Comparison of the Compact Dopplar Radar Rain Gauge and Optical

Short Paper J. Agric. Meteorol. 67 (3): 199–204, 2011

Comparison of the compact dopplar radar rain gauge and optical disdrometer Ko NAKAYA†, and Yasushi TOYODA (Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko, Chiba, 270–1194, Japan)

Abstract The operations of a compact Doppler radar rain gauge (R2S; Rufft, FRG) and optical disdrometer (LPM; THIES, FRG) are based on raindrop size distribution (DSD) measurements. We checked the instrumental error of these sensors and compared each sensor with a reference tipping-bucket rain gauge. This is because both rain gauges can detect fine particles and so they can function as rain sensors. The R2S has a measuring bias of rainfall intensity when the drop size distribution differs from the assumed statistical DSD model. The instrumental error on the LPM is small; in fact, the LPM shows good agreement with the reference rain gauge. Where the atmospheric density differs remarkably from the standard elevation, as is the case in highland areas, the R2S requires calibration using a reference rain gauge. The resultant calibration coefficient of the R2S to convert the reading into a reference tip- ping-bucket rain-gauge equivalent was 0.51 in a forest at an elevation of 1380 m. Further gathering of calibration coefficients obtained at different elevations will improve the R2S’s applicability. Key words: Doppler radar rain gauge, Drop size distribution, Optical disdrometer, Tipping-bucket rain gauge.

Although the accuracy and suggested errors of rainfall 1. Introduction observations using typical Doppler radar have been Rainfall properties, such as the intensity, amount, reviewed in many studies (Maki et al., 1998, for duration, and type, constitute important meteorological example), reviews for the R2S compared to a reference information that is useful for agriculture and forestation rain gauge are few. For snowy rainfall, Inoue et al. assessments. A compact Doppler radar rain gauge (2009) compared different rain gauges along with the (R2S, G. Rufft Mess-und Regeltechnik GmbH, FRG) R2S. The optical disdrometer (LPM) measures the size and optical disdrometer (LPM, ADOLF THIES and descent velocity of raindrops passing through a thin GmbH＆Co. KG, FRG), which are based on raindrop infrared laser beam. Based on those measurements, the size distribution (DSD) measurements, have recently device estimates the rainfall intensity by integrating the become commercially available at reasonable prices. quantity of raindrops. Lanzinger et al. (2006) reported Rain gauges of these kinds can not only sense raindrops a tendency toward overestimation by the LPM in precisely; they can also distinguish the precipitation greater rainfall intensity with a comparison of an early type as rain, snow, drizzle, or hail. The expected production lot of the LPM and reference rain gauges, applications of these rain gauges include the assessment but subsequent results announced by the manufacturer of rainfall interception, leaf wetness, soil erosion, are unclear. As described in this paper, we intend to and weather disasters. Typical vertical Doppler radar determine the effects of rainfall properties or implied observation targets the DSD, velocity, and intensity errors on measurements made using these rain gauges. of the rain from hundreds of meters above ground, We further intend to present usage notes. and is applicable to climatological studies aimed at 2. Materials and Methods understanding the microphysical processes of rainfall.

Received; May 14, 2010. 2.1 Theory The Vertical Doppler radar operation measures Accepted; May 9, 2011. both the radar reflectivity factor (Z mm6 m–2) and the †Corresponding Author: [email protected]

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Doppler velocity (V ms–1) of drops via vertically emitted and speed. Therefore, the possible error factors are as reflected microwaves. The rainfall intensity (R mm follows: (1) discrepancy of the actual DSD from the hr–1) can be estimated from Z, under the assumption assumed value, (2) the effect of atmospheric density that the drop diameter (D mm) is related directly to change on the fall speed, and (3) the effect of an the fall speed (Atlas et al., 1973), because Z reflects updraft or downdraft on the fall speed. The detailed the DSD of the raindrops. In fact, Z is related to the measurement principle of R2S was not revealed by DSD as the manufacturer. The LPM measures the number,

3 6 diameter, and fall speed of precipitation particles as ZN= # D DD dD (1) 0 ^ h they fall through the light beam (infrared, 780 nm) where ND (D) is the number concentration per unit of the sensor. The rainfall intensity is estimated by volume, per unit size interval of the particle diameter integration of raindrop volumes. The LPM outputs the (m3 mm–1). The rainfall intensity (R) is written as sphere equivalent diameter of the raindrop (D) to avoid conversion errors from the projected diameter because 1 3 3 R = r# o t D NDD D dD (2) raindrops are oblate, depending on the fall speed, as 6 0 ^h ^ h a result of the air resistance. The instruction manual where ot (D) is the terminal fall velocity of raindrops for the device describes a “hamburger” shape that is (ms–1) of the diameter D. The well known statistical presumed for raindrops. Lanzinger et al. (2006) showed DSD model reported by Marshall and Palmer (1948) that the LPM tends to measure larger rainfall amounts is written with a power law function as than reference rain gauges, up to 19.2％ when 75 mm –1 - KD hr . A possible explanation is that two (or more) NDD = N0 e (3) ^ h coincident particles in the light sheet can appear as one –4 where N0＝0.08cm , K is the function of the rainfall large particle. Therefore, the possible factors causing intensity R (mm hr–1) as K＝41R–0.21 cm–1 . errors to the LPM are (1) the assumption of a sphere The relation of the fall speed to the raindrop diameter equivalent diameter and (2) higher rainfall intensity. is expressed using empirical exponential equations. An 2.2 Comparative measurements empirical equation by Atlas et al. (1973) that matches Two R2Ss (Fig. 1 A) were set up 6 m away to avoid an experimental study by Gunn and Kinzer (1949) has mutual interference in an observation field (12 m a.s.l.) been used practically. To apply the fall speed-diameter during April-July 2009, in Central Research Institute of relations to other elevations, the equations should be Electric Power Industry (CRIEPI), Abiko, Japan. Two 0.4 multiplied by the correction factor (ρ0/ρ) , where ρ LPMs (Fig. 1 B) were set up in the same field during is the air density at the elevation of observation and June-August 2009, (Fig. 2 A). The period during which

ρ0 is the air density at the standard ground level. The all sensors were operating together was 30 June-23 relations of the fall speed-diameter change according July. The R2S outputs a pulse signal corresponding to the type of precipitation, such as snow or hail. To to a given quantity as the resolution, similar to a reduce the estimation errors of R from Z, the updraft tipping-bucket rain gauge (T-B), and 1-min integrated correction is performed using the Doppler velocity, V, values were recorded. The measurement range of –1 which is expressed with the fall speed of raindrops vt the rainfall intensity by the R2S is 0.01 mm hr to –1 and updraft ou as 200mm hr . The serial output by the LPM contains the following information: DSD spectrum, rainfall V =ot + o u (4) intensity, precipitation type, and instrumental status. The updraft or downdraft can be predicted from These serial data were collected on micro SD cards the difference between the theoretical fall speed of every minute. The LPM measures rainfall intensity –1 –1 raindrops (ot) derived from Z and the Doppler velocity from 0.05 mm hr to 450 mm hr . We referred to the (V) of raindrops. The recently released compact Dop- rain gauge (0.5 mm resolution, 10 min integration) of pler radar rain gauge (R2S) is easy to operate and has AMeDAS at the same site for comparison. One R2S a fine resolution (0.01 mm, 1 s) compared with the con- and a LPM were set up on top of a scaffolding tower ventional tipping-bucket rain gauge (T-B). According (28 m a.g.l.) in a mountainous forest (1380 m a.s.l.) to the manual, the R2S estimates the rainfall intensity in Karuizawa (Nakaya et al., 2007). Their data were (R) by means of the correlation between raindrop size compared to those obtained using a reference T-B

200 K. Nakaya et al. : Comparison of radar rain gauge and optical disdrometer

(0.2 mm resolution, Rain-Collector; Davis Instruments Corp., US) set in the middle of the tower above the forest canopy (16.5 m a.g.l.). The measurement period was 18 June-30 July 2010. The layout of the sensors is shown in Fig. 2 B.

3. Results and Discussion

3.1 Observations in a lowland area 3.1.1 Instrumental errors of the R2S and LPM There were seven rainfall events greater than a total of 1 mm recorded during the period when all sensors were operating and the maximum was a total of 34 mm. Fig. 1. Appearance of (A) the small Doppler radar A rainfall period separated by a dry period of 6 hr or rain gauge (R2S) and (B) the optical disdrometer more was counted as 1 event. Examples of the time (LPM). series variation of rainfall (29-30 June, 2009; values by single sensors) are shown in Fig. 3: The reference T-B’s (0.5 mm resolution) data are shown. Gentle rain continued for about 12 hr and the total rainfall of this event was 10.5 mm. The average temperature and wind Side view R2S speed by AMeDAS were 20℃ and 1.2 ms–1 respectively A N LPM for the event. Both the R2S and LPM estimate rainfall T-B 3.0m 0.5m 0.9m duration more comparable to the T-B data because they

3.0m 1.0m 3.0m 3.0m can sense fine raindrops. Consequently, they are useful Rain gauges as rain sensors. The relative error of the regression

9.5 results of the two LPMs for the 10-min integrated 5.5 values of all events was 6.8％, as opposed to 19.4％ for 3.0 the two R2Ss. Although the two LPMs were set nearly contiguously, the two R2Ss were located 6 m apart. 0 20 40 m AMeDAS Trees H=12 m However, the variation of the 10 min integrated value

Top view B 1 0.5 1.0m

2 4 5 tipping-bucket ]

3 -1 1 28m T-B 0.3 m intervals N Tower floor 0.5 R2S 1.8m square LPM 16m 12 1 17m Rainfall [mm 10 min

1: Anemometers (28, 17 m) 8 2: Thermometers (28, 17 m) 3: T-B (17 m) 4: R2S (28 m) 4 5: LPM (28 m) LPM R2S

Flux tower Cumulative rainfall [mm] tipping-bucket Fig. 2. Layout of sensors. 22:00 00:00 02:00 04:00 06:00 08:00 10:00 12:00 Doppler radar rain gauge (R2S); Optical disdrometer Fig. 3. Time series example of a rainfall event for (LPM); Tipping-bucket rain gauge (T-B). (A) various rain gauges. 29 June 2009 23:00-30 June Observation at lowland area in CRIEPI. White 2009 11:00; total rainfall was 10.5 mm; values by boxes indicate buildings and value in the boxes are single sensors are shown. The average temperature height of buildings (m). (B) Observation at highland and wind speed by AMeDAS were 20℃ and 1.2 forest in Karuizawa. ms–1 respectively for the event.

201 J. Agric. Meteorol. 67 (3), 2011 of rainfall is not attributable to the temporal-spatial overestimative tendency of the LPM under heavy rain distribution of rainfall, but is likely to be a result of conditions was suggested (Lanzinger et al., 2006), but the instrumental error of the R2S. results of this analysis showed no such similar trend. 3.1.2 Comparison with a tipping-bucket rain In the case of the event shown in Fig. 3, an apparent gauge underestimation by the R2S was not shown because A comparison of the LPM and T-B revealed that the rainfall intensity was small and the rainfall for 10 min integrated rainfall amounts for all events (30 every 10 min was less than 1 mm. June-23 July, 2009; 220 samples) by the LPM was 3.1.3 Influence of the raindrop size distribution about 6％ below the T-B (Fig. 4A). In contrast, the On closer inspection of the conditions, the R2S R2S measurements were about 24％ below the T-B indicates smaller values than the reference rain gauge, value (Fig. 4B). The major error factors of the T-B the rainfall intensity values of 5-9 mm during 10 min are the rainwater loss of the T-B for heavy rains (mm 10 min–1) were observed during an almost identi- and the reduced capture rate of raindrops caused by cal event. The total rainfall of this event was 33.5 mm crosswinds (Jevons effect). However, the T-B can be during 2 hr. Therefore, as a result of an investigation regarded reasonably as the reference value because of the diameter distribution of a raindrop by the LPM, neither strong winds nor greater rainfall intensity are conducted simultaneously, the greatest rainfall intensity likely to have caused the observation errors observed is shown in Fig. 5A. This event turned out to have during the analysis period. The LPM estimates the extremely numerous large raindrops compared with the rainfall intensity through integration of individual DSD model by Marshall and Palmer (1948) (shown as raindrop quantities optically, meaning that the LPM MP48 in Fig. 5). When the actual number of raindrops requires fewer assumptions than the R2S does. An is greater than the assumed number, then the rainfall quantity is underestimated. Corresponding to this 10 explanation, an overestimative tendency of the rainfall

A 4 ] 8 10 -1 103 A

6 ] 102 -1 1

mm 10

3 MP48 4 y = ax +b 0 a [m 10 = 0.94 D

b N -1

Rainfall [mm 10 min = 0.00 2 10 LPM-1 10-2 LPM-2 0 10-3 0 2 4 6 8 10 0 2 4 6 8 10 10 104 B ] 8 103 B -1

] 102 -1 6 1

mm 10

3 MP48 0

[m 10

4 D y ax b

= + N -1 a = 0.76 10 b

Rainfall [mm 10 min = 0.02 2 10-2 R2S-1 -3 R2S-2 10 0 2 4 6 8 10 0 0 2 4 6 8 10 Diameter [mm] -1 Rainfall (T-B) [mm 10 min ] Fig. 5. Drop size distributions of 10 min integrated Fig. 4. Comparison of (A) LPM or (B) R2S to value for typical rainfall events. (A) 9.0 mm 10 min–1 the reference T-B. Ten-minute integrated rainfall 23 July 2009 7:30-7:40, (B) 0.5 mm 10 min–1 17 July amounts for all events are shown (30 June-23 July, 2009 8:10-8:20. Marshall and Palmer DSD model, 2009; 220 samples). equation (3), is shown with lines as MP48.

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was found when the number of raindrops was less than -1 20 the assumed value (Fig. 5B). The total rainfall of this A event was 3.0 mm during 3 hr. The actual DSD for 15 the individual rainfall events is prone to differ from 10 the DSD model that is derived statistically. The DSD 5 is critical for the estimation of the rainfall quantity y = 1.96x 2 because the volume of rainwater is proportional to the R = 0.85 0 third power of the raindrop radius. Although it remains Rainfall (R2S) [mm 10 min 0 5 10 15 20 -1 uncertain what model the R2S uses, we should consider Rainfall (T-B) [mm 10 min ] that a bias in measurement arises for individual rain ] events. The instrumental difference between the two -1 20 LPMs was small and the LPM showed good agree- B ment with the reference rain gauge because the LPM 15 measures the individual raindrops directly. 10 3.2 Observations in a highland area 5 3.2.1 Trends of measurements y = 0.96x R2 = 0.71 There were 25 rainfall events greater than a total 0 of 1 mm during the period when all sensors were Rainfall (LPM) [mm 10 min 0 5 10 15 20 -1 operating and the maximum total rainfall was 50.8 Rainfall (T-B) [mm 10 min ] mm. The maximum wind speed measured 1.4 m away –1 ] from the T-B was 7.5 ms and the average was 1.7 -1 20 ms–1. The atmospheric pressure at the tower base varied C from 849 to 871 hPa; the average was 860 hPa. The 15 rain data with wind direction from 170 to 225 degrees 10 were excluded from the analysis to avoid distortion by 5 the tower. The results of the R2S established in the y = 1.46x R2 = 0.62 highland area tended to exceed those obtained using 0 Rainfall (R2S) [mm 10 min the reference T-B and the LPM (Figs. 6A and 6C). 0 5 10 15 20 Rainfall (LPM) [mm 10 min-1] Furthermore, the LPM had an underestimative trend to the T-B same as the result of the lowland area (Fig. Fig. 6. Comparison of rain gauges of different types 4B). The reference T-B was mounted at 17 m above at the highland area. (A) T-B to R2S, (B) T-B to the ground over a forest canopy, so the reduction of LPM, and (C) LPM to R2S. Ten-minute integrated the capture rate by crosswind (Jevons effect) was an rainfall amounts for all events are shown (18 June-30 important concern. However, the difference between July, 2010; 868 samples). the T-B and other sensors showed no remarkable trends in either the wind speed or the wind direction (Fig. 7). overestimation. When the fall speed (o), as estimated We compared the reading of the reference rain gauge from the radar reflectivity factor (Z), is less than the with the other T-B operated by Nagano prefecture Doppler velocity (V), then apparently, the updraft (ou) (Mineno-chaya Sta.), set 350 m away. There was no in equation (4) can overshoot the actual value. If the remarkable discrepancy, though the rain gauge data presumed raindrop speed is lower than the actual value, obtained by Nagano pref. were 1.13 times greater than then the rainfall intensity will be overestimated. The data from the tower-mounted rain gauge. manufacturer states that the R2S does not consider 3.2.2 Measuring bias on R2S the influence of atmospheric density changes, but The diameter-fall speed relation is proportional to the that it incorporates the updraft influence of rain and ratio of the standard atmospheric density to the actual snow. Therefore, the overestimation is likely to be atmospheric density, as multiplied by the correction induced synergistically by increased drop speed and 0.4 factor (ρ0/ρ) . However, the correction factor was updraft correction influences. Moreover, measuring not more than 1.07 for the averaged atmospheric targets of the R2S are thought to differ from other pressure at this site; so it cannot account for the R2S instruments because the R2S senses raindrops several

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10 standard value remarkably, such as in a highland area, 8 A R2S the R2S is likely to overestimate the rainfall. This 6 is because of the multiple influences resulting from 4 the increased drop speed and updraft correction. The resultant calibration coefficient of the R2S to convert 2 Rel. error to T-B the reading into a reference tipping-bucket rain gauge 0 0 1 2 3 4 5 6 7 8 equivalent was 0.51 in the forest at an elevation of 10 1380 m. 8 B LPM Acknowledgements 6 4 We would like to thank Nagano prefecture for 2 permitting us to use the weather station data as a Rel. error to T-B 0 reference value. 0 1 2 3 4 5 6 7 8 Wind speed [m s-1] References Fig. 7. Influence of wind speed on relative errors Atlas, D., Srivastava, R. C., and Sekhon, R., 1973: Dop- of the rain gauges (A) R2S to T-B and (B) LPM pler radar characteristics of precipitation at vertical to T-B. Ten-minute integrated rainfall amounts for incidence. Rev. Geophys. Space Phys., 11, 1–35. all events are shown (18 June-30 July, 2010; 868 Gunn, R., and Kinzer, G. D., 1949: The terminal samples); Wind speed at tower top (28 m). velocity of fall for water droplets in stagnant air. J. Meteorol., 6, 243–248. meters overhead. For those reasons, the manufacturer Inoue, S., Hirota, T., Iwata, Y., Suzuki, K., and Nemoto, proposes to determine the correction coefficient (1.0 M., 2009: Comparison of four instruments for mea- is default) to convert the reading into a reference T-B suring solid precipitation below the freezing point equivalent. In this study, one correction coefficient condition. J. Agric. Meteorol., 65, 77–82. 1/1.96＝0.51 for (Fig. 6A) is shown for an elevation of Lanzinger, E., Theel, M., and Windolph, H., 2006 1380 m. However, it is noteworthy that the comparison Rainfall amount and intensity measured by the was held at the tower top (28 m a.g.l.) over the forest THIES laser precipitation monitor: In TECO-2006. canopy. It is preferable to perform calibrations at the http://www.wmo.int/pages/prog/www/IMOP/publica- ground level for a sufficient period to obtain the higher tions/IOM-94-TECO2006/3(3)_Lanzinger_Germany. rainfall intensity and various DSD. The gathering of pdf coefficients obtained at various elevations will improve Maki, M., Sasaki, Y., and Iwanami, K., 1998: Ac- the R2S’s applicability. curacy of precipitation parameters by vertically pointing doppler radar observations. Rep. Natl. 4. Conclusion Res. Inst. Earth Sci. Disast. Prev., 58, 149–168 The rain gauges of two types based on raindrop (in Japanese). distribution measurement, the Doppler radar rain Marshall, J. S., and Palmer, W. M., 1948: The dis- gauge (R2S) and the optical disdrometer (LPM), tribution of raindrops with size. J. Meteorol., 5, were mutually compared and were also compared 165–166. to a reference rain gauge. Both the rain gauges can Nakaya, K., Suzuki, C., Kobayashi, T., Ikeda, H., and detect fine particles and function as rain sensors. The Yasuike, S., 2007: Spatial averaging effect on local instrumental difference on the LPM was small and the flux measurement using a displaced-beam small ap- LPM showed good agreement with the reference rain erture scintillometer above the forest canopy. Agric. gauge. When the atmospheric density differs from the For. Meteorol., 145, 97–109.

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