ADCP Data Collection during the SBI Spring Cruise, HY0201

The USCGC Healy is equipped with two Acoustic Doppler Current Profilers (ADCPs), a broad band 150 kHz (BB150) and an ocean surveyor 75 kHz (OS75). The OS75 was newly installed during the recent dry-docking period early this spring and enhancements were made to both systems to maximize their capabilities. Roughly speaking, the higher frequency BB150 unit is capable of finer vertical resolution while the lower frequency OS75 unit is capable of greater range. During the cruise the configurations of both units were varied to test the capability of different setups. When conditions were most favorable, the BB150 had a range of 150 to 200m while the OS75 had a range of over 400m.

Both units returned good data when conditions permitted. The biggest item affecting the quality of the data was breaking ice. Under those conditions only occasional pings returned data. In their sensitivity to the noise and vibration associated with ice breaking, the ADCPs were no different than the other acoustic devices on the ship. Since the ship spent most of the cruise in heavy ice conditions, he distribution of good data is heavily biased to those periods when the ship was on station with the result that while the temporal coverage looks very good, the spatial coverage appears intermittent. Data processing has not reached the stage where we are able to determine to what extent we can extract reasonable quality data from the ice breaking periods. Nor have we been able to determine whether there are differences in data quality that depend on the severity of ice breaking. Another factor affecting the data coverage was the shallowness of the Chukchi Sea. The draft of the ship plus the quiescent interval before the ADCPs can start to acquire the data from each ping means that the shallowest bins start at about 20m, a significant fraction of the water depth over the shelf (~40m). Special techniques are required to extract useable data from the narrow region before interference from the bottom limits the range. Maximizing the data under both ice breaking and shallow water conditions will be one of the major tasks during the post cruise processing.

A preliminary data processing methodology was developed during the cruise that enables a presentation of velocity data that have been roughly calibrated and undergone some quality checks. The results should be adequate for determining the overall velocity structure during the cruise. Further refinements will probably make only fine adjustments to the velocities. Although the magnitude of the adjustments is hard to anticipate, it should be small. The biggest modification anticipated is to the BB150 unit when we are able to determine the speed of sound in the transducer well. However, the lack of knowledge of the sound velocity in the well has been roughly taken into account during bottom-track calibration and since temperature is fairly constant in the fresh water/antifreeze mixture in the transducer well, big changes in the velocity data are not anticipated.

The ADCP velocity data distributed at the end of the cruise have been averaged into half-hour by 10 meter vertical bins from each instrument. These data are contained in two large ASCII data files, HY0201_BB150_ADCP_Vel.dat and HY0201_OS75_ADCP_Vel.dat, in which the velocity profiles are listed, together with the date, time and position, by time interval. Data bins where the velocity data did not pass the preliminary editing process are indicated by “99999”. These data are also presented in color contour plots of eastward and northward current components in the files: HY0201_BB150_Vel.tif and HY0201_OS75_Vel.tif, respectively.

In addition to these basic data products, velocity contours are also presented for the West and East Hanna Shoals sections. For these plots, the data were averaged into 5 km radial segments from the beginning of each section. The standard deviations of the eastward and northward binned velocities are also presented in separate plots for each ADCP. The velocity contour plots for the transects are presented in: HY0201_BB150_WHS_UV.tif, HY0201_BB150_WHS_STD.tif, HY0201_BB150_EHS_UV.tif, HY0201_BB150_EHS_STD.tif, HY0201_OS75_WHS_UV.tif, HY0201_OS75_WHS_STD.tif, HY0201_OS75_EHS_UV.tif, HY0201_OS75_EHS_STD.tif.