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Sea breeze mechanism and observations of its effects
in the Stellenbosch wine producing area
Valérie Bonnardot(1), Victoria Carey(2), Olivier Planchon(3)
and Sylvie Cautenet(4)
In order to remain competitive in an ever-expanding international wine market, the South African wine industry considers viticultural terroir identification to be a high priority. Identification and spatial characterisation of terroir units are important as a scientific basis for the delimitation of areas for the production of characteristic wines of high quality (Carey and Bonnardot, 2000; Carey, 2001). Terroir is a complex term and includes the interaction between soil, climate, topography and the plant genotype, which results in a unique product. The ARC Infruitec-Nietvoorbij automatic weather station network has been settled in the wine growing areas for a number of years, allowing climatic investigations for this purpose.
This paper deals with the sea breeze mechanism and the effect of distance from the sea on the climate. This is an important factor for the climate of the region, especially during the maturation period in February, as cool temperatures are beneficial for the development of grape aromas and wine quality, especially white cultivars (Becker, 1977; Huglin and Schneider, 1998). Producers throughout the wine producing area therefore claim that the advantage of the sea breeze and its cooling effect result in quality wine production. The difference in temperature is experienced empirically, but there is a lack of knowledge concerning the sea breeze and its precise effect on relative humidity, wind and temperature in the vineyard area. In order to better clarify the situation, the question of how far inland the wine growing areas were influenced by sea breeze, was investigated.
The sea-breeze mechanism
Sea and land breeze circulations encountered near coastlines are due to the contrasting temperature responses of the land and water surfaces due to their different properties and energy balances. Because the sea is a fluid in continual motion, any heating or cooling is distributed to a considerable depth and so the rise or fall of water surface temperature is only slight. This results in a very small diurnal variation in surface temperature. Conversely, the land has a very small thermal conductivity and heats and cools rapidly, resulting in a marked diurnal variation. The resulting land-water differences produce corresponding land-water pressure differences in the lower atmospheric layer, which result in a system of breezes along a coastline (Abbs and Physick, 1992). Onshore flow during the day is known as a sea breeze, while offshore flow at night is referred to as a land breeze (Fig 1).
In the Western Cape, there is a significant contrast between the cool ocean and the warm inland temperatures as shown on the NOAA image in Figure 2. This results in a frequent occurrence of the sea breeze, especially during the maturation period in February (Bonnardot, 1997; 2000) when land temperatures are high (mean February temperature is 22°C at Cape Town airport) and the ocean remains cool due the cold Benguela current (below 15°C in some places).
In most cases a sea breeze can be recognised by the change in the direction of the surface wind and an increase in the velocity of a light onshore air flow in the afternoon or late morning.
Materials and method
In order to study the sea breeze, hourly data (6-year mean for February 1995-2000) from the ARC Infruitec-Nietvoorbij automatic weather station network were analysed for three weather stations (T10, T01 and T30), located at 10, 20 and 50 km from False Bay respectively, covering the Stellenbosch-Paarl winegrowing area in order to show the influence of the distance from the sea (Fig 3). The weather stations are located next to a vineyard facing SW (except T30, which faces NW). Unfortunately, there is no weather station further than 50 km north recording wind direction.
Relative humidity, temperature, wind speed and direction were plotted. The most important points to note were the minimum relative humidity, the maximum temperature, the time at which they occurred, the increase in wind velocity in the morning or early afternoon, as well as the change in wind direction during the day. Wind direction is only shown for February 2000.
Numerical simulation of the air movement over the Western Cape was performed on a 5 km grid. February 2000 was selected for the simulations as the conditions were close to average with a southern synoptic wind, which was able to strengthen the sea breeze from the False Bay. The Regional Atmospheric Modelling System (RAMS), developed by the University of Colorado (Pielke et al., 1992), was used in co-operation with CNRS-France. The RAMS takes many factors into account: six hourly meteorological data at 30 levels in the atmosphere up to 9000m (temperature, humidity, wind speed and direction as well as air pressure), topography, land cover and soil data as well as sea surface temperatures. Data was supplied by the European Centre for Medium Range Weather Forecast, NOAA, ARC Institute for Soil, Climate and Water and the South African Weather Bureau. The study domain for simulation stretches as far as Worcester (Fig3). A south-north cross section at 18°47'E over the wine growing area was selected to present the results. This longitude corresponds approximately to the longitude of the weather stations mentioned above and therefore simulated results were compared to data observed in situ. Time is expressed as local time.
Results
Mean observations in situ (February 1995-2000): Characteristics of the sea breeze.
a) Diurnal wind variability
The change in wind direction in the early afternoon was observed at each station (Fig4). While the prevailing synoptic wind recorded at 13:00 for February 2000 at Cape Town airport was on average from the west (Fig4b), all three stations recorded a prevailing south-western wind (72% at 10 km, 45% at 20 km) or western wind (35% at 50 km) compared to 20 or 35% south-western winds recorded close to the sea at 05:00, as well as a small percentage of northern or north-eastern winds (Fig4a). This characterises the land breeze at night and the sea breeze from False Bay during the afternoon.
The increase in wind velocity was also generally noticed at each station in the afternoon for February (1995-2000), the two stations close to the sea recorded stronger winds than the station inland, especially in the afternoon (Fig5a). The wind speed started to increase at 09:00 near the coast (at T10 and T01), reaching 3 m.s-1 at 10:00 and 4 m.s-1 at 13:00 with a daily maximum of 5 m.s-1 at 16:00 and 17:00 at T10. At 50 km inland (at T30), the wind speed remained below 2 m.s-1 in the morning, increased at 13:00 only and remained below 4 m.s-1 during the afternoon with a daily maximum between 17:00 and 19:00.
b) Relative humidity and temperature differences
The relative humidity was higher near the sea and from there on decreased rapidly inland. The minimum relative humidity in the afternoon was 58% at 10 km, 48% at 20 km and only 38% at 50 km inland (Fig5b). A stronger gradient was noticed for the first 20 km compared to inland with a decrease of 10% over a distance of 10 km, between 10 and 20 km from the coast, while a decrease of 10% over a distance of 30 km, between 20 and 50 km inland, was experienced.
The stations within 20 km from the coast recorded cooler temperatures with an average maximum February temperature of 26,2°C at 10 km, 27,3°C at 20 km and 29,8°C at 50 km. This resulted in an approximate 4°C difference over 40 km (Fig5c). The diurnal temperature variation is affected by the penetration of the cool and moist air from the sea. Temperature had already started to decrease on the coast, while the temperature further inland continued to increase. This resulted in measuring points close to the coast experiencing smaller temperature fluctuations than those further inland. It also resulted in a time lag for the maximum temperature, which occurred earlier close to the sea (at 14:00) and a little later inland at about 15:00.
In view of the above results, a climatic transition appears to take place between approximately 20 and 50 km from the coast, and may possibly even be closer to the sea. These in situ data provide important information but are discontinuous, which is why it was necessary to perform a numerical simulation. A 5 km grid was used as a first step to get a general idea of the location of the sea breeze front and its consequences in the vineyard.
Results of simulation for February 2000
a) Alternating land and sea breeze
The modelled diurnal wind variation is shown in Figure 6. At 09:00 the land breeze rotated above False Bay where the mountains caused various local slope winds inland (Fig6a). At 17:00, the sea breeze penetrated inland from False Bay (Fig6b). The breezes with a west coast origin did not penetrate from Table Bay, but further north.
The land breeze was weak (1 to 3 m.s-1) and circulated in a 100-300 m wide layer (Fig7a). At 17:00, the sea breeze and the synoptic southerly wind blew approximately from the same direction. The sea breeze, therefore, combined with the prevailing flow and penetrated inland to at least the edge of the study domain (Fig7b). The velocity was strengthened and approached 8 m.s-1 at surface level.
b) Location of the land and sea breeze cells
The up and down circulating cells associated with the land and sea breeze circulations are shown in Figure 8. Because the land breeze blew in an opposite direction to the southerly prevailing flow of air, the convergence between the land breeze and the southerly wind (land breeze front) caused the well-defined upward motion above False Bay at 09:00 (Fig8a). The land breeze cell had a descending branch on the coast and the southern slope of Bottelary and an ascending branch at 10 km from the coast above False Bay. At 17:00, the sea breeze penetrated inland, divided into several cells due to the topography. Upward-moving cells developed above the southern slopes, which are exposed to the southerly wind. The main circulation had an ascending branch at 20 km inland on the southern slope of Bottelary hill and a descending branch over False Bay (Fig8b). The Bottelary hill is the nearest high hill to the sea along the studied cross-section, and there the up-slope winds combined with the sea breeze and the synoptic flow caused an upward motion over the southern windward slope of the hill. These results corroborate those of Alpert et al. (1982) in Israel at the same local time.
c) Consequences for relative humidity and temperature
Southern slopes obviously received air with a higher humidity than the northern slopes (Fig 9a). At 17:00, it seems that the Bottelary hill "stopped" the moist air as the relative humidity rapidly decreased on its southern slopes, then gradually on its northern slopes and further inland. As a result, the thermal gradient was found 20 km inland at 17:00, approximately north of the Bottelary hill (Fig9b). The air above the sea and the first 10 km inland was cool and homogeneous (23°C). The temperature increased rapidly after the first 10 km inland. Further inland, there was a temperature increase of 4°C over a distance of 10 km (23°C at 10 km and 27°C at 20 km), 2°C between 20 km (27°C) and 30 km (29°C) and only 1°C between 30 km (29°C) and 50 km (30°C).
Considering that the simulation was performed with a 5 km grid and did not take topoclimatic effects into account, the simulated surface temperature at 17:00 was similar to the data recorded in the vineyards for the same day. A daily maximum temperature of 30°C was recorded at T30; 27.2°C at T01 and 26.8°C at T10 at 50, 20 and 10 km from False Bay respectively.
Conclusion
Even though the sea breeze penetrates up to at least the edge of the study domain (more or less 100 km inland), the effect on relative humidity and temperature decreases rapidly with distance from the coast. Because of the complex sea breeze circulation due to the topography, the sea breeze front and its movement would be more clearly shown by studying the hourly variation between 09:00 and 17:00. Results obtained by the RAMS are confirmed with observed data in the vineyards, which validates the modelled results. Automatic weather stations situated in vineyards to determine mesoclimatic effects are invaluable for the validation of such data.
The advantage of the modelling system is its ability to focus on a large scale giving information, in this case, each 5 km and at different altitudes. A map locating the sea breeze front can be compiled using as many cross-sections as are necessary for the study area. Mean data at a 1 km scale or even 200 m scale can also be used as input for RAMS in order to draw the climatic "limit" between the coastal/maritime and inland climate, which would be of especial interest for terroir identification. Other synoptic situations during different seasons are also being considered for further investigation.
For further information, please contact Dr Valérie Bonnardot, ARC-Institute for Soil, Climate and Water, Private Bag X5026, Stellenbosch 7599, South Africa.
Tel: +27 (0) 21 809 3082, Fax: +27 (0) 21 809 3002, e-mail Valerie@ infruit.agric.za
Acknowledgements
The authors wish to thank Winetech and CNRS for funding the research as well as Prof. Guy Cautenet (LaMP, University of Clermont-Ferrand), Dr Vincent Dubreuil and Isabelle Ganzetti (COSTEL, University of Rennes II) for technical assistance.
References
Abbs, D.J. and Physick, W.L., 1992. Sea-breeze observations and modelling: a review. Aust. Met. Mag. 41, 7-19.
Alpert, P., Cohen, A., Neumann, J. and Doron, E., 1982. A model simulation of the summer circulation from the eastern Mediterranean past Lake Kinneret in the Jordan Valley. Mon. Wea. Rev., 110 (8), 994-1006.
Becker, N.J., 1977. The influence of geographical and topographical factors on the quality of the grape crop. OIV International Symposium on the quality of the vintage. Cape Town, 169-180.
Bonnardot ,V.M.F., 1997. Sea breeze effect on temperature in the Stellenbosch-Klein Drakenstein wine producing district in South Africa. Research report, ARC Infruitec-Nietvoorbij, 42p + Figures.
Bonnardot, V.M.F., 2000. Étude préliminaire des brises de mer pendant la période de maturation dans la région viticole du Cap en Afrique du Sud. AIC Publication, 12, 26-33.
Carey, V.A. and Bonnardot, V.M.F., 2000. Spatial characterisation of terrain units in the Bottelaryberg-Simonsberg-Heldeberg wine growing area (South Africa). In: Proc. 3rd Int. Sym. Viticultural zoning, May 2000, Puerto de la cruz, Tenerife.
Carey, V.A, 2001. Spatial characterisation of natural terroir units for viticulture in the Bottelaryberg-Simonsberg-Heldeberg wine growing area. MSc Agric (Viticulture), University of Stellenbosch. 90p + annexes (In publication).
Huglin, P. and Schneider C., 1998. Biologie et Écologie de la vigne. 2nd Edition, Lavoisier TEC&DOC, 370 p.
Pielke, R.A., Cotton, W.R., Walko, R.L., Tremback, C.J., Lyons, W.A., Grasso, L.D., Nicholls, M.E., Moran, M.D., Wesley, D.A., Lee, T.J. and Copeland, J.H., 1992. A comprehensive Meteorological Modeling System- RAMS. Meteorol. Atmos. Phys. 49, 69-91.
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