WHOI-89-44
IMET TR-89-01
Improved Meteorological Measurements
from Buoys and Ships (!MET):
Preliminary Comparison of Precipitation Sensors.
by
Gennaro H. Crescenti
and
Robert A. Weller
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts 02543
October 1989
Technical Report
Funding was provided by the National Science Foundation under
Grant No. OCE-87-09614
Reproduction in whole or in part is permitted for any purpose ofthe
United States Government. This report should be cited as:
Woods Hole Oceanog. Inst. Tech. Rept., WHOI-89-44, IMET TR-89-01.
Approved for publication; distribution unlimited.
Approved for Distribution:
Abstract
Rainfall data obtained from an optical rain gauge and a capacitive siphon rain
gauge are analyzed and discussed. These sensors were developed for unattended use
and are being considered for use at sea on ships and buoys.
i
Table of Contents
1 Introduction and Motivation
3
2 Instrumentation . . . . . . .
3
2.1
Optical Rain Gauge
4
2.2
Siphon Rain Gauge .
5
2.3
Plastic Rain Gauge .
7
2.4
Tipping Bucket Rain Gauge .
7
3 Tests .
8
4 Data Analysis
8
5 Discussion . . .
9
Acknowledgements .
11
References
12
Appendix A
14
Figures . ..
15
1
1
Introduction and Motivation
Very little is known about distribution patterns and rainfall intensities over the
open oceans. What data has been obtained is subject to much speculation and large
inaccuracies. The reason for this problem is an instrumental one. Aside from the fact
that meteorological surface observations are sparse over the open waters, inaccuracies
arise from the types of rain gauges and the platforms on which they reside (Skaar,
1955; World Meteorological Organization, 1962). Error sources include the disturbing
effects of a ship or buoy on the air flow near the sensor, the disturbing effect of the
sensor itself on the air flow, the effect of the rocking motion of the ship or buoy, the
effect of sea spray leading to overestimates in rainfall, and the effect of forward
motion of a ship.
Some of this error could be reduced by improving the method of sensing rainfall.
This report discusses the preliminary findings of comparison testing of an optical rain
gauge and a self-siphoning rain gauge. Both of these gauges are relatively new in
design and are substantially different from the conventional tipping bucket or
weighing bucket rain gauges. These new gauges are developed for unattended use in
remote areas including use at sea.
2
Instrumentation
Four rain gauges were obtained for intercomparison testing. Besides the optical
and self-siphoning rain gauges, a simple plastic rain gauge and a tipping bucket rain
gauge were included as standards. Photographs of the rain gauges can be seen in
Figure 1. These sensors are described below.
3
2.1
Optical Rain Gauge
The design of the ORG-705 precipitation sensor is intended to overcome the
limitations and inaccuracies of conventional precipitation sensors, especially at the
extreme ranges of rainfall rate. This particular sensor is manufactured by Scientific
Technology Inc. (S.T.I., see Appendix A for manufacturer's address and
information). It is a single unit package with no moving parts developed for
unattended use with minimal maintenance. The unit is relatively small, measuring
under 1 m long, 0.5 m high, and 17 em wide. Total weight is 4.3 kg.
The ORG-705 uses an infrared emitting diode (IRED) as a light source. As
raindrops fall through the beam of light they induce optical scintillations in the
detected light intensity. Measurement of the rainfall by scintillation technique has
been well documented (e.g., Wang and Clifford, 1975; Wang et al., 1977; Wang and
Lawrence, 1977; Wang et al., 1979). However, it has not been until recently that a
small package such as the ORG-705 has been made commercially available. Early
scintillation devices used path lengths of 100 m to measure rainfall. Clearly, such a
device is not suited for operation at sea. The statistical average of the measured
scintillation signals give a measurement of instantaneous rainfall rates. The optical
gauge is intended to be insensitive to variation in the source intensity caused by
extreme temperature variations, IRED ageing, or dirt on the lenses.
The measurement range of the ORG-705 spans from 0.1 to 1000 mm/hr. The
time resolution is a 10 second exponential average, and the output is a 0 to 5 volt
analog signal that is proportional to the log of the rainfall rate
RR = 10(Vout
-C)
(1)
where RR is the rainfall rate in mm/hr, Vout is the output signal measured by the
rain gauge, and Cis a calibration constant, nominally equal to 0.65, which is
determined by the manufacturer. Power requirements are standard 115 VAC (0.75
4
Amp). Quoted accuracies are 1% from 10 to 100 mm/hr, 4% from 1 to 500 mm/hr,
and 10% from 0.1 to 1000 mm/hr.
2.2
Siphon Rain Gauge
Conventional tipping bucket rain gauges do not work well on moving platforms
where the total acceleration vector is not constant. Many conventional rain gauges
are also constructed from metals· susceptible to corrosion in the marine environment.
An effort was initiated at the National Data Buoy Center (NDBC) to develop a
precipitation gauge for extended use on a buoy (Michelena, 1989). It was decided
that a rain gauge on a moving platform should measure the volume of rainfall caught
rather than its weight (Michelena, 1989). The result is a commercially available rain
gauge manufactured by the R. M. Young Company
~hich
is based on the NDBC
design. Early prototype designs are first described by Holmes et al. (1981) and
Holmes and Michelena (1983).
A capacitance-type, water-level measuring transducer proved to be easiest to
implement (Michelena, 1989). Precipitation is collected by a funnel which leads to a
storage container. The water level in this rain accumulation container is sensed as a
capacitance by an electronic transducer to obtain an analog signal proportional to
the height of the fluid column. A stainless steel rod inside the collection tube is
covered with a teflon sheath that serves as the dielectric. The water mass that
surrounds the probe forms the outer "plate" of the coaxial-type capacitor, and the
central metallic rod is the inner "plate." As more rain is accumulated, the water
level in the rain accumulator tube rises, thereby increasing the total capacitance.
Additional electronics are used to measure the value of the total capacitance and
output an analog voltage that is directly proportional to the amount of precipitation
collected by the rain gauge.
5
Dumping of the accumulated rain occurs when the rain storage tube is full by a
self-initiating siphon that starts when the water level reaches the upper limit of the
container. This empties the gauge for a new rain collection cycle. Any rainfall
collected during the siphon dump is not recorded which may introduce an error in
total rainfall measured. The early prototype gauge (Holmes et al., 1981) dumped the
contents of its storage tube in approximately 4 minutes. A sustained rainfall rate of
50 mm/hr would introduce an underestimate of total rainfall by approximately
3 mm. Since then, improvements have been made on siphon dump time and our
study has shown that the dump time of the Young siphon rain gauge to be on the
order of 20 seconds. The mean time series of height versus time from dumps is shown
in Figure 2. The profile can be approximated by a straight line. An error of less than
0.1 mm would be introduced for a sustained rainfall rate of 50 mm/hr. This can be
seen in Figure 3. Even in extreme rainfall rates, the associated error is still minimal.
The NDBC prototype rain gauge has been found to operate reliably on the
NDBC ocean test platform for a period of 18 months (Holmes and Michelena, 1983).
This test platform is a 10m discus buoy located in the Gulf of Mexico. The
capacitance probe used has a stable calibration and is not affected by the
environmental temperatures at the test platform. The siphon dumping technique has
been quite successful and errors introduced by siphoning are minimal.
The Young rain gauge is fairly light weight ( 4 kg) and compact, measuring
14 em across at its widest (collection funnel) and is 65 em long. The quoted
resolution of the rain gauge is 1 mm of rainfall with an accuracy of ±2 mm. The
output signal ranges from 0 to 5 volts corresponding linearly to 0 to 50 mm of rain.
Circuit power requires 8-30 volts at less than 3 rnA (unregulated). The operating
temperature is 0 to 50 C, or -20 to 50 C with an optional 28 volt heater.
6
2.3
Plastic Rain Gauge
An inexpensive rain gauge constructed of clear butyrate plastic was also used to
measure rainfall. This simple gauge has a 280 nun capacity with a 0.2 mm resolution.
The catch area of the gauge is approximately 81 cm2 • Although this gauge is not
intended for use at sea, these data were manually recorded after rainfall events for
comparison against the total rainfall recorded by the optical and siphon gauges.
2.4
Tipping Bucket Rain Gauge
An 8" tipping bucket rain gauge from Climatronics Corporation was used as a
standard with which to compare the optical and siphon rain gauges. It is one of the
standard measuring devices for rainfall used by National Weather Service observing
stations.
Operation of a tipping bucket rain gauge is quite simple. Precipitation is
channeled into a hinged dual bucket which tips back and forth every 0.254 nun
(0.01 inch) of water collected. When the bucket tips, it activates a sealed reed switch
which sends a digital signal to the data acquisition system. Upon tipping the
accumulated water is drained from one side of the bucket and the opposing bucket is
then filled and tips back upon receipt of the next 0.254 mm of rainfall.
Rainfall rate is computed by dividing 0.254 mm of water by the difference in
time between successive tips. The total rainfall is simply the number of tips
multiplied by 0.254. An Alter-Type wind screen was placed around the tipping
bucket rain gauge to help minimize the loss in precipitation catch due to streamlining
effects of strong winds around the gauge orifice. The quoted accuracies of this device
are ±1% for rainfall rates up to 75 nun/hr and ±5% for rates up to 250 nun/hr.
It was found that the tipping bucket rain gauge grossly overestimates rainfall.
These errors were due to the force of the rain funneling into the bucket which causes
premature tipping and to adhesion of water droplets (with dirt) to the sides of the
7
bucket causing an imbalance in weight of the bucket. No useful data was recorded
from this sensor for this data set.
3
Tests
Rainfall data of the optical and siphon rain gauges are recorded on a NEC
APC-IV computer using a 12-bit Metrabyte analog-to-digital (A/D) board. J?ata are
sampled at once per second and averaged over 7.5 minute blocks. The sensors are
located on a roof top in Woods Hole near the W.H.O.I. docks. Each sensor is
approximately 5 m from each other and 1 to 2 m up from the surface of the roof.
Although these sensors have an open exposure to the sky, taller buildings are found
on either side. This may lead to anomalous rainfall in high wind conditions in the
wake of these taller superstructures.
Rainfall caught by the plastic rain gauge was manually recorded usually after
each rainfall event.
4
Data Analysis
After several weeks of data collection which yielded poor agreement between the
optical rain gauge and the others, the ORG-705 was returned to the factory for
recalibration. At the factory a breach in the sensor itself was found that caused the
sensor to underestimate rainfall. The sensor housing is sealed from the environment,
and that seal must be maintained in order for the sensor to work properly. Even the
slightest of humidity increases inside the sensor will cause erroneous values.
Following the return of the ORG-705, a total of 12 rain events were recorded.
They ranged from very light rains and drizzles to moderately heavy but short
showers. Total accumulations ranged from a few millimeters up to several tens of
centimeters.
8
Figures 4a-15a depict the rainfall rates as determined by the
optic~
raii,l gauge.
Figures 4b-15b depict the cumulative rainfalls of both the optical and siphon gauges.
In most instances, the siphon rain gauge measured slightly more rainfall than the
optical rainfall. The cumulative rainfall profiles generally had the same slopes.
Offsets existed during high rainfall episodes where the cumulative rainfall of the
siphon gauge jumped more than the optical gauge. Whether or not this can be
attributed to averaging of the signal remains to be seen. However, allowing for the
sudden offsets, the slopes of both sensors match well. A relatively short but strong
rainfall episode can be seen in Figure 14a which lasted about 6 hours. Figure 14b
shows excellent agreement of all three gauges.
A comparison of rainfall totals of the optical rain gauge against the siphon rain
gauge can b e seen in Figure 16. There is a slight bias towards higher totals from the
siphon gauge. The same may also be said of the optical gauge when compared against
the plastic rain gauge (Figure 17). However, the slope of the linear least-squares
fitted line is closer to unity. As found in earlier studies, the siphon and plastic gauge
totals show a nearly one-to-one correspondence but with a slight offset (Figure 18).
It should be noted that the measurements of rainfall from the plastic gauge are
also subject to errors. Such errors may be due to evaporation (Hamilton and
Andrews, 1953), splashing of excess water into the gauge (Ashmore, 1934), and wind
effects around the gauge (Alter, 1937).
5
Discussion
The test area was not the best suited for rainfall measurement. Nearby buildings
may lead to spatial variations in rainfall. Even so, the results are encouraging and
both the optical and siphon rain gauge sensors show much promise.
The optical and siphon rain gauges have both advantages and disadvantages.
The optical rain gauge has the inherent advantage to measure rainfall rate
9
instantaneously throughout a wide range. The rainfall rate can easily be integrated
to obtain cumulative and total rainfalls. However, the major power restrictions limit
the operation of this gauge to ships only. S.T.I. is currently designing a newer optical
rain gauge which is about 1/3 the size of the ORG-705 and can operate on the power
budget available on a buoy. The other main disadvantage is the limited accuracy. As
observed in this study, many of the rainfall rates were less than 1 mm/hr which places
the accuracy of measurement at 10%. The optical rain gauge seems best suited for
moderate rainfall rates even though it can measure extremely light rainfalls. Another
problem with the optical rain gauge is that it measures the vertical component of the
falling rain. Should any sea spray cross the light beam moving vertically, that signal
would be recorded as a rainfall. Any vertical motion, either upwards and downwards ,
is observed as a rainfall. This problem should not be a major factor if the optical
rain gauge is sufficient distance from the sea surface during rough weather.
The siphon rain gauge which was developed for use on a buoy is better suited to
measuring rainfall volume rather than rate. A tube can be fitted to the siphon spout
and led to a reservoir which can be used as a check against the rain measured by the
gauge. The capacitance sensor can pick up electrical noise. A capacitor has just been
put on this gauge to solve this problem. Another problem lies with the sampling
scheme of the gauge. Should the sampling rate be sufficiently long and the rainfall
rate be high, it is possible to run through a complete siphoning cycle. However, with
an adequate sampling interval, in this case 7 and 1/2 minutes, there is sufficient time
resolution to discern individual rainfall episodes.
The siphon rain gauge is relatively immune to salt water and moderate tilting
(R. Young, personal communication) . Manufacturing tests show that moderate tilts
as observed on a buoy have minimal effect on the output signal of the sensor. Also
salt water solutions have minimal effect on the signal. The effect of funnel catch area
10
on collection efficiency (Huff, 1955) will also be addressed by work being done at the
R. M. Young Company.
Further testing will continue with these rain gauges. Future plans include more
dock side testing of the siphon and optical rain gauges, acquisition of the new S.T.I.
low power optical rain gauge, and testing of the siphon rain gauge on a buoy.
Acknowledgements
The authors wish to express their appreciation for the help given by the staffs of
the R. M. Young Company and Scientific Technology Inc. The authors would also
like to thank Barbara Gaffron for her review and suggestions for this manuscript.
This work was funded by the National Science Foundation (Grant
OCE-8709614) as a World Ocean Circulation Experiment (WOCE) long-lead time
development activity.
11
References
Alter, J . C., 1937. Shielded storage precipitation gages. Monthly Weather Review,
65, 262-265.
Ashmore, S. E ., 1934. The splashing of rain. Quarterly Journal of the Royal
Meteorological Society, 60, 517-522.
Hamilton, E. L., and L. A. Andrews, 1953. Control of evaporation from rain gauges
by oil. Bulletin of the American Meteorological Society, 34, 202-204.
Holmes, J ., B. Case, and E. Michelena, 1981. Rain gauge for NOAA data buoys.
Marine Technology Society, IEE/MTS Symposium Oceans '81 , pp. 463-467.
Holmes, J ., and E. Michelena, 1983. Design and testing of a new rain gauge for
NDBC meteorological data buoys. American Meteorological Society Fifth
Symposium, Meteorological Observations and Instrumentation, April11- 15,
pp. 34-37.
Huff, F. A., 1955. Comparison between standard and small orifice rain gauges.
Transactions, American Geophysical Union, 36, 689-694.
Michelena, E., 1989. Precipitation measurements at NDBC. NOAA National Data
Buoy Center, Technical Bulletin, 15(1), 8 pp.
Skaar, J., 1955. On the measurement of precipitation at sea. Geofysiske
Publiksjoner, 19(6), 3-32.
Wang, T. 1., and S. F. Clifford, 1975. Use of rainfall-induced optical scintillations to
measure path-averaged rain parameters. Journal of the Optical Society of
America, 65, 927-937.
12
Wang, T. I. , and R. S. Lawrence, 1977. Measurement of rain parameters by optical
scintillation: Computer simulation of the correlation method. Applied Optics,
16, 3176-3179.
Wang, T. I., G. Lerfald, R. S. Lawrence, and S. F. Clifford, 1977. Measurement of
rain parameters by optical scintillation. Applied Optics, 16, 2236-2241.
Wang, T . I., K. B. Earnshaw, and R. S. Lawrence, 1979. Path averaged
measurements of rain rate and raindrop size distribution using a fast-response
optical sensor. Journal of Applied Meteorology, 18, 654-660.
World Meteorological Organization, 1962. Precipitation measurements at sea.
W.M.O. Technical Note No . 47, Geneva, Switzerland, 18 pp.
13
Appendix A
Manufacturers:
Climatronics Corporation
Airport International Plaza
140 Wilbur Place
P.O. Box 480
Bohemia, New York 11716
(516) 56 7-7300
100097 8" Tipping Bucket Rain Gauge
$550.00
Science Associates
P.O. Box 230
Princeton, New Jersey 08542
(609) 924-44 70
6331 Plastic Rain Gauge
$ 40.00
Scientific Technology Inc.
2 Research Place
Rockville, Maryland 20850
(301) 948-6070
ORG-705 Precipitation Intensity Sensor
R. M. Young Company
2801 Aero-Park Drive
Traverse City, Michigan 49684
(616) 946-3980
50505 Precipitation Gauge
Heater (optional)
$4500.00
$548.00
$206.00
14
Figure 1: Photos of (from top left corner, clockwise) R. M. Young self-siphoning rain
gauge, Scientific Technology Inc. ORG-705 optical rain gauge, Science
Associates plastic rain gauge, and Climatronics Corporation 8" tipping bucket
ram gauge.
15
50-----------------------------------------------.
R. M. Young Siphon
Rain Gauge 50202
(S/N 00203)
40
~20
c
·-0
a::::
10
Average Siphon
Dump Profile
0~--~~r-~~~~~~-r-r~~,-~~~~~~~~
0
5
10
Time
15
(sec)
Figure 2: Average siphoning time of R. M. Young siphon rain gauge.
16
20
25
10~------------------------------------------------~
1
"0
""-
w
a.
0.1
E
::J
0
0.01
c
0
..c
a.
0.001
·(()
Siphon Dump nme:
0 .0001
0.1
1
10
20 sec
1o..o
Rainfall Rate (mm/hrJ
Figure 3: Siphon dumping error of the R. M. Young siphon rain gauge as a function of
rainfall rate. This dumping error is the amount of rainfall that enters the gauge
and is lost during the siphon dump. This curve is for a siphon dump time of 20
seconds.
17
1000
100 :
..
a
Optical Rain Gouge
~
L...
.r:.
'-....
E
E
...._.,
'
10
Q)
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.
.
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0
0::::
1 -:
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0.1
95.5
~
j
I
I
96.0
. 97.5
L.a
96.5
97.0
1989 Julian Day
20----------------------------------------------,
- - Optical 13.4 mm
- - - Siphon 1 6 . 1 mm
ccccc Plastic 17.0 mm
~
E
E
b
c
r
'-"'15
,_---
r
0
'+-
·-c0 10
0::::
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>
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::J
(.)
04--T--r-~~--~-r~~~-T--r-~~--~-r~~
95.5
96.0
96.5
97.0
97.5
1989 Julian Day
Figure 4: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical, siphon and plastic rain gauges.
18
100~-----------------------------------------------.
a
Optical Rain Gauge
Q)
+-'
0
a:::
1
0
......
c
·0
a:::
0.1
~--~---r--~--~~~._~--~--~~~~~~--r-~
97.5
98.0
98.5
99.0
1989 Julian Day
20,------------------------------------------------.
b
- - Optical 12.4 mm
- - - Siphon 1 6.5 mm
...
-:-
---------
I
I
0
......
·-c0 10
a:::
Q)
>
+-'
0
5
:3
E
:3
(.)
o;---~--~---r---r--~~~--~--~--~~~~--r-~
97.5
98.0
98.5
99.0
1989 Julian Day
Figure 5: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical and siphon rain gauges.
19
100,-----------------------------------------------.
a
Optical Rain Gauge
~
'-
.r:.
"E
E
.....__,
10
...,Q)
0
a::::
1
0
"+-
c
0
a::::
0.1
99.00
99.25
99.50
99 .75
100.00
1989 Julian Day
2.0 ----------------------------------------------,
b
~
- - Optical 1.4 mm
Siphon 0.8 mm
E
--E
.....__,1.5
0
"+-
c
·~ 1.0
Q)
...,>
·-
~
v.--------
r
.J
,.. ... I
Oo.s
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r
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u
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99.00
99.25
99.50
99.75
100.00
1989 Julian Day
Figure 6: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical and siphon rain gauges.
20
100~-----------------------------------------------.
Optical Rain Gauge
a
1
0
..,._
·-c0
a::::
0.1
105.5
106.0
106.5
107.5
107.0
1989 Julian Day
60~------------------------------------------------.
- - Optical 51 .3 mm
- - - Siphon 56.1 mm
,..-- --
_...---.
-- ----
b
0~~--~~~~~--r-~~~~~--~~--~~--~~
105.5
106.0
106.5
107.0
107.5
1989 Julian Day
Figure 7: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical and siphon rain gauges.
21
100--------------------------------------------~
a
Optical Rain Gauge
0
'+-
·-c0
a::::
o. ~ ,-'9.5-.....---r---r--,-2olr-.-o...l.-i----,----,r--,-2•o-.sL...-..,.---r---r--,-t21 .o
1989 Julian Day
30--------------------------------------------~
b
23.3 mm
17.4 mm
=2o
0
'+-
·-c0 15
a::::
Q)
.>
+J
10
0
:J
E
:J
u
120.0
120.5
121 .0
1989 Julian Day
Figure 8: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical and siphon rain gauges.
22
100
a
.
~
~
..c
...........
.
E
E
........,
10 -:
Optical Rain Gouge
.
Q.)
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"+-
c
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122.0
A" .
v . f\ A
\~ .
I
122.5
123.0
1 989 Julian Day
15~------------------------------------------------,
. -.
=
0
b
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.I
/'
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I
10-
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c
0
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.
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·->0 5-.
+I
.
:l
E
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.
u
0
I
122.0
122.5
'
123.0
1 989 Julian Day
Figure 9: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical and siphon rain gauges.
23
100
~
L...
..c
....,
Cl)
0
a
Optical Rain Gauge
-
...........
E
E
...........
--
10
-
-
a:::
1~
0
~
·-c0
a:::
--
0 .1
125.5
~
I
126.0
~
I
126.5
_!
127.5
127.0
1989 Julian Day
8~------------------------------------------------------------------------------------------------------------------------~
b
---------- Optical 5 .8 mm
- - - Siphon 4.7 mm
cccco Plastic 6.0 mm
c
r
- - J\. _ ,.
,._ I
I
I
r------------------------1 I
I
I
I
r - - - - "'-..,f.. I
126.0
126.5
127.0
127.5
1989 Julian Day
Figure 10: Time series plot of (a) rainfall rate of optical rain gauge and (b ) cumulative
rainfalls of optical, siphon and plastic rain gauges.
24
100~----------------------------------------------~
a
Optical Rain Gauge
,.......
'-
..!:
'EE
10
"'-"'
1
0
'+-
·-c0
a:::
0.1
131
130
132
1 989 Julian Day
35~----------------------------------------~~--~
, -_a
,-....
E30
E
"'-"'
- - Optical 31 .0 mm
- - - S1phon 34.2 mm
cccoc Plastic 34.4 mm
b
25
0
'+-
c
20
0
a:::
15
Q)
>
·0 10
::J
E
5
::J
u
0~~--~~~-r--~-T--~-T--~-,--~~~~~~-r~
130
131
132
1989 Julian Day.
Figure 11: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical, siphon and plastic rain gauges.
25
100
~
L..
..c
.:
.
a
Optical Rain Gauge
.
...........
E
E
............
10
.
.
Cl>
......,
~
0
0:::
1
0
~
c
0
-:
..
.
0:::
0.1
1:51.75
A
/J
I
I
1:52.50
1:52.25
1:52.00
1989 Julian Day
~
35~--------------------------------------------~
b
• --Optical 24.7 mm
• - - - S1phon 30.6 mm
29.4 mm
E JO- ooooo Plastic
E :
............ .
25~.
0
_,.,---
--- -----0
J
I
.
~
:
0:::
•
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(.)
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132:25
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132.50
1989 Julian Day
Figure 12: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical, siphon and plastic rain gauges.
26
100
:
.
.
.
~
l....
..s::
............
E
E
10 -:
....,CL>
.
a
Optical Rain Gouge
~
.
0
0:::
1
0
"+-
~
-:
.
·-c0
.
0:::
0.1
144.0
'
MA
A
'
I
144.5
145.0
145.5
1989 Julian Day
4.0 ------------------------------------------------~
- - Optical 3.1 mm
b
- - - S1phon 2.1 mm
ooooo Plastic 3.0 mm
0
~2.5
c
·~2.0
..
rI
~ 1.5
§o.5
--
I
I
....,
·-
~ 1.0
:J
\
J
1
--..,
--
r-
.,------.,,-1 -'
J
r
u
0.0 4-_.:..-r--L-...,.--.,....-or---~-r----,r----,-~---r-~--1
144.0
144.5
145.0
145.5
1 989 Julian Day
Figure 13: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical, siphon and plastic rain gauges.
27
100~
.
......-..
L...
..c:
a
Optical Rain Gauge
.
...........
.
E
E
............
10-:
Q)
..,
.
0
0:::
1-
:
...
.
0
'+-
·-0c:
0:::
i
1
0.1
I
146.5
l
14e.o
147.5
147.0
.
148.S
1989 Julian Day
30 .
~
.
E 25-
E .
--20-.
--Optical 24.9 mm
- - - S1phon 24.6 mm
ccccc Plastic 25.8 mm
b
0
'(
............
I
I
I
0
I
'+-
·-c:0 15-..
a:::
Q)
..
.>
.., 10-.
-::l0
E
::l
(.)
.
.
5-
..
0
146.5
. . 147.0
I
~
~
I
147.5
14B.o
148.5
1989 Julian Day
Figure 14: Time series plot of (a) rainfall rate of optical rain gauge and (b ) cumulative
rainfalls of optical, siphon and plastic rain gauges.
28
100
~
..r:.
......_,
Optical Rain Gouge
.
.
.
~
"EE
:
a
10 ":
Q)
+J
0
a::
--
-
1.
0
'+-
·-c:0
.
a::
0.1
150
I
..151
I
'I
152
I
I
153
154
I
I
155
156
.
'
157
1989 Julian Day
.
.
-.
.
--Optical 5.4 mm
- - - S1phon 4.9 mm
ccccc Plastic 7.4 mm
be
.
.
~5 c: .
, ,... .., -
·-04 .
a:: ::
.
Q)3 .
·-> .
~2 -.
:::s .
E1
:::s -.
u .
0
c
I
+J
150
-.
c
...151
.
----'"'_ ... -~... .. \,
~,..~ - ' - '
•••• -r
--rTm
152
-r
153
•••
I ' '
154
--
I
- · .J
! •••
155
• •!
156
.'
157
1989 Julian Day
Figure 15: Time series plot of (a) rainfall rate of optical rain gauge and (b) cumulative
rainfalls of optical, siphon and plastic rain gauges.
29
60~------------------------------------~
Total Rqinfall (mm)
c
·-0 30
0:::
0
·-
0
~
a.
010
0
Y -
0.89x
+
0.77
a~~~~~~~~~~~~~~~~~~~~
0
10
30
20
40
50
Siphon Rain Gauge
Figure 16: Scatter plot of total rainfall of optical rain gauge versus siphon rain gauge with
linear least squares best fit.
30
60
40----------------------------------~
Total Rainfall (mm)
Q)
0"30
::J
0
(.!)
c
·-o20
0:::
-0
u
·....., 10
c..
0
Y -
0.90x -
0.29
o~~~~~~~~~~~~~~~-T-r~~
0
20
10
30
Plastic Rain Gauge
Figure 17: Scatter plot of total rainfall of optical rain gauge versus plastic rain gauge with
linear least squares best fit.
31
40
40~----------------------------------~
Total Rainfall (mm)
Q)
~30
0
(.!)
c
·-020
a::
c
0
..r::.
0.10
·Ul
Y -
1 .06x -
1 .86
o~~~~~~~~~~,-~~~~~~~~
0
20
10
30
Plastic Rain Gauge
Figure 18: Scatter plot of total rainfall of siphon rain gauge versus plastic rain gauge with
linear least squares best fit .
32
40
DOCUMENT LmRARY
July 5, 1989
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REPORT DOCUMENTATION
PAGE
2.
,1. REPORT NO.
WHOI-89-44
1
4. Title and Subtitle
7. Author(s)
October, 1989
6.
8. Performing Organization Rapt. No.
Gennaro H. Crescenti and Robert A. Weller
WHOI-89-44
9. Performing Organization Nama and Address
f
3. Recipient's Accession No.
5. Report Data
Improved Meteorological Measurements from Buoys and Ships (IMEl):
Preliminary Comparison of Precipitation Sensors.
(
IMET TR-89-01
10. Project/Task/Work Unit No.
The Woods Hole Oceanographic Institution
Woods Hole, Massachusetts 02543
11. Contract(C) or Grant(G) No.
(C)
(G)
OCE 87-09614
13. Type of Report & Period Covered
12. Sponsoring Organization Nama and Address
Technical Report
The National Science Foundation
14.
15. Supplementary Notes
This report should be cited as: Woods Hole Oceanog. Inst Tech. Rept., WHOI-89-44, IMET 1R-89-0l
16. Abstract (Umlt: 200 words)
Rainfall data obtained from an optical rain gauge and a capacitive siphon rain gauge are analyzed and discussed. These
sensors were developed for unattended use and are being considered for use at sea on ships and buoys.
..
17. Document Analysis
a. Descriptors
1. precipitation sensors
2. marine meteorological measurements
3. ship and buoy instrumentation
b. ldantlfiars/Opan-Endad Terms
•
1.
c. COSATI Field/Group
18. Availability Statement
Approved for publication; distribution unlimited.
(Sea ANSI-Z39.18)
19. Security Class (This Report)
UNCLASSIFIED
20. Security Class (This Page)
21. No. of Pages
32
22. Price
OPTIONAL FORM 272 (4-77)
(Formerly NTIS-35)
Depanment of Commerce