Naslov
Spectral characteristics of the bora wind

Autori
Orlić, Mirko ; Pasarić, Miroslava ; Pasarić, Zoran

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Book of Abstracts B / . - , 2003

Skup
XXIII General Assembly of the International Union of Geodesy and Geophysics

Mjesto i datum
Sapporo, Japan, 30.06.2003. - 11.07.2003

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
.

Sažetak
Bora is a cold, dry wind that blows above the Adriatic from the northeast quadrant, most often during winter season. Its average speed is usually not high, but - due to the gustiness - the bora speed maxima may surpass 60 m/s. Thus, bora is one of the severest local winds in the world. Previous measurements of the bora speed and direction were performed either with the large sampling intervals (greater than 1 min) over a day or more or with the small sampling intervals (1-16 s) over an hour at most, thus tending to emphasize low- or high-frequency part of the spectrum, respectively. Measurements caried out recently at Senj, east Adriatic station which is notorious for its strong bora outbreaks, with a 1 s sampling interval over two months (December 2001 - January 2002), enabled for the first time broad characteristics of the wind spectrum to be determined. Micro-m-asta TM anemometer was positioned at the coast, 15 m above sea level. During the two-month measurement interval it recorded more than 20 bora episodes, the longest lasting over 82 hours. The maximum wind speeds surpassed 40 m/s, and they were - as expected - related to gusts, the low-pass filtered speeds being smaller than 20 m/s. Spectral analysis revealed (1) an increase of energy from the smallest resolved period (2 s) to about 1 min, (2) further increase of energy between 1 and 10 min with frequently occuring maxima, (3) a spectral gap between about 10 min and 1 h, and (4) renewed increase of energy from 1 h to the largest periods as determinsd by the length of bora episodes. The last two are well-known characteristics of wind spectra observed worldwide. What makes the bora wind special is the energy continuum extending from 2 s to 1 min and accounting for more than 50% of the total wind variance - an obvious sign of strongly turbulent wind regime, and significant spectral peaks observed at periods from 3 to 8 min during most bora episodes. The position and intensity of the peaks vary from one bora episode to the other, and even within an episode, but no regularity could be noticed in their occurrence. This seems to agree with the prevailing wisdom, according to which the bora gustiness is related to unstable gravity waves that develop at the upper boundary of the bora layer.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

POVEZANOST RADA