an investigation into the effect of the synoptic weather on sea breezes at whitsand bay, cornwall

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and October 2009. An Oregon Scientific WMR200 weather station was used with Weather Display software to analyse the wind speed, direction, temperature and humidity. The anemometer was mounted at a height of 4 metres from the ground on the top of 120-metre-high cliffs. The pres-sure, temperature and humidity readings were all adjusted to the sea-level equivalent. Observations were also gathered from the

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Correspondence to: J.F.P. GalvinMet Office, FitzRoy Road,Exeter, EX1 3PB, UK

jim.galvin@metoffice.gov.uk

© British Crown Copyright, 2011, published with the permission of the Controller of HMSO and the Queen΄s Printer for Scotland

DOI: 10.1002/wea.668

An investigation into the effect of the synoptic weather on sea breezes at Whitsand Bay, CornwallMichael ReedUniversity of Plymouth

A sea breeze is a phenomenon that occurs due to a horizontal land/sea pressure gradi-ent caused by the daily heating of the Earth’s surface. Land warms faster than the adjacent sea and this causes a surface pres-sure difference as the day progresses. Cool, moist, air flows from the sea driven by the pressure gradient and causes a shoreward wind. The increase in the onshore wind speed is well defined when the large-scale synoptic wind is blowing away from the land.

The aim of this project was to quantify what synoptic weather conditions were needed for a sea breeze to develop in Whitsand Bay, Cornwall, and to develop a forecast method using a regression analysis of the factors influencing the sea breeze at this location. Whitsand Bay is a horseshoe-shaped bay located in southeast Cornwall, near Plymouth, illustrated in Figure 1. It is approximately 14 kilometres long from Looe in the west to Rame Head in the east. A large tidal range of up to 6 metres during spring tides exposes long sandy beaches that are backed by 100-metre-high cliffs for almost the whole length of the bay. This

Figure 1. Map of southeast Cornwall and south Devon. The location of the weather station at Whitsand Bay and the University of Plymouth meteorological station are denoted by the red crosses.

area is of specific interest to hang glider and paraglider pilots who use sea breezes to soar along the coast.

The investigation into sea breezes at Whitsand Bay focused on the breeze that developed in the eastern side of the bay. A weather station was set up on the View Restaurant (OS Reference: SX 412 511) espe-cially for this project and a total of 20 sea-breeze days was recorded between March

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breeze remains a shallow, localised, effect (Koschmieder, 1936, cited in Simpson, 1994). The longer the sea breeze has to develop, the deeper the sea breeze can become which contributes to the distance and speed the front can travel inland. The air at the sea-breeze front is convected and flows offshore at a higher altitude to complete the sea-breeze circulation cell illustrated in Figure 4. Clouds form at the location of the sea-breeze front as the environmental air is forced over the front and reaches its dew point as it rises and cools.

Satellite images and photographs were taken to illustrate the sea-breeze front that develops over Dartmoor when the sea

ity (ms–1) and ΔT is the land/sea temperature difference (degC).

The model is limited in accuracy due to observations only being conducted over one year.

Inland frontal developmentA sea breeze not only affects the local winds but also contributes to inland cloud devel-opment which can sometimes lead to sum-mer rain and is often associated with the sea-breeze front. A sea-breeze front is the area in which the sea breeze onshore flow and the synoptic wind offshore flow meet which during the development of a sea

University of Plymouth meteorological sta-tion to identify the sea-breeze front pene-trating further inland.

Only sea breezes that reversed an off-shore synoptic surface wind were analysed during this project as sea breezes that develop when there is a light synoptic wind already blowing onshore are hard to detect.

Primary characteristics

The development of a sea breeze at Whitsand Bay was found to be primarily dependent on two variables: the strength of the synoptic wind at the surface, and the land/sea surface temperature gradient. A linear relationship was identified between the occurrence of a sea breeze and these two components: a larger land/sea tem-perature difference was needed to over-power a larger synoptic wind speed at the surface. This result is presented graphically in Figure 2. The linear correlation confirms previous research which identified a rela-tionship between the temperature differ-ence and synoptic wind speed and includes work by Brittain (1978), Simpson (1966) and Watts (1955) to name only a few.

The strength of the sea breeze was found to be influenced by the land/sea tempera-ture difference and the synoptic wind speed at the surface. This relationship was found to be proportional, such that the larger of either of these two components, the stronger the associated sea breeze. The observations suggest that the correlation between the strength of the sea breeze and the strength of the synoptic wind may be due to the need for the wind compo-nents to balance. As a result, the land/sea temperature difference needs to reach a level at which the developing sea breeze can compete with the synoptic wind speed. A statistically significant relation-ship was found such that the sea-breeze velocity could be forecast using the land/sea temperature difference and the synop-tic wind speed at the surface as predictors. The observations plotted in Figure 3 were used to develop the forecast regression model. The model output is represented as the overlaid mesh and has an accuracy of +/–0.8ms–1 with respect to the observed data. The maximum values of the predic-tors (20 degC and 6ms–1) are the largest observed values for the sea breeze at Whitsand Bay during 2009.

The model uses the following equation derived using multi-linear regression statistics:

V = 0.3195 + (0.8807 × V1) + (0.3731×ΔT)

+ (–0.0616 × V1 × ΔT)

where V is the expected sea-breeze velocity (ms–1), V1 is the synoptic surface wind veloc-

Figure 3. Plot showing the relationship between temperature gradient, synoptic wind speed at the surface and the strength of the sea breeze. The black crosses represent the observed sea breezes and the mesh represents the modelled output. The model has an accuracy of +/–0.8ms–1 with respect to the observed data.

Figure 2. Plot of land/sea temperature difference and the corresponding synoptic wind speed at the sea breeze onset for each sea breeze day recorded at Whitsand Bay.

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Bay to the University of Plymouth meteorological station with an average velocity ~7ms–1.

A good example of the sea-breeze front developing over Dartmoor was on 20 April

breeze becomes deep enough to penetrate further inland away from the coast. During the six-month recording period, the sea-breeze front was observed to take approxi-mately 50 minutes to travel from Whitsand

Figure 5. Visible satellite image from NOAA 18 taken at 1200  UTC on 20 April 2009, showing frontal cloud developing across the spine of Cornwall and Devon formed by the convergence of sea breezes from both coasts. (University of Plymouth Meteorological Department.)

Figure 6. Photograph taken at 1430  UTC on 20 April 2009, looking northwest from Whitsand Bay. The sea breeze front is clearly identified by the line of cumulus cloud caused by convection over the Moors.

2009 (Figures 5 and 6). The day began with a light northeasterly wind; there was a clear sky and the land warmed efficiently, as identified by the rapid increase in the tem-perature and decrease in the humidity (Figure 7). The sea breeze at Whitsand Bay started showing signs of development at  0930 UTC and was fully established by 1010 UTC. A maximum wind speed of 5ms–1 was recorded during the sea breeze and it had a total duration of approximately 7  hours. The humidity was observed to be 65% at the sea-breeze onset and large cumulus clouds formed at the front as the day progressed.

Tidal influences on the sea breezeThe research at Whitsand Bay showed some dependence of the sea breeze on the state of the tide. It was found that 60% of the sea breezes developed when the time of high tide was between 1200 and 1600 UTC, which is also the time of maximum solar heating. Although this result does not show a strong dependency, it does follow a trend that was clearly identified at Hayling Island in south-ern England (Simpson et al., 1977). This 12-year study found a strong connection between the penetration of a sea-breeze front inland and the phase of the tidal cycle and this can be compared with the results of this project in Figure 8.

A 10-year study of sea breezes and their interaction with the tide on the North Sea coast of Germany discovered that a flooded tidal flat during the period of maximum solar heating acted as an extension of the sea and an unflooded tidal flat caused a weakening of the sea breeze. The weaken-ing of the sea breeze when a low tide is present is thought to be caused by the pres-ence of exposed moist sand that is adjacent to relatively dry soil on the land. The moist sand takes longer to warm than the land so lowers the sharp temperature gradient that occurs when the tide is high during the period of maximum solar heating. This area of moist sand can weaken the sea-breeze flow before it reaches the land (Kessler et al., 1985).

SummaryThe development of a sea breeze at Whit-sand Bay has been found to be highly influ-enced by the synoptic weather. An offshore synoptic surface wind with a velocity in excess of 6ms–1 was found to inhibit the sea-breeze development at Whitsand Bay. It is thought that a synoptic wind greater than 6ms–1 is large enough to create substantial mixing between the warm air over the land and the cooler air over the sea, thus diffus-ing any temperature difference that may have developed whilst the wind speed was

Figure 4. A schematic diagram of a sea breeze circulation cell: the sea breeze front is identified by the area of convection. The synoptic wind flows over the sea breeze cell and is identified by the long thin arrows (Simpson, 1994, Courtesy of Cambridge University Press).

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Figure 8. Frequency table of sea breezes with the time of high water at Hayling Island in southern England (left) and Whitsand Bay (right). A significant relationship was identified between sea breeze development and the state of the tide at Hayling Island. Although the results at Whitsand Bay do not have a strong statistical relationship they do show a trend towards the results gathered at Hayling Island (Simpson, 1994, Courtesy of Cambridge University Press).

lower, but no observational evidence was gathered to confirm this. It is also possible that the sea-breeze front did not have suf-ficient time in which to become deep enough to overpower the synoptic wind on days when this was greater than 6ms–1.

On days of light winds, a temperature dif-ference of only 2.5 degC was needed for a sea breeze to develop. Furthermore, the sea-breeze wind speed would be small. The mean speed of the sea-breeze wind throughout 2009 was observed to be 4.4ms–1 and had a mean direction of 220o, which is normal to the coastline of Whitsand Bay where the weather station is located.

The results at Whitsand Bay share similar characteristics for the development of a sea breeze with the one that develops in Plymouth Sound identified during previous research (Hope-Hislop, 1974). The research in Plymouth Sound found that a sea breeze could develop with an offshore synoptic wind of up to 8ms–1. A minimum land/sea temperature difference of 2 degC was needed on light-wind days. The mean sea-breeze speed in Plymouth Sound was found to be 3.5ms–1 with a mean direction of 190o.

In conclusion the sea breeze at Whitsand Bay is most likely to develop when the synoptic wind speed is less than 6ms–1

with a minimum land/sea temperature dif-ference of 2 degC on lighter-wind days. The results suggest that the speed of the sea breeze behind the sea-breeze front will be proportional to the speed of the synoptic wind.

AcknowledgementsThe work described in this article was com-pleted by the author for the final year project of a BSc (Hons) Ocean Science degree at the University of Plymouth. The author would like to thank Dr Len Wood for helping to develop his initial ideas for this project and Dr Phil Hosegood and Dr Tim O’Hare for advising him throughout the research and helping to develop his forecast model. He would also like to thank Jim Galvin at the Met Office who helped during the construction of the forecast model and proof-read the final report, as well as Michael Connatty, Michael McCulloch, Bill Brewer, and Helen Nance for the support they have given throughout his studies. Finally, an anonymous reviewer contributed valuable comments.

ReferencesBrittain OW. 1978. Forecasting sea-breezes at Eskmeals Meteorol. Mag. 107: 88–95.Hope-Hislop J. 1974. MPhil Thesis: The Plymouth Sea Breeze. The University of Plymouth: Plymouth, UK.Kessler RC, Eppel D, Pielke RA, McQueen J. 1985. A numerical study of the effects of a large sandbar upon sea breeze development. Archive for Meteorological, Geophysical and Bioclimatology 34: 3–26.Koschmieder H. 1936. Danziger Seewindstudien, I. Dan. Meteorological Forsch. 8: 45.Simpson JE. 1966. The sea breeze at Lasham. On sea breeze forecasting techniques. Memo #12. Forecasting Techniques Branch. Met. Office, Bracknell.Simpson JE, Mansfield DA, Milford JR. 1977. Inland penetration of sea-breeze fronts. Q.J.R. Meteorol. Soc. 103: 47–76.Simpson JE. 1994. Sea Breezes and Local Winds. Cambridge University Press: Cambridge.Watts AJ. 1955. Sea breeze on Thorney Island. Meteorol. Mag. 84: 42–48.

Figure 7. Plot of weather data recorded on 20 April 2009 at Whitsand Bay. During the hours before the sea breeze onset the temperature increases whilst the humidity decreases. These changes occur as a result of morning heating and occur prior to the sea breeze development. The onset is clearly identified by the sudden decrease in temperature, increase in humidity, changing of wind direction from 045° to 190o and an increase in the wind speed. This all occurred at approximately 1000 UTC.

Correspondence to: Michael Reed,Lowertown House,St Germans,Saltash, PL12 5NS, UK

mj.reed@tiscali.co.uk

© Royal Meteorological Society, 2011

DOI: 10.1002/wea.623