OBSTACLE-INDUCED PERTURBATIONS ON TURBULENT QUANTITIES MEASURED IN AIR-FLOWS OVER THE SEA

An experimental campaign, aiming to investigate the perturbation effec ts induced by fixed obstacles on turbulence measurements in ail flows at the air-sea interface, was carried out at the marine platform of th e Italian Navy, located in the harbour of La Spezia (North Ligurian Se a, Italy), near Lerici, on 28th, 29th, and 30th June 1994. This study was prompted by the ever-growing interest in more reliable estimates o f energy, mass, and momentum exchanges between water surfaces and atmo sphere, whose measurements are severely limited by the geometrical con straints of floating or fixed platforms where they are installed. Two types of meteorological instruments have been used: fast response (20 and 21 Hz) ultrasonic anemometers and fluxmeters to measure turbulent momentum, sensible, and latent heat fluxes and slow-response sensors ( less than 4 Hz and sampled at a rate of 10(-2) Hz) to measure average wind and temperature vertical profiles in the perturbed boundary layer . Both fast- and slow-response instruments have been located a few met ers apart from each other, along horizontal and vertical directions, s o as to establish also an upper limit to the reliability of horizontal and vertical divergences and gradients of average and turbulent quant ities in the obstacle wake. It has been observed that, in the airflow perturbed by the marine platform and its fixed structures, the fast-re sponse instruments of the same type and made by the same manufacturers gave results that compared well with each other, even if they were lo cated at different positions and heights (except for the vertical comp onent of turbulent wind speed), while the comparison among different t ypes of fast instruments gave more uncertain results. On the contrary, as far as mean values of the physical quantities were concerned, the measurements of slow-response instruments in the perturbed airflow wer e always in good agreement with the averaged data of fast instruments, irrespective of their factory or construction features.