A Lake and Land Breeze Primer

by Dr. David Sills

The information in this 'primer' has been adapted from the first chapter of the following publication:

Sills, D.M.L., 1998: Lake and land breezes in southwestern Ontario: observations, analyses and numerical modeling. PhD dissertation. York University, 338 pp. Available from the Centre for Research in Earth and Space Science, York University, 4700 Keele St., Toronto, Ontario, Canada, M3J 1P3 and in PDF form
here (large document!)

The primer serves as a foundation for a more in-depth understanding (and in some cases a rethinking) of lake breezes and their impacts developed through subsequent research work that I either led or contributed to. Such research includes the following (in reverse chronological order):

Sills, D. M. L., J. R. Brook, I. Levy, P. A. Makar, J. Zhang, and P. A. Taylor, 2011: Lake breezes in the southern Great Lakes region and their influence during BAQS-Met 2007.
Atmos. Chem. Phys., 11, 7955-7973. [ACP Abstract / PDF]

Hayden, K. L., D. M. L. Sills, J. R. Brook, S.-M. Li, , P. Makar, M. Z. Markovic, P. Liu, K. G. Anlauf, J. M. O’Brien, Q. Li, and R. McLaren, 2011: Aircraft study of the impact of lake-breeze circulations on trace gases and particles during BAQS-Met 2007. Atmos. Chem. Phys., 11, 10173-10192. [ACP Abstract / PDF]

Makar, P. A., J. Zhang, W. Gong, C. Stroud, D. Sills, K. L. Hayden, J. Brook, I. Levy, C. Mihele, M. D. Moran, D. W. Tarasick, and H. He, 2010: Mass tracking for chemical analysis: the causes of ozone formation in southern Ontario during BAQS-Met 2007. Atmos. Chem. Phys., 10, 11151-11173. [ACP Abstract / PDF]

Levy, I., P. A. Makar, D. Sills, J. Zhang, K. L. Hayden, C. Mihele, J. Narayan, M. D. Moran, S. Sjostedt, and J. Brook, 2010: Unraveling the complex local-scale flows influencing ozone patterns in the southern Great Lakes of North America, Atmos. Chem. Phys., 10, 10895-10915. [ACP Abstract / PDF]

King, P. W. S., M. Leduc, D. M. L. Sills, N. R. Donaldson, D. R. Hudak, P. I. Joe, B. P. Murphy, 2003: Lake breezes in Southern Ontario and their relation to tornado climatology. Weather and Forecasting, 18, 795-807. [AMS abstract] [PDF]

Sills, D., P. Taylor, P. King, W. Hocking and I. Nichols, 2002: ELBOW 2001 - studying the relationship between lake breezes and severe weather: project overview and preliminary results. Preprints, 21st Severe Local Storms Conference, San Antonio, TX, Amer. Meteorol. Soc., 611-614. [PDF]

King, P., B. Murphy, A. Erfani and D. Sills, 2002: The Use of the GEM forecast model at very high resolution during ELBOW 2001. Preprints, 21st Severe Local Storms Conference, San Antonio, TX, Amer. Meteorol. Soc., J125-J128. [PDF]

Sills, D. M. L. and P. W. S. King, 2000: Landspouts at lake breeze fronts in southern Ontario. Preprints, 20th Conference on Severe Local Storms, Orlando, FL, Amer. Meteorol. Soc., 243-246. [HTML]

Hastie, D. R., J. Narayan, C. Schiller, H. Niki, P. B. Shepson, D. M. L. Sills, P. A. Taylor, Wm. J. Moroz, J. W. Drummond, N. Reid, R. Taylor, P. B. Roussel and O. T. Melo, 1999: Observational evidence for the impact of lake breeze circulation on ozone concentrations in southern Ontario. Atmospheric Environment, 33, 323-335. [Elsevier abstract] [PDF]

Sills, D. M. L. and P. King, 1998: The 1997 ELBOW Project: high-resolution modelling of lake breezes in a pre-storm environment. Preprints, 19th Conference on Severe Local Storms, Amer. Meteorol. Soc., Minneapolis, MN, 26-29. [HTML]

Sills, D.M.L., 1998: Lake and land breezes in southwestern Ontario: observations, analyses and numerical modeling. PhD dissertation. York University, 338 pp. Available from the Centre for Research in Earth and Space Science, York University, 4700 Keele St., Toronto, Ontario, Canada, M3J 1P3 and in PDF form
here (large document!)

Table of Contents

1.  Introduction

2.  Lake Breeze and Land Breeze Characteristics

3.  Roles in Summer Severe Weather

4.  Air Pollution Effects


1.  Introduction

The onshore surface winds of the sea breeze and the nocturnal offshore surface winds of its counterpart, the land breeze, have been studied since ancient times. Aristotle pondered the origin of sea breeze winds over two thousand years ago (see Simpson, 1994). Believing the wind to be a 'dry exhalation', he found the generation of the sea breeze difficult to explain since such an exhalation could not be expected to originate over the moist sea. Much nearer to modern times, the arrival and inland penetration of the sea breeze were investigated near Boston in 1890 by a group from Harvard College Observatory (Davis et al., 1890). Using a team of over 100 people, they tracked the progress of sea breezes as far inland as 30 km. In the last century, hundreds of papers have been written on the topic, describing observed characteristics, proposing theoretical models and, with the advent of computers, performing numerical simulations.

Though recognized in work dating as far back as 1799 (Ellicott, 1799), breezes produced by the Great Lakes of North America have been thoroughly investigated in only the last several decades. The shore of Lake Michigan was the site for several pioneering field studies of lake breezes including those of Moroz (1967) and Lyons (1972). Early lake breeze investigations were also conducted on the shores of Lake Erie (Biggs and Graves, 1962), Lake Huron (Munn and Richards, 1964), and Lake Ontario (Estoque et al., 1976). Lake and land breezes near lakes appear to have the same initiation processes as sea and land breezes near oceans. However, as will be discussed later, the evolutions of these circulations through the day can be very different.

2.  Lake Breeze and Land Breeze Characteristics

General Features

Contemporary textbooks, such as Meso-Scale Atmospheric Circulations (Atkinson, 1981), Mesoscale Meteorological Modeling (Pielke, 1984) and Sea Breeze and Local Wind (Simpson, 1994), offer detailed though oversimplified explanations of the formation of lake or sea breezes. Each shows that during clear, calm conditions along a straight coastline, daytime insolation heats a land surface more rapidly than a water surface due to differences in the thermal properties of common soils and water.

Atkinson writes that air over land expands more rapidly than air over water due to the warmer underlying surface during the day. The vertical pressure gradient over land must then decrease under hydrostatic conditions. Assuming that surface pressures over land and water are initially the same, this results in higher pressure over land than over water at constant heights above the surface causing air above the surface to flow offshore towards lower pressure during the day. This flow aloft induces a flow opposite in direction at the surface known as the 'sea breeze' or 'lake breeze'.

Pielke describes a process very similar to that of Atkinson. However, Pielke explains the formation of the pressure gradient aloft in terms of mass mixing rather than the expansion of air. That is, he states that the pressure gradient aloft forms in response to enhanced upward mass flux over land caused by turbulent mixing in the unstably stratified boundary layer.

Simpson's description involves the expansion of air over land being limited vertically by a capping stable layer. It is, therefore, the sideways expansion of air over land that results in a low pressure region near the surface. The resulting land-water pressure difference gives rise to onshore flow near the surface and a weak return flow is induced aloft to balance the system.

A more complete description of the lake breeze initiation process, modified from a 'textbook definition' suggested by Pearce (1955), is offered below with the aid of diagrams in Figure 1. It is assumed that the day is clear and calm, the shoreline is straight, and constant pressure surfaces over land and water are initially horizontal. After sunrise, the land surface begins to warm more rapidly than the water surface. This causes air over land to warm more rapidly than the air over water and thus the expansion of the air over land occurs at a relatively higher rate. This expansion causes the constant pressure surfaces over land to be forced upward through the depth of the atmosphere. The resulting pressure differences across the shoreline result in a weak, lakeward tidal motion (Figure 1a).

Figure 1.  Schematic diagrams describing the initiation process for lake breezes. The process includes three stages (a-c) that occur nearly simultaneously. In the diagrams, Pn > Pn+1 where P is pressure.

Pearce estimated that lakeward velocities would be on the order of 1 km h-1. This adjustment occurs rapidly and the pressure surfaces aloft quickly become horizontal once more. Near the surface, however, hydrostatic conditions result in lower pressure over land since the air over land is warmer than that over water from the surface to a level several hundred metres above the ground. This pressure gradient causes a flow from lake to land near the surface known as the 'lake breeze' (Figure 1b).

Near the surface, convergence occurs over land and divergence occurs over water due to this onshore flow. This induces an upward motion over land while a downward motion is induced over the water. These motions again shift the constant pressure surfaces in the vertical so that a lake-land pressure gradient is achieved aloft. The resulting flow from land to lake aloft is called the 'return layer flow' (Figure 1c) and completes the circulation. These steps occur nearly simultaneously and the entire process occurs continuously with the addition of heat to the surface.

The intensity of the lake breeze circulation, as with all circulations that are thermally forced at the surface, is directly proportional to the magnitude of the horizontal temperature gradient and the depth of the temperature perturbation in the atmosphere. This fundamental concept is shown mathematically by Pielke and Segal (1986). Thus, in the case of lake breezes, a greater temperature contrast between air over land and over water results in a more intense circulation. Conversely, a stably-stratified lower atmosphere will tend to vertically restrict the land-water air temperature perturbation resulting in a less intense circulation. The maximum intensity and depth of the lake breeze has also been found to decrease with decreasing lake size though lake breezes may still develop on lakes only a few kilometres in width (Segal et al., 1997).

Along a straight coastline in flat terrain, the lake breeze initially blows perpendicular to the shore, then may approach geostrophic balance (flow parallel to the shore) after a period of time due to the Coriolis acceleration if the circulation is long lived (Lyons, 1972). This period of time is dependent upon the latitude of the location but is on the order of several hours for the Great Lakes region. The onshore wind may exhibit significantly more complicated behaviour in the presence of complex coastal topography and/or an irregular shoreline (Simpson, 1994). Lakes breezes, especially on the Great Lakes, become nearly as well-developed as their oceanic counterparts having a typical maximum inflow wind speed of 4-7 m s-1 and a slightly weaker typical maximum return current of 2-5 m s-1 aloft (Moroz, 1967; Lyons, 1972; Lyons and Olsson, 1973; Keen and Lyons, 1978). The maximum depth of a typical lake breeze inflow layer is between 500 m and 1000 m with the return current extending to 1000-2000 m above the inflow layer (Moroz, 1967; Lyons, 1972; Lyons and Olsson, 1973; Keen and Lyons, 1978). A typical lake breeze, based on observations from the literature cited here, is illustrated in Figure 2.

Figure 2.  Idealized illustration of a typical lake breeze circulation and its associated front based on the literature referred to in the thesis. Common features are labelled. The dashed line represents the outer boundary of the inflow layer. The frontal zone is not shown to scale.

Sea breezes have been observed to penetrate inland between 40 km and over 300 km depending largely on latitude while typical penetration distances for the Great Lakes region have been found to be near 30 km (Atkinson, 1981). These circulations also extend offshore but knowledge of this aspect is minimal due to the sparsity of data over water. To the author's knowledge, no detailed measurements of the lakeward extent of lake breezes have been made though a few studies have examined this aspect of the sea breeze. For example, Finkele et al. (1995) sampled a sea breeze circulation cell with an aircraft over the southern coast of Australia and found that the over-sea extent was several times larger than the inland penetration distance.

Physick (1976) found by numerical simulation that, for gulfs and lakes roughly the size of the Great Lakes or smaller, circulations on each shore should not occur independently but interact to form a mesoscale high pressure area with associated subsidence over the water. This result has been confirmed by Estoque (1981) and Comer and McKendry (1993), among others and probably represents the most significant difference between the dynamics of lake and sea breezes. Moroz and Hewson (1966) postulated that the return flow aloft associated with a lake breeze may be much more pronounced than that which is observed along ocean coastlines due to this effect.

The lake breeze circulation usually dissipates as changes in air temperature over land and over the lake eliminate the horizontal pressure gradient. This typically occurs near sunset. Lake breezes have also been observed to retreat to the shoreline due to an increase in cloud cover or increase in the offshore gradient wind (Ryznar and Touma, 1981). Simpson (1977) suggests that in the evening when dissipative processes such as convective turbulence near the surface subside, the lake breeze frontal vortex may detach from the evanescent circulation and continue to penetrate inland. Simpson further postulates that deeply penetrating lake breeze fronts exist in most cases in the form of such a cut-off vortex.

Though lake breezes can occur any time of year as long as conditions for its development are met, they are most frequently observed in the spring and summer months. This is due to large lake-land temperature differences that typically occur in the spring and the prevalence of synoptic conditions conducive to lake breeze development during the mid-summer months. Ryznar and Touma (1981), using data collected over a period of six years, found that the occurrence of lake breezes on the eastern shore of Lake Michigan reached a maximum in August though lake breezes were recorded as late as November. Biggs and Graves (1962) found a maximum occurrence on the western shore of Lake Erie in June and July over three years of spring and summer observations. Lastly, Lyons (1972) reported the highest frequency of lake breeze occurrence in the late spring and summer months on the eastern and western shores of Lake Michigan over 10 warm-season months. Occurrences earlier and later in the year were fairly common and were recorded even in January and February.

The land breeze forms after sunset when radiational surface cooling commences. Since land surfaces cool more rapidly than water surfaces, air over land becomes cooler than air over water and begins to contract. The constant pressure surfaces over land shift downward through the depth of the atmosphere in response and the initiation process described for the lake breeze occurs, but in reverse. Thus, an offshore flow is generated near the surface while an onshore flow is induced aloft. However, radiational cooling also stabilizes the nocturnal boundary layer so that the land breeze circulation is inhibited vertically. Maximum velocities within the circulation are significantly lower than that for lake breezes due to this restriction (Pielke and Segal, 1986). Air trajectories are rarely of sufficient length for the Coriolis force to become important (Munn and Richards, 1964). Thus, the land breeze usually blows perpendicular to the coast in the presence of flat terrain and a straight coastline. As with lake breezes, complex coastal topography and/or irregularly shaped coastlines may result in a significantly different offshore wind. Currently, the spatial extent of these breezes is less well known than that of lake breezes. This is due to the difficulties inherent in measuring these weak winds over land, the lack of measurements over water, and perhaps also due to lack of motivation since summer land breezes are regarded as relatively innocuous. The author knows of no detailed studies of the spatial characteristics of land breezes associated with lakes. Keen and Lyons (1978), however, included a diagram summarizing the structure of the land breeze according to 'the recent literature'. Additionally, at least two studies have documented the spatial characteristics of land breezes associated with an ocean or sea. Wexler (1946) briefly mentions a study that found the average outflow depth of the land breeze on the Black Sea to be 180 m. Land breezes on two successive days at Wallop's Island in the northeastern United States were investigated by Meyer (1971) using ultra-sensitive radar. It was found that the maximum depth of the land breeze outflow was near 90 m with a return flow extending to 800 m above the outflow layer. A typical land breeze, based on observations from the literature cited here, is illustrated in Figure 3.

Figure 3.  Idealized illustration of a typical land breeze circulation and its associated front based on the literature referred to in the thesis. Common features are labelled. The dashed line represents the outer boundary of the outflow layer. The frontal zone is not shown to scale.

As with lake breezes, land breezes can occur any time of year that suitable meteorological conditions exist. However, a detailed study of land breeze occurrence frequency has yet to be undertaken.

Lake and Land Breeze Fronts

After the initial development of a lake breeze circulation, the leading edge of the lake breeze may begin to show many similarities in temperature, humidity, pressure and wind changes to a scaled-down synoptic-scale cold front. This occurs primarily by the tightening of horizontal gradients at the lake breeze convergence zone. There can be a sharp decrease in temperature, increase in moisture content and change in wind velocity as the front passes. However, these changes may not always be observed due to inland modification of air mass properties such as temperature and humidity, or an onshore gradient wind that reduces gradients of these properties at the leading edge of the circulation. Lyons (1972) found that shifts in wind direction are the best markers for inland lake breeze tracking. The rate of inland advance is highly dependent upon the gradient wind. However, lake breeze winds behind the front are usually greater than the propagation speed of the front. Thus, upward vertical motion occurs at the frontal convergence zone. Lyons and Olsson (1973) observed that the Lake Michigan lake breeze front was 1-2 km wide and calculated maximum vertical velocities of over 1 m s-1 and peak convergence values greater than 200 x 10-5 s -1 using a two-dimensional form of the continuity equation. Simpson (1977) has pointed out various similarities between lake breeze fronts and laboratory gravity currents including the presence of a 'head' at the leading edge. These characteristics have also been observed with thunderstorm outflow boundaries (Simpson, 1977). A typical lake breeze front, based on observations from the literature cited here, is illustrated in Figure 2.

Since a land breeze front moves out over the lake, it is rarely observed. However, many characteristics similar to the lake breeze front can be assumed. Since the land breeze circulation is weaker than that of the lake breeze, the land breeze front will likely possess less intense gradients of temperature, moisture and wind. Meyer's radar study of the land breeze near Wallop's Island found that the land breeze front penetrated up to 25 km seaward (Meyer, 1971) with a propagation speed of about 1 m s-1. A typical land breeze front, based on observations from the literature cited here, is illustrated in Figure 3.

Synoptic Environments

In order for local thermal forcing to predominate over synoptic-scale dynamics, the synoptic-scale pressure gradients must be relatively weak in the coastal region and clear to partly clear skies must be present so that insolation may heat the land surface. By definition, the air over the land surface must be warmer than air over the lake. Sensitivity tests conducted by Arritt (1987) show that details of the water surface temperature have little effect on the simulated lake breeze as long as the water surface is cool enough to stably stratify the atmosphere over the lake. However, several studies have shown that a greater land-water temperature difference is required for the development of a lake breeze as the gradient wind increases (Watts, 1955; Biggs and Graves, 1962).

The gradient wind plays a very important role in the evolution of sea and lake breezes. This role has been investigated through numerical simulations conducted by Estoque (1962), Savijärvi and Alestalo (1988), Bechtold et al. (1991), Arritt (1993), and Comer and McKendry (1993), among others. In a calm or very light gradient wind, there is little to hinder the development of lake breezes which may exist along the entire perimeter of the lake. An offshore gradient wind can result in a strong lake breeze front due to increased convergence at the lake breeze boundary but can overpower a lake breeze if it is strong enough to overcome thermally-forced flow (typically 7-10 m s-1, Atkinson, 1981). Estoque (1962) found that a gradient wind parallel to the shore can either strengthen or weaken the lake breeze depending on the direction it blows along the shore. With a gradient wind that blows parallel to the shore with the lake to the left (in the northern hemisphere), offshore flow near the surface induced by friction differences between land and lake strengthens the pressure gradient and thus the lake breeze circulation. In the opposite case of a gradient wind blowing parallel to the shore with the lake on the right (in the northern hemisphere), frictional differences result in an onshore flow near the surface that weakens the lake breeze circulation. Similar behaviour is noted by Bechtold et al. (1991) though they show that changes in circulation intensity due to these frictional differences are small. Light onshore gradient winds reduce the gradients between air over the land and that over the lake and result in weaker lake breeze circulations. In this case, the lake breeze front may form some distance inland. However, an onshore flow of more than a few metres per second may suppress development of the lake breeze.

Land breezes have requirements for their formation similar to those for lake breezes: light winds associated with a weak pressure gradient, clear to partly clear skies, and a lake-land temperature gradient. To the author's knowledge, no detailed study of the effect of the gradient wind on the land breeze circulation has been conducted though Bechtold et al. (1991) included several land breeze hours in their numerical simulations of sea breezes. The effect of the gradient wind is likely to be opposite to that found with lake breezes. That is, for a given gradient wind speed, onshore winds should tend to increase the strength of the land breeze circulation while offshore winds result in a weak or absent circulation. Winds parallel to the shore would likely have less effect on circulation strength. This appears to be supported by Bechtold et al. (1991) though they show that the land breeze is suppressed in the northern hemisphere when the gradient wind is coast-parallel with land to the left due to frictional drag causing an onshore deviation of the coast-parallel wind. Generally, land breeze circulations are weaker than those of lake breezes and are thus expected to have lower thresholds of suppression by the gradient wind.

Effects on Coastal Climatology

Onshore lake breeze winds near the surface deliver cool, moist marine air to locations at and near the coast. In the spring and summer months when lake breezes are most active, these locations have lower mean daily maximum temperatures than locations farther inland (Brown et al., 1980). Moisture content in the air remains relatively high through the day behind the lake breeze front. Wind speed may increase or decrease behind the lake breeze front depending on the synoptic conditions. Also, stable stratification of air over water and subsidence that caps the lake breeze inflow layer serve to restrict convection over lakes and coastal regions resulting in decreased cloudiness and increased insolation. Conversely, enhanced upward vertical motion at the lake breeze front very often results in a band of cumulus clouds parallel to the shore and can even initiate deep moist convection (defined here as convective activity resulting in precipitation at the surface) including showers and thunderstorms. This process has been studied extensively over the Florida peninsula (Byers and Rodebush, 1948; Pielke, 1974; Blanchard and Lopez, 1985; Wilson and Megenhardt, 1997) and, to a far lesser extent, in the vicinity of Lake Michigan (Moroz and Hewson, 1966; Chandik and Lyons, 1971) (the relationship between lake breezes and thunderstorms is discussed in more detail in a later section). Coastal regions with high lake breeze frequencies will have a local climatology more strongly influenced by the above effects than those with lower frequencies.

The effects of land breezes on coastal climatology are less well known than those for lake breezes. Shallow offshore winds near the surface can be expected to deliver land-cooled air over the lake. However, over land, these winds enhance mechanical mixing. Thus, the surface air temperature should be warmer and the relative humidity lower at land locations within the land breeze circulation when compared to those stations farther inland under strong stable stratification. Indeed, mean daily minimum temperatures at coastal locations in southern Ontario have been found to be slightly higher than those at locations farther inland during the spring and summer months (Brown et al., 1980). The increased minimum temperatures during land breezes and the decreased maximum temperatures during lake breezes result in a reduced diurnal temperature range at coastal locations. This is important for some types of agriculture (Brown et al., 1980). The general effect on wind speed is not clearly known. The land breeze front is capable of producing cumulus clouds and even initiating deep moist convection (Neumann, 1951). As with lake breezes, coastal regions with higher land breeze frequencies will have a local climatology more greatly influenced by the above effects than those with lower frequencies.

3.  Roles in Summer Severe Weather

Lake breezes can affect the occurrence of summer severe weather in several ways. First, as was mentioned previously, stable stratification of air over water disables the convective boundary layer while the convective boundary layer is greatly limited by subsidence that caps the lake breeze inflow layer over land. Also, desiccation due to this subsidence increases the height of the lifting condensation level. Thus, the requirement that the lifting condensation level be less than or equal to the convective boundary layer depth for cumulus cloud development may not be met (Segal et al., 1997). Indeed, on many lake breeze days, the sky over the lake and areas inland behind the lake breeze front are completely cloud-free. This convective suppression results in reduced frequencies of thunderstorms over lakes such as the Great Lakes (see Kendall and Petrie, 1962). It is likely then that severe weather elements such as large hail, damaging winds and tornadoes are also less frequent over lakes and nearby shore areas. Indeed, Newark (1984) found that tornado occurrence appeared to be inhibited in land areas adjacent to the Great Lakes.

Second, lake breeze circulations can aid in the development and maintenance of severe convective storms. Three ingredients are necessary for the development of severe thunderstorms and associated severe weather elements including some or all of damaging winds, heavy rain, large hail and frequent lightning. The required ingredients are large quantities of low-level moisture, deep layers of conditional instability and significant upward vertical motion. The synoptic-scale environment can usually provide the first two ingredients. However, the third ingredient is not always available on the synoptic scale. Occasionally, the lake breeze is the only forcing mechanism available to provide the lift required to initiate development. In other cases, the lake breeze front may serve to enhance lift in a column caused by synoptic-scale dynamics. Enhanced upward vertical motion can also result from the collision of two lake breeze fronts (Simpson, 1994). Outflow boundaries emanating from beneath subsequent showers and thunderstorms can interact with lake breeze fronts or other outflow boundaries to initiate even more intense thunderstorms. Deep moist convection occurring along a lake breeze front can evolve into a quasi-stationary storm with intense lightning and high potential for flash flooding. Several studies (Clodman and Chisholm, 1994; Murphy, 1991) have investigated cases when very high to extreme rainfall accumulations occurred with a quasi-stationary thunderstorm on a lake breeze front. Clodman and Chisholm found that these storms typically have the following features: an area of about 200 km2, duration of about two hours, slow movement in any direction, and the frequent occurrence of two or more storms of varying intensity separated by clear areas. They suggest that the lake breeze is able to provide the required strong, moist, low-level inflow required for the development of these severe storms. Low-level, vertical wind shear produced by the lake breeze circulation might also play a role in sustaining a strong thunderstorm updraft.

Lastly, Wakimoto and Wilson (1989) have shown that vertical vorticity produced by shearing (Helmholtz) instability in the horizontal shear zone of a surface boundary such as a lake breeze front can contribute to tornadogenesis. It has been thought that a mesocyclone at the mid-levels of a thunderstorm is required to produce a tornado at the surface (Wakimoto and Wilson, 1989). However, a new class of tornadoes has recently been identified (Bluestein, 1985; Brady and Szoke, 1989; Wakimoto and Wilson, 1989) that is forced by mesoscale boundary-layer interactions and generally develop in benign environments with low to moderate instability and relatively weak shear. These weak, small and short-lived (about twenty minutes maximum) tornadoes are surface-based and occasionally extend to the base of the parent cloud. The tornado is the result of intensification of a low-level vertical vortex at a mesoscale boundary such as a lake breeze front. When the pre-existing vortex becomes co-located with the updraft of a parent cloud experiencing rapid development, it undergoes vortex tube stretching, intensifying the vorticity to tornadic levels. This type of tornado shows waterspout-type characteristics and thus has been called a 'landspout'. Recent high-resolution numerical modelling work done by Lee and Wilhelmson (1997) suggests that there is less chance involved in the co-location of the updraft and the low-level vortex (which they call a 'misocyclone' after Fujita, 1981) than previously thought. Their simulations show that misocyclones forming along surface boundaries due to shearing instability can act to initiate updrafts and deep convection on their own. Tornadoes of this type may account for a significant percentage of those ranking F0 and F1 on the Fujita scale of tornado intensity (Fujita, 1981) and have been also observed to reach F2 and even F3 intensities. Tornadoes with surface-based or 'landspout' characteristics have been recorded in Ontario on several occasions:

An F0 tornado touched down in Albuna, Ontario southeast of Windsor in the late afternoon of May 31, 1991 (Murphy, 1991). The tornado occurred when the parent thunderstorm was in the developing stage with tops at only 6.7 km and had a track of only a few kilometres. The parent thunderstorm was initiated at the intersection of a weak surface front and a Lake Erie lake breeze front.
On August 14, 1984 in western North York, an  F1 tornado was observed to move slowly from northeast to southwest along a track 2.8 km long (Bertolone, 1984). Though the author does not mention lake breezes as a possible factor, time series of wind, temperature and dew point temperature data included with the report strongly suggest that the tornado occurred near the Lake Ontario lake breeze front.

On August 28, 1992, an F1 tornado in Newcastle, Ontario near the north shore of Lake Ontario was observed on King City Doppler radar (Joe et al., 1995). The tornado lasted less than ten minutes and could only be seen by using the radar's lowest elevation angle (0.5 degrees) suggesting a surface-based phenomenon.

In Etobicoke, Ontario, just northeast of Pearson International Airport, a short-lived F0 tornado was spawned from a low-topped, weak reflectivity thunderstorm in the late afternoon of September 17, 1988 (Hogue et al., 1989). The tornado occurred as the parent cumulonimbus was experiencing rapid growth. Low-level convergence at a weak surface trough was thought to be the principal contributing factor supporting the event.

In the first two cases, the tornadic storms apparently occurred in association with a lake breeze convergence zone. Lake breezes are not explicitly implicated with the latter two cases, though in both cases tornadoes occurred within 15 km of the Lake Ontario shoreline. Many more tornadoes of weak to moderate strength that have occurred in southern Ontario may well have been initiated on boundaries such as a lake breeze front.

There is recent evidence that even the initiation process for violent (F4-F5) tornadoes may not be directly related to mesocyclones at mid-levels in the parent thunderstorm (Fujita and Wakimoto, 1982; Wakimoto and Wilson, 1989). An emerging concept is that all tornadogenesis occurs in the planetary boundary layer by the processes described above but a violent tornado is possible if the parent thunderstorm possesses a mesocyclone. The link between the tornado at the surface and the mesocyclone aloft has yet to be understood. If this is the case, then one might be tempted to speculate that a large number of tornadoes may be influenced by lake breeze activity in regions such as the Great Lakes region and other regions with large lakes such as southern Manitoba.

4.  Air Pollution Effects

Many studies have demonstrated the exacerbation of air pollution problems in coastal areas. Some of these effects are the result of the onshore flow of relatively cool marine air in either the absence or presence of a lake breeze. Other effects are related specifically to mesoscale circulations such as lake and land breezes. One of the generally occurring effects of the onshore flow of marine air is fumigation of pollutants downwind of the shoreline. Effluent from a smokestack at the shore blown inland by onshore winds may be confined to a plume in the stably stratified marine air. However, as this plume intersects the convective boundary layer inland, pollutants can be mixed down to the surface resulting in fumigation (Lyons and Cole, 1973).

Another commonly occurring effect in coastal areas is plume trapping. Stably stratified marine air moving onshore can have a mean mixing depth that is 10% of that existing away from the influence of the lake (Lyons and Cole, 1973). Thus, effluent that is ejected into this layer is effectively trapped and high concentrations of pollutants can subsequently reach the surface.

Fumigation and plume trapping commonly occur in association with lake breezes. However, lake and land breezes can introduce unique problems. The first is the ability of lake and land breezes to transport pollutants in three dimensions. Lake and land breezes are quasi-closed circulations and pollutants emitted into them can be recirculated several times over the near-shore area (Lyons, 1972). That is, pollutants emitted into the inflow layer get lofted in the frontal regions and disperse into the return flow aloft. A fraction of these pollutants are forced into the inflow layer again by the descending branch of the circulation. Remaining pollutants reside in an elevated layer aloft. Lyons and Olsson (1973) observed a helical trajectory within a lake breeze circulation and suggested that the motion of pollutants might include an along-coast component in addition to the cross-coast components. Lyons et al. (1995) have successfully simulated this three-dimensional behaviour using a numerical model. Also, during periods of stagnant synoptic conditions, lake and land breezes can occur nearly continuously, effectively confining pollutants to coastal regions and causing the accumulation of pollutants over periods of several days (Simpson, 1994; Lu and Turco, 1995). Despite apparently adequate ventilation with onshore winds, rapidly deteriorating air quality can result.

Another effect on air pollution peculiar to lake breezes involves the enhanced production of ground-level ozone or 'photochemical smog'. The ingredients for enhanced ground-level ozone concentrations include: a plentiful supply of reactive hydrocarbons (RHC) and nitrogen oxides (NOX), strong insolation, relatively high air temperatures, light wind speeds and limited mixing depths (Lyons and Cole, 1976). Three of these ingredients - strong insolation, high temperatures and light winds - are also conditions conducive to the development of lake breezes. When a lake breeze occurs, enhanced insolation is common over the lake and at inland locations affected by the circulation and can result in increased ozone production there.

Thus, the occurrence of high concentrations of pollutants, especially ground-level ozone, in coastal regions is highly correlated with the occurrence of lake and sea breezes and, to a lesser extent, land breezes. Examples of major coastal cities suffering from the above air quality problems include Los Angeles, Athens, Chicago, Tokyo and, in Canada, Vancouver and Toronto.


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