The dynamics of pulsations in the bora flow

Severe episodes of the bora wind are characterized by the mean wind speeds close to 20 m/s, while gusts may reach 60 m/s. Data analysis of the previously measured high resolution (sampling intervals from 1–16 s) time series of the bora wind revealed the existence of pulsations with periods ranging from 1 to 10 min, similarly to downslope winds in other parts of the world. However, the length of those time series did not exceed a few hours in length and thus could not provide a sufficiently long database for inspection of the possible variations throughout episodes. During the 2-month period, from 1 December 2001 to 31 January 2002, wind speed and direction were measured with a 1 s sampling interval at Senj (east Adriatic) and a number of strong bora episodes were recorded. Spectral analysis of the data pointed to three distinct possibilities of the behavior of the bora gusts: a) presence of the pulsations through the entire episode, b) possibility of cessation and reappearance of the pulsations over the episode and c) fully non-periodic situations (i.e. without the pulsations). The latter two situations have not been reported so far and are thus interesting for additional study. In order to find possible causes for the observed behavior the correlation between 10-min wind speeds and the variances resulting from the spectra of 1 s wind speed has been inspected. Also, the relationship with the upstream tropospheric profiles has been examined and several interesting results came into view.