Drifters in Hurricanes Frances and Jeanne, September 2004

Quality controlled data files for all 66 drifters are available here: Frances Drifter Data . In addition, data from two deployed floats are available from here: Frances Float Data .

For the following figures, click on the images for enlarged versions of plots.

1. Drifter Tracks during Hurricanes Frances and Jeanne

2. Drifter Data Sampling Frequency

In the following figures, "Time of observation", the drifter observations are plotted as day vs. hour of day: blck dots indicate position data only (possibly SST and pressure), blue indicates direction data available, and red means direction and speed available. Note, that QSCAT observations in the area of the drifters occur around 10:30 and 22:30.

3. Time Series of drifter observed air pressure and SST

3.1 Metocean (9 drifters with speed, SST, and air pressure)

3.2 ADOS (8 drifters with speed, SST, and air pressure)

3.3 Minimet (14 drifters with SST and air pressure)

3.4 SVP-15m (3 drifters with SST and air pressure)

3.5 SVP-100m (12 drifters with SST only)

3.6 SST (17 drifters with SST only)

3.7 SST&pres (3 drifters with SST and air pressure)

4. Drifter observed wind speed and direction

4.1 Time series of speed and direction

4.2 ADOS wind speed data from drifter 41921

Comparisons with collocated QSCAT data are presented below. Because of the sampling frequency of the drifters and the resulting time differences with local overflights of the scatterometer (see fig. 5.1.1-5.1.5), it was necessary to use a maximum time difference of 4hr for collocations. The maximum distance allowed was 100km. The resulting average time difference of the collocations is ca. 140min (2:20hr); and the average distance is 17km (ranging from 1 to 95km).
Presented are timeseries of wind speed: drifter speeds derived from the 6-10kHz frequency and standard QSCAT. Also shown are the wind speed differences and the time differences of the two collocated measurements. Note that the time period after day 281 (when the relative magnitudes of the three sound channels change) does not exhibit anomolous differences with respect to QSCAT.

Scatterplots of wind speeds from drifter and QSCAT are presented below. Rain-flagged data are marked in red.

The few rain-flagged data (Nrain=4) do not exhibit very large wind speed differences with respect to QSCAT, and, therefore, will not be excluded from the comparisons.
About a third of the drifter measurements tend to overestimate wind speed. Breaking surface waves produce large acoustic noise that result in larger measured sound levels than only due to wind speed at the surface of the ocean. That contamination of the acoustic signal would bias the results towards higher wind speeds.

Also plotted are scatterplots of wind speed differences vs. time differences and separation distances.

In the time difference plot, the average speed differences for 4 time difference (Tdif) intervals are drawn with blue lines. While the speed differences for Tdif = 0-60min are the smallest (average=0.7m/s), the averages for longer Tdif do not change very much for time differences from 60 to 230min (average speed differences vary between 1.7 and 2.1m/s). Therefore, it seems reasonable to include the relatively large time differences (up to 230min = 3:50hr) in the collocation data sets.
Distances between collocated data tend to be within 20km. The standard QSCAT product is provided on a 25x25km grid. Larger distances only occur in patches of missing QSCAT wind vector cells (WVC),or when the drifters were at the edge of the scatterometer swath.

The next figure presents the functional fit between sound pressure level (spl) and wind speed. Black dots indicate the wind speeds derived from Equation I (a=20., b=104.5, c=53.91). When compared with collocated QSCAT, the average speed difference is 1.2m/s (Stdev=2.1m/s). Using the collocated QSCAT wind speeds, one can derive an estimate of the conversion coefficients based on a least-square-fit: a=39.21, b=1.1, c=2.84. This conversion function and the resulting wind speed estimates are plotted in red.

The data are limited to wind speeds of less than 15m/s. For small wind speeds (< 7m/s), the best-fit function agrees very well with the function from Equation I. For stronger winds, the best-fit function would result in lower estimates of wind speeds.

Hourly estimates of drifter derived wind speeds are used in the following figures with collocated QSCAT. The hourly estimates are computed by averaging only the two lowest sound pressure levels in every hour. The wind speed satterplots show that the correlation betwen drifter and QSCAT speeds gets better: the average speed difference drops from 1.2 to 0.6m/s, and stdev shrinks from 2.1 to 1.6m/s. Also the best-fit conversion function moves closer to the theoretical function.

From the two figures above it is easy to see that most of the speed differences have been reduced, when comparing the hourly with the 15min data. In particular, it is striking how speed differences within 20km separation distances have greatly been reduced. In the 15min data, 27% (17 out of 63) of the data within 20km had speed differences of more than 2.2m/s (max=7.7m/s). In the hourly data, this percentage has been reduced to 8% (5 out of 59, max=5.5m/s).
There are seven outliers in the hourly data set with relatively large speed differences (2.2-5.0 m/s). These outliers are either associated with large time differences (5 with Tdif>90min) or large separation distances (1 with dist>80km). In the next section, a sub-sample of the hourly is investigated, from which these seven outliers have been removed.

A subsample of hourly estimates is presented below. For the subsample large speed differences with either large time differences (speed diff>2.2m/s & Tdiff>90min) or large separation distances (speed diff>2.2m/s, and dist>80km) were eliminated (7 out of 67 samples were removed).
For the resulting 60 collocated pairs of drifter derived wind speeds and QSCAT measurements, the average speed difference is 0.4m/s, with Stdev = 1.0m/s.

The best-fit conversion function, based on this subsample, requires coefficients of (20.3, 98.0, 53.2), for which the theoretical values are (20.0, 104.5, 53.9). The best-fit average speed difference is 0.0m/s, with Stdev = 1.0m/s.

4.3 Wind direction vs. QSCAT scatterplots

4.4 Wind speed vs. QSCAT scatterplots

5. QSCAT wind fields

5.1 Standard QSCAT surface winds compared with drifter observations in Hurricane Frances

In the next section QSCAT surface wind data for Frances are shown for five days during 9/1 - 9/5/2004. In all figures rain-flagged vectors are plotted in red, rain-flagged vectors with speeds &ge 15m/s are in green. Drifter observations within a few minutes of the scatterometer data are shown in blue. Drifter vectors do not represent observed speeds: ADOS and Metocean drifters are plotted, and only Metocean drifters have speed observations. The last two digits of the drifter ID are written on the figures. Also shown are the NOAA best track hurricane centers (red crosses), interpolated to the QSCAT data time.

5.2 High-Resolution Surface Windspeeds from Drifters and QSCAT in Hurricane Jeanne

The subtitles below the figures on the right list the time differences between drifter and satellite observations (drifter minus QSCAT, in hours); and the the last two digits of the drifter IDs are printed. Drifter dots with no arrows indicate observed speeds of less than 5 knots. Drifter wind arrows with no barbs indicate a valid direction but no speed.

6. Drifter observations relative to hurricane center

6.1 SST relative to hurricane center of Frances

To generate a contour plot, all data are averaged on a 50x50km grid. In fig. 6.1.1(b) the grid cells with averages are marked with "x". Then points are filled in by averaging all surrounding 9 points (marked with solid dots). This step is repeated one more time, to create extra coverage in the south (marked with "+").

The contoured SST map is shown below. The figure on the left shows all drifters, fig. 6.1.2(a). For the figure on the right, 3 drifters, which are the farthest from the hurricane array, are omitted: Metocean drifters 41540 and 41646 at (282° E,28° N), and SST drifter 41612 at (300° E, 20° N). The data from 30 drifters show a very well defined narrow wake of SST (ca. 200km wide), where the surface temperature has cooled up tp -2.5° C. The -1° C contour line is shown with a black line.

6.2 Air pressure relative to hurricane center of Frances

The drifter observed air pressure values are plotted as a function of distance from the hurricane center. As an approximation to the functional behavior of pressure vs. distance, the following formula is used, and the coefficients are determined by curve-fitting to the data:

where p0 is the lowest pressure at the center, and pe is the pressure in undisturbed environment, or background pressure. P0 is pre-set at 940mb, as NOAAA's "best track" pressure data has a minimum of 937mb on 9/1/2004 12:00UTC. Pe is determined by the drifter data. All drifter observations are combined, both from the north-west and south-east quadrants. The resulting best-fit coefficients are pe=1016.77, a=49.27, and b=1.13.

6.3 Wind direction relative to hurricane center of Frances

Available wind direction data within 800km of the hurricane center are plotted below on a map, relative to the center of Frances. Shown are 7 ADOS and 7 Metocean drifters. The plotted vectors are of uniform lengths, and do not reflect the measured speeds. Metocean data are marked with a dot at the base of the arrows. As the hurricane is approaching the drifters, its northwest quadrant is measured. Here the wind directions are close to radial (i.e. circular around the center) within 400km. Further away, the winds tend to blow increasingly away from the center. In the southeast quadrant , measured as the hurricane was moving away from the drifters, the drifter-measured winds tend to blow towards the center. At very large distances (800km), the winds are directed almost straight towards the center.