Available potential energy of the daily coastal circulation at Zadar (Croatia) |
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| Authors: | T Tro?i? N ?inik ? Tro?i? |
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| Institution: | (1) Ruđer Bošković Institute, Center for Marine and Environmental Research, Zagreb, Croatia;(2) Department of Geophysics, Faculty of Science, Zagreb, Croatia;(3) Faculty of Maritime Studies, Split, Croatia |
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| Abstract: | Summary The aim of this study is the evaluation of the sea breeze speed on the basis of its energy. Energetics of the sea breeze can
be studied by means of the available potential energy (APE). Part of this energy is transformed into the kinetic energy of
the sea breeze. Some similarity exists between the large scale processes of the circulation and the small coastal air circulation
due to the fact that both circulations are triggered by the same physics, i.e., solenoidal activity of the baroclinic atmosphere.
To evaluate the sea breeze speed, APE was calculated by use of the Lorenz’s equation (1955), and which is possible if the
coastal circulation is considered to be a closed system in a hydrostatic equilibrium. For calculations and verifications hourly
sea-surface temperatures, near-ground air temperatures and wind speed measurements, as well as the radio-sounding measurements
at 12 UTC were used at the Zadar station (ϕ = 44° 08′ N, λ = 15° 13′ E), which is situated in the central part of the eastern
Adriatic coast. Two days with an undisturbed sea breeze circulation were extracted using the methods for minimizing other
atmospheric influences. Calculated hourly near ground sea breeze speeds obtained in this way were higher than the measured
ones. With the assumption that some of the APE is transformed into the kinetic energy it is possible to obtain characteristic
speed of the developed sea breeze with small discrepancies to the near-ground measurements. If 6.6% of the mean daily near
ground APE was taken to be transformed to the mean daily kinetic sea breeze energy on the 29th and 4.2% on the 30th September
2002, the best agreement was obtained with the mean daily measured near ground sea breeze speed. This range of values can
be attributed to inability to extract precise values for the lapse-rate needed in the APE sea breeze calculations. Results
show similarities to the general circulation of the atmosphere, since about 10% of the APE is transformed to the kinetic energy
of the sea breeze. On the other hand calculated wind speed at the lower branch of the borderline coastal circulation was not
dependent on the integral value of the APE over the land, but on its value at the near-ground level. |
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