Comparison of the Compact Dopplar Radar Rain Gauge and Optical
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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] 199 J. Agric. Meteorol. 67 (3), 2011 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.