Tropical storm Jeanne. Wind Vel. and platform oscillation
speed versus time for 17 hours starting at 14:30 AST 15sep04 (.ps)
in tiedown 12 versus hour of day for 15sep04 (.ps). (.pdf)
of the oscillations(.ps) (.pdf)
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 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 (.ps)
(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
Platform vertical oscillations.
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 tenstion in tiedown 12 versus hour of day for 15sep04 (.ps)
The 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 tiedown).
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
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 .57 hz oscillation. The .450 oscillation on 15sep04 is actually
.55 hz aliased down to .45 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 maxima 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
processing: x101/040916/doit.pro, td.pro, doit.pro
|date of measurement
after undoing alias.