The Ocean Acoustics Division (OAD) of the Atlantic Oceanographic and Meteorological Laboratory (AOML), in cooperation with NOAA/NESDIS and NASA, has deployed six rain gauges for calibration and intercomparison purposes. These instruments include: (1) a weighing rain gauge, (2) a RM Young Model 50202 capacitance rain gauge, (3) a ScTI ORG-705 (long path) optical rain gauge, (4) a ScTI ORG-105 (mini-ORG) optical rain gauge, (5) a Belfort Model 382 tipping bucket rain gauge, and (6) a Distromet RD-69 disdrometer. The system has been running continuously since July 1993. During this time period, roughly 150 events with maximum rainfall rate over 10 mm/hr and 25 events with maximum rainfall rates over 100 mm/hr have been recorded. All rain gauge types have performed well, with intercorrelations 0.9 or higher. However, limitations for each type of rain gauge have been observed

Nystuen, Jeffrey A.

English

NASA Technical Reports Server

-N95 18204
Inter-Comparison of Automatic Rain Gauges
Jeffrey A. Nystuen
Cooperative Institute for Marine and Atmospheric Studies
University of Miami, Miami, Florida
and
Ocean Acoustics Division
Atlantic Oceanographic and Meteorological Laboratory
Miami, Florida
The Ocean Acoustics Division (OAD) of the Atlantic Oceanographic and Meteorological
Laboratory (AOML), in cooperation with NOAA/NESDIS and NASA, has deployed six rain
gauges for calibration and inter-comparison purposes. These instruments include: 1) a weighing
rain gauge, 2) a RM Young Model 50202 Capacitance Rain Gauge, 3) a ScTI ORG-705 (long-
path) Optical Rain Gauge, 4) a ScTI ORG-105 (mini-ORG) Optical Rain Gauge, 5) a Belfort
Model 382 Tipping Bucket Rain Gauge, and 6) a Distromet RD-69 Disdrometer. The system has
been running continuously since July 1993. During this time period, roughly 150 events with
maximum rainfall rate over 10 mm/hr and 25 events with maximum rainfall rates over 100 mm/hr
have been recorded. All rain gauge types have performed well, with inter-correlations 0.9 or
higher. However, limitations for each type of rain gauge have been observed.
OAD is interested in determining the accuracy of the rainfall rate measurement over
relatively short time intervals. The optical rain gauges (ORGs) are designed to measure rainfall
rate directly via optical scintillation, however there is an Automatic Gain Control (AGC) with an
exponential time filter constant of 10 seconds built into the mini-ORGs. This filter limits the
temporal resolution of the mini-ORGs to roughly 20 seconds. The weighing and capacitance
gauges' measurement of rainfall rate is controlled by the flow rate (or dripping) of rainwater
from the catchment basin into the measurement reservoir for each intstrument. The nature of this
"dripping" controls the accuracy of the rainfall rate measurement. It was found that a smoothing
filter with a time constant of about one minute was necessary to remove noise associated with
the dripping. The one minute rainfall rate accuracy for the weighing and capacitance rain gauges
is about 1 mm/hr, although the capacitance rain gauge recorded isolated rainfall rate errors
associated with large "drips" of more than 10 mm/hr. The tipping bucket rain gauge has a
minimum one minute rainfall rate precision of_+ 12 mm/hr (one tip in one minute) and was not
used to study rainfall rate. The disdrometer has a built-in time resolution of one minute. To
facilitate inter-comparison of data, all rainfall rate data were processed to one minute time
resolution.
15
https://ntrs.nasa.gov/search.jsp?R=19950011789 2020-06-16T09:35:32+00:00Z
=One feature tmique to the ORGs was the background voltage level. The ORG rainfall rate
measurement is based on the scintillation of an optical NIR beam (2, = 0.85 _tm wavelength). In
fact, the total voltage variance measured is the sum due to rainfall, background turbulence, and
electronic noise. In the absence of rainfall, the voltage variance due to background turbulence
and electronic noise can be falsely interpreted as rainfall. By properly chosing a minimum
threshold, one should be able to avoid false data, however the upper limit of the equivalent
rainfall rate due to noise is variable. In January, this level exceeded 1 mm/hr on 3 occasions for
over 6 consecutive hours (Fig. 1). Dr. Wang (ScTI) suggested that this was due to dew collecting
on the receiver lens. Dew on the receiver lens would cause attenuation of the optical beam and
cause the AGC circuit to amplify the receiver's signal, amplifying the noise and resulting in a
high equivalent rainfall rate. Note that ScTI does not believe that a variable AGC level should
affect the rainfall rate measurement. This is an issue which should be examined more carefully.
=-. , , : =
Figure 2 shows a comparison of accumulation totals for 33 rain events during September
and October. The accumulation totals are highly correlated with the capacitance and tipping
bucket rain gauges biased slightly low relative to the weighing rain gauge and the ORGs biased
high by 10-20%. The tendency of the ORG s to kbias high is also evident in Figure 3. F{g. 3
shows a comparison of 1 minute rainfall rate estimates between the ORG-105 (mini-ORG) and
the weighing rain gauge during 5 convective events. Note the high con_elation coefficient (r =
0.98). The slope of the regression is 1.14 (14% bias high). For the ORG-705 (long-path), the
correlation coefficient was r = 0.97 and the regression slope was 1.23. For both instruments the
scatter about the mean regression was roughly _+20%. Figure 4 shows that the regression slopes
for several individual events (indicated by Julian date) are widely scattered about the September
mean value. This scatter is unrelated to the background voltages (AGC values?) of the ORGs
on the days of the rain events. This result suggests that after the ORGs are corrected for a mean
bias, for any given event, the rainfall estimate is still + 20%. Within a single event, this
statement still holds. Figure 5 shows rainfall rate estimates during Event 275. The capacitance,
weighing, disdrometer agree closely, while the short and long-path optical gauges show more
variance.
It has been suggested that variations in the drop size distribution are responsible for the
+ 20% disagreement between the ORGs and the other gauges. To investigate this possibility, the
disdrometer data from Event 275 was examined. During the first minutes of this event (see Fig.
5), the rainfall rate increased rapidly with many very large (over 3 mm diameter) raindrops
present (Minutes 2-8). During Minutes 8-20, very. heavy (convective) rainfall was present, which
was followed by low rainfall rates (stratiform rain) from Minutes 21-100. Figure 6 shows the
percentage rainfall rate error [(ORG-105 minus Weighing)/Weighing] of the mini-ORG compared
to the weighing rain gauge. During the initial minutes (Min 2-8) of the event, the error is + 50%.
During the heaviest rainfall rates (Min 8-20), the error is _+20%. During the light drizzle (Min
21-100), the relative error can be very high (+_ 300%), however the absolute error is relatively
small. The tendency of the ORGs to overestimate light rainfall rates is possibly due to the
background noise levels (the AGC issue). ScTI suggested that the optimal dynamic range of the
ORGs could be adjusted to provide better "low end" sensitivity.
16
I| I(
Using the disdrometer data, it is possible to calculate different moments of the drop size
distribution. These moments are given by:
2O
Mx = E DI_'dsd (D i)
i=i
where dsd(D_) are the disdrometer data in 20 drop size categories. Some of the moments are
proportional to physically significant quantities. For example, M0 is the number of drops per unit
volume, M 2 is proportional to the cross-sectional area of the rain (mm2/m3), M 3 = M is the liquid
water volume, M3. 6 is proportional to R, the rainfall rate, and M6 is reflectivity (radar). By
theory, the ORGs should correlate most highly to the moment associated with rainfall (x = 3.6)
(Wang, pers. comm.). Fig. 7 (ORG-105) and Fig. 8 (ORG-705) show the correlation between
the moments of the drop size distribution and the ORG rainfall rate estimate for six individual
rainfall events. The mini-ORG (Fig. 7) tends to correlate most highly to a lower moment of the
drop size distribution than rainfall, while the long-path ORG correlates most highly to a higher
moment. On an event by event basis, the ORG-105 correlates most highly to a lower moment
of the drop size distribution than the ORG-705. ScTI noted that the optical source used in the
ORG-105 (mini-ORG) is less coherent than for the ORG-705. A less coherent source would
imply a more diffuse shadow and thus a smaller signal per raindrop. Apparently the incoherent
optical source affects the signal from the larger raindrops more than that of the smaller raindrops.
It should be noted that the overall correlation levels in Figs. 7 and 8 are very high for all events
(r ~ 0.95).
17
1.4
Event 037 - Comparison of Optical and Capacitance Gauges
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Event Time (min after 0103)
i
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Figure i. A time series showing the temporal variation in the equivalent rainfall rate dueto background
turbulence and electronic no_ise The ORG-105 (short-path) is reading abov e 1 mm/hr. _ScTI suggests _-_
dew collecting on the receiver lens causes the AGC circuit to amplify the background noise levds. The
ORG-705 (long-path) does not show high levels. The ORG-705 has a heater on the lens to prevent dew
build up, The acoustic rainfall sensor indicated that no rain was present during this 400+ minute record.
18
-ilill
Comparison of Accumulation between Weighing, Capacitance, Tipping and Disdrometer
3O
25
E
E20
O
E
_15
10
0
cap
+ tip
I 1 I I0 ' 1
0 5 10 15 20 25 30
Accumulation of Weighing Rain Gauge (mm)
35
Figure 2a. A comparison of accumulation totals for 33 rainfall events in September and October 1993.
The regression slope between the weighing rain gauge and the disdrometer (disd, *), capacitance (cap, 0)
and tipping bucket (tip, +) rain gauges is 0.97.
19
_jutr _:
45
Comparison of Accumulation between Weighing and Optical Rain Gauges
I I t I I !
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E
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"_ 25
0
0
o 20
0
"515
E
0
5 0
0 I I I | I ,I
0 5 10 15 20 , 25 30
Accumulation of Weighing Rain Gauge (mm)
35
Figure 2b. A comparison of accumulation totals for 33 rainfall events in September and October 1993.
The regression slope betw-een the v_'eighing rain gauge and the ORG-105 (sp,0) is 1.12 and for the ORG-
705 (lp, +) is 1.19.
20
-q_ | I
One Minute Rainfall Rates
T i i i 1
160_
2O
•+ ". 4"
• +
+
+" 4-
O
+
X
×
I I I I
0 20 40 60 80 100 120 140
Disdrometer Rainfall Rate (mm/hr)
Figure 3. One minute rainfall rate estimates from the disdrometer and the ORG-105 optical rain gauges.
The first order regression (dash-dot line) is shown. Data from five events (Event 271, *; Event 272, o;
Event 275, x; Event 287, +; Event 289, *) are shox_n. Note the high correlation (r = 0.98) and the scatter
about the mean regression (+_ 20%).
21
z_
g.
Slope of First Order Regression with Weighing Rain Gauge
I I I l I
1.8
r--
Q.
c160 "
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(.9
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272287 289 +
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+ o 277
Sept +
032
+
+
275
I I l I I I
0.8 1 1.2 1.4 1.6 1.8
Slope of Regression for ORG Shorl-path
Figure 4. The slope of the first order regression bct_'een the weighing rain gauge and the ORG-105
(abscissa) and the ORG-705 (ordinate) for individual rain events. The events are identified by Julian date.
The event on JD 032 (1994) is a light rainfall event (maximum rainfall rate 5 ram/h0. The other events
arc heavy mint:all events _vith maximum rainfall rates near 100 mmPar.
2
Z2
Rainfall Rates during Event 275 @0909
l i , v , , ,
/
i'°°I "• " F" /
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Event Time (min) for Event 275@0909
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Event Time (min) for Event 275@0909
30 35
Figure 5. One minute rainfall rate estimates during Event 275. The upper panel shows that the rainfall
rate measurements from the disdrometer (solid line), the capacitance rain gauge (dashed line) and the
weighing rain gauge (dash-dot line) are in exccilcnt agreement. The lower panel shows the rainfall rate
measurements from the disdrometer (solid line), the ORG- !05 short-path (dash-dot line) and the ORG-705
long-path (dashed line). While the ORGs tend to overestimate rainfall rate relative to the other gauges,
however the)' are occasionally in agreement x_ith or underestimate the rainfall rate relative to the other
gauges.
23
3OO
Event 275 - Short Path Optical Raingauge Error (vs Weighing)
I Ib I I 1 I I I
25O
2OO
150
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100
o_
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Min 2:8
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0 I I I I I I I I
-10 0.5 1 1.5 2 2.5 3 3.5 4
Median Drop Diameter, Dzero (mm)
45
Figure 6. The percentage rainfall rate error bet_'een the ORG-105 (short-path) and the weighing rain
gauge. The abscissa shows the median drop size (by liquid water volume, D 0, calculated from the
disdrometer data. During minutes 2-8 (O symbol), the rainfall rate is increasing and the rain contains
relatively more very large raindrops (over 3 mm diameter). During minutes 8-20 (+ symbol), the rainfall
rate is very high (60 - 100 mm/hr). During the remainder of the event (minutes 21-100) (o symboi), the
rainfall rate is low (stratiform rain). No clear trend is evidelat in the percentage error values.
24
0.98
0.96
0.84
0.82
Correlations to Moments of the Drop Size Distribution - Short-path ORG-105
I I I I I
275
0.8 i
0 1
\
\
M R __ I
I I I
2 3 4 5
Moment of the Drop Size Distribution
6
Figure 7. Correlation between the moments of the drop size distribution and the ORG-105 (short-path)
rainfall rate estimate for 6 individual rain events. The rain events are identified by their Julian date. The
highest correlation tends to be at a moment of the distribution that is less than rainfall rate (R, the 3.6th
moment). Note that all of the correlation values are very high.
25
E
(1)
.o
0
0
E
0
L..
o
o
0.98
0.96
0.94
0.92
0.9
0.88
0.86
0.84
0.82
Correlations to Moments of the Drop Size Distribution - Long-path ORG-705
I I I I I
I
I
/
I
/
I
/
/
/
/ I
I
l
/
i
I
t
I / k
/ /
/ /
M R
0.8 i J I J
0 1 2 3 4 5
Moment of the Drop Size Distribution
F
6
Figure 8. Correlation bet'wccn the moments of the drop size distribution and the ORG-705 (long-path)
rainfall rate estimate for 6 individual rain events. The rain events are identified by their Julian date. The
highest correlation tends to be at a moment of the drop size distribution that is higher than rainfall (R is
the 3.6th moment). For each event the ORG-705 is most highly correlated to a higher moment of the drop
size distribution than the ORG-105 (Fig. 7). Note that all of the correlation values are very high.
26


Inter-comparison of automatic rain gauges

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