Numerical Simulation of Sea Breeze Convergence over Antarctic Peninsula
Offshore extents in the order of km were simulated by both Arritt () The way in which either the background wind or the land–sea. Sea breezes play an important role in inland transport of moisture especially of the shallow sea breeze despite the overestimate of temperature over land in both simulations. basin at 1°/16° resolution, dating back to , and accu-. Sea breezes play an important role in inland transport of moisture The coupled simulation provides a more realistic representation of During daytime ( nighttime), land temperature exceeds (is lower than) the sea surface temperature. .. dating back to , and accurately represents the Mediterranean.
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 km depending largely on latitude while typical penetration distances for the Great Lakes region have been found to be near 30 km Atkinson, 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. Physick 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 and Comer and McKendryamong others and probably represents the most significant difference between the dynamics of lake and sea breezes. Moroz and Hewson 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, Simpson 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 Toumausing 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 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 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, Air trajectories are rarely of sufficient length for the Coriolis force to become important Munn and Richards, Thus, the land breeze usually blows perpendicular to the coast in the presence of flat terrain and a straight coastline.
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 Lyonshowever, included a diagram summarizing the structure of the land breeze according to 'the recent literature'.
A Lake and Land Breeze Primer
Additionally, at least two studies have documented the spatial characteristics of land breezes associated with an ocean or sea. Wexler briefly mentions a study that found the average outflow depth of the land breeze on the Black Sea to be m.
Land breezes on two successive days at Wallop's Island in the northeastern United States were investigated by Meyer 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 m above the outflow layer. A typical land breeze, based on observations from the literature cited here, is illustrated in Figure 3.
Idealized illustration of a typical land breeze circulation and its associated front based on the literature referred to in the thesis. The dashed line represents the outer boundary of the outflow layer. 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 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 observed that the Lake Michigan lake breeze front was km wide and calculated maximum vertical velocities of over 1 m s-1 and peak convergence values greater than x s -1 using a two-dimensional form of the continuity equation.
Simpson 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, 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, with a propagation speed of about 1 m s 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 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, ; Biggs and Graves, The gradient wind plays a very important role in the evolution of sea and lake breezes.
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 m s-1, Atkinson, Estoque 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 hemisphereoffshore 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 hemispherefrictional differences result in an onshore flow near the surface that weakens the lake breeze circulation.
Similar behaviour is noted by Bechtold et al. 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: 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.
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. 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.Land and Sea Breeze
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. 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, ; Pielke, ; Blanchard and Lopez, ; Wilson and Megenhardt, and, to a far lesser extent, in the vicinity of Lake Michigan Moroz and Hewson, ; Chandik and Lyons, 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. 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. 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, 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. 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. 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, 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 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, 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, ; Murphy, 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: 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 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, However, a new class of tornadoes has recently been identified Bluestein, ; Brady and Szoke, ; Wakimoto and Wilson, that is forced by mesoscale boundary-layer interactions and generally develop in benign environments with low to moderate instability and relatively weak shear.
Source As their names suggest, land breeze comes from land while sea breeze comes from water. Depth Since the cooling of the air over land typically occurs within a shallower layer at night, the land breeze is likewise shallower as compared to the sea breeze.
Season Sea breezes are more often experienced during spring and summer because of the significant temperature differences between land and water. On the other hand, land breezes are most common in autumn and winter due to the cooler nights. Strength Generally, sea breezes are stronger than land breezes due to the bigger temperature differences.
Moisture Sea breezes have more moisture due to the absorbed particles from the bodies of water. On the other hand, land breezes are often dry winds. Occurrence during Winter Months Unlike land breezes, sea breezes are not often observed during winter season.
During such cold months, land breezes are predominant in influencing weather; particularly when a strong change of wind direction occurs at night. Benefit to glider pilots Glider pilots particularly take significant advantage of the sea breeze to steer in higher altitudes. Unlike land breezes, sea breezes are strong enough to influence such aircraft operations.
Decrease in Air Temperature A decrease in air temperature is most likely due to sea breezes whereas land breezes do not essentially cause temperature changes.
Land Breeze vs Sea Breeze: Land and Sea breezes affect precipitation rates, humidity levels, and atmospheric temperature. Land breezes come from land while sea breezes come from the ocean or other large bodies of water. The key difference is due to the property of water to retain heat and warm up longer as compared to land. Land breezes are also known as off-shore winds while sea breezes are also called on-shore winds.
While a land breeze is formed at night, a sea breeze is formed at daytime. Land breezes are more often experienced during autumn and winter while sea breezes often occur during spring and summer months. Land breezes are generally weaker than sea breezes. This makes sea breezes quite significant to glider pilots and other related aircraft operators.
Lake and Land Breeze Primer
Typically, land breezes are slower than sea breezes. Land breezes are generally dry winds while sea breezes have more moisture.
Unlike sea breezes, land breezes are predominant during winter months. Unlike land breezes, sea breezes can significantly lower air temperature. If you like this article or our site.