Excitement at the front: one island, two sea breezes

Science on a beach. Photo: KramG.

Science on a beach. Photo: KramG.

The dark days of winter may have you longing for warmer and sunnier times. For the sake of science, let us get away from this misery. Imagine a nice summer day and place yourself on a beach in Florida.

The sun is out warming our bodies, minds and the land and sea surrounding us. The beach heats faster than the sea due to different heat capacities of land and ocean. Air temperatures follow the surface temperatures, the warmer, less dense air over land rises and a local low develops. A horizontal pressure gradient is formed and the resulting ‘sea breeze‘ circulation brings colder air from the ocean to the land, preventing us from overheating on the beach.

At the sea breeze front, relatively cold sea air meets warmer air from the land. At this convergence line rising air may form cumulus clouds. If the rising motion is strong enough even cumulonimbus clouds and thunderstorms might develop. In the satellite picture the sea breeze front can be tracked inland by the clouds forming around the lakes Erie and Ontario.

Lake breeze around Lake Erie and Lake Ontario, 24 May 2012 from NASA’s AQUA satellite. Note the convergence cloud line between the two lakes. Source: http://blogs.agu.org/wildwildscience/2012/05/24/lake-breeze-around-lake-erie-and-lake-ontario/.

Lake breeze around Lake Erie and Lake Ontario, 24 May 2012 from NASA’s AQUA satellite. Note the convergence cloud line between the two lakes. Source: http://blogs.agu.org/wildwildscience/2012/05/24/lake-breeze-around-lake-erie-and-lake-ontario/.

We are in Florida for a reason. Summer there is generally better than in the UK, with more frequent hot and sunny days. Also, closer to the equator sea breezes have the potential to travel further inland, up to 400 km in Australia [1]! The large horizontal extent implies that for narrow peninsulas or islands converging sea breezes from opposing shorelines can meet. In this case, the horizontal convergence is much stronger than for a single sea breeze and can directly initiate deep convection [2].

In southern Florida converging sea breezes are one of the dominating controls of the location of thunderstorm complexes [3]. The ‘Morning Glory‘, a north Australian cloud line, is linked to two meeting sea breezes and their relative asymmetry [4]. Also ‘Hector the Convector‘, an almost daily recurring thunderstorm over the Tiwi islands, is the result of radial converging sea breezes [5].

In the examples above, field campaigns, satellite imagery and numerical studies have given insight in the sea breeze convergence. I investigated the convergence in the laboratory during the Geophysical Fluid Dynamics program at the Woods Hole Oceanographic Institute this summer. Sea breezes are an example of gravity currents, horizontal circulations driven by density differences. By means of ‘lock-exchange’ experiments [6] we created two colliding gravity currents in a tank. Lab experiments are useful because they can be specifically designed to test, for example, the influence of density differences separate from other conditions like frontal heights, background winds, surface roughness, etc.

From our chair on the beach we can feel the sea breeze front and see the exciting weather develop further inland. However, the actual collision will not be visible, the thunderstorms are merely a tracer of the collision. The experiments visualise this collision rather than the weather it triggers. The development of the collision front in time and space proves to be equally interesting. The shape, height and horizontal propagation speed of this front changes for different experimental setups.

So, if winter or upcoming deadlines get to you, book a flight to a tropical island or peninsula. Spend the morning hours relaxing on the beach and study the onset of the sea breeze. In the afternoon travel inland and find the collision front. Exciting weather in combination with nice temperatures might be just what you need.

Excitement at the front.
Colliding gravity currents at WHOI’s GFD laboratory.

[1] R. H. Clarke (1983): Fair weather nocturnal inland wind surges and atmospheric bores: Part I Nocturnal wind surgesAustralian Meteorological Magazine, 31, pp. 133–45.
[2] R. E. Carbone, J. W. Wilson, T. D. Keenan, and J. M. Hacker (2000): Tropical island convection in the absence of significant topography. Part I: Life cycle of diurnally forced convection. Monthly Weather Review, 128, pp. 3459–3480.
[3] H. R. Byers and H. R. Rodebush (1948): Causes of thunderstorms of the Florida peninsula. Journal of Meteorology, 5, pp. 275–280.
[4] M. J. Reeder, R. K. Smith, J. R. Taylor, D. J. Low, S. J. Arnup, L. Muir and G. Thomsen (2013): Diurnally forced convergence lines in the Australian Tropics. Q.J.R. Meteorol. Soc., 139, pp. 1283–1297.
[5] N. A. Crook (2001): Understanding Hector: The dynamics of island thunderstorms. Monthly Weather Review, 129, pp. 1550–1563.
[6] J. O. Shin, S. B. Dalziel, and P. F. Linden (2004): Gravity currents produced by lock exchange. Journal of Fluid Mechanics, 521, pp. 1–34.


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Karin van der Wiel

I recently (June 2015) completed my PhD in meteorology at the University of East Anglia (UK). Since then I have moved to the USA to work at NOAA's Geophysical Fluid Dynamics Laboratory.
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