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1 1999

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HIROMB forecasts versus spatial measurements of the currents by means of rdi's BBADCP during the POLRODEX'97 experiment

Krzymiński W.

Biuletyn Instytutu Morskiego w Gdańsku

1999

Vol. 26, No. 1

77--86

The paper presents the observations of the flow field within central part of the Gulf of Gdansk, which have been carried out during POLRODEX'97 experiment in September 1997 by means of BBADCP (Broad Band Acoustic Doppler Current Profiler). In the paper some results of comparison between predicted values and measured are presented as well. The goal of the experiment it was to collect the data in order to verify numerical model calculations and to study the method of the flow field reconstruction. The cruise took place on the RV BALTICA, which carried a shipboard RDI' s BBADCP measuring device. The measurements were carried out along parallel transects with 30 s frequency of the collected averaged bins, while velocities of the current were computed relative to the bottom. The flow fields were examined at alti-meter layers from 8 meters depth down to the bottom layer on 22 -24 of September 1997. For the analysis, the data for one layer (mid value 12 m.) were selected and prepared for the comparison. On the basis of the results of the measurements, quasicsynoptic flow fields were reconstructed. It made possible to compare them with the results of the HIROMB forecasts. In pre-analysis phase the two forecasts, i.e.: 24 hours for 22nd and 6 hours for 23rd forecast were compared with the measurements. It showed us that the best result would be obtained for shortest time span between measurements and forecast. So, for further analysis the 6 hours forecast for 23rd was used. Then, the statistical analysis of differences was carried out. In order to find any regularity of the differences distribution it is discussed their spatial layout as well. In general, the most significant differences occurred in the northern part of the investigated area. Differences increased from the centre of the gulf toward east and north. It was clearly seen that model reproduce current velocities and directions well mainly along the Hel Peninsula and in central part of the Gulf In other areas, velocities of the predicted currents are of another order of magnitude than measured values. Despite the results of the comparison between measured and modelled values, the collected data showed well-expressed pattern of the flow in the gulf what could be studied further.