Acoustic Rain Gauges
Acoustic Rain Gauges (ARGs) have been designed and built at the Applied
Physics Laboratory for autonomous deployment on ocean surface moorings. An ARG contains an ITC-8263 hydrophone,
signal pre-amplifiers and a recording computer (Tattletale-8). The nominal
sensitivity of these instruments is -160 dB relative to 1 V/mPa and the
equivalent oceanic background noise level of the pre-amplifier system is about
28 dB relative to 1 mPa2Hz-1. Band-pass filters are present to reduce
saturation from low frequency sound (high pass at 300 Hz) and aliasing from
above 50 kHz (low pass at 40 kHz). The ITC-8263 hydrophone sensitivity also
rolls off above its resonance frequency, about 40 kHz. A data collection
sequence consists of four 1024 point time series collected at 100 kHz (10.24 ms
each) separated by 5 seconds. Each time
series is fast Fourier transformed (FFT) to obtain a 512-point (0-50 kHz) power
spectrum. These four spectra were averaged together and spectrally compressed
to 64 frequency bins, with frequency resolution of 200 Hz from 100-3000 Hz and
1 kHz from 3-50 kHz. These spectra are evaluated individually to detect the
acoustic signature of rainfall and then are recorded internally.
The overall
temporal sampling strategy is designed to allow the instrument to record data
for up to one year without servicing and yet detect the relatively short
rainfall events present in the tropics (Nystuen, 1998). In order to achieve this, the ARG is designed
to enter a low power mode "sleep mode" between each data sample. For these deployments, the ARGs
"sleep" for 9 minutes and then sample the sound field. If
"rain" is detected, the sampling rate changes to 1 minute (or 3
minutes if "drizzle" is detected) and stays at the higher sampling
rate until rain is no longer detected.
Some "noise" will trigger the high sampling mode and must be
removed from the data.
