Raindrop Axis Ratio Research Articles

Data for "Revisiting the raindrop axis ratio based on 3D ellipsoidal reconstruction from 2DVD observations"

Data for "Revisiting the raindrop axis ratio based on 3D ellipsoidal reconstruction from 2DVD observations"

Data for "Revisiting the raindrop axis ratio based on 3D ellipsoidal reconstruction from 2DVD observations"

Data for "Revisiting the raindrop axis ratio based on 3D ellipsoidal reconstruction from 2DVD observations"

The Effect of Drop Shape, Sensing Volume and Raindrop Size Statistics to the Scattered Field on 300 GHz

Scattering and attenuation properties of rain using various drop models are calculated using Mie theory, the T-matrix method and numerical technique respectively. Ellipsoidal and oblate drops use the axis ratio of raindrops reported in literature. Scattering are presented both for small volumes containing a single drop and for large volumes with multiple drops in which the statistics of drop size distribution need to be taken into account. The angular dependence of scattering, absorption and scattering cross sections and polarization ratios are investigated. For single scattering the angular dependence and cross sections of nonspherical drops differed from those of a spherical drop. Differences between ellipsoid and oblate drops were minimal. Drop shape affected the polarization mostly at 40-140 degree detection angles. The averaged linear attenuation and rain induced cross channel signal were studied. Attenuation was close to the ITU-R.838 model and in the same order of magnitude as the attenuation due to atmospheric moisture. The cross channel signal was calculated as a function of rain rate and transmitter/receiver angle. Vertical polarization was observed to produce a higher cross channel signal than horizontal polarization. It was concluded that drop shape is not an important factor at 300 GHz due to small drops dominating the scattering signal. Overall, the results showed that in single scattering, the ellipsoid is a good approximation for raindrops and that for large volumes, a spherical drop approximation and a Mie solution may be sufficient at frequencies of 300 GHz and higher.

Measuring Hydrometeors with a Precipitation Microphysical Characteristics Sensor: Calibration and Field Measurements

Aiming at the simultaneous measurement of the size, shape, and fall velocity of precipitation particles in the natural environment, we present here a new ground-based precipitation microphysical characteristics sensor (PMCS) based on the particle imaging velocimetry technology. The PMCS can capture autocorrelated images of precipitation particles by double-exposure in one frame, by which the size, axis ratio, and fall velocity of precipitation particles can be calculated. The PMCS is calibrated by a series of glass balls with certain diameters under varying light conditions, and a self-adaptive threshold method is proposed. The shape, axis ratio, and fall velocity of raindrops were calculated and discussed based on the field measurement results of PMCS. The typical shape of large raindrop is an oblate ellipsoid, the axis ratio of raindrops decreases linearly with the diameter, the fall velocity of raindrops approaches its asymptote, and the above observed results are in good agreement with the empirical models; the synchronous observation of a PMCS, an OTT PARSIVEL disdrometer, and a rain gauge shows that the PMCS is able to measure the rain intensity, accumulated rainfall, and drop size distribution with high accuracy. These results have validated the performance of PMCS.

A Bayesian Approach for Integrated Raindrop Size Distribution (DSD) Retrieval on an X-Band Dual-Polarization Radar Network

AbstractA raindrop size distribution (DSD) retrieval method for a weather radar network consisting of several X-band dual-polarization radars is proposed. An iterative maximum likelihood (ML) estimator for DSD retrieval in a single radar was developed in the authors’ previous work, and the proposed algorithm in this paper extends the single-radar retrieval to radar-networked retrieval, where ML solutions in each single-radar node are integrated based on a Bayesian scheme in order to reduce estimation errors and to enhance accuracy. Statistical evaluations of the proposed algorithm were carried out using numerical simulations. The results with eight radar nodes showed that the bias and standard errors are −0.05 and 0.09 in log(Nw); and Nw (mm−1 m−3) and 0.04 and 0.09 in D0 (mm) in an environment with fluctuations in dual-polarization radar measurements (normal distributions with standard deviations of 0.8 dBZ, 0.2 dB, and 1.5° in ZHm, ZDRm, and ΦDPm, respectively). Further error analyses indicated that the estimation accuracy depended on the number of radar nodes, the ranges of varying μ, the raindrop axis ratio model, and the system bias errors in dual-polarization radar measurements.

Algorithm Development for the Optimum Rainfall Estimation Using Polarimetric Variables in Korea

In this study, to get an optimum rainfall estimation using polarimetric variables observed from Bislsan radar which is the first polarimetric radar in Korea, rainfall cases for 84 hours caused by different conditions, which are Changma front and typhoon, Changma front only, and typhoon only, occurred in 2011, were analyzed. And rainfall algorithms were developed by using long period drop size distributions with six different raindrop axis ratio relations. The combination of the relations betweenRandZ,ZDR,RandKDP,ZDR, andRandKDPwith different rainfall intensity would be an optimum rainfall algorithm if the reference of rainfall would be defined correctly. In the case the reference is not defined adequately, the relation betweenRandZ,ZDR,KDP,AHandRandZ,KDP,AHcan be used as a representative rainfall relation. Particularly if the qualifiedZDRis not available, the relation betweenRandZ,KDP,AHcan be used as an optimum rainfall relation in Korea.

이중편파레이더 시뮬레이터 개발을 위한 2차원 영상우적계 관측자료의 활용가능성 연구

The KMA has cooperated with the Oklahoma University in USA to develop a Polarimetric Radar Data (PRD) simulator to improve the microphysical processes in Korea Local Analysis and Prediction System (KLAPS), which is critical for the utilization of PRD into Numerical Weather Prediction (NWP) field. The simulator is like a tool to convert NWP data into PRD, so it enables us to compare NWP data with PRD directly. The simulator can simulate polarimetric radar variables such as reflectivity (Z), differential reflectivity (), specific differential phase (), and cross-correlation coefficient () with input of the Drop Size Distribution (DSD) and scattering calculation of the hydrometeors. However, the simulator is being developed based on the foreign observation data, therefore the PRD simulator development reflecting rainfall characteristics of Korea is needed. This study analyzed a potential application of the 2-Dimension Video Disdrometer (2DVD) data by calculating the raindrop axis ratio according to the rain-types to reflect Korea`s rainfall characteristics into scattering module in the simulator. The 2DVD instrument measures the precipitation DSD including the fall velocity and the shape of individual raindrops. We calculated raindrop axis ratio for stratiform, convective and mixed rainfall cases after checking the accuracy of 2DVD data, which usually represent the scattering characteristics of precipitation. The raindrop axis ratio obtained from 2DVD data are compared with those from foreign database in the simulator. The calculated the dual-polarimetric radar variables from the simulator using the obtained raindrop axis ratio are also compared with in situ dual-polarimetric observation data at Bislsan (BSL). 2DVD observation data show high accuracies in the range of 0.7~4.8% compared with in situ rain gauge data which represents 2DVD data are sufficient for the use to simulator. There are small differences of axis ratio in the diameter below 1~2 mm and above 4~5 mm, which are more obvious for bigger raindrops especially for a strong convective rainfall case. These differences of raindrop axis ratio between domestic and foreign rainfall data base suggest that the potential use of disdrometer observation can develop of a PRD simulated suitable to the Korea precipitation system.

Raindrop axis ratios, fall velocities and size distribution over Sumatra from 2D-Video Disdrometer measurement

Raindrop axis ratio, falling velocity and size distribution are important in broad list of applications. However, they are not frequently observed in the equatorial region. This paper elucidated the characteristics of raindrop axis ratio, falling velocity and size distribution based on 2D-Video Disdrometer (2DVD) data that have been collected in the equatorial Indonesia, particularly at Kototabang (hereafter called KT), west Sumatra, Indonesia (0.20°S, 100.32°E, 864m above sea level). A comprehensive follow-up of the previous study on the natural variability of raindrop size distributions (DSDs) is presented. Precipitation was classified through 1.3-GHz wind profiler observation. The dependence of raindrop falling velocity and axis ratio on rainfall type was not clearly observed. Overall, measured raindrop fall velocities were in good agreement with Gunn–Kinzer's data. Raindrop axis ratio at KT was more spherical than that of artificial rain and equilibrium model, and close to the values reported in the turbulent high shear zone of surface layer which can be partially due to the effect of the instrument errors (e.g., location and container shape). Of some natural variations of DSD investigated, the dependence of DSD on rainfall rate and rainfall type as well as diurnal variation was clearly visible. A striking contrast between the stratiform and convective rains is that the size distributions from the stratiform (convective) rains tend to narrow (broaden) with increasing rainfall rates. For rainfall rate R<10mm/h, the size distribution of stratiform was broader than that of convective. On the other hand, at higher rainfall rate more large-sized drops were found in convective rain. During the convective rain, very large-sized drops were found mainly at the very start of rain event while for the stratiform they were found to be associated with a strong bright band. In diurnal basis, the DSDs in the morning hours were narrower than those in the evening which was indicated by smaller Dm values in the morning hours than their counterparts in the evening. Rainfall type dependence and diurnal variation of DSD lead to significant variation of Z–R relations so that they must be considered to increase the accuracy of Z–R conversion from weather radar in this region. Consistent with the previous study, lack of seasonal DSD variability was also found in this work that would be due to significant local convective and orographic effect at this region throughout the year. However, Dm values in our result were larger than the typical orographic DSD.

Rainfall Estimation from C-Band Polarimetric Radar in Okinawa, Japan: Comparisons with 2D-Video Disdrometer and 400 MHz Wind Profiler

This paper presents an inter-comparison of rainfall parameters (median volume diameter and rain rate) using C-band polarimetric radar, a 2D-video disdrometer and a 400 MHz profiler for the Baiu front event of 8-9 June 2005 in Okinawa, Japan. These instruments are part of the Okinawa Sub-Tropical Environment Remote Sensing Center, operated by the National Institute of Information and Communications Technology (NICT). The 2D-video disdrometer is used to derive the mean axis ratio of raindrops versus drop diameter, as well as the drop size distribution for the Baiu event. The data are then used to simulate various relations between polarimetric scattering parameters such as: specific attenuation (Ah), and specific differential attenuation (Adp), versus specific differential phase (Kdp) which are required to correct the measured reflectivity at horizontal polarization (Zh), and the differential reflectivity (Zdr) for rain attenuation. The 2D-video disdrometer data are also used to arrive at retrieval formulas for median volume diameter (D0) from radar Zdr and rain rate from radar Kdp.The intense Baiu event of 8-9 June 2005 was composed of heavy convective rain cells embedded in large areas of stratiform rain. The inter-comparison of D0 and rain rate (R) between instruments was conducted for 12 hours (03:00-07:00, 11:00-19:00 UTC on 8th June 2004). The C-band radar retrievals were found to be in excellent agreement with the 2D-video disdrometer for the entire period. The 400 MHz profiler retrievals of D0 and R were in good agreement with 2D-video disdrometer during the more steady rain periods, with more scatter observed during the heavier convective rain periods. These inter-comparisons demonstrate the accuracy of C-band polarimetric radar to retrieve important rainfall parameters, as well as the accurate correction for rain attenuation using differential propagation phase.

Laboratory Measurements of Axis Ratios for Large Raindrops

The oscillations of moderate to large raindrops are investigated using a seven-story fall column with shape data obtained from multiple-strobe photographs. Measurements are made at a fall distance of 25 m for drops of D = 2.5-, 2.9-, 3.6-, and 4.0-mm diameter, with additional measurements at intermediate distances to assess the role of aerodynamic feedback as the source of drop oscillations. Oscillations, initiated by the drop generator, are found to decay during the first few meters of fall and then increase to where the drops attained terminal speed near 10 m. Throughout the lower half of the fall column, the oscillation amplitudes are essentially constant. These apparently steady-state oscillations are attributed to resonance with vortex shedding. For D = 2.5 and 3.6 mm, the mean axis ratio is near the theoretical equilibrium value, a result consistent with axisymmetric (oblate/prolate mode) oscillations at the fundamental frequency. For D = 2.9 and 4.0 mm, however, the mean axis ratio is larger than the theoretical equilibrium value by 0.01 to 0.03, a characteristic of transverse mode oscillations. Comparison with previous axis ratio and vortex-shedding measurements suggests that the oscillation modes of raindrops are sensitive to initial conditions, but because of the prevalence of off-center drop collisions, the predominant steady-state response in rain is expected to be transverse mode oscillations. A simple formula is obtained from laboratory and field measurements to account for the generally higher average axis ratio of raindrops having transverse mode oscillations. In the application to light to heavy rainfall, the ensemble mean axis ratios for raindrop sizes of D = 1.5–4.0 mm are shifted above equilibrium values by 0.01–0.04, as a result of steady-state transverse mode oscillations maintained intrinsically by vortex shedding. Compared to the previous axis ratio formula based on wind tunnel measurements, the increased axis ratios for oscillating raindrops amount to a reduction of 0.1–0.4 dB in radar differential reflectivity ZDR, and an increase of about 0.5 mm for a reflectivity-weighted mean drop size of less than about 3 mm.

A New Model for the Equilibrium Shape of Raindrops

Abstract The equilibrium shape of raindrops has been determined from Laplace's equation using an internal hydrostatic pressure with an external aerodynamic pressure based on measurements for a sphere but adjusted for the effect of distortion. The drop shape was calculated by integration from the upper pole with the initial curvature determined by iteration on the drop volume. The shape was closed at the lower pole by adjusting either the pressure drag or the drop weight to achieve an overall force balance. Model results provide bounds on the axis ratio of raindrops with an uncertainty of about 1% and very good agreement with extensive wind tunnel measurements for moderate to large water drops. The model yields the peculiar asymmetric shape of raindrops: a singly curved surface with a flattened base and a maximum curvature just below the major axis. A close match was found between model shapes and profiles obtained from photos of water drops for diameters up to 5 mm. Coefficients are provided for computing...

Microphysical Interpretation of Multi-Parameter Radar Measurements in Rain. Part I: Interpretation of Polarization Measurements and Estimation of Raindrop Shapes

The backscattered electric field received by a radar is related to the transmitted field by a backscatter matrix. In the case of single raindrops the elements in this matrix are determined by raindrop size and shape. At long radar wavelength certain combinations of elements, however, only depend on raindrop shape. These combinations form the basis of current linear and circular polarization measurables. An ensemble of raindrops will usually exhibit a variety of shapes because of the variation in raindrop size and because of oscillations induced by drop oscillations. This shape variation can be described by an axis ratio distribution. Since at long wavelengths in rain the radar polarization measurables are largely determined by shape, they are sensitive to this axis ratio distribution. In this study, the sensitivity of some linear and circular polarization measurables to the mean and variance of the raindrop axis ratio distribution is investigated. These measurables are then used to formulate estimators of radar-power-weighted mean axis ratio and variance. These estimators appear to be applicable to a wide variety of drop size distributions.