Tropical storm Jeanne. Wind Vel. and platform oscillation
wind speed versus time for 17 hours starting at 14:30 AST
15sep04 (.ps) (.pdf)
tension in tiedown 12 versus hour of day for 15sep04 (.ps).
magnitude and frequency of the oscillations(.ps)
Tropical storm jeanne passed thru the observatory on
15sep04 around 17:00 hours. The wind speed was recorded using john
hagen's wind monitor on the platform. The vertical
oscillation frequency of the platform was measured using the load
cells on tiedown 12.
The wind speed:
Jon hagen's wind speed monitor was running during
the storm. The program that normally monitors the data was not
writing to disk. I took the analog front panel output and sampled it
using the radar interface. The setup was :
The plots show the
wind speed versus time for 17 hours starting at 14:30 AST 15sep04
(extending till 7:45 16sep04).
- front panel output (14 volts = 140 mph) to a 4db attenuator,
then to an opamp with times 2 voltage gain, and then into the
radar interface which is a 50 ohm load.
- The data was sampled at 10 hz using the 12 bit a/d. It has a
maximum input voltage of 2.5 volts.
- To calibrate the measurement I read the voltage off a dvm
while the operator called out the wind speed. A linear fit of
windspeed vs a/d count was done to 25 of these readings.
- The top plot is the wind speed versus hour of day. 24 Hours
is the start of 16sep04. The data has been smoothed to 1 second.
The wind meter peak hold showed a maximum value of 60 Mph so the
calibration may have been off by 30% or the peak hold may have a
longer time constant on it. The dropout at 16:15 to 17:00 was
caused by someone removing the cable.
- The bottom plot shows the calibration data and the linear fit
to the data. The fit coef's were vel=1.743+.0365*AtoDcounts.
The tiedowns have loadcells connected to each
cable (2 cables per tiedown). This data is read at a 1 hz rate by
the computer. The tiedowns had been released to 3 inches to lower
the tension on the platform in preparation for the storm.On 15sep04
at 14:00 tiedown 4 and tiedown 8 went off line when the breakers on
tiedown 4 tripped. I used the tension readings from tiedown 12 since
they were available for the entire day.
The azimuth arm was locked in an east-west position for the storm.
Td 12 is perpendicular to this in the north corner of the triangle.
The first plot shows the
tension in tiedown 12 versus hour of day for 15sep04 (.ps)
tension was slowly varying (from temperature) until 8 am. After
8am, the tension began to jump around. This jumping reached a
maximum at 17:30. The deviations got up to 8 kips (in this 1
To compute the oscillation strength, 1 hour at
a time (3600 1 second samples) was transformed and then the
magnitude was computed: osc=abs(fft(tiedownTensionFor1Hour)). The
second set of plots shows
the strength of the oscillations (.ps) (.pdf).
When hurricane george passed in 1998, the oscillation
frequencies were measured with all of the tiedowns, and with
the tilt sensors in the dome (they sampled at 5 hz). The tiedowns
measured a .35 hz oscillation while the tilt sensors found a .63
hz oscillation. The .35 Hz td freq (1 hz sampling) is most
likely an alias of 1-.35=.65 hz/
- Fig 1 plots the oscillation strength versus frequency for 1
hour steps starting at 8:30 (bottom) and going to 23:30 (top).
The frequencies .328, .354, and .450 Hz have been flagged for
reference. Power in these frequencies start to increase at 8:30
am and continue until they hit a maxi ma at 18:30.
- Fig 2-4 plots the oscillation strength from 6:30 thru 23:30 .
The scale on the left is kips (kilo pounds). The frequency
resolution is .2 millihz.
processing: x101/040916/doit.pro, td.pro, doit.pro
|date of measurement
after undoing alias.