XBT fall-rate variation in waters of extreme temperature and the resulting depth error has been addressed using controlled XBT–CTD datasets collected from two cruises in the Southern Ocean. Mean depth errors deduced from both the datasets are significantly different from those reported earlier for tropical and subtropical regions. The comprehensive study of Hanawa et al. (making use of controlled XBT–CTD data, mostly from tropical and subtropical waters) showed that the manufacturer's equation underestimates the probe's fall rate. This is manifested by the mean negative depth error reported from this region. However, results from the present study show that the manufacturer's equation slightly overestimates the fall rate in this region, as indicated by the small positive error (5–10 m).
In order to provide theoretical support to the observed depth error, an analytical approach is adopted based on the viscosity effect on the probe's fall rate. Observed as well as analytical results suggest that the probe has a decelerating tendency due to the viscosity effect in high-latitude waters, and the existing correction scheme is not appropriate for XBT data from regions of such extreme low temperature.
The existing correction scheme is valid for tropical and subtropical waters of negative depth error zones. However, for XBT data from high-latitude waters it is reasonable not to correct XBT data based on the existing scheme until the exact nature of depth error from this region is known. Though the mean depth errors from both the datasets show nearly identical values, it is necessary to conduct more controlled XBT–CTD experiments in this region in order to substantiate the exact nature of error for this region and then develop an appropriate depth-correction scheme.
Abstract:An oil spill occurred off Goa, west coast of India, on 23 March 2005 due to collision of 2 vessels. In general, fair weather with weak winds prevails along the west coast of India during March. In that case, the spill would have moved slowly and reached the coast.
NCMRWF (National Centre for Medium Range Weather Forecast) winds assimilated with MSMR (Multi-channel Scanning Microwave Radiometer) winds are used as input to MIKE21 Offshore Spectral Wave model (OSW), which takes into account wind-induced wave growth, nonlinear wave-wave interaction, wave breaking, bottom friction and wave refraction. The model domain covers the north Indian Ocean, bounded by the equator and to 30u N, and 50u to 100u E. An experiment has been conducted to find out improvement in wave prediction when NCMRWF winds blended with MSMR winds are utilized in the wave model. A comparison between buoy and TOPEX wave heights of May 2000 at four buoy locations provides a good match, showing the merit of using altimeter data, wherever it is difficult to obtain measured data for comparison with model wave heights. This is further confirmed when model wave heights for January 2001 in the entire model domain were compared with TOPEX altimeter data; there was a good match for the full range of wave heights considered. Model wave heights, periods and directions have been compared with the data of deep and shallow water buoys. The correlation coefficients between model and measured wave heights are in the range 0.85-0.91. The model slightly over-predicts wave periods, but match is reasonably good. In general, model wave periods are of the order of 6-9 s during June-July 2001 and 4-8 s during May, August and September 2001. The wave periods clearly indicate that waves during June-September are generated by monsoon wind forcing, and dominated by 'wind waves'.
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