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What is a Seabreeze?
Do Seabreezes Cause Any Problems Along the Coast?
How are Seabreezes Detected and Analyzed?
The Role of Coastal Upwelling in Seabreeze Frontogenesis Along the NJ Shore
A seabreeze is a common phenomenon during the spring and summer months along the coastline of New Jersey . It is the sudden reversal of winds from an offshore wind to an onshore wind. It is formed due through a complex ocean-atmosphere interaction wherein the differential heating of the land and sea results in pressure and density gradients which permits frontogenesis to occur right along the shoreline. Meteorologically speaking...it is a mesoscale version of a cold front which forms at sea and slowly propagates inland.
During the spring and early summer months the temperature of the ocean off the coast of New Jeresy is still rather cold compared to the land. Due to the high heat capacity of the ocean, it will take much longer for it to warm in relation to the land. On warm spring and summer days, the land is heated by incoming solar radiation, which in turn heats up parcels of air by convection. These parcels of air will rise until it reaches a stable layer of the atmosphere a few thousand feet in altitude. Since the rising air cannot continue rising and it cannot sink, then to maintain mass continuity it must spread outward, or diverge. When the warm divergent air moves over the cooler air residing over the ocean it sinks back down to the surface of the ocean.
The rising motion of the air over the land results in the formation of a weak thermal low pressure area. in contrast, the sinking motion of the air over the water results in the formation of a weak high pressure area. This difference in pressure is know as a pressure gradient. Since the pressure will try to stabilize itself, a fetch of wind will blow from the high to the low, resulting in a onshore wind, or a seabreeze.
The leading edge of the seabreeze is known as the seabreeze front. It is in actuality a mesoscale, or small-scale, version of a cold front. As such, there are several similarities between a seabreeze front and a synoptic-scale cold front. Most notably it is colder on the back side of the front. As the seabreeze propagates inland, the winds behing the front are from off of the cool ocean, so therefore the air being advecting inland is as cool as the air just above the ocean surface. Tempertaures may decrease by more than 5F in only a few minutes depending on the difference between the tempertaures of the land and the ocean. Historical records show that some seabreeze events have dropped temperatures as much as 25F degrees.
Another change during a seabreeze frontal passage is a change in the relative humidity. As the front passes, the relative humidity will first drop about ten percent, and then will rapidly rise by approximately fifteen percent.
Also, there is a significant change in wind direction. Normally the wind comes from a westerly or southwesterly direction during the spring and summer months. A seabreeze frontal passage will quickly shift the wind direction to a northeasterly, easterly, or southeasterly direction. In most cases the wind direction will be nearly perpependicular to the seabreeze front as it moves inland.
Besides ruining your day at the beach, most people do not believe that seabreezes can be in any way harmful. This is not necessarily true. Seabreezes have been known to create problems for low-flying aircraft. The seabreeze cell itself can create a dangerous phenomenon known as wind shear in the lowest 3,000 feet of the atmosphere along coastal sections. At the surface seabreezes generally produce an easterly wind flow, however at the top of the seabreeze circulation cell, the wind flow is directly opposing the surface flow. This sharp difference in wind direction with height is known as wind shear. Wind shear has been blamed on many airline crashes over the years.
A seabreeze may also create hazardous conditions for mariners. Since seabreeze develop fairly rapidly and without warning, the winds near the shoreline can increase rather dramatically.
Another potential hazard resulting from seabreeze development is thunderstorm formation. When there is a seabreeze interacting with another source of atmospheric lift, say from the outflow boundary of another thunderstorm, new thunderstorms may develop along the seabreeze front.
It is also possible that seabreezes may help to enhance low level moisture which could lead to excessive humidity on hot days, leading to dangerous heat indices.
There are several methods in use to detect and analyze seabreezes. The first is to monitor weather observations from stations along the coast. For the case studies presented here, several weather stations along the coast were used. These stations in New Jersey include (for current weather conditions click on station name)...Tuckerton, Long Beach Island, Avalon, Atlantic City, Atlantic City International Airport (Pomona), and Wildwood.
Doppler Radar is the best way in which to document and analyze seabreezes. The National Weather Service has deployed a series of WSR-88 Doppler Radars throughout the United States. In our area of interest, the Doppler Radar images that will be looked at come from the radars deployed at Fort Dix, NJ (KDIX) and Dover, DE (KDOV). Doppler radar is famous for its ability to detect wind speed by utilizing the doppler shift.
Doppler Radar also has the ablity to shift between two operating modes: Precipitation Mode and Clear-Air Mode. As its name states Precipitation mode is most useful in detecting the intensity of precipitation echoes. Clear-Air Mode is useful in seabreeze detection. The radar is at its most sensitive in Clear-Air Mode. It is able to detect temperature discontinuities as well as the scattering of the radar signal off birds and insects. The area of the greatest temperature discontinuity in a seabreeze event is right along the front. Also right along the frontal boundary, the onshore wind can carry birds and insects inland where they can be detected by the backscattering given off as the radar pulse is reflected off their bodies and back to the receiver.
An example of a seabreeze front, a baybreeze from the Delaware Bay, and the temperature discontinuity from the Atlantic City Expressway can be seen in the image below.
A better representation of the seabreeze front can be seen in the radar base reflectivity loop from KDIX below. The seabreeze is represented by the higher reflectivities (red line) along the NJ coast that moves inland.
For more information about using Clear-Air Mode to analyze seabreezes CLICK HERE.
CODAR (Coastal Ocean Dynamics Applications Radar) is a system designed by oceanographers to measure surface ocean currents offshore. Such a system is deployed along the NJ coast. Site 1 is located in Brant Beach on Long Beach Island. Site 2 is located in Brigantine.
A sample image of CODAR surface velocities in the LEO-15 research area, overlayed on a SST image is shown below.
Since the surface currents are produced by wind stress, a change in wind direction over the coastal waters should be visible using CODAR. Therefore, as a seabreeze develops and the winds over the ocean change directions, the current direction displayed by CODAR should also change accordingly.
CODAR data can be invaluable in analyzing the beginning stages of a forming seabreeze before it has been detected on land. It is also useful in showing how the changing currents, as a proxy for changing wind stresses, affect sea surface temperature changes along the coast.
As the seabreeze develops, the ocean currents become light and variable, and then quickly shift to an onshore direction. As the winds behind the seabreeze front change the ocean currents should change with them, but with a lag time of up to 1 or 2 hours. As the seabreeze circulation breaks apart or reverses, the currents should switch back to an offshore direction.
For more information about how CODAR works CLICK HERE.
To go to the RU C.O.O.L. CODAR webpage CLICK HERE.
Since seabreeze frontogenesis depends a great deal on the difference between the land temperature and the ocean temperature, then coastal upwelling may play a part in the initiation of seabreeze events. Upwelling is when the upper layer of ocean water is transported offshore right along the beaches, so that in order to maintain mass continuity water must come up from below the surface to replace the displaced water. Since this water comes from below the surface and has not been directly heated by the incoming radiation from the sun it is much colder than the displaced surface water.
Coastal upwelling is explained through the process of Ekman pumping. When a wind blows across the ocean's surface, the ocean current moves perpendicular to the wind stress. Along the coast of New Jersey coastal upwelling will occur with a southerly or southwesterly wind which will result in an Ekman transport of the ocean's surface waters to the east or southeast.
Coastal upwelling is a common occurrance along the coast of New Jersey during the spring and summer months due in part to the Bermuda High. The Bermuda High is a subtropical high that is a permanent feature in the North Atlantic that is usually located close to the island of Bermuda. Due to the anti-cyclonic flow around high pressure systems, the Bermuda High normally produces a warm and humid southwesterly wind along the East Coast of the United States.