M.R. DRINKWATER
In terms of different mean ice drift speeds Fig. 15 indicates a sample ranging from 2 to
10 cm s·'. Figure 15a shows the west-east (i.e., x or u component) and Fig.15b south-north
(i.e. y or v) components of velocity, while Fig.15C indicates the magnitude of the velocity.
Ice drift is largely westward and northward during this period. Though a small sample,
the plot shows a high degree of accuracy in deriving both the direction and the magnitude of ice drift. Although spatial mean SAR velocities are used in the regression analysis, as opposed to values of the SAR-derived velocity interpolated to the ISW floe location, the result in Fig.15C indicates a correlation of 0.98 (illustrated by the dotted regression line). The rms error in SAR motion tracking is computed to be less than 0.5 cm s·'
for 3-day motion tracking. This translates into an error of approximately 100 m, or equivalently 1 pixel, after accounting for error variance generated by temporal interpolation of
GPS locations and spatially averaged mean SAR velocity vectors.
The largest outlying vector in Fig. 15 is identified throughout by a curly arrow, indicating that the motion field displays a large amount of rotation or vorticity. As Fig. 12
shows, the mean velocity components of such a field are not sufficiently representative
of any single point in space for a good comparison to be made. On the other hand, it is
felt that spatial interpolation of the velocity field is not representative of true inter-floe
motions, and so further attempts are not made to try to improve this comparison.
8.6.1.2
Time-Series Observations of Ice Floes
Results of processing all 3-day or I-day repeat overlapping "pairs" into ice kinematics
information are shown as a velocity time series in Fig. 16 together with surface wind
speed and the GPS monitored drift of the ISW camp floe. In Figure 16a ISW measurements of the wind speed are shown as a dotted line. Prior to completion of the meteorological mast on day 57, once- or twice-daily wind speeds were recorded. The 3-day
smoothed solid line indicates Finnish Salargos buoy (no. 5908) wind speed measurements prior to day 57 (courtesy Timo Vihma), whereafter the solid line is a 3-day running mean of the hourly means recorded by the ISW met station anemometer.
Figure 16b and Fig.16c show the mean SAR instantaneous velocity components joined
by a dotted trend line. Horizontal bars indicate the period separating the tracked image
pair, and the vertical bar indicates one standard deviation about the mean velocity vector (spatially averaged over a 100 x 100 km scene). An overlapping solid line indicates
the 3-day smoothed velocity of ISW and begins upon installation of a fully functional
GPS receiver at ISW (on day 57). Hollow diamond symbols represent the velocity computed from distances traveled between instantaneous GPS locational "fixes" at SAR
imaging times (shown in Fig. 15). Where diamonds overlap mean SAR velocity measurements, the spatially averaged SAR velocity is representative of a single GPS velocityvector.In a spatial context, where the mesoscale velocity field does not represent translational motion, the ISW GPS velocity diamond does not overlap the mean SAR velocity. Thus, in the case of day 70 the discrepancy shown in Fig. 16b,c is explained by strong
anticyclonic or cyclonic motion, and is similar to the example in Fig. 12. A mean velocity of a spatial field of cyclonic or anticyclonic flow is not representative of any single vector within that field, and such spatial means cannot be directly compared to the "ensemble-averaged" ISW GPS-derived velocity. Nevertheless, smoothed 3-day velocity components indicate responses to sustained bursts of wind shown in the solid line in Fig.
In terms of different mean ice drift speeds Fig. 15 indicates a sample ranging from 2 to
10 cm s·'. Figure 15a shows the west-east (i.e., x or u component) and Fig.15b south-north
(i.e. y or v) components of velocity, while Fig.15C indicates the magnitude of the velocity.
Ice drift is largely westward and northward during this period. Though a small sample,
the plot shows a high degree of accuracy in deriving both the direction and the magnitude of ice drift. Although spatial mean SAR velocities are used in the regression analysis, as opposed to values of the SAR-derived velocity interpolated to the ISW floe location, the result in Fig.15C indicates a correlation of 0.98 (illustrated by the dotted regression line). The rms error in SAR motion tracking is computed to be less than 0.5 cm s·'
for 3-day motion tracking. This translates into an error of approximately 100 m, or equivalently 1 pixel, after accounting for error variance generated by temporal interpolation of
GPS locations and spatially averaged mean SAR velocity vectors.
The largest outlying vector in Fig. 15 is identified throughout by a curly arrow, indicating that the motion field displays a large amount of rotation or vorticity. As Fig. 12
shows, the mean velocity components of such a field are not sufficiently representative
of any single point in space for a good comparison to be made. On the other hand, it is
felt that spatial interpolation of the velocity field is not representative of true inter-floe
motions, and so further attempts are not made to try to improve this comparison.
8.6.1.2
Time-Series Observations of Ice Floes
Results of processing all 3-day or I-day repeat overlapping "pairs" into ice kinematics
information are shown as a velocity time series in Fig. 16 together with surface wind
speed and the GPS monitored drift of the ISW camp floe. In Figure 16a ISW measurements of the wind speed are shown as a dotted line. Prior to completion of the meteorological mast on day 57, once- or twice-daily wind speeds were recorded. The 3-day
smoothed solid line indicates Finnish Salargos buoy (no. 5908) wind speed measurements prior to day 57 (courtesy Timo Vihma), whereafter the solid line is a 3-day running mean of the hourly means recorded by the ISW met station anemometer.
Figure 16b and Fig.16c show the mean SAR instantaneous velocity components joined
by a dotted trend line. Horizontal bars indicate the period separating the tracked image
pair, and the vertical bar indicates one standard deviation about the mean velocity vector (spatially averaged over a 100 x 100 km scene). An overlapping solid line indicates
the 3-day smoothed velocity of ISW and begins upon installation of a fully functional
GPS receiver at ISW (on day 57). Hollow diamond symbols represent the velocity computed from distances traveled between instantaneous GPS locational "fixes" at SAR
imaging times (shown in Fig. 15). Where diamonds overlap mean SAR velocity measurements, the spatially averaged SAR velocity is representative of a single GPS velocityvector.In a spatial context, where the mesoscale velocity field does not represent translational motion, the ISW GPS velocity diamond does not overlap the mean SAR velocity. Thus, in the case of day 70 the discrepancy shown in Fig. 16b,c is explained by strong
anticyclonic or cyclonic motion, and is similar to the example in Fig. 12. A mean velocity of a spatial field of cyclonic or anticyclonic flow is not representative of any single vector within that field, and such spatial means cannot be directly compared to the "ensemble-averaged" ISW GPS-derived velocity. Nevertheless, smoothed 3-day velocity components indicate responses to sustained bursts of wind shown in the solid line in Fig.