Drifters in Hurricanes Frances and Jeanne, September 2004


Drifters in Hurricanes Frances and Jeanne, September 2004

On August 31, 2004, 39 drifters were deployed ahead of Hurricane Frances around 70°W and 24°N (CBLAST array): 14 Minimets, 8 ADOS, 14 SVP-100m, and 3 SVP-15m. 36 drifters were sending SST data (two SVP-100m and one Minimet drifter failed to transmit SST data) and 25 were also collecting pressure data (Minimets, ADOS, and SVP-15m). In addition, 29 other drifters were in the vicinity to provide additional SST data (3 of them have also pressure data): 9 Metocean, 17 SST, and 3 SST&pres drifters. The Minimet, ADOS, and Metocean drifters are also measuring surface wind direction and wind speed.

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

Drifters measured wind direction, wind speed, SST, and air pressure. During the passage of hurricanes Frances and Jeanne, there were a total of 66 drifters with useful data in the area of roughly 280-300°E and 20-30°N: 9 Metocean (dir, speed, SST, pres), 8 ADOS (dir,speed,SST,pres), 14 Minimet (SST,pres), 12 SVP-15m (SST), 3 SVP-15m (SST,pres), 17 SST drifters (SST), and 3 SST&pres drifters (SST,pres). Below are plots of the drifter tracks, for the time period 8/31-9/5/2004. Solid dots indicate deployment locations or beginning of data records. The track of Frances is superimposed. Plots for each group of drifters follow below.

Fig. 1.1 Minimet, ADOS, Metocean, SVP-100m and SVP-15m drifter tracks, during Frances, 8/31-9/5/2004.


Fig. 1.2 Drifter tracks, 8/25-10/1/2004, for (a) Metocean, and (b) ADOS drifters.
Fig. 1.3 Drifter tracks, 8/25-10/1/2004, for (a) Minimet, and (b) SVP-15m drifters.
Fig. 1.4 Drifter tracks, 8/25-10/1/2004, for (a) SVP-100m, and (b) SST drifters.
Fig. 1.5 Drifter tracks, 8/25-10/1/2004, for SST&pres drifters.


The drifter IDs, time period for observations, and type of observations for each drifter are summarized in Table 1.1.

Table 1.1: List of all Drifters in Frances and Jeanne
Type Drifter dates days N dir N speed N SST N pres
SST (17) 13518 8/25-10/01 37 306
13594 8/25-10/01 37 318
13595 8/31-10/01 31 220
13608 8/25-09/24 30 244
13610 8/25-10/01 37 292
13921 8/25-10/01 37 341
13923 8/25-10/01 37 360
31921 8/25-10/01 37 335
41552 8/25-10/01 37 364
41554 8/25-10/01 37 333
41555 9/10-10/01 21 200
41559 9/10-10/01 21 199
41575 8/25-10/01 37 250
41577 8/25-10/01 37 363
41591 8/25-10/01 37 352
41612 8/25-10/01 37 319
41616 8/25-10/01 37 360
SST&pres (3) 13533 8/25-10/01 37 880 879
41520 8/25-10/01 37 1454 1454
41631 8/25-10/01 37 535 537
Metocean (9) 41539 8/25-10/01 37 682 696 692 699
41540 8/25-09/09 15 262 276 278 280
41541 8/25-10/01 37 568 647 647 654
41542 8/25-10/01 37 643 667 670 671
41543 8/25-10/01 37 662 671 672 676
41544 8/25-10/01 37 645 665 666 672
41545 8/25-10/01 37 650 674 679 685
41590 8/25-10/01 37 381 642 641
41646 8/25-10/01 37 511 515 515
ADOS (8) 41919 8/31-10/01 30 193 66 238
41920 8/31-10/01 30 201 227 240
41921 9/04-10/01 27 196 207 56 209
41922 9/01-10/01 30 232 251 218 250
41923 9/03-10/01 28 131 141 146
41924 9/04-10/01 27 192 316 215
41925 9/01-10/01 30 228 264 236
41926 8/31-10/01 30 251 338 259
Minimet (14) 41927 9/01-10/01 30 176 176
41928 8/31-10/01 30 163 161
41929 8/31-10/01 30 170 169
41930 8/31-10/01 30 156
41931 9/04-10/01 26 133 134
41932 9/01-10/01 30 161 160
41933 8/31-10/01 30 159 159
41934 9/01-10/01 30 181 180
41935 9/01-10/01 30 176 177
41936 9/01-10/01 30 186 188
41937 8/31-10/01 30 177 176
41938 8/31-09/15 14 75 76
41939 8/31-10/01 30 168 169
41941 8/31-10/01 30 188 186
SVP-15m (3) 41942 9/01-10/01 30 269 268
41943 8/31-10/01 30 269 277
41944 9/01-10/01 30 248 251
SVP-100m (12) 41617 8/31-10/01 30 336
41618 8/31-10/01 30 327
41619 8/31-10/01 30 313
41620 8/31-10/01 30 321
41622 8/31-10/01 30 312
41623 8/31-10/01 30 334
41668 8/31-10/01 30 338
41669 8/31-10/01 30 316
41671 8/31-10/01 30 314
41852 8/31-10/01 30 330
41853 8/31-10/01 30 132
41854 8/31-10/01 30 326


A summary of number of drifters for each type of observation is listed in Table 1.2. Not all drifter records extend to 10/1/2004: one SST drifter ends on 9/24, one Metocean ends on 9/9, and one Minimet ends on 9/15.

Table 1.2: Number of Drifters with Data
Type Total dir speed SST pres
SST 17 - - 17 -
SST&pres 3 - - 3 3
Metocean 9 9 7 9 9
ADOS 8 8 2 8 8
Minimet 14 - - 13 14
SVP-15m 3 - - 3 3
SVP-100m 12 - - 12 -
Total 66 17 9 65 37


2. Drifter Data Sampling Frequency

Nine Metocean drifters (ID 4139-45, 41590, and 41646) and eight ADOS drifters (ID 41919-41926) are examined.

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.

Fig. 2.1 Time of observations for ADOS drifters (a) 41919, and (b) 41920.


Fig. 2.2 Time of observations for Metocean drifters (a) 41539, and (b) 41541.





3. Time Series of drifter observed air pressure and SST

In the following figures, the "distance" panel shows distance between drifter and the center of the hurricane (AOML data), speed shows drifter measured speeds, if available, in black, hurricane "maximum sustained winds" from AOML in blue, and collocated QSCAT speeds (in red). The figures in this section are based on the quality-controlled, corrected data.
The applied air pressure offsets are only 1 to 3mb (except for drifter 41919, which requires a -20mb offset). SST corrections vary between 0.1 and 1.6°C, but are mostly in the order of +/- 0.3°C. In addition, there are a few obvious spikes that can be removed. Those corrections have been applied to the data shown below.
Hurricane data are available every 3 hours, for almost the entire data period, and when missing were interpolated to every 3 hours.
9 Metocean, 8 ADOS, 14 Minimet, 3 SVP-15m, 12 SVP-100m, 17 SST , and 3 SST&pres drifters are plotted below.

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

A few spikes in SST, pressure, and speed were removed, and pressure was corrected for the following drifters: 41539 (-1mb), 41542 (-2mb), 41545 (-1mb), and 41590 (-3mb).

Fig. 3.1.1 Drifter wind speed, SST, and air pressure for Metocean drifters (a) 41539, and (b) 41540.


Fig. 3.1.2 Drifter wind speed, SST, and air pressure for Metocean drifters (a) 41541, and (b) 41542.


Fig. 3.1.3 Drifter wind speed, SST, and air pressure for Metocean drifters (a) 41543, and (b) 41544.


Fig. 3.1.4 Drifter wind speed, SST, and air pressure for Metocean drifters (a) 41545, and (b) 41590.


Fig. 3.1.5 Drifter wind speed, SST, and air pressure for Metocean drifter 41646.

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

The corrected ADOS figures include SST data from a secondary sensor. Pressure was corrected for two drifters: 41919 (-20mb) and 41922 (-2mb).

Fig. 3.2.1 Drifter wind speed, SST, and air pressure for ADOS drifters (a) 41919, and (b) 41920.


Fig. 3.2.2 Drifter wind speed, SST, and air pressure for ADOS drifters (a) 41921, and (b) 41922.


Fig. 3.2.3 Drifter wind speed, SST, and air pressure for drifter (a) 41923, and (b) 41924.


Fig. 3.2.4 Drifter wind speed, SST, and air pressure for ADOS drifters (a) 41925, and (b) 41926.


3.3 Minimet (14 drifters with SST and air pressure)

A few spikes in SST and pressure were removed, and pressure was corrected for the following drifters: 41928 (+1mb), 41931 (-1mb), and 41939 (+1mb).

Fig. 3.3.1 Drifter SST and air pressure for Minimet drifters (a) 41927, and (b) 41928.


Fig. 3.3.2 Drifter SST and air pressure for Minimet drifters (a) 41929, and (b) 41930.


Fig. 3.3.3 Drifter SST and air pressure for Minimet drifters (a) 41931, and (b) 41932.


Fig. 3.3.4 Drifter SST and air pressure for Minimet drifters (a) 41933, and (b) 41934.


Fig. 3.3.5 Drifter SST and air pressure for Minimet drifters (a) 41935, and (b) 41936.


Fig. 3.3.6 Drifter SST and air pressure for Minimet drifters (a) 41937, and (b) 41938.


Fig. 3.3.7 Drifter SST and air pressure for Minimet drifters (a) 41939, and (b) 41941.

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

All three SVP-15m drifters stayed very close together during Frances and Jeanne. It is not possible to decide which drifter(s) is the "truth", but the records can be corrected so that there are no offsets among these three drifters. The initial SST from drifter 41942 is closest to other surrounding drifters (Metocean, ADOS, and Minimet), and was used to correct the other two drifters. Drifters 41943 and 41944 have also sudden shifts in their SST records that were corrected. These shifts resulted from the SST measurement cycle, which was too short to get a correct SST reading. Drifter 41943 was corrected by +1.2°C before day 249.0, and by +0.6°C afterwards. Drifter 41944 was corrected by +0.8°C before day 260.5, and by +0.4°C afterwards.
In addition, a few spikes were removed in SST and all pressure data were shifted by -1mb to become similar to other neighboring drifters.

Fig. 3.4.1 Drifter SST and air pressure for SVP-15m drifters (a) 41942, and (b) 41943.


Fig. 3.4.2 Drifter SST and air pressure for SVP-15m drifter 41944.

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

12 of 14 deployed SVP-100m drifters have usable SST observations.

Fig. 3.5.1 Drifter SST for SVP-100m drifters (a) 41617, and (b) 41618.


Fig. 3.5.2 Drifter SST for SVP-100m drifters (a) 41619, and (b) 41620.


Fig. 3.5.3 Drifter SST for SVP-100m drifters (a) 41622, and (b) 41623.


Fig. 3.5.4 Drifter SST for SVP-100m drifters (a) 41668, and (b) 41669.


Fig. 3.5.5 Drifter SST for SVP-100m drifters (a) 41671, and (b) 41852.


Fig. 3.5.6 Drifter SST for SVP-100m drifters (a) 41853, and (b) 41854.


3.6 SST (17 drifters with SST only)

Only very few SST spikes were removed in the SST drifters.

Fig. 3.6.1 Drifter SST for SST drifters (a) 13518, and (b) 13594.


Fig. 3.6.2 Drifter SST for SST drifters (a) 13595, and (b) 13608.


Fig. 3.6.3 Drifter SST for SST drifters (a) 13610, and (b) 13921.


Fig. 3.6.4 Drifter SST for SST drifters (a) 13923, and (b) 31921.


Fig. 3.6.5 Drifter SST for SST drifters (a) 41552, and (b) 41554.


Fig. 3.6.6 Drifter SST for SST drifters (a) 41555, and (b) 41559.


Fig. 3.6.7 Drifter SST for SST drifters (a) 41575, and (b) 41577.


Fig. 3.6.8 Drifter SST for SST drifters (a) 41591, and (b) 41612.


Fig. 3.6.9 Drifter SST for SST drifter 41616.

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

A few spikes in SST and pressure were removed, and pressure was corrected for only one drifter: 41631 (-3mb).

Fig. 3.7.1 Drifter SST and air pressure for SST&pres drifters (a) 13533, and (b) 41520.


Fig. 3.7.2 Drifter SST and air pressure for SST&pres drifter 41631.



4. Drifter observed wind speed and direction

4.1 Time series of speed and direction

The following figures are as above, but observed wind directions are shown, and collocated QSCAT data are superimposed with solid dots. rain-flagged QSCAT data are marked with open squares.

Fig. 4.1.1 Drifter wind speed and direction for Metocean drifters (a) 41539, and (b) 41540.


Fig. 4.1.2 Drifter wind speed and direction for Metocean drifters (a) 41541, and (b) 41542.


Fig. 4.1.3 Drifter wind speed and direction for Metocean drifters (a) 41543, and (b) 41544.


Fig. 4.1.4 Drifter wind speed and direction for Metocean drifters (a) 41545, and (b) 41590.


Fig. 4.1.5 Drifter wind speed and direction for Metocean drifter 41646.


Fig. 4.1.6 Drifter wind speed and direction for ADOS drifters (a) 41919, and (b) 41920.


Fig. 4.1.7 Drifter wind speed and direction for ADOS drifters (a) 41921, and (b) 41922.


Fig. 4.1.8 Drifter wind speed and direction for ADOS drifters (a) 41923, and (b) 41924.


Fig. 4.1.9 Drifter wind speed and direction for ADOS drifters (a) 41925, and (b) 41926.


4.2 ADOS wind speed data from drifter 41921

The sound pressure data from ADOS drifter 41921 is analyzed in detail, in order to derive wind speed data. During each data transmission 4 records of sound pressure level (spl) from four 15min sampling intervals are transmitted. Each measurement is based on 75sec of acoustic data.
Drifter 41921 transmitted useful data from 9/4 to 10/23/2004. Sound pressure levels for three frequency bands are presented: 0-2kHz (green), 6-10kHz (red), and 14-20kHz (blue). Wind speeds are computed based on the following formula, which should only be used for the 6-10kHz band:

wind speed (m/s) = { 10 (spl/20) + 104.5 } / 53.91 (Equation I)


Fig. 4.2.1 Derived wind speed and sound pressure levels for drifter 41921, 9/4-10/23/2004.

It is clear from the above figure that the Pacific Gyre speed values are based on the 6-10kHz band. It is also noteworthy that the sound pressure levels from all three frequency bands run almost parallel to each. Something changes in the acoustic noise on day 281 (10/6/2004): all frequency levels drop to lower values. This period will be scrutinized further in the comparison with collocated QSCAT data, in order to determine if the wind regime changed or whether the acoustic measurements cannot be trusted anymore. In the following figure, a few spikes in the 6-10kHz band have been removed; and in order to emphasize the similarities between all three frequency bands, the 0-2 and 14-20kHz bands have been shifted to match the 6-10kHz levels as best as possible by using the difference in mean spl level for the entire time period shown.

Fig. 4.2.2 Wind speed and spl; spikes removed in 6-10kHz, and shifted 0-2kHz and 14-20kHz.

Further down on this page the drifter observed wind speeds will be compared with collocated QSCAT. This comparison will be done for the 15min data, as well as for hourly estimates. It is useful to know that measured sound pressure levels do not change very much during four 15min intervals. In the following figure, histograms are presented for all three frequency bands that show the distribution of differences between the four 15min measurements. When no records are missing, there are six differences among the four values. Only 5-8% of the data have differences larger than 5 among the four measurements per hour. This fact could be used as a quality control measure to remove spiky measurement errors. Note that a change in spl of 5 units results in a change of wind speed of 1.5 m/s in wind regimes of 3-5m/s, but 5m/s in wind regimes of 8-12m/s, and 8m/s in wind regimes of 12-20m/s.
In the following figure, the number of differences ("Ndat") are listed at the top when the values exceed the maximum plot value. When the number of data exceed 0.5%, the percentage of data are listed as well.

Fig. 4.2.3 Histogram of differences among, usually 4, observed sound pressure levels per 1 hour.

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.

Fig. 4.2.4 Drifter 41921 6-10kHz derived wind speeds and collocated QSCAT (red dots).

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

Fig. 4.2.5 Drifter wind speed vs. QSCAT.

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.

Fig. 4.2.6 Wind speed differences vs. (a) time differences, and (b) 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.

Fig. 4.2.7 Conversion function between sound pressure and wind speed.

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.

Fig. 4.2.8 Hourly drifter 6-10kHz derived wind speeds and collocated QSCAT.

Fig. 4.2.9 Hourly wind speed differences vs. (a) time differences, and (b) distances.

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.

Fig. 4.2.10 (a) Hourly drifter wind speed vs. QSCAT, and (b) conversion function.


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.

Fig. 4.2.11 Subsample of hourly drifter 6-10kHz derived wind speeds and collocated QSCAT.

Fig. 4.2.12 Subsample of hourly wind speed differences vs. (a) time differences, and (b) distances.

Fig. 4.2.13 (a) Subsample of hourly drifter wind speed vs. QSCAT, and (b) conversion function.


4.3 Wind direction vs. QSCAT scatterplots

The drifter observed wind directions are compared with standard QSCAT direction. The average differences and rms are calculated separately for non-rain flagged QSCAT data, and rain-flagged QSCAT. The 1:1 line is plotted, as well as the non-rain rms lines.

Fig. 4.3.1 Wind direction scatterplot for Metocean drifters (a) 41539, and (b) 41540.


Fig. 4.3.2 Wind direction scatterplot for Metocean drifters (a) 41541, and (b) 41542.


Fig. 4.3.3 Wind direction scatterplot for Metocean drifters (a) 41543, and (b) 41544.


Fig. 4.3.4 Wind direction scatterplot for Metocean drifters (a) 41545, and (b) 41590.


Fig. 4.3.5 Wind direction scatterplot for Metocean drifter 41646.


Fig. 4.3.6 Wind direction scatterplot for ADOS drifters (GTS data) (a) 41919, and (b) 41920.


Fig. 4.3.7 Wind direction scatterplot for ADOS drifters (GTS data) (a) 41921, and (b) 41922.


Fig. 4.3.8 Wind direction scatterplot for ADOS drifters (GTS data) (a) 41923, and (b) 41924.


Fig. 4.3.9 Wind direction scatterplot for ADOS drifters (GTS data) (a) 41925, and (b) 41926.


The above figures for ADOS wind directions are based on GTS data. The GTS data have only been corrected for the battery effect. PACIFICGYRE data have also been used to derive wind speeds. These data include all 15min records, not just the latest transmission as in the GTS data. PACIFICGYRE data have been corrected for the battery effect and also for magnetic declination. The resulting scatterplots with collocated QSCAT are presented below.

Fig. 4.3.10 Wind direction scatterplot for ADOS drifters (PAC data) (a) 41919, and (b) 41920.


Fig. 4.3.11 Wind direction scatterplot for ADOS drifters (PAC data) (a) 41921, and (b) 41922.


Fig. 4.3.12 Wind direction scatterplot for ADOS drifters (PAC data) (a) 41923, and (b) 41924.


Fig. 4.3.13 Wind direction scatterplot for ADOS drifters (PAC data) (a) 41925, and (b) 41926.



4.4 Wind speed vs. QSCAT scatterplots

Drifter observed wind speeds are compared with standard QSCAT. The average differences and rms are calculated separately for non-rain QSCAT flagged data, and rain-flagged QSCAT. The 1:1 line is plotted, as well as the non-rain rms lines. Drifters 41646, 41919, 41920, 41923, 41924, and 41925 do not have good speed data. Drifters 41921, 41922, and 41926 are questionable.

Fig. 4.4.1 Wind speed scatterplot for Metocean drifters (a) 41539, and (b) 41540.


Fig. 4.4.2 Wind speed scatterplot for Metocean drifters (a) 41541, and (b) 41542.


Fig. 4.4.3 Wind speed scatterplot for Metocean drifters (a) 41543, and (b) 41544.


Fig. 4.4.4 Wind speed scatterplot for Metocean drifters (a) 41545, and (b) 41590.


Fig. 4.4.5 Wind speed scatterplot for ADOS drifters (a) 41921, and (b) 41922.


Fig. 4.4.6 Wind speed scatterplot for ADOS drifter 41926.


The wind speed statistics for drifter vs. QSCAT collocations are summarized in the next table. The comparisons are only shown for those Metocean drifter which are used to plot wind speed near the hurricane center ( Drifter data in Hurricane Momentum Equation ). Sub-samples are generated by only including differences within two standard deviations of the average difference of each drifter.

Table 4.4.1: METOCEAN collocated wind speed differences
No-rain data Sub-sample
Drifter days Ndat av Diff st.dev. Nsub av Diff st.dev.
41539 38 51 -1.0 1.7 49 -1.3 1.0
41540 15 19 -0.3 1.8 17 -0.8 1.2
41541 38 53 -0.4 3.3 47 -1.5 1.2
41542 37 47 -0.5 3.2 45 -1.1 1.5
41543 38 49 -0.7 2.2 48 -0.9 1.7
41544 37 57 -0.2 1.3 55 -0.3 1.2
41545 37 50 -0.4 2.8 49 -0.7 1.5
average -0.9 1.3


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.

Fig. 5.1.1 QSCAT wind vectors during Frances, on (a) 9/1/2004 at 10:30, and (b) at 22:55.

Fig. 5.1.2 QSCAT wind vectors during Frances, on (a) 9/2/2004 at 10:10, and (b) at 22:30.

Fig. 5.1.3 QSCAT wind vectors during Frances, on (a) 9/3/2004 at 9:40, and (b) at 23:00.

Fig. 5.1.4 QSCAT wind vectors during Frances, on (a) 9/4/2004 at 10:50, and (b) at 23:20.

Fig. 5.1.5 QSCAT wind vectors during Frances, on (a) 9/5/2004 at 10:30, and (b) at 22:50.


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

David Long from BYU has created high-resolution images from QSCAT surface observations for several hurricanes at ftp://ftp.scp.byu.edu/outgoing/data/pn/. In the following figures, wind barbs are conventional (JPL L2B) 25 km winds on a 25 x 25 km grid. Black barbs are no-rain while white are flagged as rain by the QuikSCAT processing code. The rain-flagging is not meant as an accurate representation of all rain, but only as an indication of suspicious wind retrievals. The ultra-high resolution wind speeds are computed on a 2.5 x 2.5 km grid.

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.

Fig. 5.2.1 9/16/2004 10:45 UTC, QSCAT high-res wind speed and drifter obs (a), and drifters obs only (b).


Fig. 5.2.2 9/16/2004 22:42 UTC, QSCAT high-res wind speed and drifter obs (a), and drifters obs only (b).


Fig. 5.2.3 9/17/2004 10:12 UTC, QSCAT high-res wind speed and drifter obs (a), and drifters obs only (b).


Fig. 5.2.4 9/17/2004 22:09 UTC, QSCAT high-res wind speed and drifter obs (a), and drifters obs only (b).


Fig. 5.2.5 9/20/2004 10:46 UTC, QSCAT high-res wind speed and drifter obs (a), and drifters obs only (b).


In fig.5.2.5 and following figures, the location of drifters with SST and air pressure measurements are also indicated, in addition to drifters with wind direction and speed measurements.

Fig. 5.2.6 9/20/2004 22:42 UTC, QSCAT high-res wind speed and drifter obs (a), and drifters obs only (b).



6. Drifter observations relative to hurricane center

6.1 SST relative to hurricane center of Frances

All drifter observed SST data are plotted relative to the hurricane center. SST is shown as SST change, which is defined as observed SST minus pre-hurricane SST. All available drifters with good SST change data within the vicinity of the hurricane center are used: 13 Minimet, 5 ADOS, 7 Metocean, 12 SVP-100m, 3 SVP-15m, 9 SST, and 1 SST&pres: Total = 50 drifters. The first plot shows the data locations relative to hurricane center, fig. 6.1.1(a). Printed are the last 2 digits of the drifter ID. The color indicates the SST change: red > -0.3, orange -0.3 to -0.9, green -1.0 to -1.4, and blue < -1.5. The drifter SST data in these plots cover about 7 days, from 8/31 - 9/7/2004.

Fig. 6.1.1 Drifter SST change during passage of Frances, (a) drifter ID, and (b) averaged data on 50x50km grid.

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.

Fig. 6.1.2 Drifter SST change during passage of Frances, (a) all 50 drifters, and (b) only 47 drifters.

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:

pressure = f(dist) = p0 + (pe-p0) · e-(a/distb) ,

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.

Fig. 6.2.1 Pressure curve fits during Frances, combined north-west and south-east quadrants.

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.

Fig. 6.3.1 Wind directions relative to hurricane center Frances, 0-800km.


last modified on March 11, 2009
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