is 2 K greater.Shortly after 0800 EST the mixed layer begins growingin both the observations and model, reaching a height inthe mid-afternoon of nearly 3 km in the model comparedwith about 2 km in the radiosonde cross section. While themixed layer in the model is much deeper than observed,the radiosondes show that the lapse rate is relatively smallabove the mixed layer, but not zero. In both the modeland observations, the atmosphere is strongly stable aboveabout 3.5 km and the mid-afternoon mixed-layer temper-ature is 304 K. Although the front arrives at 0930 EST,its passage is obscured by the daytime heating and asso-ciated mixing which opposes the local cooling from the

advection of cold air behind the front. The airmass changebecomes apparent only after the diabatic heating and mix-ing begins to wane in the late afternoon. The inversioncapping the cold air is more pronounced in the observa-tions than in the model, which is a common limitationof numerical models which seldom have adequate verticalresolution to capture sharp inversions.

3.5 Development of the bore

The potential temperature, horizontal wind vectors andhorizontal convergence from the inner grid of the MM5simulation at 0600 EST 10 September are shown in Fig.6. At this time, two curved convergence lines spaced




Hei

ghta

bove

grou

nd[m

]

Time (EST)

Pseu

do-h

eigh

tabo

vegr

ound

[m]

Time (EST)

Figure 5. Event 1. (a) Time-height cross section of potential tem-perature from radiosonde ascents at Mt Isa. Time increases fromright to left along the abscissa. The height in km is plotted alongthe ordinate. Contour interval is 2 K. Reproduced from Smith et al.(1995). The time of the frontal passage is indicated by a verticalline. (b) Time-height cross section of potential temperature from theMM5 inner domain. Contour interval is 2 K. Pseudo-height in km is

plotted along the ordinate.

approximately 60 km apart lie across the inner modeldomain, extending from about 20◦S in the western partto the southeastern corner. The wind ahead of the leadingline is northwesterly, but backs and becomes westerlyon the southern side. The wind backs again across thesecond line, becoming southerly in the colder air. Themagnitude of the convergence varies considerably alongthe cold front, reflecting the variability in the boundarylayer and underlying topography. Away from the front,short convergence lines aligned roughly parallel to thelow-level winds appear to be attempts by the model toproduce boundary layer rolls.

Figure 7 shows vertical cross sections of potentialtemperature, vertical motion and relative horizontal windnormal to the front on the inner grid at the same timeas Fig. 6. The speed c is determined graphically bycomparing position of the leading edge of the front atconsecutive times, and the relative horizontal wind normalto the front is defined as un − c, where un is the horizontalwind in the plane of the cross section. The position of thevertical cross section is marked in Fig. 6. The structure of

Figure 6. Event 1. Potential temperature and horizontal windvectors on the σ = 0.955 surface from the MM5 inner domain at0600 EST 10 September. Contour interval for potential temperatureis 2 K. The arrow below each panel indicates 10 m s−1. Shadingmarks those regions where the horizontal convergence larger inmagnitude than 1.5 ×10−4 s−1 on the σ = 0.9975 surface. Thethick lines mark the location of the vertical section plotted in Fig. 7.

the two convergence lines identified in Fig. 6 is bought outmore clearly in these cross sections.

The first change is located at 120 km, at whichpoint the isentropes are displaced sharply upward. Forexample, the height of the 301 K contour changes abruptlyby about 600 m. The change propagates to the right(downstream) into a low-level stable layer. To the left ofthe change (upstream), the change the low-level isentropesbulge upward before almost returning to their downstreamheight. This kind of structure is characteristic of a solitarywave. At the leading edge of the jump there is strongupward motion and a surge in the horizontal wind in thelowest 1 km. The second change is located at 80 km. Onceagain the isentropes are displaced upward. This time the301 K contour changes by about 400 m and does notreturn to its upstream elevation. This sharp, permanent,isentropic displacement, is suggestive of a bore. On theupstream side of the jump, especially at heights aboveabout 2 km, the isentropes have a wave-like structure,which is the defining characteristic of an undular bore.Atmospheric bores are invariably undular (Christie 1992).There is a strong updraught at the leading edge of thebore. The horizontal temperature gradient associated withboth changes is very weak at the surface, which agreeswell with Smith et al. (1995) who reported that, whileEvent 1 was characterized by a strong wind change, thetemperature decreased little at the surface with the passageof the change. The simulation produces also a very stronglow-level surge in the wind behind the second change,which gives rise to mechanically-generated turbulenceand vigorous mixing in the cold air. The dynamics of thebore is discussed further in Section 5. In the NOAA-12Advanced Very High Resolution Radiometer (AVHRR)visible satellite image at 0818 EST (Fig. 8), the leading




line appears prominently as a rope cloud. The satelliteimage shows also cloud lines in the cold air orientedperpendicular to the rope cloud with some similarity tothe boundary layer rolls developed by the model.

(a)

(b)

Figure 7. Vertical cross sections from the MM5 inner domain. Event1 at 0600 EST 10 September. (a) Potential temperature and verticalmotion. Contour interval for potential temperature is 1 K. Darkshaded regions represent vertical wind speeds greater than 10 cms−1. Light shaded regions represent vertical wind speeds less than-10 cm s−1. (b) Relative horizontal wind speed in the plane of thecross section un − c, where c = 11 m s−1. The bold line marks the

0 m s−1 contour.

A time-series of the surface observations taken atUrandangi between 0500 EST and 1100 EST on 10September is plotted in Fig. 9a. The singular feature ofthe time-series, and a signature common to cold frontsover subtropical Australia, is the double change. Thefirst change at 0700 EST is a propagating bore, theleading edge of which is marked by a pressure jumpof 1.8 mb and a sharp change in wind direction fromnorthwesterly to westerly. This change is followed by aseries of pressure oscillations coinciding with fluctuationsin the wind speed and direction. The wind and pressuresignatures are typical of a series of amplitude-orderedsolitary waves (Smith 1988, Christie 1992). The secondchange at 0815 EST marks the cold front. At this boundarythe pressure rises slightly, the wind backs from westerlyto southerly, and the wind speed increases notably. Boththe dew-point temperature and mixing ratio (not shown)fall sharply with the second change. The dashed lines inFig. 9a are the corresponding curves taken from the model

simulation. While the model produces two changes also(as shown in Figs. 6 and 7), the double structure is lessapparent at the surface than in the observations. Thesetime-series show a double change also with a pressurejump and wind change (speed increase and directionchange) at 0600 EST followed by a further fresheningof the wind, a wind direction change and a pressure risecommencing at about 0715 and finishing at about 0830EST. The second change brings about a steady fall indewpoint temperature, but little change in temperature.

Figure 8. Event 1. NOAA-12 Advanced Very High ResolutionRadiometer (AVHRR) visible satellite image at 0818 EST 10

September 1991. Reproduced from Smith et al. (1995).

While not central to the main themes of the paper, it isworth noting the series of convergence lines in the north-eastern part of the inner model domain in Fig. 6. Thesemark another bore, which has propagated southwestwardinto the domain from the middle part of the Cape YorkPeninsula where it was generated (see Figs. 3b-d). Dur-ing the late dry season, southwestward propagating boresare commonly generated over the Cape York Peninsula inthe late evening or early hours of the morning as the east-coast and west-coast sea breezes meet over the peninsula(Clarke 1984, Noonan and Smith 1986, Smith and Noo-nan 1998, Menhofer et al. 1997a, b, Goler and Reeder2004, Thomsen and Smith 2006; see also the reviews bySmith 1988, Christie 1992, and Reeder and Smith 1998).Striking roll clouds, known in Australia as northeasterlymorning glories, are observed regularly as moist low-levelair is lifted and cooled adiabatically in the crest of the indi-vidual bore waves and warms as it descends in the wavetroughs that follow.

4 Event 2

The second cold front, Event 2, passed through MountIsa just after midnight, at about 0020 EST 17 Septem-ber. There are three important differences between theEvents 1 and 2. First, Event 2 passed rapidly across theobservational network towards the gulf in the late evening




Figure 9. Event 1. Observed time series of temperature T and dew-point temperature Td, wind speed V, wind direction at a height of 2m Dirn, and surface pressure ps at Urandangi between 0000 and1000 EST on 10 September. Reproduced from Smith et al. (1995).The dashed curves show the corresponding curves from the MM5

inner domain, except that the wind data are extracted at 10 m.

and early hours of the morning. In contrast, Event 1reached Mount Isa mid-morning, weakened and decel-erated, finally reaching the gulf in the early hours ofthe following morning. Second, sharp temperature fallsaccompanied the passage of Event 2, whereas the temper-ature changed little with the arrival of Event 1. Third, anamplitude-ordered family of solitary waves was generatedand propagated ahead of Event 1. In contrast, the undularbore that developed with Event 2 did not separate from thecold front until after sunrise, when front began to weaken.The brief analysis presented now focusses mainly on thedifferences between the two events.

4.1 Synoptic environment for Event 2

The ECMWF analyses at 2200 EST 16 September isshown in Fig. 10. The synoptic environment is very sim-ilar to Event 1 (Fig. 1). Indeed, the synoptic environmentis similar to all the subtropical cold fronts studied in thefour field experiments held in the region. A high to thewest and a weak low centred on the southeastern coastlinedirects a cold southerly airflow across the central part ofthe continent. The cold front (Event 2) is oriented south-east to northwest from the northern part of the low andalong the northern boundary of the high. A heat trough islocated to the north and parallel to the cold front.

4.2 Mesoscale structure and frontogenesis during Event2

After sunset, as the turbulent mixing weakens, the low-level winds accelerate toward the heat trough. Figures

Figure 10. Event 2. ECMWF analyses at 2200 EST 16 September.MSLP (dashed lines), horizontal wind vectors and virtual potentialtemperature (shaded) at 850 mb. Contour intervals are 3 mb and 4K. The two boxes centred over northeastern Australia show the outer

and inner domains used in the simulations with MM5.

11 shows the potential temperature, horizontal divergenceand horizontal wind fields at 2100 EST 16 September onthe σ = 0.995 surface and on the outer grid. At this time,two principal lines of elevated relative vorticity lay north-west to southeast across the middle of the domain. Themore northeasterly of the two lines marks the northeast-ern side of heat trough. The more southwesterly of thetwo lines marks the cold front, and the two lines bounda tongue of warm air. Although not shown, both linesare accompanied by pronounced changes in the equivalentpotential temperature. Strong southwesterlies and cold airlie to the southwest of the cold front. Subsequently, thecold front strengthens and accelerates, overtaking the firstline and evolving into a propagating bore (Section 4.3). Inthe northern part of the domain at 2100 EST another lineof elevated relative vorticity, aligned perpendicular to thelow-level flow, corresponds to the sea breeze front, anda northeasterly morning glory is located over Cape YorkPeninsula.

Cross sections of the potential temperature and theterms that contribute to the frontogenesis function (Eq.1) at 2100 EST 16 September are shown in Fig. 12. Thetitling term is not plotted as its contribution is insignif-icant. In the plane of the vertical cross section, the coldfront, as defined by the second line of vorticity, is locatedat about 400 km. The first, more northeasterly, line of vor-ticity is located at about 580 km along the cross sectionat the southwestern edge of a low-level cool anomaly. Thesea breeze front is at about 750 km. The contribution tothe frontogenesis function from deformation and conver-gence is large and positive through a depth of 1-2 km inthe neighbourhood of the front (Fig. 12b). It is strong alsoin shallower layers centred on the heat trough, the firstline of vorticity, and the sea breeze front. The diabaticcontribution to the frontogenesis function is more com-plicated than that from deformation and convergence and




Figure 11. Event 2. Potential temperature and horizontal windvectors on the σ = 0.955 surface from the MM5 outer domain at2100 EST, 17 September. Contour interval for potential temperatureis 2 K. The arrow below each panel indicates 10 m s−1. Shadingmarks those regions where the magnitude of the cyclonic relativevorticity exceeds 7.5 × 10−5 s−1 on the same σ-surface. The thick

line marks the location of the vertical sections plotted in Fig. 12.

is much more variable in time and space (Fig. 12c). It con-tributes to frontolysis at the cold front through mixing,but is frontogenetic far ahead of the cold front where themodel produces shallow cloud. The net result is that thefrontogenesis function is large and postitive in the lowest1.5 km in the region of the cold front and in a deeper layerabove first line of vorticity (Fig. 12a). Between the twolines, the frontogensis is function is negative. The fron-togenesis function is large and positive also at the at theleading edge of the sea breeze.

4.3 Comparison between the observations and simula-tion of Event 2 on the inner grid

Figure 13 shows time-height cross sections of potentialtemperature from the radiosonde ascents made at MountIsa and from the corresponding model profiles. While awell-mixed layer forms during the day in both the modeland observations, there is a large discrepancy in theirdepths. The depth of the mixed layer in the model isnot much more than 2 km deep whereas the observationsshow that it is closer to 3.5 km. As in Event 1 themodel fails to capture the very strong inversions observedat the top of the mixed layer and capping the cold,postfrontal air. The sensitivity of the depth of the mixedlayer to the soil moisture was checked by rerunning MM5with the soil moisture data from the Australian Bureauof Meteorology’s limited area forecast model, which isdrier than that from the ECMWF analyses. In this case,the mixed layer depth increased by only a few hundredmetres. An intense, but shallow radiation inversion isobserved to form beneath a deep well mixed-layer priorto the arrival of the front. The model simulation producesthe same features, although they are weaker and more

(a)

(b)

(c)

Figure 12. Event 2. Vertical cross section of the potential tempera-ture and contributions to the frontogenesis function from the MM5outer domain at 2100 EST 16 September along the line marked inFig. 11. (a) Frontogenesis function D|∇hθ|/Dt. (b) Convergenceand deformation term − 1

2|∇hθ| (D − E′ cos (2β)). (c) Differen-

tial diabatic heating term n · ∇hQ. Contour interval for potentialtemperature is 2 K. Contour interval for frontogenesis is 5× 10−10

K m−1 s−1. In panel (c) the bold contour marks cloud.

diffuse. The modelled front arrives at Mt Isa about 2 hoursearlier than observed. Both the observed and modelledfronts are shallow, no more than 1.5 km deep. The coldestair arrives at Mount Isa at about 0900 EST, after whichtime the potential temperature increases during the dayas the warming associated with diabatic heating exceedsthe cooling produced by the post-frontal advection of coldair. The model captures this effect also, but produces astronger gradient at the leading edge of the cold air at 0600EST. The pressure jump in the model is approximately 1mb, which agrees reasonably well with the observations.




Hei

ghta

bove

grou

nd[m

]

Time (EST)

Pseu

do-h

eigh

tabo

vegr

ound

[m]

Time (EST)

Figure 13. Event 2. (a) Time-height cross section of potentialtemperature from radiosonde ascents at Mount Isa. Time increasesfrom right to left along the abscissa. The height in km is plottedalong the ordinate. Contour interval is 2 K. Reproduced from Smithet al. (1995). The time of the frontal passage is indicated by avertical line. (b) Time-height cross section of potential temperaturefrom the MM5 inner domain. Contour interval is 2 K. Pseudo-height

in km is plotted along the ordinate.

4.4 Development of the bore

Figures 14 and 15 show the modelled fields at 0600EST. Plotted are the potential temperature, divergence andhorizontal wind on the σ = 0.955 surface, and verticalcross sections of potential temperature and relative windnormal to the front. At this time, the cold front advancesat 11 m s−1 and is oriented almost east-west. It is markedby a band of convergence, a wind shift from northeastto south, and the arrival of the cold air (Fig. 14). In thevertical cross sections (Fig. 15), the front is located at130 km and propagates to the right into a low-level stablelayer produced by radiative cooling and the decaying seabreeze circulation from the Gulf of Carpentaria. The frontis accompanied by a shallow cold pool immediately tothe rear of the front and strong upward motion, and hasfeatures in common with a propagating undular bore. Forexample, the isentropes comprising the low-level stablelayer are displaced sharply upward through a depth ofabout 1 km with the arrival of the front, while behind thefront the isentropes undulate. The bore is not confined tothe surface, but extends to a height of more than 3 km,

and the low-level relative inflow is every negative at itsleading edge, which is also one of the properties of abore. The bore formation is consistent with observationsof a southerly morning glory that reached Burketown(139.5◦E, 17.7◦S), located on the southeastern coastlineof the Gulf of Carpentaria, at 0638 EST 17 September.This disturbance propagated at a speed of 15.3 m s−1

from a direction of 205◦ and was marked by a pressurejump of 1.4 mb. The NOAA-12 AVHRR visible satelliteimage at 0720 EST (Fig. 16) shows a pair of wave cloudspropagating ahead of a field of shallow cumuli, whichpresumably marks the cold air.

Figure 14. Event 2. Potential temperature and horizontal windvectors on the σ = 0.955 surface from the MM5 inner domain at0600 EST 17 September. Contour interval for potential temperatureis 2 K. The arrow below each panel indicates 10 ms−1. Shadingmarks those regions where the convergence exceeds 1.5 × 10−4

s−1 on the same σ-surface. The thick line marks the location of thevertical section plotted in Fig. 15

5 A comparison of the bores in Event 1 and Event 2

It is common for strong fronts to be interpreted locally assteady gravity currents (see Smith and Reeder 1988 andreferences therein). Haase and Smith (1989) reported aseries of numerical experiments in which a gravity currentadvances into an environment that is neutrally stratifiedabove a shallow, stable layer. In this configuration thekey nondimensional parameter controlling the behaviourof the flow is µ = c0/cgr. Here c0 = 2Nh/π is the phasespeed at which the small amplitude waves propagate onthe stable layer, cgr is the speed of the gravity current, Nis the Brunt-Vaisala frequency in the stable layer aheadof the gravity current, and h is the depth of the stablelayer. In the experiments of Haase and Smith, the gravitycurrent head breaks away from the flow that advects coldfluid towards it and advances as a horizontal roll vortex.When the stable layer is sufficiently deep and strongto enable waves to propagate faster than the speed ofthe gravity current, the roll vortex evolves into a large-amplitude solitary-wave-like disturbance that propagates




(a)

(b)

Figure 15. Vertical cross sections from the MM5 inner domain.Event 2 at 0600 EST 17 September. (a) Potential temperature andvertical motion. Contour interval for potential temperature is 1 K.Dark shaded regions represent vertical wind speeds greater than 10cm s−1. Light shaded regions represent vertical wind speeds lessthan -10 cm s−1. (b) Relative horizontal wind speed in the plane ofthe cross section un − c, where c = 11 m s−1. The bold line marks

the 0 m s−1 contour.

Figure 16. Event 2. NOAA-12 Advanced Very High ResolutionRadiometer (AVHRR) visible satellite image at 0730 EST 17

September 1991. Reproduced from Smith et al. (1995).

ahead of the gravity current on the stable layer. In contrast,when the stable layer is comparatively shallow or weak,

the gravity current head becomes wavelike, but the wavesremain attached to the gravity current and advect a smallvolume of cold air. The first case is said to be subcriticaland the second case supercritical. Haase and Smith foundthat the transition from subcritical to subcritical flowoccured at about µ = 0.7. The transition point is less thanone because, according to weakly nonlinear theory, thephase speed of the waves which propagate on the stablelayer increases with increasing amplitude.

The two paradigms reported by Haase and Smith(1989) appear to be relevant to Events 1 and 2. How-ever, Haase and Smith used an empirical formula to deter-mine cgr, which is probably not appropriate here. For thisreason and for simplicity, we define µ = c0/Uf , where Uf

is the typical value of the horizontal wind in the cold airimmediately behind the cold front. For Event 1 at 0600EST (Fig. 7), µ = 0.3± 0.1. Like the subcritical case ofHaase and Smith, the solitary-wave-like disturbance sep-arates from the cold front and propagates ahead of it onthe stable layer in both the model and the observations(Figs. 7 and 9). In contrast, at 0600 EST for Event 2,µ = 0.9± 0.3 (Fig. 15). This event produced a particularlypronounced bore, which, in agreement with the observa-tions and the calculation of the µ, remained attached tothe cold front, advecting cold air and developing a seriesof large-amplitude waves at its leading edge. Event 2 issimilar to the Haase and Smith’s supercritical case. Theuncertainty attached to the calculations of µ arises becauseof the difficulties in determining U , N and h in a timevarying, spatially complex flow. The likely errors in muhave been estimated graphically.

In a steady, idealised gravity current, dense fluidis advected towards the head. Consequently, the low-level flow behind the head, normal to the direction ofpropagation, un, exceeds the translation speed of thehead, c, so that un − c > 0. In contrast, the low-level flowbehind an idealised, steadily propagating bore is less thanthe translation speed, so that un − c < 0. At 0200 EST10 September, un − cc > 0 to the south of the front (Fig.17a), where cc is the speed of the convergence line at theleading edge of the front. Positive relative flow impliesthat cooling accompanying the passage of the disturbanceis produced principally by the advection of cold air fromthe southwest. From this perspective, the front has at leastone of the characteristics of a steady gravity current. Atthis time Event 1 comprises a single change with thelines of maximum convergence, cyclonic relative vorticityand potential temperature gradient almost coincident. Sixhours later Event 1 has separated into two changes (Figs.6 and 7). As discussed earlier, the first line has similaritieswith a solitary-wave-like disturbance, while the secondline, which coincides with the onset of strong southerlies,has similarities with a propagating bore. The distributionof un − cc is dramatically different at this time (Fig. 17b).The relative flow is almost everywhere negative, whichmeans that the leading disturbance moves faster than thewind in the direction of translation. This is one of thecharacteristics of a wave, and it implies that any cooling




(a) (b)

(c) (d)

Figure 17. un − cc on the σ = 0.955 surface from the MM5 inner domain, where cc is the speed of the leading convergence line. Event1 at (a) 0200 EST and (b) 0900 EST 10 September, where cc = 4.4ms−1 and cc = 11.0ms−1 respectively. Event 2 at (c) 0300 EST and(d) 0900 EST 17 September, where cc = 8.8ms−1 and cc = 13.2ms−1 respectively. Shading indicates those regions where the relative

flow is positive.

must be produced principally as air parcels are lifted andadiabatically cooled at the leading edge of the disturbance.

Likewise, for Event 2, the model shows that between0300 and 0900 EST 17 September the relative flowchanges radically (Fig. 17c and d). Behind the leadingedge of the cold front at 0300 EST, the flow in the direc-tion of the disturbance is almost everywhere faster thanthe disturbance itself (un − c > 0), indicating that coldair advection is responsible for the cooling associatedwith the passage of the front. In contrast, there are onlysmall regions of positive relative flow at 0900 EST, whichis consistent with the interpretation that the leading dis-turbance has transformed into a bore (un − c < 0) as itaccelerates ahead of the original cold front. Although notshown, the original cold front weakens rapidly after sun-rise and is hardly detectable by 0900 EST.

Caution is needed when interpreting plots of un − cas the condition un − c < 0 does not by itself demonstratethat the leading change line is a solitary wave or propa-gating bore. This is because under certain circumstances,cold fronts may propagate, in which case un − c < 0 also(Reeder and Smith, 1986, 1987, 1988, Smith and Reeder

1988). However, both Events 1 and 2 developed many ofthe structural characteristics of solitary waves or propagat-ing bores, including a sharp vertical isentropic displace-ment followed by a series of undulations in the wind andpotential temperature. Thus, it appears that, in the casesexamined, the change in sign of un − c signals a funda-mental change in the dynamics of the cold front.

6 Conclusions

This study has examined the effect of strong diurnal heat-ing and cooling on the structure and evolution of coldfronts in the Australian subtropics and has sought toexplain the observations made during a series of fieldexperiments. The paper has focussed on numerical sim-ulations of the two main cold fronts documented duringthe 1991 Central Australian Fronts Experiment, whichhave provided a more complete picture of the structureand evolution of the fronts and bore-like disturbances atscales smaller than those observable by the instrumentarray available at the time. Notwithstanding the differ-ences in the timing of the cold front and in the depth of




the mixed layer compared with observations, the modelcaptured the main observed features of the cold fronts andbores, demonstrating the capability of MM5 initialisedwith the routine ECMWF analyses to forecast the diur-nal evolution of subtropical cold fronts. The contributionsto explaining the observations are listed below.

Cold fronts in the Australian subtropics are observedto decelerate and rapidly weaken during the day, oftenbecoming stationary and difficult to detect by the routineobservational network. Our modelling study has shownthat:

• While the daytime cross-isobaric flow towards thetrough produces deformation and convergence inthe boundary layer, the enhanced daytime turbulentstress acts to decelerate the low-level winds, thenet result being a reduction of the deformation andconvergence.• Although the low-level wind field still acts to

strengthen cold fronts during the day, they ulti-mately weaken and stall because the frontoge-netic effect of deformation and convergence is out-weighed by the frontolytic effect of daytime radia-tive heating and the associated turbulent mixing.• Daytime turbulent mixing is frontolytic because the

air in contact with the heated surface is rapidlymixed through the boundary layer, warming it ata rate which is inversely proportional to the depthof the boundary layer. Consequently, the cold sideof the front warms faster than the warm side as themixed layer is shallower in the cold air.

The cold fronts are observed to rapidly redevelopand accelerate at night once the boundary layer turbulencesubsides. Our modelling study has shown that after sunsetthe low-level winds accelerate towards the centre of thetrough. As the low-level winds strengthen, so do thecontributions made to frontogenesis by deformation andconvergence.

Observations have shown that subtropical cold frontsare shallow. Our modelling study indicates that this isbecause the mechanisms that lead to frontogenesis areconfined to the boundary layer.

Observations have shown also that subtropical frontsinvariably produce bores overnight. Our modelling studyhas shown that:

• Large regions of positive flow relative to the frontdevelop behind the front following periods of fron-tolysis.• After a period of strong frontogenesis during the

night, large and abrupt vertical displacements ofthe isentropes form at the leading edge of the fronttogether with regions of negative flow relative to themotion of these jumps. These results support theinterpretation that the isentropic displacements areassociated with propagating bores.• The model reproduces the bore-like structure

observed to develop during the early hours of themorning as the front strengthens and encounters the

developing radiation inversion and cool air advectedinland by the sea breeze from the Gulf of Carpen-taria. In agreement with the observations, the borepropagates ahead of the airmass change and devel-ops a series of large-amplitude waves at its leadingedge.

While the sequence of events described here is com-mon over the Australian interior, it is not known howfrequent it is elsewhere. Understanding the behaviour ofcold fronts as they advance into subtropics in other partsof the world, such as over north Africa, remains a topic forfurther research.

Acknowledgement

We are grateful to the ECMWF for providing analysisdata. The first author gratefully acknowledges receipt ofa research scholarship from the University of Munich,which supported this study, as well as a Rupert Ford TravelAward from the Royal Meteorological Society, whichenabled him to visit research facilities in Australia, wherepart of this work was conducted.

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the diurnal evolution of cold fronts in the …roger/...morning glories generated by cold fronts...

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