Drifters in Hurricanes Fabian and Isabel, September 2003


On September 3, 2003 eleven Minimet drifters were successfully deployed at a distance of about 18 hours in front of the projected path of a category-4 hurricane, Fabian, in the vicinity of 25°N, 65°W. Eight of them provided useful data: seven drifters transmitted data for about 5 months, one drifter for 1.5 months. SST, air pressure, ambient noise (to derive wind speed), wind direction, and position data have been analyzed. Wind directions are compared with scatterometer observations from SeaWinds on QuikSCAT and ADEOS-II (SWSA2). Drifter measured ambient noise has been used to derive wind speed by calibrating against collocated scatterometer data. Detailed comparisons are made for the passage of hurricanes Fabian and Isabel over or near the Minimet drifters.

Quality-controlled data files for 8 drifters are available for SST, air pressure, wind direction and wind speed. Each data file contains interpolated drifter positions.
The data files are available here: Fabian Drifter Data .

Table of Contents
Summary of Data
1. Drifter Tracks
2. Drifter Tracks and Track of Hurricane Fabian
3.a Satellite Winds and Hurricane Fabian
3.b Satellite Winds and Hurricane Isabel
4. Drifter Air Pressure
5. Drifter Sea Surface Tempearture
6. Drifter Wind Directions
7. Drifter Data relative to Hurricane Center

Click on images for enlarged versions of plots.

Summary of Data

A summary of all drifter measured data (after quality control) is presented in the following table: Pos = position, Pres = air pressure, SST = sea surface temperature, Dir = wind direction, Noise = acoustic noise (to derive speed), Min = last date when all sensors were still working, Days = number of days corresponding to Min.

Table 1.1: Summary of all Drifter Data
Ending Dates Number of Obs
Drifter Pos Pres SST Dir Noise Min Days Pos Pres SST Dir Noise
41578 2/07 2/03 2/06 2/02 2/06 2/02 151 1098 6879 1744 6951 6846
41581 2/04 2/04 2/04 2/04 2/04 2/04 154 1147 6965 1747 6926 6934
41582 2/04 2/02 2/04 2/04 2/04 2/02 152 1309 7272 1836 7326 7310
41583* 10/24 10/04 10/25 10/25 10/22 10/04 30 105 1177 375 1477 1415
41584 2/14 2/11 2/06 2/13 2/13 2/06 155 1231 7335 1861 7404 7406
41585 1/22 1/13 1/22 1/22 1/22 1/13 132 1038 6061 1599 6352 6261
41587 2/25 2/23 2/25 2/25 1/09 1/09 129 1311 7884 1998 7948 5999
41588 1/30 1/28 1/30 1/30 1/30 1/28 146 1151 6417 1714 6808 6811
av days* 146
Nobs/day* 8 46 11 46 45

* Note: Drifter 41583 is not included in the averages.

During each successful data transmission, drifters usually report four 15-min averages from the last hour for pressure, direction, and acoustic noise. SST gets reported only once, from the last 15 min. So on average, there are about 11 transmissions per day, with about 8 successful positionings per day.

The daily sampling frequency is quite complex, and depends mostly on successful communications with the ARGOS network. The sampling pattern for drifter 41578 is depicted in Figures 1.0 a&b, which are representative of all drifters. Note, that after day 310 (11/6/2003), there start to be sampling gaps between 8-12 and 19-23 UTC, corresponding to local times of 3-7am and 2-6pm.

Fig. 1.0 Sampling from drifter 41578, for (a) wind direction, and (b) SST.

1. Drifter Tracks

Below are plots of eight drifter tracks: 41578, 41581, 41582, 41583 (stops after 51 days: 9/3-10/24/03), 41584, 41585, 41587, and 41588. All data have been processed. On average, for each drifter there are about 8 position data per day, except for drifter 41583 which has only about 2 positions per day. Every day is indicated with a black dot (interpolated in time), every 10 days are marked in red and labeled. For quality control, all instances when the drift velocity exceeded 100 cm/s are marked with small red numbers. None of those were eliminated because they do not seem to indicate large erratic movement of the drifters. For collocation purposes, the drifter locations will be compared with satellite observations that have a resolution of only 25km x 25km.

Three pairs of drifters were deployed in close vicinity to each other, and some of them followed similar paths for several days. The three pairs are: 41578 & 41585, 41581 & 41583, and 41584 & 41588. In the following pictures, the latitude and longitude limits are chosen so that the resolution and lon/lat ratio is the same for all plots.

Fig. 1.1 (a) Drifter 41578, and (b) Drifter 41585.

Fig. 1.2 (a) Drifter 41581, and (b) Drifter 41583.

Fig. 1.3 (a) Drifter 41584, and (b) Drifter 41588.

Fig. 1.4 (a) Drifter 41582, and (b) Drifter 41587.

2. Drifter Tracks and Track of Hurricane Fabian

In the plots below, drifter tracks are shown relative to hurricane locations. Open squares indicate starting positions, and solid dots are the last reported positions. The data time period for each drifter is provided below the the plots, in days starting on 1/1/2003.

The track of the hurricane is indicated by two different data sets. The red track (with crosses at data locations) is from NOAA's preliminary Tropical Cyclone Report (for reference, see Unisys' 2003 Hurricane/Tropical Data for Atlantic). Those location data are available about every 3 hours. They are labeled with the day in September, hr Z, and the reported wind speed (in m/s). The blue track, with diamonds, indicates approximate hurricane locations inferred from SeaWinds swaths. At those locations, the time is in days in 2003, hr:min Z; satellite names (QSCAT or SWSA2),\ and revolution number are also provided. In addition, there are lines plotted connecting the SeaWinds hurricane center locations with NOAA centers, interpolated to the SeaWinds time. Plots of the surface wind field provided by these satellite swaths are provided further below on this page.

Fig.2.1 Drifter tracks for 9/4/2003 to 1/6/2004 (124 days).

Fig.2.2 (a) Drifter tracks for the first 20 days, and (b) the first 2 days.

Fig.2.3 Drifter tracks for the first 20 days, and paths of hurricanes Fabian, Isabel, and Juan.

3.a. Satellite Winds and Hurricane Fabian

The scatterometer overflight data swaths in the area of the drifters during Fabian, a category-4 hurricane with maximum winds of 64m/s, are presented below. The data come from SeaWinds on QSCAT and SeaWinds on ADEOS-II (SWSA2). Each satellite measured the center of the hurricane about every 7 revolutions, i.e. every 11:47 hr (except after the first rev, when the next two revs missed the center). The coverage of the two satellites in the area of interest is almost identical: QSCAT's swath area is shifted by only 1° to the west of SWSA2's swath. QSCAT follows SWSA2 7:20 hr later. The more recently deployed SWSA2 instrument has been tuned to perform as close to QSCAT measurements as possible.

Drifter observations are shown (with blue vectors), when there were observations within 60min of the satellite data. The time differences (drifter-scatterometer) are indicated below the figures. These preliminary plots do not include any drifter wind speed data (inferred from measured ambient noise). In thses plots the drifter wind vectors are drawn with a speed of 20m/s.

The satellite data are either from ascending tracks (Southeast to Northwest, e.g. SWSA2 3741 and QSCAT 21898) or descending tracks (Northeast to Southwest, e.g. SWSA2 3763 and QSCAT 21820).

Also plotted are the hurricane centers inferred from the scatterometer wind vectors (black crosses) and the time-interpolated hurricane centers from NOAA (red crosses). Those locations are used to plot the hurricane tracks in Figures 2.1 and 2.2. Note, that in almost all cases the scatterometer locations are to the Southwest of the NOAA locations.

Both scatterometers employ a rain detection algorithm that is based on the radar backscatter signal. Rainflagged vectors are plotted in red and green. Previous analysis has indicated that the rain effect is diminished above wind speeds of 15m/s. Rain-flagged vectors with speeds of less than 15m/s are plotted in red, vectors with speeds &ge 15m/s are plotted in green. It appears, however, from these figures that for many wind vector cells (WVC) the rain-flagged vectors fit nicely into the surrounding non-flagged wind field. The rain-effect is most strongly observed north of the hurricane center where the wind vectors appear to be more East-West than expected. Those erroneous directions are due to the rain effect on the radar backscatter which tends to align wind vectors in a cross-track direction. It is also apparent that the abrupt (seemingly non-physical) change in the measured flow field to the Northwest of the hurricane center seems to be caused by the rain effect: in this area the scatterometer wind vectors turn abruptly from a East-West direction to a North-South direction. The same blocky flow happens to the Southwest of the hurricane center, but not quite as badly as Northwest of the center. There are instances, however, when the the circular nature of the wind field around the hurricane center is quite well represented by the scatterometer measurements, see e.g. QSCAT rev 21948, SWSA2 3791, and QSCAT 21927.

The maximum windspeeds measured by the scatterometers are much less than NOAA's reported maximum speeds for the hurricane, as indicated on the figures. On average, the reported maximum wind speeds are 16m/s smaller for QSCAT, and 19m/s smaller for SWSA2.

Fig. 3.1 (a) SWSA2 rev 3741 on 9/2/2003 at 14:25, and (b) QSCAT rev 21898 at 21:41.

Fig. 3.2 (a) SWSA2 rev 3763 on 9/4/2003 at 2:51, and (b) QSCAT rev 21920 at 10:15.

Fig. 3.3 (a) SWSA2 rev 3770 on 9/4/2003 15:13, and (b) QSCAT rev 21927 at 22:34.

Fig. 3.4 (a) SWSA2 rev 3777 on 9/5/2003 at 2:25, and (b) QSCAT rev 21934 at 9:46.

Fig. 3.5 (a) SWSA2 rev 3784 on 9/5/2003 at 14:47, and (b) QSCAT rev 21941 at 22:07.

Fig. 3.6 (a) SWSA2 rev 3791 on 9/6/2003 at 2:02, and (b) QSCAT rev 21948 at 9:22.

3.b. Satellite Winds and Hurricane Isabel

During the time period of 9/12 to 9/18/2003 the category-5 hurricane Isabel (with maximum wind speeds of 72m/s) crossed the North Atlantic very close to the deployed drifters. Collocated wind vector plots are presented below. The plots are similar to the fabian plots, except that there are no scatterometer inferred hurricane centers, i.e. there are no black crosses marking these locations. It appears that the scatterometer derived wind fields indicate hurricane centers very close to the centers identified by NOAA (red crosses).

From drifter-to-drifter comparisons (see Fig. 6.15a), it has become clear that the wind direction measurements on drifter 41583 are not error free. It appears that there is an offset that grows over time. When compared with the nearby deployed drifter 41581, this offset has been determined to be 7.9° for days 247-252.4. It is 14.3° for days 252.4-258, and 32.4° for days 258-267. The original measurements from drifter 41583 are drawn in magenta, and the corrected directions are plotted in light blue. The correction amounts to a counter-clockwise rotation of about 8-32°. Drifter 41583 appears in 15 plots with wind vectors from QSCAT and ADEOS-II. Assuming that the scatterometer wind fields are close to truth (in the vicinity of the difters), the corrected 41583 wind directions are a better match in 12 cases, are about the same in 2 cases, and look worse in only one case.

Fig. 3.7 (a) QSCAT rev 22034 on 9/12/2003 at 10:08, and (b) SWSA2 rev 3884 at 15:17.

Fig. 3.8 (a) QSCAT rev 22041 on 9/12/2003 at 22:32, and (b) SWSA2 rev 3891 on 9/13 at 2:28.

Fig. 3.9 (a) QSCAT rev 22048 on 9/13/2003 at 9:39, and (b) SWSA2 rev 3898 at 14:49.

Fig. 3.10 (a) QSCAT rev 22055 on 9/13/2003 at 22:03, and (b) SWSA2 rev 3905 on 9/14 at 2:00.

Fig. 3.11 (a) QSCAT rev 22062 on 9/14/2003 at 9:13, and (b) SWSA2 rev 3912 at 14:24.

Fig. 3.12 (a) QSCAT revs 22069 & 22070 on 9/14/2003 at 21:37-23:20, and (b) SWSA2 revs 3919 & 3920 on 9/15 at 1:35-3:18.

Fig. 3.13 (a) QSCAT rev 22077 on 9/15/2003 at 10:28, and (b) SWSA2 rev 3927 at 15:39.

Fig. 3.14 (a) QSCAT rev 22084 on 9/15/2003 at 22:51, and (b) SWSA2 rev 3934 on 9/16 at 2:50.

Fig. 3.15 (a) QSCAT rev 22091 on 9/16/2003 at 10:02, and (b) SWSA2 rev 3941 at 15:14.

Fig. 3.16 (a) QSCAT rev 22098 on 9/16/2003 at 22:26, and (b) SWSA2 rev 3948 on 9/17 at 2:25.

Fig. 3.17 (a) QSCAT rev 22105 on 9/17/2003 at 9:36, and (b) SWSA2 rev 3955 at 14:49.

4. Drifter Air Pressure

All drifter measured air pressure data have been processed. On average, for each drifter there are about 46 air pressure observations per day. Whenever a contact is made with ARGOS, the most recent 4 selected values (from four 15min measuring intervals) are transmitted for pressure (as well as for wind direction and acoustic noise). For SST, only the most recent value is sent. That means for pressure (and wind direction and speed), about every other hour is sampled continuously for four 15min intervals.
Table 4.1: Pressure Quality Control (statistics for first 112 days)
Drifter N read N repeat N elim N bad N plot
41578 9501 4425 168 15 4893
41581 20778 15437 389 11 4941
41582 26325 20729 303 6 5287
41583 2083 1064 60 28 931
41584 22739 17332 330 1 5076
41585 22375 16934 421 11 5009
41587 20580 15153 388 13 5026
41588 20907 15922 287 27 4671

Fig. 4.1 Air Pressure 41578, pre-quality control first 112 days, including "Nbad" spikes.

Six drifters were released in pairs and are plotted next to each other: 41578 & 41585, 41581 & 41583, and 41584 & 41588.

Fig. 4.2 (a) Air Pressure 41578 and (b) 41585.

Fig. 4.3 (a) Air Pressure 41581 and (b) 41583.

Fig. 4.4 (a) Air Pressure 41584 and (b) 41588.

Fig. 4.5 (a) Air Pressure 41582 and (b) 41587.

Intercomparison of all air pressure data shows that drifter 41585 needs to adjusted by subtracting 1mb. This has been done for the final data release ("41585.pres10.pos").

5. Drifter Sea Surface Temperature

On average, for each drifter there are about 11 SST observations per day, or about one every 2 hours.
Table 5.1: SST Quality Control (statistics for first 118 days)
Drifter N read N min N repeat N elim N bad N plot
41578 2513 4 1210 7 7 1285
41581 5501 13 4152 29 4 1303
41582 6910 4 5496 17 5 1388
41583 678 3 353 6 0 316
41584 5986 3 4603 46 3 1331
41585 5865 6 4502 39 1 1317
41587 5436 4 4073 34 1 1324
41588 5956 11 4596 27 3 1319

Fig. 5.1 SST 41578, pre-quality control, first 118 days, including "Nbad" spikes.

Fig. 5.2 (a) SST 41578, and (b) SST 41585.

Fig. 5.3 (a) SST 41581 and (b) 41583.

Fig. 5.4 (a) SST 41584 and (b) 41588.

Fig. 5.5 (a) SST 41582 and (b) 41587.

5.a SST immediately after hurricane Fabian

The following plots examine SST during passage of hurricane Fabian and for several days afterwards. The hurricane produces a cooling of the SST, and this cooling persists for at least seven days in the wake of the hurricane trail.
Fig. 5.6 SST of eight drifters during the first eleven days.

Three pairs of drifters stay relatively close together for several days after the passage of the hurricane. In the drifter plot below, the coloring scheme is the same as in Fig. 5.6 for SST. Every day is marked with a small circle. Day 250 is labeled. Also indicated is the track of hurricane Fabian.

Fig. 5.7 Drifter tracks during the first 10 days after deployment (squares mark start of tracks).

The SST measurements of three pairs of drifters are illustrated below. The three pairs are 41578 & 41585, 41581 & 41583, and 41584 & 41588. The top panel shows SST measurements when recorded within 30min by both drifters. The difference is shown in the middle panel (with the 2-day average of the difference superimposed in red). The bottom panel depicts the distance between the drifters (in km), with the 2D tracks superimposed. Squares mark the deployment locations. The North & East vectors indicate geographic orientation, and provide a 20km scale length for the tracks. On the 2D tracks every 5 days are labeled (on the black tracks); the corresponding day on the red track is marked with a red dot only.

The 41581&41583 pair remained within 50km of each other for 50 days, when drifter 41583 stopped reporting data. The data for this pair is presented in two figures: for the first 30 days, and also for the entire 50 day record.

Fig. 5.8 SST of nearby drifters: (a) 41578 & 41585, and (b) 41584 & 41588.

Fig. 5.9 SST of nearby drifters 41581 & 41583: (a) first 30 days, and (b) 50 days.

Relatively constant temperature differences between nearby drifters might indicate erroneous offsets of some or all instruments. Drifter 41585 is about 0.2°C warmer than 41578, until day 252.5, when the drifters are starting to move further apart than 5km.

SST from drifter 41588 is a relatively constant 0.25°C warmer than 41584, up to a distance of 30km on day 257, when the drifters are starting to move into different directions.

The relationship between 41583 and 41581 is a little more complicatead. The two drifters stay within 20km for the entire data record of 41583, but the temperature difference between the two records changes from ca. 0.5°C during the first 11 days (days 247-259) to 0.2 °C during the last 32 days of drifter 41583 (days 265-297). Note that the largest diurnal SST change observed by both drifters (at day 270), of about 3.5°C, is measured by both drifters within 0.4°C.

In order to illustrate the SST cooling from the hurricane, and the slow warming of SST in the wake of the hurricane, the 2-day averaged SST are plotted below for the first 11 days. Fig. 5.10a shows the 2-day averages for all drifters. In Fig. 5.10b nearby drifters have been averaged.

Fig. 5.10 2-day averaged SST: from (a) all drifters, and (b) averaged pairs.

To illustrate the geographic pattern of SST changes relative to the track of the Fabian, the 2-day averaged SST data are plotted vs. distance from the track and vs. time, for the first 7 days after the hurricane passes through the deployment array.

Fig. 5.11a shows (SST-27) for all drifters. This figure shows how close several drifters stayed to each other during the first week of measurements. In fig.5.11b nearby drifter pairs have been averaged. It becomes apparent that the largest cooling occurs to the right of the hurricane track, of at least 1.5°C. To the left of the track the cooling amounts to only 0.6°C.

Fig. 5.11 2-day averaged SST relative to hurricane track: from (a) all drifters (SST-27),
and (b) averaged pairs.

6. Drifter Wind direction

On average, for each drifter there are about 46 wind direction observations per day. As in the case of pressure, that corresponds to 4 measurements (sampling an hour continuously for four 15min averaging intervals) about every other hour. Drifter 41577 recorded wind directions for only 1.5 hr on September 4.

Table 6.1: Wind Direction Quality Control (statistics for first 124 days)
Drifter N read N 0 N 360 N repeat N elim N plot
41578 10684 156 36 5023 91 5379
41581 23180 176 56 17323 227 5398
41582 28952 176 43 22770 185 5778
41583 2812 76 12 1401 61 1262
41584 25156 204 69 19135 213 5535
41585 24572 168 77 18541 316 5470
41587 23052 148 90 16981 312 5521
41588 24948 144 72 19040 200 5492
41577 96 68 0 16 4 8

Fig. 6.1 U (black) and V (red) components of wind directions from 41578.

Fig. 6.2 Wind direction from drifter 41578, (a) days 245-285, and (b) days 285-325.
Fig. 6.3 Wind direction from drifter 41578, (a) days 325-365, and (b) days 365-405.

Fig. 6.4 Wind direction from drifter 41581, (a) days 245-285, and (b) days 285-325.
Fig. 6.5 Wind direction from drifter 41581, (a) days 325-365, and (b) days 365-405.

6.a Collocated Scatterometer wind directions

In the following figures, the 1-hourly averaged drifter wind directions are plotted with collocated QSCAT and ADEOS-II scatterometer data (black solid dots) for the first 14 days. Rain-flagged scatterometer are marked with open squares.

Fig. 6.6 1-hourly averaged wind directions, for drifters (a) 41578, and (b) 41585.

Fig. 6.7 1-hourly averaged wind directions, for drifters (a) 41581, and (b) 41583.

Fig. 6.8 1-hourly averaged wind directions, for drifters (a) 41584, and (b) 41588.

Fig. 6.9 1-hourly averaged wind directions, for drifters (a) 41582, and (b) 41587.

Below are presented scatterplots of collocated drifter and scatterometer wind direction observations. Any pair of observations within 1hr and 100km are allowed. On average, the separation was 12-19km and 3-9min. All rain-flagged scatterometer data are marked in red. The dashed lines indicate the rms of the data around the 1:1 fit (for the no-rain data only).

Fig. 6.10 Drifter vs. scatterometer wind directions, for drifters (a) 41578, and (b) 41585.

Fig. 6.11 Drifter vs. scatterometer wind directions, for drifters (a) 41581, and (b) 41583.

Fig. 6.12 Drifter vs. scatterometer wind directions, for drifters (a) 41584, and (b) 41588.

Fig. 6.13 Drifter vs. scatterometer wind directions, for drifters (a) 41582, and (b) 41587.

The collocation statistics for SeaWinds on QSCAT and ADEOS-II are compared in table 6.2: days = number of collocation days, Ndat = number of collocation pairs, av Dist = average separation distance (km), av Tdiff = average time difference (scat-drifter, minutes), av Diff = average wind direction difference, rms = rms of wind direction differences, scat = scatterometer (Q=QSCAT, A = ADEOS-II). SeaWinds on ADEOS-II data are only available until Oct 24, 2003, so that the ADEOS-II collocation data set is shorter than the QSCAT data set.

Table 6.2: QSCAT and ADEOS-II collocated Wind Directions
Drifter days Ndat av Dist av Tdiff av Diff rms scat
41578 155 563 12.1 -8.6 -3.4 34.3 Q
50 277 16.8 -7.7 -3.7 39.9 A
41581 153 563 13.6 -9.1 3.7 36.2 Q
50 247 12.9 -5.3 9.0 41.0 A
41582 153 558 13.5 -9.2 0.1 43.7 Q
50 354 11.9 -7.2 -1.6 47.0 A
41583 50 253 12.9 -4.5 26.2 54.3 Q
50 116 15.5 4.7 27.8 40.5 A
41584 162 497 13.2 -8.1 0.5 40.5 Q
50 302 12.4 -4.1 -5.8 23.3 A
41585 140 492 12.4 -7.5 1.2 29.5 Q
50 277 12.9 -4.6 -4.5 26.6 A
41587 174 559 14.6 -9.8 0.0 37.0 Q
50 330 12.0 -6.6 -1.5 28.2 A
41588 148 523 16.4 -6.6 2.7 36.3 Q
50 312 19.2 -5.5 -11.4 46.7 A

A summary of the combined QSCAT and ADEOS-II collocated data is presented in Table 6.3, contrasting no-rain data with rain-flagged data. Drifter 41583 has a bias in its wind direction measurements, and presents an outlier among this group of drifters. It transmitted wind direction data for only 50 days, while all other drifters were active for about five months. The average difference of all other 7 drifters in wind direction of no-rain data is -0.6°, with an rms of 35°. Rain-flagged data have a bias of -12°(drifter - scat), i.e. drifter wind directions are, on average, 12° counterclockwise from the scatterometer observations; with an rms of 54°.

Table 6.3: Collocated Wind Direction Differences
No Rain Rain
Drifter av Diff rms N av Diff rms N
41578 -3 34 421 -20 69 44
41581 5 35 409 -5 49 34
41582 -1 40 433 -8 62 62
41583* 23 51 207 50 60 17
41584 0 32 419 -18 67 40
41585 -1 29 379 -4 23 40
41587 -1 35 453 -5 45 28
41588 -3 38 427 -18 42 45
av/total* -1° 35° 2941 -12° 54° 293

* Note: Drifter 41583 is not included in the averages and totals.

When the drifter wind directions are averaged with a sliding 1-hour window, the comparisons with collocated scatterometer data do not change very much. Average differences and rms are only slightly reduced.

Table 6.4: Summary of Collocated 1-hourly Averaged Wind Direction Differences
No Rain Rain
Drifter av Diff rms N av Diff rms N
av/total* 33° 2941 -10° 50° 293

6.b Drifter-to-Drifter Wind Direction Comparisons

Wind direction data of adjacent drifters are compared for 3 pairs of drifters: 41578 & 41585 , 41584 & 41588 , and 41581 & 41583 . The next three figures show the two time series in the top panel, the direction differences in the central panel (with the 2-day running average in red, and the average wind direction difference and rms values printed on the panel), and the distance between the drifters in the lower panel (in km). Data pairs were only plotted when the distance was less than 50km, and when the time difference between two drifter measurements was less than 7.5 min. Each drifter measures wind directions every 15min. Depending on the internal clock of the drifter measuring device, those times do not fall onto the same real time for all drifters.

Drifters 41581 and 41583 stayed within 50km of each other for the entire time that drifter 41583 was operational (9/4 - 10/24/2003). Drifter 41583 shows a growing bias with respect to drifter 41581, which is asumed to be error free. The bias grows from 7.9°(deployment until day 252.4) to 14.3°(days 252.4-258), and then increases further to 32.4°(days 258-267). After that day the differences are less systematic, and appear to depend on the prevailing wind direction measured by drifter 41583.

The (drifter-scatterometer) wind direction difference for 41583 is 23° and for 41581 it is 5°. The (drifter-drifter) difference for (41583-41581) is 19°. This agrees well with the scatterometer comparions, from which a value of 18° (= 23°-5°) would be expected.

Fig. 6.14 Wind direction differences, for (a) 41585-41578 (16days), and (b) 41588-41584 (13 days).

Fig. 6.15 Wind direction differences, 41583-41581, for (a) first 21 days, and (b) all 50 days.

7. Drifter Obervations relative to Huricane Center

7.1 Surface Air Pressure relative to Huricane Center

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 taken from the data to be 940mb, and pe to be 1020mb. All drifter observations are combined, both from the north-west and south-east quadrants. The resulting best-fit coefficients for Fabian are a=90.4 and b=1.18; and for Isabel a=30.7, b=0.96. Those data fits are shown here.

Fig. 7.1.1 Pressure curve fits for (a) Fabian, and (b) Isabel.

The pressure fit to just the south-east quadrant is shown below.

Fig. 7.1.2 Fabian pressure curve fits, south-east quadrant only.

The next figures show pressure data plotted on a lon/lat map relative to the hurricane center. Pressure data are written on the plots, and colored according to magnitude: lowest values are in red, largest in black. On the last figure, the Fabian data has been contoured by hand, to emphasize the distribution of the data.

Fig. 7.1.3 Pressure relative to hurricane center, for (a) Fabian, and (b) Isabel.
Fig. 7.1.4 Pressure relative to Fabian, with contour lines.

This plot indicates that the region of lowest pressure is found slightly to the south-east of the hurricane center.

7.2 Wind Directions relative to Huricane Center

Drifter observed wind directions are shown here.

Fig. 7.2.1 Wind direction relative to Fabian center.

last modified on march 12, 2009
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