Aust. Met. Mas. 4l (lgg2) 7-lg

Sea-breeze observations and modelling:a revrew

Deborah J. Abbs and William L. PhysickCSIRO Division of Atmospheric Research, Australia(Manuscript received April 1992; revised JuIy 1992)

We present an overview ofsea-breeze characteristics and behaviour and then discusssome more important aspects in greater detail. These include the fine-scale structureof the sea-breeze in the head region, its behaviour in complex terraino and its abilityto penetrate hundreds of kilometres inland. These properties are then considered ina discussion of the role of sea-breezes in the dispersion of pollutants. Finally weexamine the historical milestones of sea-breeze modelling, as it progressed from theearly linear analytic models to the complex non-hydrostatic numerical models of thenineties.

IntroductionThe sea-breeze has been studied extensively in thepast, especially so since the end of World War II.Neumann (1973) refers to the Greek philosophersAristotle, Plutarch and Theophrastus, and thelrideas on the sea and land-breeze. They consideredthe sea-breeze to be a reflection from obstaclessuch as islands and coastal hills; onshore windsfrom the open ocean were not regarded as sea-breezes by them as there are no objects in the opensea from which the land-breeze could rebound.Jehn (1973) refers to Dampier (1697, 1701 and1705) who wrote at length on winds, includingland and sea-breezes. In his writings he gave a cor-rect description of the sea-breeze which was ig-nored by the theoreticians for nearly 200 years.

Before the more detailed aspects of the sea-breeze can be investigated, it is necessary todescribe the basic processes involved in its devel-opment. These processes are discussed in most ofthe elementary meteorology texts, the followingbeing based on the account presented by Clarke( l 95s).

The sea and land-breeze circulations encoun-tered near shorelines are due to the contrastingthermal responses of the land and water surfacesbecause of their different properties and energy

Corresponding author address: D. Abbs,Atmospheric Research, Private Bag No.3195. Austra l ia.

CSIRO Division ofl , Mordia l loc, Vic

balances. Because the sea is a fluid in continualmotion, any heating or cooling is distributed to aconsiderable depth and so the rise or fall ofwatersurface temperature is only slight. This results, forexample, in a very small diurnal variation in sur-face temperature. Conversely, the land has a verysmall thermal conductivity and so heats and coolsrapidly, resulting in a marked diurnal variation.The resulting land-water temperature diferencesand their diurnal reversal produce correspondingland-water pressure differences in the atmospherewhich result in a system of breezes across thecoast. Onshore flow during the day is known as asea-breeze while offshore flow at night is referredto as a land-breeze.

During the day the heating of the nearJand sur-face is distributed upwards by mixing. The rise inair temperature results in expansion, so that aloft,individual particles over the land are lifted rela-tive to those over the sea. Level for level, thepressure aloft, but not at the surface, becomeshigher over the land than over the sea, and thisresults in a compensating outflow of air fromnear-coastal areas over the land to those over thesea. As a result the surface pressure falls over acoastal strip ofland and rises over a coastal stripof sea. The pressure gradient thus set up results ina lowJevel onshore flow with a return flow aloft.These factors operate simultaneously, not con-secutively.

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Australian Meteorological Magazine 4l December 1992

The sea-breeze circulation cell begins near thecoast in the morning and expands both landwardand seaward. If there is no gradient wind the ex-pansion is greater over the water due to a smallerfrictional force at the sea surface than at the landsurface. The effect ofnon-zero synoptic flow is toeither accelerate or delay the development of thesystem. In some cases strong oflshore gradientwinds result in the sea-breeze developing over thesea with little or no inland progression during theday. In cases where the gradient wind is very lightor has a moderate onshore component, the sea-breeze front tends to form l0 to 30 km inland inthe afternoon.

The depth ofthe sea air, usually defined by thelevel of the zero onshore wind component, is oftenless than half the vertical extent of the sea-breezesystem. At any given location the depth of the seaair varies with the time of day due to the migrationof the system inland and to a thickening of thelayer ofsea air as the day progresses. On averagethe vertical extent of the sea-breeze is greater intropical areas than in temperate zones. In temper-ate regions the sea-breeze inflow is usually be-tween 200 and 500 m deep. These depths increasefrom 1000 m in moderately warm climates to1400 m in tropical coastal regions. Maximumheights of 2 km have been observed in India(Defant 1951; Atkinson l98l).

The return flow is often difficult to observe as itis often 'masked' by the synoptic conditions.Lyons (1972) found that the return flow layer ofLake Michigan breezes was typically twice as deepand with peak velocities half those of the inflowlayer. Atkinson (198 l) summarises observationswhich show the depth of the sea-breeze returnflow ranging from less than 0.5 to nearly 3 km.

The sea-breeze is usually accompanied by a de-crease in the temperature and a correspondingincrease in the humidity of the atmosphere.Atkinson (1981) presents data for the sea-breezein India, showing a mean monthly temperaturefall due to the sea-breeze varying from 1.l'C to4.4'C. These same data show that the range ofrelative humidity rises is from five to 30 per centwith an average value of about ten per cent. Theobservations of Simpson et aL (1977) show thatthe main humidity rise takes place within 200-250 m ofthe leading edge ofthe front. They alsofound a minimum value of humidity directlybehind the front, indicating dry air which has de-scended behind the front and which will sub-sequently be mixed with the moist sea air. Thisminimum most often occurs about 800 m fromthe front. The transport of moister air over theland is regarded as a typical characteristic ofthesea-breeze, although this is by no means defini-tive. Observations presented by Physick (1982)and Abbs (1986) document cases in which the re-verse was observed to occur.

Flohn (1969) gives examples of the averagehorizontal extent ofthe sea-breeze for various cli-

matic regimes. In mid-latitudes the horizontalextent of the sea-breeze rarely exceeds 40 to50 km, while in tropical and subtropical areas itreaches 100 to 150 km on both the landward andseaward sides ofthe coast, resulting in a total ex-tent of 200 to 300 km. Some of the most extensivestudies of deeply penetrating sea-breezes havebeen carried out in Australia by Clarke (1955) andReid (1957). Clarke has recorded cases ofthe sea-breeze reaching Kalgoodie, 300 km from thecoast. Clarke (1983a) also reports results whichindicate that the sea-breeze penetrates 500 km in-land in northern Australia and has also beenrecorded 320 km seaward ofBorneo.

In the following sections, we examine some as-pects ofthe sea-breeze in greater detail, includingits deep inland penetration and its role in deter-mining air quality in coastal regions. We concludewith a summary of the more important milestonesas sea-breeze modelling progressed from theearly linear analytic models to the complex non-hydrostatic numerical models of the nineties.

Fine-scale structure of sea-breezesThe similarities between density currents pro-duced in the laboratory, and atmospheric ex-amples such as sea-breezes and thunderstormoutflows, were first shown at Reading Universityby retired school master John Simpson. Denser(sea) air flows towards the sea-breeze front at lowlevels and is swept up and backwards at the lead-ing edge in the region referred to as the head(Simpson 1969, 1912). Mixing between coolmoist sea air and warm dry land air takes place atthe rear ofthe head through breaking billows, gen-erated at the front of the head by the Kelvin-Helmholtz shear instability mechanism. Lidarobservations of such billows, with a wavelength ofabout 2 km, can be found in Nakane and Sasano(1986). The mixed air flows back towards thecoastline above the inflow and constitutes thereturn flow ofthe sea-breeze.

Later experiments by Simpson and Britter(1919) examined the relation between the depthsof the boundary layer, the inflow and the raisedhead. Nakane and Sasano (1986) measured a1300 m head with a following flow of 300 m, butno mixedJayer height ahead of the sea-breeze isgiven (the Simpson and Britter experiments implyit should be about 5000 m, which is surely toolarge). For the well-documented sea-breeze de-scribed by Simpson et al. (19'17), also with a300 m deep inflow and with a mixed layer of1400 m, the head was found to be only twice thedepth ofthe 300 m deep inflow and extended backabout one to two kilometres from the leadingedge. Aircraft measurements through this sea-

Abbs and Physick: Sea-breeze observations and modelling

Fig. I Plot of vertical velocity, hurnidity mixing ratioand potential temperature made during a traverseofa sea-breeze front. The traverse was made at aheight of 600 rn. The sea-breeze front occurs at0 km on the horizontal axis. 'Ihe captions'rough', 'bubbly' and 'steady, smooth' indicatethe degree of turbulence (Simpson et al. 1977).

breeze (Fig. l) show mixing taking place in thehead region with a billow wavelength of about200 m. Points marked A and B, 50 m apart, de-note adjacent data points characteristis of undi.luted sea and land air respectively.

Along the front during the day, the leading edgeconsists ofa continually changing pattern oflobesand clefts about one kilornetre across. These arisefrom convective instability caused by the over-running ofwarm land air at the ground by densersea air (Britter and Simpson 1978), and disappearat night when radiative cooling rapidly cools thenear-surface layers.

In a summary paper of the laboratory exper-iments on density currents, Simpson and Britter( I 980) point out that no mixing takes place at theinterface between the sea-breeze inflow and thereturn flow above. This implies that emissions ofpollutants into the sea-breeze will be transportedinland virtually undiluted, rise and mix with am-bient air at the front, and head back towards thecoast in the return flow. Furthermore, a'clean'sea-breeze will sweep polluted ambient air up-wards and remove it seawards at higher levelswhile maintaining unpolluted air at the ground.The role ofsea-breezes in air quality is discussedin a later section.

A recent aircraft study of sea-breezes on thestraight South Australian coastline by Kraus et al.( I 990) obtained d^ta at 3 m resolution at variouslevels through the fronts. The various terms in thefrontogenesis equation were calculated on a 2 kmlength scale, with confluence, shear and diabaticprocesses contributing in equal magnitude to

frontogenesis. The raised head in this study isabout twice as deep as the 250 m deep sea-breezeinflow.

The first fine-resolution modelling study of thesea-breeze to appear in the literature was pub-lished by Sha et al. (1991). Using a non-hydro-static model and a horizontal resolution of 100 rn.they were able to simulate all the fine-scalefeatures found in the laboratory experiments andverify the empirical relations involving the inflowand head depths, and billow amplitudes andwavelengths. They also found that Kelvin-Helmholtz instability (KHI) does not occur in theinitial or final stages ofthe sea-breeze, and that theKHl-induced mixing in the middle stage (after-noon) is responsible for slowing the inland pen-etration during this period. This finding mayexplain the failure of coarser-grid models to simu-late the observed middle-of-the-day slowing ofthe 14 June 1973 sea-breeze (Carpenter 1979;Physick 1980).

Behaviour in complex terrainThe topography of the coastline has an importantcontrolling effect on the sea-breeze. As well as in-fluencing the inland penetration ofthe sea-breezeit affects convergence/divergence which in turnaffects associated weather. It may also afect thenumber ofsea-breezes experienced and there areindications that it may have an effect on the onsettime. The classical model of the sea-breeze, as out-lined in the introduction, has been developed inuncomplicated terrain, usually along a lqngstraight coastline with a flat hinterland. Laterstudies conducted in complex terrain indicatethat these sea-breezes do not conform to thesimple model.

Irregular coastlines

Curved coastlines. Neumann ( l95l ) showed thatthe convergence or divergence ofthe land and sea-breezes depends on the curvature ofthe coastline.Where the coast is concave seawards the sea-breeze diverges in contrast to the converging land-breeze, while at convex coastlines the reverseoccurs. He also showed that the variation in thepercentage of thunderstorms within coastal areasis connected in part with the convergent ordivergent nature of the sea and land-breezesinvolved.

In a study of the meteorological conditions as-sociated with Los Angeles air pollution, Edingerand Helvey (1961) discovered that on some sum-mer afternoons, narrow zones of convergingwinds extend across the inland valleys. The con-tinuous records from a group ofobserving stationsin the San Fernando valley revealed abrupt shifts

l 0 Australian Meteorological Magazine 4l December 1992

from the usual sea-breeze from the south-east to awesterly sea-breeze later in the day. These con-verging winds are the result of the sea-breezesformed along a convex section of the Californiacoast. As the sea-breezes converge, both easterliesand westerlies are deflected upwards in the con-vergence zone and become strong updrafts. Theysuggest that this mechanism may help to act as agenerator of upper level pollution by pumpingaloft Los Angeles air pollution. Similarly, in theElsinore Valley, glider pilots have reported theoccurrence of a 'smog front' on many summerafternoons. This results from the convergence oftwo air masses of markedly different historiesover the land - one a polluted breeze from LosAngeles, the other an unpolluted breeze from thecoast. They also report that the line along whichthe two sea-breezes converge is the site of strongvertical currents.

The convergence of sea-breezes from differentcoastlines has also been studied by Physick andAbbs ( 1991, 1992). Their analysis of data from theLatrobe Valley surface network (Fig. 2) revealedthat sea-breezes from the south and east coasts ofthe region were the dominant wind features overthe region during the study period. In this case theeast coast sea-breeze travels farther inland and isthe stronger of the two breezes, pushing thesouthern sea-breeze'back'{ifwards the coast afterthe two meet in the late afternoon. This behaviourwas reproduced in their numerical modellingstudy.

The first three-dimensional modelling study ofthe sea-breeze (McPherson 1970) was motivatedby a desire to assess quantitatively the effect ofcoastline curvature.,on'the evolution of the breeze.Ih his simulations he was able to show that theeffect of the bay is evident immediately. Both thesea-breeze convergence zone and the thermalgradient were distorted around the model bay andwere symmetric with respect to the bay. As thesimulation progressed the degree of distortiondiminished and an asymmetry in the thermal andvertical motion fields developed. This asymmetryis due to the Coriolis acceleration.

Bay and ocean breezes. In regions where thecoastline changes direction suddenly, such as agulfor deep bay, it is possible to produce two sea-breezes. The first of these we will refer to as thebay-breeze. This breeze is a response to the tem-perature diference between the waters ofthe bay(or gulf) and the surrounding coastline. The sec-ond is the ocean-breeze which originates along the'large scale' coastline.

Physick and Byron-Scott (1977) observed theexistence of sea-breezes on each side of St Vin-cent's Gulf and an ocean-breeze which moves upthe Gulf from the Southern Ocean and reachesAdelaide during the afternoon. The ocean-breezeis evident in the wind records only under near-calm synoptic conditions. This is supported by

Fig. 2 Mean arrival time (in the form of isochrones)throughout the Latrobe Valley for the period 8-12 March 1985 for (a) the sea-breeze from theeast coast and (b) the sea-breeze from the southcoast (Physick and Abbs 1992).

lsochrones East Coast Sea Breeze

lsochrones South Coast Sea Breeze

examples of days when the gradient wind is east tonortheast. the resolved wind at the western coast-line is observed to be from the southeast in theafternoon whereas the Gulf sea-breeze compon-ent and the gradient wind component are bothfrom the northeast quadrant.

Abbs and Phvsick: Sea-breeze observations and modelling l l

Barbato (1978) observed the generation of abay-breeze and an ocean-breeze in Boston. Bostonis situated in a concave, hemispheric-shapedbasin with an escarpment rising to 100 m locatedalong the west and northwest. He found that thebay-breeze was often superseded later in the dayby the ocean-breeze. Ocean-breezes of long dul-ation penetrated inland with well defined charac-teristics.

A similar situation has been observed by Abbs(1986) for Port Phillip Bay, Melbourne. She ob-served situations in which the ocean-breeze fromBass Strait penetrated inland above the Port Phil-Iip Bay bay-breeze but was not recorded at thesurface. Under these conditions the ocean-breezeascends the b ay-breeze and penetrates inland withit. This situation occurs when the ocean-breeze iswarmed from below by the land surface overwhich it passes, hence becoming more buoyant.Because of this excess buoyancy the ocean-breezeis able to mount the bay-breeze as it moves inland.On many occasions when the ocean-breeze doesoccur at the surface, its onset is accompanied byan increase in dry-bulb temperature and a de-crease in humidity. This is due to the advection ofwarmer, drier, land-modified air replacing thecool, moist bay-breeze of Port Phillip Bay. It hasalso been shown, both observationally and nu-merically, that the interaction of these two breezesmay produce a mesoscale cyclonic circulation.

OrographyDoes the presence of coastal orography act as abarrier to the inland penetration of the sea-breeze? Numerical experiments by Mahrer andPielke (1977) using a 900 m high bell-shapedmountain extending from the coast to about75 km inland found that the sea-breeze andmountain circulations acting together produce amore intense circulation than they do acting sep-arately, and that by early afternoon the sea-breezehad reached the lee side of the mountains. Thisgeneral result was also found for the Kanto Plainregion of Japan by Kikuchi et al. (1981) whoshowed that the extended sea-breeze (over 200 kmhorizontally) observed in that region could onlybe produced numerically when the orography wasincluded. Similarly, strong afternoon winds regu-larly observed over Lake Kinneret in Israel wereshown by Alpert et al. ( I 982) to be the sea-breezefrom the Mediterranean coast 45 km away. Dur-ing itsjourney to the lake (210 m below sea level),the sea-breeze passed over hills 400 m high. Intheir numerical experiments for the Perth regionof Western Australia, Manins et al. (1992) simi-larly found that the presence of the escarpment,30 km from and parallel to the coast, acts to accel-erate the inland movement of the sea-breeze.

Observations from coastal valleys running par-allel to the shoreline, and opening to the sea atsome location, have shown that the sea-breeze willeither travel up the valley from the mouth or spill

over the coastal ridge into the valley, dependingon the direction of the synoptic wind (SumnerI 977; Physick 1 982). Studies on the west coast ofthe United States have shown that the inland pen-etration of marine air is strongly dependent ontopographic channels (Fosberg and Schroeder1966; Olsson et al. 1,973). In the San Franciscoregion, Fosberg and Schroeder (l 966) have foundthat the sea-breeze moves from the coast to theinland valley through the natural passes such asthe Golden Gate, Petaluma Gap and CarquinezStrait. They also found that these passes and de-flecting barriers such as hills create vortices in thesea-breeze flow. During the Atmospheric Studiesin Complex Terrain (ASCOT) program in theGeysers area of northern California, the sea-breeze was found to have a strong influence on theinitiation and evolution of evening drainagewinds (Neffand King 1987).

Neumann and Savijarvi (1986) modelled thesituation where mountains rise steeply from theshoreline. They were prompted by two sets of l9thcentury sea-breeze observations from the moun-tainous coasts of Chile and Peru, involving sailingships. In one ofthese, the vesselln lucolay in portready to sail for four days 'while the water abouther was mirror-smooth'. About 4-6 km offshore,the sea was disturbed each day by a moderate sea-breeze. In their experiments they varied themountain height and the Brunt-Viiisiilii fre-quency, finding that as each is increased theblocking effect on the sea-breeze intensifies. Thiseffect is most pronounced in the vicinity of theshoreline, where the sea-breeze breaks down intotwo cells (Fig. 3), one over the sea and another

Fig. 3 Isotachs of the onshore component (solid lines, inm s-r) and ofthe vertical velocity (dashed lines,in cm s-r) of the sea-breeze flow in the presenceof a 1000 m high barrier. Note that in the de-tached cell over the lower slope of the mountainthe maximum seaward velocity is - 3 m s- I andthe maximum downward velocity is -28 cm s-r(Neumann and Savijarvi 1986).

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over the windward side of the mountain. For thesailing ship case, their predictions were inagreement with the observations of the ship'scaptain.

Inland penetrationWhile working as a forecaster in Canberra in the1940s, Reg Clarke became interested in a lateafternoon eastedy wind which was often observedin Canberra during the warmer months (Clarke,personal communication). Certain that it was asea-breeze from the east coast (l l2 km away) butunable to convince any ofhis colleagues, Reg usedcar and aeroplane observations to prove that thiswas indeed the case. Data from these experiments,in which it was also claimed the sea-breeze pro-gressed to Wagga Wagga (270 km inland), werepublished much later and favourably comparedto Reg's numerical predictions of the inlandpenetration (Clarke I 983a).

To avoid any orographic effects, and interestedin the claim of Hounam (1945) that sea-breezesoccasionally reached Kalgoorlie (345 km inland),he later mounted a sea-breeze experiment in thefeatureless terrain of southern Western Australia(Clarke 1955). To prove his assertion that sea-breezes were notjust coastal phenomena (the pre-vailing view at the time), Reg and his brothertracked several sea-breezes by car from the southcoast through to Kalgoorlie, where they arrivedaround midnight. Since this pioneering work, sea-breezes have been observed by Reid (1957) atRenmark (217 km inland), by Garratt andPhysick (1985) at Daly Waters (280 km) and byClarke et al. (1981) across Cape York Peninsula.Elsewhere, deeply penetrating sea-breezes havebeen observed up to 200 km from the coast inCalifornia (Carroll and Basket 1979), in Japan(Kurita et al. 1985), and in Saudi Arabia (Steed-man and Ashour 1976).

One of the most interesting aspects of the pen-etration ofthe sea-breeze is the change in the rateofadvance inland ofthe front, although conflict-ing reports on this characteristic appear in theliterature. Sha et al. (1991) identified three dis-tinct stages in the penetration of their modelledsea-breeze front. Initially the front moves at aspeed of approximately 5 km h-t before decelerat-ing in the early afternoon. Later, the sea-breezefront accelerates before decelerating and decayinglate at night. The afternoon deceleration of thesea-breeze was observed by Pedgley (1958) forsea-breezes in Egypt and by Simpson et al. (1977)for their well documented southern England sea-breeze. The evening acceleration ofthe sea-breezefront has been observed and modelled by a num-ber of authors (Clarke I 95 5; Simpson et al. 197 7 ;Clarke 1984; Physick and Smith 1985).

Simpson et al. (1977) were able to show, using asimple numerical model, that the inland advanceofthe sea-breeze is governed locally by the densitydifferences across the front. Under conditions inwhich the synoptic flow across the coastline isweak, a sea-breeze will develop at the coast assoon as the land-sea temperature difference islarge enough. As the sea air moves inland its tem-perature rises more rapidly than that of the landair as the heat is distributed over a much smallerdepth. Consequently, the temperature contrastacross the front decreases and the sea-breeze frontdecelerates. In addition Kelvin-Helmholtz insta-bility increases the top drag through strong turbu-lent mixing and this increase in the top friction isalso associated with the deceleration of the sea-breeze in the middle of the day (Sha et al. I 99 I ). Inthe late afternoon the solar heating decreases butthe temperature of the land air still increasesgradually. At the same time, the temperature ofthe sea air is becoming cooler as the sea air arriv-ing at the front has had less heat added to it sincecrossing the coast. As a result, the temperaturedifference across the front increases and the sea-breeze begins to accelerate. In the nocturnal stageofthe sea-breeze, horizontal advection ofcool seaair is instrumental in maintaining the frontal den-sity gradient. This advection ofcool sea air ceasesonce air which crossed the coast near sunsetreaches the front. Physick and Smith (1985)suggest that the further inland a sea-breeze is atsunset, the longer it will survive before dissipatingin the manner described below.

Clarke (1984) used a two-dimensional hydro-static numerical model to investigate further thebehaviour of sea-breezes when the geostrophicwind is onshore. He identified five stages in thedevelopment of the sea-breeze. During the 'im-

mature stage' the inland penetration increasesmonotonically with time. At this time the relativeflow is through the isentropes and the flow is farfrom steady state. Convective overturning main-tains the frontal boundary in a near-vertical pos-ition. In the'early mature stage', corresponding tolate afternoon, the sea-breeze surge is still not asteady-state gravity current but acceleration ofthesurge has commenced. During early evening thesupply ofcool air from the sea eeases, decouplingof the frictional link with the surface occurs andthe sea-breeze continues to accelerate. Clarkeidentifies this as the 'late mature stage'of the de-velopment of the sea-breeze surge. At this time theleading isentropes are still steeply inclined but arebeginning to be flattened and the centre of thehorizontal vortex shifts to the leading edge ofthecirculation. The surge front is still sharp and ac-tive during this stage. In the next stage, the 'early

degenerate stage', the layer ofcold air flattens andspreads inland propagating as an unsteady gravitycurrent. During this period the sea-breeze steadilydecelerates and has the form ofa degenerate grav-ity current or a steadily diminishing vortex com-

Abbs and Physick: Sea-breeze observations and modelling I J

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pletely detached from the coastline. Figure 4shows a vortex about l0 km in length observedduring the last stages of a sea-breeze surge. In thisvortex the air is moving faster than the front.Physick and Smith (1985) showed (using a two-dimensional model) that once the vortex hasformed, the main heating/cooling process that oc-curs is the subsidence warming in the descendingarm ofthe circulation. This leads to the eventualdecay ofthe density gradient and the vortex itself'In the late degenerate stage there is no longer aclosed circulation near the leading edge.

Fig. 4 Relative streamlines for a dying sea-breeze surgeobserved during the Coonalpyn Downs ex-pedition in South Australia. The streamlines aredet€rmined from serial balloon flights centred on2200 CST at a point 165 km inland (Clarke1983a).

Clarke et al, (1981) present four possible mech-anisms for the production of the pressure jumpassociated with the morning glory. Their datasuggested that the dominant mechanism for for-mation of the pressure jumps was the interactionof the east coast sea-breeze gravity current withthe nocturnal inversion. The sea-breeze developsa front as it progresses inland assisted by the pre-vailing light onshore geostrophic flow, With time,the front may develop into a closed vortex whichmay persist for many hours following sunset. Thisexplanation was supported by observations ofdeeply penetrating sea-breezes (Clarke I 96 5). Thenumerical modelling studies of Crook and Miller(1985) and Sha et al. (l 991) confirm that it is poss-ible for an undular bore to be produced by agravity current moving into a stably stratifi.edfluid. Other suggested mechanisms were the pro-duction ofan internal bore on a katabatic flow, thecollapse of lee waves or standing eddies in thevicinity ofhigh ground and the head-on collisionof sea-breezes from the east and west coast ofCape York Peninsula.

After further analysis of the field data, Clarke(1983b) concluded that the morning glory is pro-duced by the interaction ofsea-breezes from bothsides of the Peninsula. The first necessary con-dition for bore formation is that a sea-breeze surgeshould move inland from the east coast of CapeYork Peninsula and interact with the weaker sea-breeze on the west coast of the Peninsula. Thewest coast sea-breeze acts to modify the atmos-phere in a coastal strip about 100 km wide, pro-ducing a nocturnal boundary layer over both seaand land on which the internal bores can propa-gate. For this sequence ofevents to occur a modesteasterly geostrophic wind is necessary.

This description ofthe formation ofthe internalbore was later confirmed by the two-dimensionalmodelling studies of Clarke (1984) and Noonanand Smith (t986). Clarke showed that the colli-sion ofthe two sea-breezes from either side ofthePeninsula raises cool air in a sharp, upward bulge.One or two bores may then propagate away fromthe genesis site at the same speed as the originalsea-breezes. The evolution ofthe vertical velocityand potential temperature fields associated withmorning glory genesis are shown in Fig. 5.Noonan and Smith also showed that sea-breezecollision must occur in the late evening for a sus-tained bore to be formed. This ensures that airassociated with the east coast sea-breeze hasstabilised at low levels and so there is a local up-ward flux of negative buoyancy on collision. Ifcollision takes place earlier, the lowlevel air oftheeast coast sea-breeze is still well mixed and it isthis air that is lifted at collision. Under these con-ditions a sustained bore is not produced. Thethree-dimensional studies of Noonan and Smith(1987) showed that the sea-breeze convergence ismaximised in the late evening at about the timeand place morning glories are observed to

Role in internal bore generation

The 'morning glory' of northern Australia is themost widely documented example of internal un-dular bores occurring in the atmosphere, and theimportance of the sea-breeze in its formation hasbeen accepted since the early 1980s. A completedescription of the 'morning glory' is presented inthis volume by Christie (1992). Analysis of thefield data from the 1978 and 1979 expeditions,enabled Clarke et al. (1981) to show that themorning glory is not a gravity current, a charac-teristic of which is lowJevel feeder flow towardthe leading edge of the disturbance in a frame ofreference moving with the front. Their obser-vations showed that the relative flow was throughthe system and contained very little advectedfluid. The streamline patterns show that the dis-turbance has the characteristics of a travellingwave (or group of waves) but the pressure riseaccompanying the disturbance is sustained forover three hours. Thus, they conclude that the dis-turbance is an atmospheric undular bore propa-gating on the nocturnal or maritime inversion.

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Australian Meteorological Magazine 4l December 1992

Fig. 5 Vertical cross-sections of the vertical velocity and potential temperature (a) before, (b) at the time of, and (c) aftermorning glory genesis. In (d) the bore-like disturbance has begun to move westwards (Noonan and Smith1986).

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l .

0 .zCC, 300. 400. 500. 500.

0lsTiNcS rKr'rl

5 .

; i .

;= 2 ,

0 .4CA. 500. 600.

OISTNNC= (KHI{0c. 500. 500.

OISTFNCE IKH'

200.

3 1 7 _

-

?00. 800. 2C0. 300. 800.

Sea-breeze observations

originate and a sustained westward propagatingdisturbance is produced. The disturbance is ap-proximately parallel with the east coast of thePeninsula, reflecting the orientation of the eastcoast sea-breeze front. They conclude that thereason that morning glories are confined to thesouthern part of the Peninsula is due to the factthat by late evening the disturbance produced bythe colliding sea-breezes has significant ampli-tude only in that region.

Sea-breezes and air quality

When assessing the impact of pollutant emittersin coastal regions, it is essential to consider severalof the sea-breeze characteristics discussed inearlier sections. We examine the air quality inboth the near and far field.

Near-source dispersionThe thermal internal boundary layer (TIBL) whichdevelops in the stable sea air as the sea-breezepasses over the heated land can be responsible forenhanced surface concentrations downwind of acoastal source. Figure 6 illustrates how a plumeinitially travelling in stable air above the TIBL isbrought to ground (fumigated) further inland byconvective eddies in the growing TIBL. Probablythe most comprehensive data set on TIBL be-haviour and plume fumigation was gathered in1978 at Nanticoke on the shore of Lake Erie,Canada (Portelli 1982; Kerman et al. 1982; Hoffet al. 1982). Over the eight days of the experiment,fumigation of the plume from the coal-fired powerplant (final-rise height about 3 50-400 m above theground) usually began between 1100 and 1300hours local time and continued till about 1700-I 900 hours. The plume was first brought to groundat distances ranging from 7 to 20 km from thestack, but this distance varied during the day as theTIBL grew and decayed. The fumigation zone wasalso found to rotate about the stack as the winddirection changed, primarily due to the Coriolisforce acting on the sea-breeze.

Continuous fumigation of shoreline plumeswas also examined by Lyons and Cole (1973) forthe Milwaukee area who found that while the on-shore lake-breeze was cleanat the shoreline, elev-ated plumes at the shore were brought to groundin the TIBL within 3-5 km inland. A similar studyby Lyons and Olsson (1973) on the western shoreof Lake Michigan in the Chicago area found thatnot only does the same fumigation mechanismoccur there, but the air quality problem is exacer-bated by recirculation of pollutants within thelake-breeze circulation. Pollutants fumigated intothe inflow rise at the front and move back over thelake in the return flow. However the larger pol-lutant particles subside into the top region oftheinflow and are brought back over land where theyare fumigated to the ground once again, along

Fig. 6 Schematic diagram illustrating fumigation of acoastal plume by the thermal internal boundarylayer. This diagram is from Portelli (1982) andillustrates the data network for the Lake Eriefumigation experiment.

with the fresh emissions from the coastal sources,leading to an ever-increasing loading of the atmos-phere in the shoreline region.

Our discussion so far has concerned elevatedsources, primarily emitting oxides of sulfur andnitrogen. We now turn to surface emissions, ofreactive pollutants, and show that another aspectofthe sea-breeze is frequently responsible for highlevels of photochemical smog, of which ozone isthe major component.

Ozone is formed on a time-scale of severalhours through reactions involving non-methanehydrocarbons and nitrogen oxides in the presenceof ultraviolet radiation. The dominant source ofthese ozone precursors in populated areas is theinternal combustion engine, although various in-dustries also contribute. In coastal cities such asAthens (Lalas et al. 1987; Gusten et al. 1988),Melbourne (Evans et al. l98l; Cope et al. 1990;Johnson and Cope 1990), Sydney (Hyde et al.1978; Hawke et al. 1983) and Perth (Manins et al.1992), the local meteorology in summertime issuch that land-breezes or light offshore synopticwinds advect emissions from the morning peaktraffic period out over the sea. Here the mixture isable to 'cook' for a few hours unhindered bysources of fresh nitric oxide (NO) which are likelyto reduce the ozone levels. When the sea-breezedevelops in the late morning, the ozone is trans-ported onshore and over the city and suburbsresponsible for its precursors. Under these con-ditions, the highest ozone levels (in late after-noon) are usually found within 30-40 km of thecoastal source. In the following section on long-range transport, we find that daily maximumreadings of'city-produced'ozone can occur as faraway as 200 km from the source.

t 6 Australian Meteorologica I Magazine 4 t Decembei I 992

Long-range transportThe deep inland penetration achieved by somesea-breezes suggests that pollution from coastalregions may be transported large distances fromthe source. Such evidence has been found in anumber of Californian localities includingYosemite National Park where the sea-breeze andupslope circulation combine to import pollutantsfrom the San Francisco Bay area 200 km away(Carroll and Baskett 1979), and in Palm Springswhere Los Angeles smog arrives from 180 kmaway around midnight (Grosjean 1983). LosAngeles is also responsible at times for high ozonereadings in the Santa Barbera region to the north-west (Moore et al. 199 l; Hanna et al. 199 l).

In Japan, the highest ozone concentrations arenot usually found in Tokyo where the precursorsare produced, but in the mountainous region150 km to the northwest in the early eveninghours. The smog is transported there by a circu-lation resulting from interaction between thesea-breeze and thermally and topographically-induced flow, known as the extended sea-breeze(Kurita et al. 1985; Chang et al. 1989; Kondo1990). By analysing meteorological and air qual-ity data for the region and aircraft data along theroute of a polluted air mass from Tokyo, Ueda etal. (1988) found that the highest surface concen-trations of oxidants occurred at night-time in themountains, at the rear edge of the cut-off vortexwhere the internal circulation brings upper-layeroxidants to the ground. This vortex, about 25 kmin length in the Tokyo cases, forms at the leadingedge of the sea-breeze as it encounters a stableatmosphere after sunset.

So far our discussion has concentrated on casesin which the sea-breeze exacerbates pollutionproblems. We now examine a situation in which itis very effective in replacing polluted surface airwith clean air. The Latrobe Valley in southeasternAustralia contains coal-fired power stations lo-cated about 90 km up the valley from the coast.Under clear-sky summertime conditions, thestation plumes become well mixed throughout theconvective layer and drift down the valley, meet-ing the sea-breeze about 25 km from the coast. Airin the polluted mixed layer rises at the sea-breezefront, is mixed with incoming sea air at the topand rear ofthe head region and continues its pass-age towards the coast in the return flow at higherlevels. Clean air is brought in at lower levels as thesea-breeze continues its movement inland. By thetime it reaches the power stations in late after-noon, convection has decayed and the plumes areno longer brought to ground. They are takenfurther inland during the night at about 500 mabove the ground, while at higher levels towardsthe coast the sea-breeze return flow transportsafternoon emissions out to sea. For the LatrobeValley region, it appears that the emitters mayvery well be located at the optimum location, as

far as minimising air pollution on sea-breeze daysis concerned. Further discussion on the meteor-ology and air quality of the Latrobe Valley regioncan be found in Physick and Abbs (1991,t992).

Sea-breeze modellingWhilst the dynamics of the sea-breeze must haveoccupied the minds of many natural philosophersduring the last few hundred years, one of theearliest worthwhile theoretical studies of thephenomenon appears to be that ofJeffreys (1922),who considered it to be a type of antitriptic wind.Since then, a number of linear models of the sea-breeze have been produced, including that ofHaurwitz (1941). He showed that the incorpor-ation of friction into the model enabled the theor-etical results to resemble the observations moreclosely, insofar as it caused the sea-breeze to de-crease earlier. He also demonstrated that the ob-served veering ofthe sea-breeze during the day rsdue to the Coriolis force. Schmidt (1947) devel-oped a linear model in which the velocity fieldswere calculated from an atmospheric temperaturedistribution which was specified as a function oftlme.

The linear models reproduced certain observedfeatures ofthe sea-breeze, such as the depth oftheflow and the rotation of the wind vector. How-ever, ir was not unti l Pearce (1955) performedthe first successful numerical integration of thenonlinear sea-breeze equations, that models wereconstructed which exhibited the oft-observed sea-breeze front or convergence zone.

In a mesoscale phenomenon such as the sea orlake-breeze, there is a significant transport ofmo-mentum and heat by turbulence, and thus it isimportant that this aspect of the numerical modelbe simulated as accurately as possible. Pearce(t 955) prescribed an artificial heating mechanismof the atmosphere unrelated to the predicted tem-perature and wind fields, and also neglected sur-face drag. In one of the first attempts at a morerealistic simulation of the momentum and heatexchange, Fisher (1961) used a gradient transferapproach and assumed a constant profile for theeddy exchange coefficients (K, and Ks) through-out the day and night, with a maximum valueoccurring at a height of 50 m. It is well known,however, that these transfer coefficients are atleast an order of magnitude less in stable than inunstable conditions.

Estoque (1961) divided the modelled regioninto a constant-flux surface layer of 50 m, in whichKu (:Ki is a function of stability and windshear, and a transition sub-layer, in which Kaadecreases in a linear fashion from its value at the

Abbs and Physick: Sea-breeze observations and modelling 1',l

interface to zero at the top of the model at 2 km.This model represents a landmark in sea-breezemodelling and all advances since then, mainly inparametrisation ofthe boundary layer, have reallybeen incremental. McPherson (1970) showed thatthe manner in which Kp, decreases to zero at thetop of the model is a'significant factor in thedetermination of the intensity and extent ofthe computed sea-breeze circulation. An expo-nential decrease was considered by McPhersonto give results in reasonable agreement withsea-breeze observations.

The next step forward followed the significantadvances made in boundaryJayer meteorology inthe fifties and sixties. In the numerical model de-veloped by Pielke (1974) to study sea-breezes inFlorida, the non-dimensional wind and tempera-ture profiles based on Monin-Obukhov similaritytheory were used to calculate fluxes in the surfacelayer. The heights ofthe surface layer and bound-ary layer were also allowed to vary over a diurnalcycle. Surface temperature (Te) had always beenspecified as a sinusoidal function of time in sea-breeze models to this stage, but Physick (197 6)employed a surface energy balance equation tocalculate Te, thus allowing horizontal gradients ofTs and surface heat flux to exist across the front.Development of this model benefited from dis-cussions with Reg Clarke who was designing amodel to help explain his observations of sea-breezes up to 300 km from the coastline. Simi-larity theory and a local scheme for the exchangecoefficients throughout the boundary layer, devel-oped from analysis of the Wangara data (Clarke197 4),werc used in his model but details were notpublished until a decade later in a study of noc-turnal wind surges (Clarke 1983a). However someresults on deeply penetrating sea-breezes werepresented at a Fire Weather Conference atMonash University (Clarke 1973).

Monin-Obukhov similarity theory in the sur-face layer and an energy balance equation for thecomputation of surface temperature are now stan-dard fare in models used for the simulation ofsea-breezes. The main variation between modelsoccurs in the parametrisation of turbulent ex-change in the transition layer above the surfacelayer. These range from local (Kondo 1990) andnon-local (Pielke 1974) K{heory to first-order(Arritt 1987), second-order (Schumann et al.I 987) and third-order (Briere I 987) turbulent kin-etic energy (TKE) schemes. A comparative studyofa numberof these parametrisations by Mahfoufet al. ( I 987) has found that the mean fields of windand temperature are relatively insensitive to theturbulence specification, but that the more de-tailed schemes predict the turbulence statisticsmore accurately, an important finding for airquality models in which the dispersion is drivenby winds and turbulence fields from a mesoscaiemodel.

Virtually all mesoscale models used for sea-breeze simulations employ the hydrostatic as-sumption in the vertical equation of motion anduse a horizontal grid spacing ranging from 2 to l0km. Pielke (1972), Martin and Pielke (1983) andSong et al. (1985) have all examined the effect ofthe hydrostatic assumption in sea-breeze modelsby deriving an estimate for the non-hydrostaticpressure, based purely on results from a hydro-static simulation. As well as concluding that thehydrostatic assumption is adequate for grid spac-ings as small as I km, they also investigated thedependence of the difference on the scale of theheating, atmospheric stability and synoptic windspeed, finding that non-hydrostatic effects gener-ally tend to weaken the sea-breeze when comparedto a hydrostatic simulation. Their conclusionswere corroborated by Yang ( I 99 I ) who comparedhydrostatic and non-hydrostatic simulationsfrom the same model. Yang also stressed the im-portance of the turbulence parametrisation andrecommended that higher order closure schemesshould be used in non-hydrostatic models.

Perhaps the final stage in sea-breeze modellingwas reached recently with the publication of apaper by Sha et al. (1991). Using a non-hydro-static model and a horizontal grid spacing of only100 m, they were able to resolve explicitly themixing process in the head region, wherebywarmer ambient air is entrained into cooler sea airby the breaking of Kelvin-Helmholtz billows. Thisfine structure of the sea-breeze head as modelledby Sha et al. is shown in Fig. 7.

Fig.7 Vertical cross-section showing the fine structureof the sea-breeze head. The numbers on thehorizontal axis are distance from the coastline.The solid line indicates the zero velocity bound-ary. The front position is also shown. Horizontalvortices are labelled, a, b, c (Sha et al. 1991).

l ro r l LAND l lK i l )

ConclusionIn this review we have presented early resultswhich described the structure of the sea-breezeand its efect on the wind, temperature and hu-midity fields at coastal locations. The various

l 8 Australian Meteorological Magazine 4l December 1992

effects of orography and coastline curvature onthe inland penetration of sea-breezes are also pre-sented. More recent studies ofthe sea-breeze are,in general, the result of air quality research forcoastal regions and the role of the sea-breeze inpollutant dispersal has been discussed.

Advances in computer power have been paral-leled by an increasing sophistication in numericalmodels of the sea-breeze. These models have al-lowed researchers to obtain a greater understand-ing ofthe sea-breeze and are nowjust beginning tobe used by regulatory authorities for air qualityproblems.

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